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SWUPDATE(1) Embedded Software Update Documentation SWUPDATE(1)


swupdate - SWUpdate Documentation [image]

SWUpdate provides a reliable way to update the software on an embedded system. Sources are hosted at


As Embedded Systems become more and more complex, their software reflects the augmented complexity. It is vital that the software on an embedded system can be updated in an absolutely reliable way, as new features and fixes are added.

On a Linux-based system, we can find in most cases the following elements:

  • the boot loader.
  • the kernel and the DT (Device Tree) file.
  • the root file system
  • other file systems, mounted at a later point
  • customer data, in raw format or on a file system
  • application specific software. For example, firmware to be downloaded on connected micro-controllers, and so on.

Generally speaking, in most cases it is required to update kernel and root file system, preserving user data - but cases vary.

In only a few cases it is required to update the boot loader, too. In fact, updating the boot loader is quite always risky, because a failure in the update breaks the board. Restoring a broken board is possible in some cases, but this is not left in most cases to the end user and the system must be sent back to the manufacturer.

There are a lot of different concepts about updating the software. I like to expose some of them, and then explain why I have implemented this project.

Updating through the boot loader

Boot loaders do much more as simply start the kernel. They have their own shell and can be managed using a processor's peripheral, in most cases a serial line. They are often script-able, letting possible to implement some kind of software update mechanism.

However, I found some drawbacks in this approach, that let me search for another solution, based on an application running on Linux:

Boot loaders have limited access to peripherals

Not all peripherals supported by the kernel are available with the boot loader. When it makes sense to add support to the kernel, because the peripheral is then available by the main application, it does not always make sense to duplicate the effort to port the driver to the boot loader.

Boot loader's drivers are not updated

Boot loader's drivers are mostly ported from the Linux kernel, but due to adaptations they are not later fixed or synchronized with the kernel, while bug fixes flow regularly in the Linux kernel. Some peripherals can then work in a not reliable ways, and fixing the issues can be not easy. Drivers in boot loaders are more or less a fork of the respective drivers in kernel.

As example, the UBI / UBIFS for NAND devices contains a lot of fixes in the kernel, that are not ported back to the boot loaders. The same can be found for the USB stack. The effort to support new peripherals or protocols is better to be used for the kernel as for the boot loaders.

Reduced file systems

The number of supported file systems is limited and porting a file system to the boot loader requires high effort.

Network support is limited

Network stack is limited, generally an update is possible via UDP but not via TCP.

Interaction with the operator

It is difficult to expose an interface to the operator, such as a GUI with a browser or on a display.

A complex logic can be easier implemented inside an application else in the boot loader. Extending the boot loader becomes complicated because the whole range of services and libraries are not available.

Boot loader's update advantages

However, this approach has some advantages, too:

software for update is generally simpler. - smaller footprint: a stand-alone application only for software management requires an own kernel and a root file system. Even if their size can be trimmed dropping what is not required for updating the software, their size is not negligible.

Updating through a package manager

All Linux distributions are updating with a package manager. Why is it not suitable for embedded ?

I cannot say it cannot be used, but there is an important drawback using this approach. Embedded systems are well tested with a specific software. Using a package manager can put weirdness because the software itself is not anymore atomic, but split into a long list of packages. How can we be assured that an application with library version x.y works, and also with different versions of the same library? How can it be successfully tested?

For a manufacturer, it is generally better to say that a new release of software (well tested by its test engineers) is released, and the new software (or firmware) is available for updating. Splitting in packages can generate nightmare and high effort for the testers.

The ease of replacing single files can speed up the development, but it is a software-versions nightmare at the customer site. If a customer report a bug, how can it is possible that software is "version 2.5" when a patch for some files were sent previously to the customer ?

An atomic update is generally a must feature for an embedded system.

Strategies for an application doing software upgrade

Instead of using the boot loader, an application can take into charge to upgrade the system. The application can use all services provided by the OS. The proposed solution is a stand-alone software, that follow customer rules and performs checks to determine if a software is installable, and then install the software on the desired storage.

The application can detect if the provided new software is suitable for the hardware, and it is can also check if the software is released by a verified authority. The range of features can grow from small system to a complex one, including the possibility to have pre- and post- install scripts, and so on.

Different strategies can be used, depending on the system's resources. I am listing some of them.

Double copy with fall-back

If there is enough space on the storage to save two copies of the whole software, it is possible to guarantee that there is always a working copy even if the software update is interrupted or a power off occurs.

Each copy must contain the kernel, the root file system, and each further component that can be updated. It is required a mechanism to identify which version is running.

SWUpdate should be inserted in the application software, and the application software will trigger it when an update is required. The duty of SWUpdate is to update the stand-by copy, leaving the running copy of the software untouched.

A synergy with the boot loader is often necessary, because the boot loader must decide which copy should be started. Again, it must be possible to switch between the two copies. After a reboot, the boot loader decides which copy should run. [image]

Check the chapter about boot loader to see which mechanisms can be implemented to guarantee that the target is not broken after an update.

The most evident drawback is the amount of required space. The available space for each copy is less than half the size of the storage. However, an update is always safe even in case of power off.

This project supports this strategy. The application as part of this project should be installed in the root file system and started or triggered as required. There is no need of an own kernel, because the two copies guarantees that it is always possible to upgrade the not running copy.

SWUpdate will set bootloader's variable to signal the that a new image is successfully installed.

Single copy - running as standalone image

The software upgrade application consists of kernel (maybe reduced dropping not required drivers) and a small root file system, with the application and its libraries. The whole size is much less than a single copy of the system software. Depending on set up, I get sizes from 2.5 until 8 MB for the stand-alone root file system. If the size is very important on small systems, it becomes negligible on systems with a lot of storage or big NANDs.

The system can be put in "upgrade" mode, simply signaling to the boot loader that the upgrading software must be started. The way can differ, for example setting a boot loader environment or using and external GPIO.

The boot loader starts "SWUpdate", booting the SWUpdate kernel and the initrd image as root file system. Because it runs in RAM, it is possible to upgrade the whole storage. Differently as in the double-copy strategy, the systems must reboot to put itself in update mode.

This concept consumes less space in storage as having two copies, but it does not guarantee a fall-back without updating again the software. However, it can be guaranteed that the system goes automatically in upgrade mode when the productivity software is not found or corrupted, as well as when the upgrade process is interrupted for some reason. [image]

In fact, it is possible to consider the upgrade procedure as a transaction, and only after the successful upgrade the new software is set as "boot-able". With these considerations, an upgrade with this strategy is safe: it is always guaranteed that the system boots and it is ready to get a new software, if the old one is corrupted or cannot run. With U-Boot as boot loader, SWUpdate is able to manage U-Boot's environment setting variables to indicate the start and the end of a transaction and that the storage contains a valid software. A similar feature for GRUB environment block modification as well as for EFI Boot Guard has been introduced.

SWUpdate is mainly used in this configuration. The recipes for Yocto generate an initrd image containing the SWUpdate application, that is automatically started after mounting the root file system. [image]

Something went wrong ?

Many things can go wrong, and it must be guaranteed that the system is able to run again and maybe able to reload a new software to fix a damaged image. SWUpdate works together with the boot loader to identify the possible causes of failures. Currently U-Boot, GRUB, and EFI Boot Guard are supported.

We can at least group some of the common causes:

  • damage / corrupted image during installing. SWUpdate is able to recognize it and the update process is interrupted. The old software is preserved and nothing is really copied into the target's storage.
  • corrupted image in the storage (flash)
  • remote update interrupted due to communication problem.
  • power-failure

SWUpdate works as transaction process. The boot loader environment variable "recovery_status" is set to signal the update's status to the boot loader. Of course, further variables can be added to fine tuning and report error causes. recovery_status can have the values "progress", "failed", or it can be unset.

When SWUpdate starts, it sets recovery_status to "progress". After an update is finished with success, the variable is erased. If the update ends with an error, recovery_status has the value "failed".

When an update is interrupted, independently from the cause, the boot loader recognizes it because the recovery_status variable is in "progress" or "failed". The boot loader can then start again SWUpdate to load again the software (single-copy case) or run the old copy of the application (double-copy case).

Power Failure

If a power off occurs, it must be guaranteed that the system is able to work again - starting again SWUpdate or restoring an old copy of the software.

Generally, the behavior can be split according to the chosen scenario:

  • single copy: SWUpdate is interrupted and the update transaction did not end with a success. The boot loader is able to start SWUpdate again, having the possibility to update the software again.
  • double copy: SWUpdate did not switch between stand-by and current copy. The same version of software, that was not touched by the update, is started again.

To be completely safe, SWUpdate and the bootloader need to exchange some information. The bootloader must detect if an update was interrupted due to a power-off, and restart SWUpdate until an update is successful. SWUpdate supports the U-Boot, GRUB, and EFI Boot Guard bootloaders. U-Boot and EFI Boot Guard have a power-safe environment which SWUpdate is able to read and change in order to communicate with them. In case of GRUB, a fixed 1024-byte environment block file is used instead. SWUpdate sets a variable as flag when it starts to update the system and resets the same variable after completion. The bootloader can read this flag to check if an update was running before a power-off. [image]

What about upgrading SWUpdate itself ?

SWUpdate is thought to be used in the whole development process, replacing customized process to update the software during the development. Before going into production, SWUpdate is well tested for a project.

If SWUpdate itself should be updated, the update cannot be safe if there is only one copy of SWUpdate in the storage. Safe update can be guaranteed only if SWUpdate is duplicated.

There are some ways to circumvent this issue if SWUpdate is part of the upgraded image:

  • have two copies of SWUpdate
  • take the risk, but have a rescue procedure using the boot loader.

What about upgrading the Boot loader ?

Updating the boot loader is in most cases a one-way process. On most SOCs, there is no possibility to have multiple copies of the boot loader, and when boot loader is broken, the board does not simply boot.

Some SOCs allow one to have multiple copies of the boot loader. But again, there is no general solution for this because it is very hardware specific.

In my experience, most targets do not allow one to update the boot loader. It is very uncommon that the boot loader must be updated when the product is ready for production.

It is different if the U-Boot environment must be updated, that is a common practice. U-Boot provides a double copy of the whole environment, and updating the environment from SWUpdate is power-off safe. Other boot loaders can or cannot have this feature.


SWUpdate is Free Software. It is copyrighted by Stefano Babic and many others who contributed code (see the actual source code and the git commit messages for details). You can redistribute SWUpdate and/or modify it under the terms of version 2 of the GNU General Public License as published by the Free Software Foundation. Some files can also be distributed, at your option, under any later version of the GNU General Public License -- see individual files for exceptions.

To make this easier, license headers in the source files will be replaced with a single line reference to Unique License Identifiers as defined by the Linux Foundation's SPDX project [1]. For example, in a source file the full "GPL v2.0 only" header text will be replaced by a single line:

SPDX-License-Identifier:        GPL-2.0-only

Ideally, the license terms of all files in the source tree should be defined by such License Identifiers; in no case a file can contain more than one such License Identifier list.

If a "SPDX-License-Identifier:" line references more than one Unique License Identifier, then this means that the respective file can be used under the terms of either of these licenses, i. e. with

SPDX-License-Identifier:        GPL-2.0-only OR BSD-3-Clause

you can choose between GPL-2.0-only and BSD-3-Clause licensing.

We use the SPDX Unique License Identifiers (SPDX-Identifiers)


Full name SPDX Identifier OSI Approved
GNU General Public License v2.0 only GPL-2.0-only Y
GNU Lesser General Public License v2.1 or later LGPL-2.1-or-later Y
BSD 1-Clause License BSD-1-Clause Y
BSD 2-Clause License BSD-2-Clause Y
BSD 3-Clause "New" or "Revised" License BSD-3-Clause Y
MIT License MIT Y
Creative Commons Zero 1.0 Universal (CC0) CC0-1.0 N
Creative Commons Attribution Share Alike 4.0 CC-BY-SA-4.0 Y
ISC License (ISC) ISC Y



This project is thought to help to update an embedded system from a storage media or from network. However, it should be mainly considered as a framework, where further protocols or installers (in SWUpdate they are called handlers) can be easily added to the application.

One use case is to update from an external local media, as USB-Pen or SD-Card. In this case, the update is done without any intervention by an operator: it is thought as "one-key-update", and the software is started at reset simply pressing a key (or in any way that can be recognized by the target), making all checks automatically. At the end, the updating process reports only the status to the operator (successful or failed).

The output can be displayed on a LCD using the frame-buffer device or directed to a serial line (Linux console).

It is generally used in the single copy approach, running in an initrd (recipes are provided to generate with Yocto). However, it is possible to use it in a double-copy approach by use of collections.

If started for a remote update, SWUpdate starts an embedded Web-server and waits for requests. The operator must upload a suitable image, that SWUpdate checks and then install. All output is notified to the operator's browser via AJAX notifications.


General Overview

  • Install on embedded Media (eMMC, SD, Raw NAND, NOR and SPI-NOR flashes)
  • check if an image is available. The image is built in a specified format (cpio) and it must contain a file describing the software that must be updated.
  • SWUpdate is thought to update UBI volumes (mainly for NAND, but not only) and images on devices. Passing a whole image can still be updated as a partition on the SD card, or a MTD partition.
  • new partition schema. This is bound with UBI volume. SWUpdate can recreate UBI volumes, resizing them and copying the new software.
  • support for compressed images, using the zlib and zstd library. tarball (tgz file) are supported.
  • support for partitioned USB-pen or unpartitioned (mainly used by Windows).
  • support for updating a single file inside a filesystem. The filesystem where to put the file must be described.
  • checksum for the single components of an image
  • use a structured language to describe the image. This is done using the libconfig library as default parser, that uses a JSON-like description.
  • use custom's choice for the description of the image. It is possible to write an own parser using the Lua language. An example using a XML description in Lua is provided in the examples directory.
  • Support for setting / erasing U-Boot variables
  • Support for setting / erasing GRUB environment block variables
  • Support for setting / erasing EFI Boot Guard variables
  • Support for pre and post update commands run before the update starts processing data and after the update has finished successfully.
  • Support for lua hooks, executed before any handler runs.
  • Support for preinstall scripts. They run after streamed handlers have handled their data, and before regular handlers.
  • Support for postinstall scripts. They run after updating the images.
  • Network installer using an embedded Web-server (Mongoose Server was chosen, in the version under Lua license). A different Web-server can be used.
local Storage: USB, SD, UART,..

  • integrated Web-Server
  • pulling from remote Server (HTTP, HTTPS, ..)
  • using a Backend. SWUpdate is open to talk with back end servers for rolling out software updates. Current version supports the hawkBit server, but other backend can be added.

  • Can be configured to check for compatibility between software and hardware revisions. The software image must contain an entry declaring on which HW revision the software is allowed to run. SWUpdate refuses to install if the compatibility is not verified.
  • support for image extraction. A manufacturer can require to have a single image that contains the software for more as one device. This simplifies the manufacturer's management and reduces their administrative costs having a single software product. SWUpdate receives the software as stream without temporary storing, and extracts only the required components for the device to be installed.
  • allow custom handlers for installing FPGA firmware, micro-controller firmware via custom protocols.
  • Features are enabled / disabled using "make menuconfig". (Kbuild is inherited from busybox project)
  • Images are authenticated and verified before installing
  • Power-Off safe

Single image delivery

The main concept is that the manufacturer delivers a single big image. All single images are packed together (cpio was chosen for its simplicity and because can be streamed) together with an additional file (sw-description), that contains meta information about each single image.

The format of sw-description can be customized: SWUpdate can be configured to use its internal parser (based on libconfig), or calling an external parser in Lua. [image]

Changing the rules to accept images with an external parser, let to extend to new image types and how they are installed. In fact, the scope of the parser is to retrieve which single images must be installed and how. SWUpdate implements "handlers" to install a single image: there are handlers to install images into UBI volumes, or to a SD card, a CFI Flash, and so on. It is then easy to add an own handler if a very special installer is required.

For example we can think at a project with a main processor and one or several micro-controllers. Let's say for simplicity that the main processor communicates with the micro-controllers via UARTS using a proprietary protocol. The software on the micro-controllers can be updated using the proprietary protocol.

It is possible to extend SWUpdate writing a handler, that implements the part of the proprietary protocol to perform the upgrade on the micro-controller. The parser must recognize which image must be installed with the new handler, and SWUpdate will call the handler during the installation process.

Streaming feature

SWUpdate is thought to be able to stream the received image directly into the target, without any temporary copy. In fact, the single installer (handler) receive as input the file descriptor set at the beginning of the image that must be installed.

The feature can be set on image basis, that means that a user can decide which partial images should be streamed. If not streamed (see installed-directly flag), files are temporary extracted into the directory pointed to by the environment variable TMPDIR with /tmp as fall-back if TMPDIR is not set. Of course, by streaming it is not possible to make checks on the whole delivered software before installing. The temporary copy is done only when updated from network. When the image is stored on an external storage, there is no need of that copy.

Images fully streamed

In case of remote update, SWUpdate extracts relevant images from the stream and copies them into the directory pointed to by the environment variable TMPDIR (if unset, to /tmp) before calling the handlers. This guarantee that an update is initiated only if all parts are present and correct. However, on some systems with less resources, the amount of RAM to copy the images could be not enough, for example if the filesystem on an attached SD Card must be updated. In this case, it will help if the images are installed directly as stream by the corresponding handler, without temporary copies. Not all handlers support to stream directly into the target. Streaming with zero-copy is enabled by setting the flag "installed-directly" in the description of the single image.

Configuration and build


There are only a few libraries that are required to compile SWUpdate.

  • mtd-utils: internally, mtd-utils generates libmtd and libubi. They are commonly not exported and not installed, but they are linked by SWUpdate to reuse the same functions for upgrading MTD and UBI volumes.
  • openssl / wolfssl / mbedtls (optional) for cryptographic operations
  • p11-kit & wolfssl (optional) for PKCS#11 support
  • Lua: liblua and the development headers.
  • libz is always linked.
  • libconfig (optional) for the default parser
  • libarchive (optional) for archive handler
  • librsync (optional) for support to apply rdiff patches
  • libjson (optional) for JSON parser and hawkBit
  • libubootenv (optional) if support for U-Boot is enabled
  • libebgenv (optional) if support for EFI Boot Guard is enabled
  • libcurl used to communicate with network

New handlers can add some other libraries to the requirement list - check if you need all handlers in case you get build errors, and drop what you do not need.

Building with Yocto

See corresponding chapter how to build in Yocto.

Configuring SWUpdate

SWUpdate is configurable via "make menuconfig". The small footprint is reached using the internal parser and disabling the web server. Any option has a small help describing its usage. In the default configuration, many options are already activated.

To configure the options:

make menuconfig


  • to cross-compile, set the CC and CXX variables before running make. It is also possible to set the cross-compiler prefix as option with make menuconfig.
  • generate the code


The result is the binary "swupdate". A second binary "progress" is built, but it is not strictly required. It is an example how to build your own interface to SWUpdate to show a progress bar or whatever you want on your HMI. The example simply prints on the console the current status of the update.

In the Yocto buildsystem,:

bitbake swupdate

This will build the package

bitbake swupdate-image

This builds a rescue image. The result is a Ramdisk that can be loaded directly by the bootloader. To use SWUpdate in the double-copy mode, put the package swupdate into your rootfs. Check your image recipe, and simply add it to the list of the installed packages.

For example, if we want to add it to the standard "core-image-full-cmdline" image, we can add a recipes-extended/images/core-image-full-cmdline.bbappend


swupdate \
swupdate-www \

swupdate-www is the package with the website, that you can customize with your own logo, template ans style.

Building a debian package

SWUpdate is thought for Embedded Systems and building in an embedded distribution is the first use case. But apart the most used buildsystems for embedded as Yocto or Buildroot, in some cases a standard Linux distro is used. Not only, a distro package allows one to run SWUpdate on Linux PC for test purposes without having to fight with dependencies. Using the debhelper tools, it is possible to generate a debian package.

Steps for building a debian package

./debian/rules clean
./debian/rules build
fakeroot debian/rules binary

The result is a "deb" package stored in the parent directory.

Alternative way signing source package

You can use dpkg-buildpackage:

dpkg-buildpackage -us -uc
debsign -k <keyId>

Running SWUpdate

What is expected from a SWUpdate run

The whole update process can be seen as a set of pipelines. The incoming stream (the SWU file) is processed by each pipe and passed to the next step. First, the SWU is streamed from one of the interfaces : local (USB, filesystem), Webserver, suricatta (one of the backend), etc. The incoming SWU is forwarded to the installer to be examined and installed. A run of SWUpdate consists mainly of the following steps:

  • extracts sw-description from the stream and verifies it It parses sw-description creating a raw description in RAM about the activities that must be performed.
  • if Signed Images is activated, extracts sw-description.sig and validate sw-description.
  • check for hardware-software compatibility, if any, reading hardware revision from hardware and matching with the table in sw-description.
  • Parse sw-description to determine which artefacts in the incoming SWU are required. Not required artifacts are simply skipped. If an "embedded-script" is defined, it is executed at this point before parsing files. If "hooks" are defined, they are executed as each file is parsed, even if they will be skipped. At the end of the parsing, SWUpdate builds an internal mapping for each artifact to recognize which handler should be called for each of them.
  • runs the pre update command, if set
  • runs partition handlers, if required.
  • execute handlers for each file marked as "installed-directly". checksum is checked while the data is streamed to handler, and copy will be marked as having failed if checksum was not correct failing the rest of the install.
  • copy other files to a temporary location while checking checksums, stopping if there was a mismatch.

  • iterates through all scripts and call the corresponding handler for pre-install scripts. Please note: if artifacts are streamed, they will be extracted before this runs. If earlier execution is required, please use the "embedded-script" or hooks features to ensure code is run before installation takes place.
  • iterates through all images and call the corresponding handler for installing on target.
  • iterates through all files and call the corresponding handler for installing on target.
  • iterates through all scripts and call the corresponding handler for post-install scripts
  • iterates through all bootenv and updates the bootloader environment.
  • reports the status to the operator through the notification interface (logging, traces) and through the progress interface.
  • runs the post update command, if set.

The first step that fails, stops the entire procedure and an error is reported.

To start SWUpdate expecting the image from a file:

swupdate -i <filename>

To start with the embedded web server:

swupdate -w "<web server options>"

The main important parameters for the web server are "document-root" and "port".

swupdate -w "--document-root ./www --port 8080"

The embedded web server is taken from the Mongoose project.

The list of available options (depending on activated features) is shown with:

swupdate -h

This uses as website the pages delivered with the code. Of course, they can be customized and replaced. The website uses AJAX to communicate with SWUpdate, and to show the progress of the update to the operator.

The default port of the Web-server is 8080. You can then connect to the target with:


If it works, the start page should be displayed as in next figure. [image]

If a correct image is downloaded, SWUpdate starts to process the received image. All notifications are sent back to the browser. SWUpdate provides a mechanism to send to a receiver the progress of the installation. In fact, SWUpdate takes a list of objects that registers itself with the application and they will be informed any time the application calls the notify() function. This allows also for self-written handlers to inform the upper layers about error conditions or simply return the status. It is then simply to add own receivers to implement customized way to display the results: displaying on a LCD (if the target has one), or sending back to another device via network. An example of the notifications sent back to the browser is in the next figure: [image]

Software collections can be specified by passing --select command line option. Assuming sw-description file contains a collection named stable, with alt installation location, SWUpdate can be called like this:

swupdate --select stable,alt

Command line parameters

Parameter Type Description
-f <file> string SWUpdate configuration file to use. See examples/configuration/swupdate.cfg in the source code for details.
-b <string> string Available if CONFIG_UBIATTACH is set. It allows one to blacklist MTDs when SWUpdate searches for UBI volumes. Example: U-Boot and environment in MTD0-1: swupdate -b "0 1".
-B <loader> string Override the default bootloader interface to use loader instead.
-e <sel> string sel is in the format <software>,<mode>. It allows one to find a subset of rules in the sw-description file. With it, multiple rules are allowed. One common usage is in case of the dual copy approach. Example: -e "stable, copy1" ==> install on copy1 -e "stable, copy2" ==> install on copy2
0.0 --excluded <sel> 168u string sel is in the format <software>,<mode>. It sets a blacklist of selections that cannot be used for an update. Selections can be activated not only with -e, but also via IPC. Multiple --excluded are allowed
-h 0.0 • 2 168u Run usage with help.
-k <file> string Available if CONFIG_SIGNED is set. Filename with the public key.
-K <file> string Available on CONFIG_ENCRYPTED_IMAGES set. Filename with the symmetric key to be used for decryption.
--cert-purpose <purpose> string Available if CONFIG_SIGNED_IMAGES is set. Set expected certificate purpose.
--forced-signer-name <cn> string Available if CONFIG_SIGNED_IMAGES is set. Set expected common name of signer certificate.
--ca-path <file> string Available if CONFIG_SIGNED_IMAGES is set. Path to the Certificate Authority (PEM).
--get-root Detect and print the root device and exit
-l <level> int Set loglevel.
-L 0.0 • 2 168u Send LOG output to syslog (local).
-i <file> string Run SWUpdate with a local .swu file.
-n 0.0 • 2 168u Run SWUpdate in dry-run mode.
-N <version> string The minimum required version of software. This will be checked with the version of new software and forbids downgrading. Version consists of either 4 numbers ( with each field in the range 0..65535) or it is a semantic version.
0.0 -m ax-version <version> 168u string The maximum required version of software. This will be checked with the version of new software. Version consists of either 4 numbers ( with each field in the range 0..65535) or it is a semantic version.
-R <version> string The current installed version of software. This will be checked with the version of new software and forbids reinstalling.
-o <file> string Save the stream (SWU) to a file.
-v 0.0 • 2 168u Activate verbose output.
-M 0.0 • 2 168u Disable setting the bootloader transaction marker.
-m 0.0 • 2 168u Disable setting the update state in the bootloader.
-w <parms> string Available if CONFIG_WEBSERVER is set. Start internal webserver and pass to it a command line string.
-d <parms> string Available if CONFIG_DOWNLOAD is set. Start internal downloader client and pass to it a command line string. See below the internal command line arguments for the downloader.
-u <parms> string Available if CONFIG_SURICATTA is set. Start internal suricatta client daemon and pass to it a command line string. See below the internal command line arguments for suricatta.
-H <board:rev> string Available on CONFIG_HW_COMPATIBILITY set. Set board name and hardware revision.
-c 0.0 • 2 168u Check *.swu file. It ensures that files referenced in sw-description are present. Usage: swupdate -c -i <file>
-P <cmd> string Execute pre-update command.
-p <cmd> string Execute post-update command.

Downloader command line parameters

Example: swupdate -d "-u"

Mandatory arguments are marked with '*':

Parameter Type Description
-u <url> string * This is the URL where new software is pulled. URL is a link to a valid .swu image
-r <retries> integer Number of retries before a download is considered broken. With "-r 0", SWUpdate will not stop until a valid software is loaded.
-t <timeout> integer Timeout for connection lost downloader or Webserver
-a <usr:pwd> string Send user and password for Basic Auth

Suricatta command line parameters

Example: swupdate -u "-t default -u localhost:8080 -i 1B7"

Note that different suricatta modules may have different parameters. The below listed options are for SWUpdate's hawkBit support.

Mandatory arguments are marked with '*':

Parameter Type Description
-t <tenant> string * Set hawkBit tenant ID for this device.
-u <url> string * Host and port of the hawkBit instance, e.g., localhost:8080
-i <id> integer * The device ID to communicate to hawkBit.
-c <confirm> integer Confirm update status to server: 1=AGAIN, 2=SUCCESS, 3=FAILED
-x 0.0 • 2 168u Do not abort on flawed server certificates.
-p <polldelay> integer Delay in seconds between two hawkBit poll operations (default: 45s).
-r <retry> integer Resume and retry interrupted downloads (default: 5 tries).
-w <retrywait> integer Time to wait prior to retry and resume a download (default: 5s).
-y <proxy> string Use proxy. Either give proxy URL, else {http,all}_proxy env is tried.
-k <targettoken> string Set target token.
-g <gatewaytoken> string Set gateway token.
-f <interface> string Set the network interface to connect to hawkBit.
-e 0.0 • 2 168u Daemon enabled at startup (default).
-d 0.0 • 2 168u Daemon disabled at startup.
--disable-token-for-dwl 0.0 • 2 168u Do not send authentication header when downloading SWU.
--cache-file string This allows one to resume an update after a power cut. If the SWU is saved in a file, SWUpdate can reuse the file and download just the remaining part of the SWU.
-m <seconds> integer Delay in seconds between re-trying to send initial feedback specified with "-c" option. Default value is 10 seconds. If Suricatta is started with initial state of STATE_WAIT ("-c 6"), this value is ignored.
-s <seconds> integer Connection timeout to use in seconds. If user doesn't set this option, default libcurl connection timeout value of 300 seconds is used. NOTE: it is not possible for Suricatta to respond to external program API requests during this period - adapt this value to your use case!
-a <name> <value> strings Custom HTTP header with given name and value to be sent with every HTTP request made.
-n <value> string Maximum download speed to be used. Value be specified in kB/s, B/s, MB/s or GB/s. Examples: -n 100k : Set limit to 100 kB/s. -n 500 : Set limit to 500 B/s. -n 2M : Set limit to 1 M/s. -n 1G : Set limit to 1 G/s.

Webserver command line parameters

Example: swupdate -w "-r /www -p 8080"

Mandatory arguments are marked with '*':

Parameter Type Description
-r <document root> string * Path where the web app is stored.
-p <port> integer * TCP port to be listened if not set, 8080 is used
-s <ssl> * Enable SSL support. Note: it must be configured with CONFIG_MONGOOSESSL
--ssl-cert <cert> string Path to the certificate to present to clients
-K <key> string Path to key corresponding to ssl certificate
-t <timeout> integer Timeout to consider a connection lost if clients stops to send data. If hit, an update is aborted. Default=0 (unlimited)
--auth-domain <string> string Set authentication domain Default: none
0.0 --global-auth-file <string> 168u string Set authentication file if any Default: none

systemd Integration

SWUpdate has optional systemd support via the compile-time configuration switch CONFIG_SYSTEMD. If enabled, SWUpdate signals systemd about start-up completion and can make optional use of systemd's socket-based activation feature.

A sample systemd service unit file /etc/systemd/system/swupdate.service may look like the following starting SWUpdate in suricatta daemon mode:

Description=SWUpdate daemon
ExecStart=/usr/bin/swupdate -u '-t default -u http://localhost -i 25'

Started via systemctl start swupdate.service, SWUpdate (re)creates its sockets on startup. For using socket-based activation, an accompanying systemd socket unit file /etc/systemd/system/swupdate.socket is required:

Description=SWUpdate socket listener

On swupdate.socket being started, systemd creates the socket files and hands them over to SWUpdate when it starts. So, for example, when talking to /tmp/swupdateprog, systemd starts swupdate.service and hands-over the socket files. The socket files are also handed over on a "regular" start of SWUpdate via systemctl start swupdate.service.

Note that the socket paths in the two ListenStream= directives have to match the socket paths CONFIG_SOCKET_CTRL_PATH and CONFIG_SOCKET_PROGRESS_PATH in SWUpdate's configuration. Here, the default socket path configuration is depicted.

Changes in boot-loader code

The SWUpdate consists of kernel and a root filesystem (image) that must be started by the boot-loader. In case using U-Boot, the following mechanism can be implemented:

  • U-Boot checks if a sw update is required (check gpio, serial console, etc.).
  • the script "altbootcmd" sets the rules to start SWUpdate
  • in case SWUpdate is required, U-boot run the script "altbootcmd"

Is it safe to change U-Boot environment ? Well, it is, but U-Boot must be configured correctly. U-Boot supports two copies of the environment to be power-off safe during an environment update. The board's configuration file must have defined CONFIG_ENV_OFFSET_REDUND or CONFIG_ENV_ADDR_REDUND. Check in U-Boot documentation for these constants and how to use them.

There are a further enhancement that can be optionally integrated into U-boot to make the system safer. The most important I will suggest is to add support for boot counter in U-boot (documentation is in U-Boot docs). This allows U-Boot to track for attempts to successfully run the application, and if the boot counter is greater as a limit, can start automatically SWUpdate to replace a corrupt software.

GRUB by default does not support double copies of environment as in case of U-Boot. This means that there is possibility that environment block get's corrupted when power-off occurs during environment update. To minimize the risk, we are not modifying original environment block. Variables are written into temporary file and after successful operation rename instruction is called.

Building a single image

cpio is used as container for its simplicity. The resulting image is very simple to be built. The file describing the images ("sw-description", but the name can be configured) must be the first file in the cpio archive.

To produce an image, a script like this can be used:

FILES="sw-description image1.ubifs  \

image2.gz.u-boot uImage.bin myfile sdcard.img" for i in $FILES;do
echo $i;done | cpio -ov -H crc > ${PRODUCT_NAME}_${CONTAINER_VER}.swu

The single images can be put in any order inside the cpio container, with the exception of sw-description, that must be the first one. To check your generated image you can run the following command:

swupdate -c -i my-software_1.0.swu

Support of compound image

The single image can be built automatically inside Yocto. meta-swupdate extends the classes with the swupdate class. A recipe should inherit it, and add your own sw-description file to generate the image.


SWUpdate is a building block and it allows one to design and implementing its own update strategy. Even if many projects have common ways for updating, it is possible to high customize the update for each project. The most common strategies (single-copy and dual-copy) were already described at the beginning of this documentation and of course are well supported in SWUpdate.

Single copy - running as standalone image

See single_copy.

Double copy with fall-back

See double_copy.

Combine double-copy with rescue system

This provides a recovery procedure to cover update failure in severe cases when software is damaged. In case none of the copy can be started, the bootloader will start the rescue system (possibly stored on another storage as the main system) to try to rescue the board. [image]

The rescue system can be updated as well during a standard update.

Split system update with application update

Updating a whole image is quite straightforward, but this means to transfer bigger amount of data if just a few files are updated. It is possible to split theupdate in several smaller parts to reduce the transfer size. This requires a special care to take care of compatibility between system and application, that can be solved with customized Lua scripts in the sw-description file. SWUpdate supports versioning for each artefact, and anyone can add own rules to verify compatibility between components. [image]

Configuration update

Thought to update the software, SWUpdate can be used to install configuration data as well. Build system can create configuration SWU with files / data for the configuration of the system. There is no requirements what these SWU should contains - it is duty of the integrator to build them and make them suitable for his own project. Again, configuration data can be updated as separate process using one of the above scenarios.



SWUpdate uses the library "libconfig" as default parser for the image description. However, it is possible to extend SWUpdate and add an own parser, based on a different syntax and language as the one supported by libconfig. In the examples directory there is the code for a parser written in Lua, with the description in XML.

Using the default parser, sw-description follows the syntax rules described in the libconfig manual. Please take a look at for an explanation of basic types. The whole description must be contained in the sw-description file itself: using of the #include directive is not allowed by SWUpdate.

The following example explains better the implemented tags:

software =

version = "0.1.0";
description = "Firmware update for XXXXX Project";
hardware-compatibility: [ "1.0", "1.2", "1.3"];
/* partitions tag is used to resize UBI partitions */
partitions: ( /* UBI Volumes */
name = "rootfs";
device = "mtd4";
size = 104896512; /* in bytes */
name = "data";
device = "mtd5";
size = 50448384; /* in bytes */
images: (
filename = "rootfs.ubifs";
volume = "rootfs";
filename = "swupdate.ext3.gz.u-boot";
volume = "fs_recovery";
filename = "sdcard.ext3.gz";
device = "/dev/mmcblk0p1";
compressed = "zlib";
filename = "bootlogo.bmp";
volume = "splash";
filename = "uImage.bin";
volume = "kernel";
filename = "fpga.txt";
type = "fpga";
filename = "bootloader-env";
type = "bootloader";
files: (
filename = "README";
path = "/README";
device = "/dev/mmcblk0p1";
filesystem = "vfat"
scripts: (
filename = "erase_at_end";
type = "lua";
filename = "display_info";
type = "lua";
bootenv: (
name = "vram";
value = "4M";
name = "addfb";
value = "setenv bootargs ${bootargs} omapfb.vram=1:2M,2:2M,3:2M omapdss.def_disp=lcd"
); }

The first tag is "software". The whole description is contained in this tag. It is possible to group settings per device by using Board specific settings.

Handling configuration differences

The concept can be extended to deliver a single image containing the release for multiple devices. Each device has its own kernel, dtb, and root filesystem, or they can share some parts.

Currently this is managed (and already used in a real project) by writing an own parser, that checks which images must be installed after recognizing which is the device where software is running.

Because the external parser can be written in Lua and it is completely customizable, everybody can set his own rules. For this specific example, the sw-description is written in XML format, with tags identifying the images for each device. To run it, the liblxp library is needed.

<?xml version="1.0" encoding="UTF-8"?>
<software version="1.0">

<name>Update Image</name>
<description>Firmware for XXXXX Project</description>
<image device="firstdevice" version="0.9">
<stream name="dev1-uImage" type="ubivol" volume="kernel" />
<stream name="dev1.dtb" type="ubivol" volume="dtb" />
<stream name="dev1-rootfs.ubifs" type="ubivol" volume="rootfs"/>
<stream name="dev1-uboot-env" type="uboot" />
<stream name="raw_vfat" type="raw" dest="/dev/mmcblk0p4" />
<stream name="sdcard.lua" type="lua" />
<image device="seconddevice" version="0.9">
<stream name="dev2-uImage" type="ubivol" volume="kernel" />
<stream name="dev2.dtb" rev="0.9" type="ubivol" volume="dtb" />
<stream name="dev2-rootfs.ubifs" type="ubivol" volume="rootfs"/>
</images> </software>

The parser for this is in the /examples directory. By identifying which is the running device, the parser return a table containing the images that must be installed and their associated handlers. By reading the delivered image, SWUpdate will ignore all images that are not in the list processed by the parser. In this way, it is possible to have a single delivered image for the update of multiple devices.

Multiple devices are supported by the default parser, too.

software =

version = "0.1.0";
target-1 = {
images: (
target-2 = {
images: (
}; }

In this way, it is possible to have a single image providing software for each device you have.

By default, the hardware information is extracted from /etc/hwrevision file. The file should contain a single line in the following format:

<boardname> <revision>


  • <revision> will be used for matching with hardware compatibility list
  • <boardname> can be used for grouping board specific settings

Software collections

Software collections and operation modes can be used to implement a dual copy strategy. The simplest case is to define two installation locations for the firmware image and call SWUpdate selecting the appropriate image.

software =

version = "0.1.0";
stable = {
copy-1: {
images: (
device = "/dev/mtd4"
copy-2: {
images: (
device = "/dev/mtd5"
}; }

In this way it is possible to specify that copy-1 gets installed to /dev/mtd4, while copy-2 to /dev/mtd5. By properly selecting the installation locations, SWUpdate will update the firmware in the other slot.

The method of image selection is out of the scope of SWUpdate and user is responsible for calling SWUpdate passing proper settings.

Priority finding the elements in the file

SWUpdate search for entries in the sw-description file according to the following priority:

Try <boardname>.<selection>.<mode>.<entry>
Try <selection>.<mode>.<entry>
Try <boardname>.<entry>
Try <entry>

Take an example. The following sw-description describes the release for a set of boards.

software =

version = "0.1.0";
myboard = {
stable = {
copy-1: {
images: (
device = "/dev/mtd4"
copy-2: {
images: (
device = "/dev/mtd5"
stable = {
copy-1: {
images: (
device = "/dev/mtd6"
copy-2: {
images: (
device = "/dev/mtd7"
} }

On myboard, SWUpdate searches and finds myboard.stable.copy1(2). When running on different boards, SWUpdate does not find an entry corresponding to the boardname and it falls back to the version without boardname. This allows to realize the same release for different boards having a completely different hardware. myboard could have an eMMC and an ext4 filesystem, while another device can have raw flash and install an UBI filesystem. Nevertheless, they are both just a different format of the same release and they could be described together in sw-description. It is important to understand the priorities how SWUpdate scans for entries during the parsing.

sw-description can become very complex. Let's think to have just one board, but in multiple hw revision and they differ in Hardware. Some of them can be grouped together, some of them require a dedicated section. A way (but not the only one !) could be to add mode and selects the section with -e stable,<rev number>.

software =

version = "0.1.0";
myboard = {
stable = {
hardware-compatibility: ["1.0", "1.2", "2.0", "1.3", "3.0", "3.1"];
rev-1.0: {
images: (
scripts: (
rev-1.2: {
hardware-compatibility: ["1.2"];
images: (
scripts: (
rev-2.0: {
hardware-compatibility: ["2.0"];
images: (
scripts: (
rev-1.3: {
hardware-compatibility: ["1.3"];
images: (
scripts: (
hardware-compatibility: ["3.0"];
images: (
scripts: (
hardware-compatibility: ["3.1"];
images: (
scripts: (
} }

If each of them requires an own section, it is the way to do. Anyway, it is more probable than revisions can be grouped together, for example board with the same major revision number could have the same installation instructions. This leads in the example to 3 groups for rev1.X, rev2.X, and rev3.X. Links allow one to group section together. When a "ref" is found when SWUpdate searches for a group (images, files, script, bootenv), it replaces the current path in the tree with the value of the string. In this way, the example above can be written in this way:

software =
version = "0.1.0";
myboard = {
stable = {
hardware-compatibility: ["1.0", "1.2", "2.0", "1.3", "3.0", "3.1"];
rev-1x: {
images: (
scripts: (
rev1.0 = {
ref = "#./rev-1x";
rev1.2 = {
ref = "#./rev-1x";
rev1.3 = {
ref = "#./rev-1x";
rev-2x: {
images: (
scripts: (
rev2.0 = {
ref = "#./rev-2x";
rev-3x: {
images: (
scripts: (
rev3.0 = {
ref = "#./rev-3x";
rev3.1 = {
ref = "#./rev-3x";
} }

The link can be absolute or relative. The keyword "ref" is used to indicate a link. If this is found, SWUpdate will traverse the tree and replaces the current path with the values find in the string pointed by "ref". There are simple rules for a link:

  • it must start with the character '#'
  • "." points to the current level in the tree, that means the parent of "ref"
  • ".." points to the parent level in the tree
  • "/" is used as filed separator in the link

A relative path has a number of leading "../" to move the current cursor to the parent leaf of the tree. In the following example, rev40 sets a link to a "common" section, where images is found. This is sets via a link, too, to a section in the parent node. The path software.myboard.stable.common.images is then replaced by software.myboard.stable.trythis

software =

version = {
ref = "#./commonversion";
hardware-compatibility = ["rev10", "rev11", "rev20"];
commonversion = "0.7-linked"; pc:{
images =
ref = "#./../trythis";
filename = "rootfs1.ext4";
device = "/dev/mmcblk0p8";
type = "raw";
} ,
filename = "rootfs5.ext4";
device = "/dev/mmcblk0p7";
type = "raw";
filename = "rootfs.ext3"; device = "/dev/mmcblk0p2";}
{ name = "bootpart";
value = "0:2";}
pdm3rev11 =
ref = "#./pdm3rev10";
pdm3rev20 =
ref = "#./pdm3rev10";
pdm3rev40 =
ref = "#./common";
}; }

Each entry in sw-description can be redirect by a link as in the above example for the "version" attribute.


hardware-compatibility: [ "major.minor", "major.minor", ... ]

This entry lists the hardware revisions that are compatible with this software image.


hardware-compatibility: [ "1.0", "1.2", "1.3"];

This defines that the software is compatible with HW-Revisions 1.0, 1.2, and 1.3, but not with 1.1 or any other version not explicitly listed here. In the above example, compatibility is checked by means of string comparison. If the software is compatible with a large number of hardware revisions, it may get cumbersome to enumerate all compatible versions. To allow more compact specifications, regular expressions (POSIX extended) can be used by adding a prefix #RE: to the entry. Rewriting the above example would yield:

hardware-compatibility: [ "#RE:^1\.[023]$" ];

It is in the responsibility of the respective project to find the revision of the board on which SWUpdate is running. No assumptions are made about how the revision can be obtained (GPIOs, EEPROM,..) and each project is free to select the most appropriate way. In the end the result must be written to the file /etc/hwrevision (or in another file if specified as configuration option) before SWUpdate is started.

partitions : UBI layout

This tag allows one to change the layout of UBI volumes. Please take care that MTDs are not touched and they are configured by the Device Tree or in another way directly in kernel.

partitions: (

name = <volume name>;
size = <size in bytes>;
device = <MTD device>;
} );

All fields are mandatory. SWUpdate searches for a volume of the given name and if necessary adjusts size or type (see below). If no volume with the given name is found, a new volume is created on the UBI device attached to the MTD device given by device. device can be specified by number (e.g. "mtd4") or by name (the name of the MTD device, e.g. "ubi_partition"). The UBI device is attached automatically.

The default behavior of swupdate is to create a dynamic UBI volume. To create a static volume, add a line data = "static"; to the respective partition entry.

If a size of 0 is given, the volume will be deleted if it exists. This can be used to remove orphan volumes possibly created by older software versions which are not required anymore.


The tag "images" collects the image that are installed to the system. The syntax is:

images: (

filename[mandatory] = <Name in CPIO Archive>;
volume[optional] = <destination volume>;
device[optional] = <destination volume>;
mtdname[optional] = <destination mtd name>;
type[optional] = <handler>;
/* optionally, the image can be copied at a specific offset */
offset[optional] = <offset>;
/* optionally, the image can be compressed if it is in raw mode */
/* Next Image */
..... );

volume is only used to install the image in a UBI volume. volume and device cannot be used at the same time. If device is set, the raw handler is automatically selected.

The following example is to update a UBI volume:


filename = "core-image-base.ubifs";
volume = "rootfs"; }

To update an image in raw mode, the syntax is:


filename = "core-image-base.ext3";
device = "/dev/mmcblk0p1"; }

To flash an image at a specific offset, the syntax is:


filename = "u-boot.bin";
device = "/dev/mmcblk0p1";
offset = "16K"; }

The offset handles the following multiplicative suffixes: K=1024 and M=1024*1024.

However, writing to flash in raw mode must be managed in a special way. Flashes must be erased before copying, and writing into NAND must take care of bad blocks and ECC errors. For these reasons, the handler "flash" must be selected:

For example, to copy the kernel into the MTD7 of a NAND flash:


filename = "uImage";
device = "mtd7";
type = "flash"; }

The filename is mandatory. It is the Name of the file extracted by the stream. volume is only mandatory in case of UBI volumes. It should be not used in other cases.

Alternatively, for the handler “flash”, the mtdname can be specified, instead of the device name:


filename = "uImage";
mtdname = "kernel";
type = "flash"; }


It is possible to copy single files instead of images. This is not the preferred way, but it can be used for debugging or special purposes.

files: (

filename = <Name in CPIO Archive>;
path = <path in filesystem>;
device[optional] = <device node >;
filesystem[optional] = <filesystem for mount>;
properties[optional] = {create-destination = "true";}
} );

Entries in "files" section are managed as single files. The attributes "filename" and "path" are mandatory. Attributes "device" and "filesystem" are optional; they tell SWUpdate to mount device (of the given filesystem type, e.g. "ext4") before copying "filename" to "path". Without "device" and "filesystem", the "filename" will be copied to "path" in the current rootfs.

As a general rule, swupdate doesn't copy out a file if the destination path doesn't exists. This behavior could be changed using the special property "create-destination".

As another general rule, the raw file handler installs the file directly to the specified path. If the target file already exists and the raw file handler is interrupted, the existing file may be replaced by an empty or partially written file. A use case can exist where having an empty or corrupted file is worse than the existing file. For this reason, the raw file handler supports an "atomic-install" property. Setting the property to "true" installs the file to the specified path with ".tmp" appended to the filename. Once the contents of the file have been written and the buffer is flushed, the ".tmp" file is renamed to the target file. This minimizes chances that an empty or corrupted file is created by an interrupted raw file handler.


Scripts runs in the order they are put into the sw-description file. The result of a script is valuated by SWUpdate, that stops the update with an error if the result is <> 0.

They are copied into a temporary directory before execution and their name must be unique inside the same cpio archive.

If no type is given, SWUpdate default to "lua".


scripts: (

filename = <Name in CPIO Archive>;
type = "lua";
} );

Lua scripts are run using the internal interpreter.

They must have at least one of the following functions:

function preinst()

SWUpdate scans for all scripts and check for a preinst function. It is called before installing the images.

function postinst()

SWUpdate scans for all scripts and check for a postinst function. It is called after installing the images.


scripts: (

filename = <Name in CPIO Archive>;
type = "shellscript";
} );

Shell scripts are called via system command. SWUpdate scans for all scripts and calls them before and after installing the images. SWUpdate passes 'preinst' or 'postinst' as first argument to the script. If the data attribute is defined, its value is passed as the last argument(s) to the script.


scripts: (

filename = <Name in CPIO Archive>;
type = "preinstall";
} );

preinstall are shell scripts and called via system command. SWUpdate scans for all scripts and calls them before installing the images. If the data attribute is defined, its value is passed as the last argument(s) to the script.


scripts: (

filename = <Name in CPIO Archive>;
type = "postinstall";
} );

postinstall are shell scripts and called via system command. SWUpdate scans for all scripts and calls them after installing the images. If the data attribute is defined, its value is passed as the last argument(s) to the script.

Update Transaction and Status Marker

By default, SWUpdate sets the bootloader environment variable "recovery_status" to "in_progress" prior to an update operation and either unsets it or sets it to "failed" after the update operation. This is an interface for SWUpdate-external tooling: If there is no "recovery_status" variable in the bootloader's environment, the update operation has been successful. Else, if there is a "recovery_status" variable with the value "failed", the update operation has not been successful.

While this is in general essential behavior for firmware updates, it needn't be for less critical update operations. Hence, whether or not the update transaction marker is set by SWUpdate can be controlled by the boolean switch "bootloader_transaction_marker" which is global per sw-description file. It defaults to true. The following example snippet disables the update transaction marker:

software =

version = "0.1.0";
bootloader_transaction_marker = false;

It is also possible to disable setting of the transaction marker entirely (and independently of the setting in sw-description) by starting SWUpdate with the -M option.

The same applies to setting the update state in the bootloader via its environment variable "ustate" (default) to STATE_INSTALLED=1 or STATE_FAILED=3 after an installation. This behavior can be turned off globally via the -m option to SWUpdate or per sw-description via the boolean switch "bootloader_state_marker".


There are two ways to update the bootloader (currently U-Boot, GRUB, and EFI Boot Guard) environment. First way is to add a file with the list of variables to be changed and setting "bootloader" as type of the image. This informs SWUpdate to call the bootloader handler to manage the file (requires enabling bootloader handler in configuration). There is one bootloader handler for all supported bootloaders. The appropriate bootloader must be chosen from the bootloader selection menu in menuconfig.

images: (

filename = "bootloader-env";
type = "bootloader";
} )

The format of the file is described in U-boot documentation. Each line is in the format

<name of variable>=<value>

if value is missing, the variable is unset.

The format is compatible with U-Boot "env import" command. It is possible to produce the file from target as result of "env export".

Comments are allowed in the file to improve readability, see this example:

# Default variables
## Board Revision dependent

The second way is to define in a group setting the variables that must be changed:

bootenv: (

name = <Variable name>;
value = <Variable value>;
} )

SWUpdate will internally generate a script that will be passed to the bootloader handler for adjusting the environment.

For backward compatibility with previously built .swu images, the "uboot" group name is still supported as an alias. However, its usage is deprecated.

Board specific settings

Each setting can be placed under a custom tag matching the board name. This mechanism can be used to override particular setting in board specific fashion.

Assuming that the hardware information file /etc/hwrevision contains the following entry:

my-board 0.1.0

and the following description:

software =

version = "0.1.0";
my-board = {
bootenv: (
name = "bootpart";
value = "0:2";
bootenv: (
name = "bootpart";
value = "0:1";
); }

SWUpdate will set bootpart to 0:2 in bootloader's environment for this board. For all other boards, bootpart will be set to 0:1. Board specific settings take precedence over default scoped settings.

Software collections and operation modes

Software collections and operations modes extend the description file syntax to provide an overlay grouping all previous configuration tags. The mechanism is similar to Board specific settings and can be used for implementing a dual copy strategy or delivering both stable and unstable images within a single update file.

The mechanism uses a custom user-defined tags placed within software scope. The tag names must not be any of: version, hardware-compatibility, uboot, bootenv, files, scripts, partitions, images

An example description file:

software =

version = "0.1";
hardware-compatibility = [ "revA" ];
/* differentiate running image modes/sets */
images: (
filename = "rootfs.ext3";
device = "/dev/mmcblk0p2";
bootenv: (
name = "bootpart";
value = "0:2";
images: (
filename = "rootfs.ext3";
device = "/dev/mmcblk0p1";
bootenv: (
name = "bootpart";
value = "0:1";
}; }

The configuration describes a single software collection named stable. Two distinct image locations are specified for this collection: /dev/mmcblk0p1 and /dev/mmcblk0p2 for main mode and alt mode respectively.

This feature can be used to implement a dual copy strategy by specifying the collection and mode explicitly.

Versioning schemas in SWUpdate

SWUpdate can perform version comparisons for the whole Software by checking the version attribute in the common part of sw-description and / or for single artifacts. SWUpdate supports two different version schemas, and they must be followed if version comparison is requested.

Numbering schema (default)

SWUpdate supports a version based on the schema:


where each field is a plain number (no alphanumeric) in the range 0..65535. User can add further fields using the dot separator, but they are not considered for version comparison. SWUpdate will check if a version number is set according to this rule and fall back to semantic version upon failure. The version is converted to a 64 bit number (each field is 16 bit) and compared against the running version of the same artifact.

Please consider that, because additional fields are descriptive only, for the comparison they are not considered. This example contains version numbers that are interpreted as the same version number by SWUpdate:

But the following is different:

And it is treated as semantic version.

Semantic version

SWUpdate supports semantic version. See official documentation for more details.

Checking version of installed software

SWUpdate can optionally verify if a sub-image is already installed and, if the version to be installed is exactly the same, it can skip to install it. This is very useful in case some high risky image should be installed or to speed up the upgrade process. One case is if the bootloader needs to be updated. In most time, there is no need to upgrade the bootloader, but practice showed that there are some cases where an upgrade is strictly required - the project manager should take the risk. However, it is nicer to have always the bootloader image as part of the .swu file, allowing to get the whole distro for the device in a single file, but the device should install it just when needed.

SWUpdate searches for a file (/etc/sw-versions is the default location) containing all versions of the installed images. This must be generated before running SWUpdate. The file must contain pairs with the name of image and version, as:

<name of component>     <version>

In sw-description, the optional attributes "name", "version", and "install-if-different" provide the connection. Name and version are then compared with the data in the versions file. install-if-different is a boolean that enables the check for this image. It is then possible to check the version just for a subset of the images to be installed.

If used with "install-if-different", then version can be any string. For example:

bootloader              2015.01-rc3-00456-gd4978d
kernel                  3.17.0-00215-g2e876af

There is also an attribute "install-if-higher" that checks if the version of the new software is higher than the version of the installed software. If it's false, the new software isn't installed. The goal is to avoid installing an older version of software.

In this case, version can be any of 2 formats. Either the version consists of up to 4 numbers in the range 0..65535 separated by a dot, e.g. <major>.<minor>.<rev>.<build>, or it is a semantic version.

bootloader              2018.03.01
kernel                  3.17.0-pre1+g2e876af
rfs                     0.17-foo3.bar5+2020.07.01
app                     1.7

It is advised not to mix version formats! Semantic versions only support 3 numbers (major, minor, patch) and the fourth number will be silently dropped if present.

Embedded Script

It is possible to embed a script inside sw-description. This is useful in a lot of conditions where some parameters are known just by the target at runtime. The script is global to all sections, but it can contain several functions that can be specific for each entry in the sw-description file.

These attributes are used for an embedded-script:

embedded-script = "<Lua code">

It must be taken into account that the parser has already run and usage of double quotes can interfere with the parser. For this reason, each double quote in the script must be escaped.

That means a simple Lua code as:

print ("Test")

must be changed to:

print (\"Test\")

If not, the parser thinks to have the closure of the script and this generates an error. See the examples directory for examples how to use it. Any entry in files or images can trigger one function in the script. The "hook" attribute tells the parser to load the script and to search for the function pointed to by the hook attribute. For example:

files: (

filename = "examples.tar";
type = "archive";
path = "/tmp/test";
hook = "set_version";
preserve-attributes = true;
} );

After the entry is parsed, the parser runs the Lua function pointed to by hook. If Lua is not activated, the parser raises an error because a sw-description with an embedded script must be parsed, but the interpreter is not available.

Each Lua function receives as parameter a table with the setup for the current entry. A hook in Lua is in the format:

function lua_hook(image)

image is a table where the keys are the list of available attributes. If an attribute contains a "-", it is replaced with "_", because "-" cannot be used in Lua. This means, for example, that:

install-if-different ==> install_if_different
installed-directly   ==> installed_directly

Attributes can be changed in the Lua script and values are taken over on return. The Lua function must return 2 values:

  • a boolean, to indicate whether the parsing was correct
  • the image table or nil to indicate that the image should be skipped


function set_version(image)

for k,l in pairs(image) do
swupdate.trace(\"image[\" .. tostring(k) .. \"] = \" .. tostring(l))
image.version = \"1.0\"
image.install_if_different = true
return true, image end

The example sets a version for the installed image. Generally, this is detected at runtime reading from the target.

Attribute reference

There are 4 main sections inside sw-description:

  • images: entries are images and SWUpdate has no knowledge about them.
  • files: entries are files, and SWUpdate needs a filesystem for them. This is generally used to expand from a tar-ball or to update single files.
  • scripts: all entries are treated as executables, and they will be run twice (as pre- and post- install scripts).
  • bootenv: entries are pair with bootloader environment variable name and its value.

Attributes in sw-description

Name Type Applies to Description
filename string images files scripts filename as found in the cpio archive
volume string images Just if type = "ubivol". UBI volume where image must be installed.
ubipartition string images Just if type = "ubivol". Volume to be created or adjusted with a new size
device string images files devicenode as found in /dev or a symlink to it. Can be specified as absolute path or a name in /dev folder For example if /dev/mtd-dtb is a link to /dev/mtd3 "mtd3", "mtd-dtb", "/dev/mtd3" and "/dev/mtd-dtb" are valid names. Usage depends on handler. For files, it indicates on which device the "filesystem" must be mounted. If not specified, the current rootfs will be used.
filesystem string files indicates the filesystem type where the file must be installed. Only used if "device" attribute is set.
path string files For files: indicates the path (absolute) where the file must be installed. If "device" and "filesystem" are set, SWUpdate will install the file after mounting "device" with "filesystem" type. (path is always relative to the mount point.)
preserve-attributes bool files flag to control whether the following attributes will be preserved when files are unpacked from an archive (assuming destination filesystem supports them, of course): timestamp, uid/gid (numeric), perms, file attributes, extended attributes
type string images files scripts string identifier for the handler, as it is set by the handler when it registers itself. Example: "ubivol", "raw", "rawfile",
compressed string images files string to indicate the "filename" is compressed and must be decompressed before being installed. the value denotes the compression type. currently supported values are "zlib" and "zstd".
compressed bool (dep recated) images files Deprecated. Use the string form. true is equal to 'compressed = "zlib"'.
installed-directly bool images flag to indicate that image is streamed into the target without any temporary copy. Not all handlers support streaming.
name string bootenv name of the bootloader variable to be set.
value string bootenv value to be assigned to the bootloader variable
name string images files name that identifies the sw-component it can be any string and it is compared with the entries in sw-versions
version string images files version for the sw-component it can be any string and it is compared with the entries in sw-versions
description string user-friendly description of the swupdate archive (any string)
install-if-different bool images files flag if set, name and version are compared with the entries in sw-versions
install-if-higher bool images files flag if set, name and version are compared with the entries in sw-versions
encrypted bool images files scripts flag if set, file is encrypted and must be decrypted before installing.
ivt string images files scripts IVT in case of encrypted artefact It has no value if "encrypted" is not set. Each artefact can have an own IVT to avoid attacker can guess the the key. It is an ASCII string of 32 chars
data string images files scripts This is used to pass arbitrary data to a handler.
sha256 string images files scripts sha256 hash of image, file or script. Used for verification of signed images.
embedded-script string Lua code that is embedded in the sw-description file.
offset string images Optional destination offset
hook string images files The name of the function (Lua) to be called when the entry is parsed.
mtdname string images name of the MTD to update. Used only by the flash handler to identify the the mtd to update, instead of specifying the devicenode


It is becoming very important that a device must not only be safely updated, but also that it can verify if the delivered image is coming from a known source and it was not corrupted introducing some malware.

To achieve this goal, SWUpdate must verify the incoming images. There are several ways to do this. Should the compound image be signed ? Or some parts of it ?

Advantages and disadvantages are described in the following chapter.

Signing the compound image

It looks quite straightforward if the whole compound image is signed. However, this has some heavy drawbacks. It is not possible to know if the image is verified until the whole image is loaded. This means that verification can be done after installing the single images instead of doing it before touching the device. This leads to have some uninstall procedure if part of a not verified image is already installed, procedures that cannot be safe in case of power off letting some unwanted piece of software on the device.

Signing the sub-images

If each sub-image is signed, the verification is done before calling the corresponding hardware. Only signed images can be installed. Anyway, this remains unbound with the description of the release in sw-description. Even if sw-description is signed, an attacker can mix signed images together generating a new compound image that can be installed as well, because all sub-images are verified.

Combining signing sw-description with hash verification

To avoid the described drawbacks, SWUpdate combines signed sw-description with the verification of hashes for each single image. This means that only sw-description generated by a verified source can be accepted by the installer. sw-description contains hashes for each sub-image to verify that each delivered sub-image really belongs to the release.

Choice of algorithm

The algorithm chosen to sign and verify the sw-descrription file can be selected via menuconfig. Currently, the following mechanisms are implemented:

  • RSA Public / private key. The private key belongs to the build system, while the public key must be installed on the target.
  • CMS using certificates

Key or certificate is passed to SWUpdate with the -k parameter.

Tool to generate keys / certificates

The openssl tool is used to generate the keys. This is part of the OpenSSL project. A complete documentation can be found at the openSSL Website.

Usage with RSA PKCS#1.5 or RSA PSS

Generating private and public key

First, the private key must be created:

openssl genrsa -aes256 -out priv.pem

This asks for a passphrase. It is possible to retrieve the passphrase from a file - of course, this must be protected against intrusion.

openssl genrsa -aes256 -passout file:passout -out priv.pem

The private key is used to export the public key with:

openssl rsa -in priv.pem -out public.pem -outform PEM -pubout

"public.pem" contains the key in a format suitable for swupdate. The file can be passed to swupdate at the command line with the -k parameter.

How to sign with RSA

Signing the image with rsa-pkcs#1.5 is very simple:

openssl dgst -sha256 -sign priv.pem sw-description > sw-description.sig

Signing the image with rsa-pss is also very simple:

openssl dgst -sha256 -sign priv.pem -sigopt rsa_padding_mode:pss \

-sigopt rsa_pss_saltlen:-2 sw-description > sw-description.sig

Usage with certificates and CMS

Generating self-signed certificates

openssl req -x509 -newkey rsa:4096 -nodes -keyout mycert.key.pem \

-out mycert.cert.pem -subj "/O=SWUpdate /CN=target"

Check the documentation for more information about parameters. The "mycert.key.pem" contains the private key and it is used for signing. It is not delivered on the target.

The target must have "mycert.cert.pem" installed - this is used by SWUpdate for verification.

Using PKI issued certificates

It is also possible to use PKI issued code signing certificates. However, SWUpdate uses OpenSSL library for handling CMS signatures and the library requires the following attributes to be set on the signing certificate:


It is also possible to completely disable signing certificate key usage checking if this requirement cannot be satisfied. This is controlled by CONFIG_CMS_IGNORE_CERTIFICATE_PURPOSE configuration option.

How to sign with CMS

Signing the image is simple as in the previous case:

openssl cms -sign -in  sw-description -out sw-description.sig -signer mycert.cert.pem \

-inkey mycert.key.pem -outform DER -nosmimecap -binary

Building a signed SWU image

There are two files, sw-description and its signature sw-description.sig. The signature file must always directly follow the description file.

Each image inside sw-description must have the attribute "sha256", with the SHA256 sum of the image. If an image does not have the sha256 attribute, the whole compound image results as not verified and SWUpdate stops with an error before starting to install.

A simple script to create a signed image can be:

IMAGES="rootfs kernel"
FILES="sw-description sw-description.sig $IMAGES"
#if you use RSA
if [ x"$MODE" == "xRSA-PKCS-1.5" ]; then

openssl dgst -sha256 -sign priv.pem sw-description > sw-description.sig elif if [ x"$MODE" == "xRSA-PSS" ]; then
openssl dgst -sha256 -sign priv.pem -sigopt rsa_padding_mode:pss \
-sigopt rsa_pss_saltlen:-2 sw-description > sw-description.sig else
openssl cms -sign -in sw-description -out sw-description.sig -signer mycert.cert.pem \
-inkey mycert.key.pem -outform DER -nosmimecap -binary fi for i in $FILES;do
echo $i;done | cpio -ov -H crc > ${PRODUCT_NAME}_${CONTAINER_VER}.swu

Example for sw-description with signed image

The example applies to a Beaglebone, installing Yocto images:

software =

version = "0.1.0";
hardware-compatibility: [ "revC"];
images: (
filename = "core-image-full-cmdline-beaglebone.ext3";
device = "/dev/mmcblk0p2";
type = "raw";
sha256 = "43cdedde429d1ee379a7d91e3e7c4b0b9ff952543a91a55bb2221e5c72cb342b";
scripts: (
filename = "test.lua";
type = "lua";
sha256 = "f53e0b271af4c2896f56a6adffa79a1ffa3e373c9ac96e00c4cfc577b9bea5f1";
); }

Running SWUpdate with signed images

Verification is activated by setting CONFIG_SIGNED_IMAGES in SWUpdate's configuration. If activated, SWUpdate will always check the compound image. For security reasons, it is not possible to disable the check at runtime. The -k parameter (public key file) is mandatory and the program stops if the public key is not passed.


SWUpdate allows one to symmetrically encrypt update images using the AES block cipher in CBC mode. The following shows encryption with 256 bit key length but you may use other key lengths as well.

Building an Encrypted SWU Image

First, create a key; for aes-256-cbc we need 32 bytes of key and 16 bytes for an initialisation vector (IV). A complete documentation can be found at the OpenSSL Website.

openssl rand -hex 32
# key, for example 390ad54490a4a5f53722291023c19e08ffb5c4677a59e958c96ffa6e641df040
openssl rand -hex 16
# IV, for example d5d601bacfe13100b149177318ebc7a4

Then, encrypt an image using this information via

openssl enc -aes-256-cbc -in <INFILE> -out <OUTFILE> -K <KEY> -iv <IV>

where <INFILE> is the unencrypted source image file and <OUTFILE> is the encrypted output image file to be referenced in sw-description. <KEY> is the hex value part of the 2nd line of output from the key generation command above and <IV> is the hex value part of the 3rd line.

Then, create a key file to be supplied to SWUpdate via the -K switch by putting the key and initialization vector hex values on one line separated by whitespace, e.g., for above example values

390ad54490a4a5f53722291023c19e08ffb5c4677a59e958c96ffa6e641df040 d5d601bacfe13100b149177318ebc7a4

Previous versions of SWUpdate allowed for a salt as third word in key file, that was never actually used for aes and has been removed.

You should change the IV with every encryption, see CWE-329. The ivt sw-description attribute overrides the key file's IV for one specific image.

Encryption of UBI volumes

Due to a limit in the Linux kernel API for UBI volumes, the size reserved to be written on disk should be declared before actually writing anything.

See the property "decrypted-size" in UBI Volume Handler's documentation.

Example sw-description with Encrypted Image

The following example is a (minimal) sw-description for installing a Yocto image onto a Beaglebone. Pay attention to the encrypted = true; setting.

software =

version = "0.0.1";
images: ( {
filename = "core-image-full-cmdline-beaglebone.ext3.enc";
device = "/dev/mmcblk0p3";
encrypted = true;
ivt = "65D793B87B6724BB27954C7664F15FF3";
); }

Running SWUpdate with Encrypted Images

Symmetric encryption support is activated by setting the ENCRYPTED_IMAGES option in SWUpdate's configuration. Use the -K parameter to provide the symmetric key file generated above to SWUpdate.

Decrypting with a PKCS#11 token

PKCS#11 support is activated by setting the PKCS11 option in SWUpdate's configuration. The key file has to have a PKCS#11 URL instead of the key then, containing at least the elements of this example:

pkcs11:slot-id=42;id=%CA%FE%BA%BE?pin-value=1234&module-path=/usr/lib/ 65D793B87B6724BB27954C7664F15FF3



It is quite difficult to foresee all possible installation cases. Instead of trying to find all use cases, SWUpdate let the developer free to add his own installer (that is, a new handler), that must be responsible to install an image of a certain type. An image is marked to be of a defined type to be installed with a specific handler.

The parser make the connection between 'image type' and 'handler'. It fills a table containing the list of images to be installed with the required handler to execute the installation. Each image can have a different installer.

Supplied handlers

  • flash devices in raw mode (both NOR and NAND)
  • UBI volumes
  • UBI volumes partitioner
  • raw flashes handler (NAND, NOR, SPI-NOR, CFI interface)
  • disk partitioner
  • raw devices, such as a SD Card partition
  • bootloader (U-Boot, GRUB, EFI Boot Guard) environment
  • Lua scripts handler
  • shell scripts handler
  • rdiff handler
  • readback handler
  • archive (zo, tarballs) handler
  • remote handler
  • microcontroller update handler

For example, if an image is marked to be updated into a UBI volume, the parser must fill a supplied table setting "ubi" as required handler, and filling the other fields required for this handler: name of volume, size, and so on.

Creating own handlers

SWUpdate can be extended with new handlers. The user needs to register his own handler with the core and he must provide the callback that SWUpdate uses when an image required to be installed with the new handler.

The prototype for the callback is:

int my_handler(struct img_type *img,

void __attribute__ ((__unused__)) *data)

The most important parameter is the pointer to a struct img_type. It describes a single image and inform the handler where the image must be installed. The file descriptor of the incoming stream set to the start of the image to be installed is also part of the structure.

The structure img_type contains the file descriptor of the stream pointing to the first byte of the image to be installed. The handler must read the whole image, and when it returns back SWUpdate can go on with the next image in the stream.

The data parameter is usually a pointer that was registered with the handler. For script handlers it is instead a pointer to a struct script_handler_data which contains a script_fn enum value, indicating the current installation phase, and the registered data pointer.

SWUpdate provides a general function to extract data from the stream and copy to somewhere else:

int copyfile(int fdin, int fdout, int nbytes, unsigned long *offs,

int skip_file, int compressed, uint32_t *checksum, unsigned char *hash);

fdin is the input stream, that is img->fdin from the callback. The hash, in case of signed images, is simply passed to copyfile() to perform the check, exactly as the checksum parameter. copyfile() will return an error if checksum or hash do not match. The handler does not need to bother with them. How the handler manages the copied data, is specific to the handler itself. See supplied handlers code for a better understanding.

The handler's developer registers his own handler with a call to:

void my_handler_init(void)

register_handler("mytype", my_handler, my_mask, data); }

SWUpdate uses the gcc constructors, and all supplied handlers are registered when SWUpdate is initialized.

register_handler has the syntax:

register_handler(my_image_type, my_handler, my_mask, data);


  • my_image_type : string identifying the own new image type.
  • my_handler : pointer to the installer to be registered.
  • my_mask : HANDLER_MASK enum value(s) specifying what input type(s) my_handler can process.
  • data : an optional pointer to an own structure, that SWUpdate saves in the handlers' list and pass to the handler when it will be executed.

UBI Volume Handler

The UBI volume handler will update UBI volumes without changing the layout on the storage. Therefore, volumes must be created/adjusted beforehand. This can be done using the partitions tag (see partitions-ubi-layout).

The UBI volume handler will search for volumes in all MTD devices (unless blacklisted, see UBIBLACKLIST) to find the volume into which the image shall be installed. For this reason, volume names must be unique within the system. Two volumes with the same name are not supported and will lead to unpredictable results (SWUpdate will install the image to the first volume with that name it finds, which may not be right one!).

When updating volumes, it is guaranteed that erase counters are preserved and not lost. The behavior of updating is identical to that of the ubiupdatevol(1) tool from mtd-utils. In fact, the same library from mtd-utils (libubi) is reused by SWUpdate.

atomic volume renaming

The UBI volume handler has basic support for carrying out atomic volume renames by defining the replaces property, which must contain a valid UBI volume name. After successfully updating the image to volume, an atomic swap of the names of volume and replaces is done. Consider the following example


filename ="u-boot.img";
volume ="u-boot_r";
properties: {
replaces = "u-boot";
} }

After u-boot.img is successfully installed into the volume "u-boot_r", the volume "u-boot_r" is renamed to "u-boot" and "u-boot" is renamed to "u-boot_r".

This mechanism allows one to implement a simple double copy update approach without the need of shared state with the bootloader. For example, the U-Boot SPL can be configured to always load U-Boot from the volume u-boot without the need to access the environment. The volume replace functionality will ensure that this volume name always points to the currently valid volume.

However, please note the following limitations:

  • Currently the rename takes place after each image was installed successfully. Hence, it only makes sense to use this feature for images that are independent of the other installed images. A typical example is the bootloader. This behavior may be modified in the future to only carry out one atomic rename after all images were installed successfully.
  • Atomic renames are only possible and permitted for volumes residing on the same UBI device.

There is a handler ubiswap that allow one to do an atomic swap for several ubi volume after all the images were flashed. This handler is a script for the point of view of swudate, so the node that provide it the data should be added in the section scripts.

scripts: (

type = "ubiswap";
properties: {
swap-0 = [ "boot" , " boot_r" ];
swap-1 = [ "kernel" , "kernel_r" ];
swap-2 = [ "rootfs" , "rootfs_r" ];
}, );

WARNING: if you use the property replaces on an ubi volume that is also used with the handler ubiswap, this ubi volume will be swapped twice. It's probably not what you want ...

volume auto resize

The UBI volume handler has support to auto resize before flashing an image with the property auto-resize. When this property is set on an image, the ubi volume is resized to fit exactly the image.


filename = "u-boot.img";
device = "mtd0";
volume = "u-boot_r";
properties: {
auto-resize = "true";
} }

WARNING: when this property is used, the device must be defined.

volume always remove

The UBI volume handler has support to always remove ubi volume before flashing with the property always-remove. When this property is set on an image, the ubi volume is always removed. This property should be used with property auto-resize.


filename = "u-boot.img";
device = "mtd0";
volume = "u-boot_r";
properties: {
always-remove = "true";
auto-resize = "true";
} }

size properties

Due to a limit in the Linux kernel API for UBI volumes, the size reserved to be written on disk should be declared before actually writing anything. Unfortunately, the size of an encrypted or compressed image is not known until the decryption or decompression finished. This prevents correct declaration of the file size to be written on disk.

For this reason UBI images can declare the special properties "decrypted-size" or "decompressed-size" like this:

images: ( {

filename = "rootfs.ubifs.enc";
volume = "rootfs";
encrypted = true;
properties: {
decrypted-size = "104857600";
filename = "homefs.ubifs.gz";
volume = "homefs";
compressed = "zlib";
properties: {
decompressed-size = "420000000";
} );

The real size of the image should be calculated and written to the sw-description before assembling the cpio archive. In this example, 104857600 is the size of the rootfs after the decryption: the encrypted size is by the way larger. The decompressed size is of the homefs is 420000000.

The sizes are bytes in decimal notation.

Lua Handlers

In addition to the handlers written in C, it is possible to extend SWUpdate with handlers written in Lua that get loaded at SWUpdate startup. The Lua handler source code file may either be embedded into the SWUpdate binary via the CONFIG_EMBEDDED_LUA_HANDLER config option or has to be installed on the target system in Lua's search path as swupdate_handlers.lua so that it can be loaded by the embedded Lua interpreter at run-time.

In analogy to C handlers, the prototype for a Lua handler is

--- Lua Handler.
--- @param  image  img_type  Lua equivalent of `struct img_type`
--- @return number           # 0 on success, 1 on error
function lua_handler(image)

... end

where image is a Lua table (with attributes according to sw-description's attribute reference) that describes a single artifact to be processed by the handler (also see the Lua Handler Interface Specification in handlers/swupdate.lua).

Note that dashes in the attributes' names are replaced with underscores for the Lua domain to make them idiomatic, e.g., installed-directly becomes installed_directly in the Lua domain.

For a script handler written in Lua, the prototype is

--- Lua Handler.
--- @param  image     img_type  Lua equivalent of `struct img_type`
--- @param  scriptfn  string    Type, one of `preinst` or `postinst`
--- @return number              # 0 on success, 1 on error
function lua_handler(image, scriptfn)

... end

where scriptfn is either "preinst" or "postinst".

To register a Lua handler, the swupdate module provides the swupdate.register_handler() method that takes the handler's name, the Lua handler function to be registered under that name, and, optionally, the types of artifacts for which the handler may be called. If the latter is not given, the Lua handler is registered for all types of artifacts. The following call registers the above function lua_handler as my_handler which may be called for images:

swupdate.register_handler("my_handler", lua_handler, swupdate.HANDLER_MASK.IMAGE_HANDLER)

A Lua handler may call C handlers ("chaining") via the swupdate.call_handler() method. The callable and registered C handlers are available (as keys) in the table swupdate.handler. The following Lua code is an example of a simple handler chain-calling the rawfile C handler:

--- Lua Handler.
--- @param  image  img_type  Lua equivalent of `struct img_type`
--- @return number           # 0 on success, 1 on error
function lua_handler(image)

if not swupdate.handler["rawfile"] then
swupdate.error("rawfile handler not available")
return 1
image.path = "/tmp/destination.path"
local err, msg = swupdate.call_handler("rawfile", image)
if err ~= 0 then
swupdate.error(string.format("Error chaining handlers: %s", msg))
return 1
return 0 end

Note that when chaining handlers and calling a C handler for a different type of artifact than the Lua handler is registered for, the image table's values must satisfy the called C handler's expectations: Consider the above Lua handler being registered for "images" (swupdate.HANDLER_MASK.IMAGE_HANDLER) via the swupdate.register_handler() call shown above. As per the sw-description's attribute reference, the "images" artifact type doesn't have the path attribute but the "file" artifact type does. So, for calling the rawfile handler, image.path has to be set prior to chain-calling the rawfile handler, as done in the example above. Usually, however, no such adaptation is necessary if the Lua handler is registered for handling the type of artifact that image represents.

In addition to calling C handlers, the image table passed as parameter to a Lua handler has a image:copy2file() method that implements the common use case of writing the input stream's data to a file, which is passed as this method's argument. On success, image:copy2file() returns 0 or -1 plus an error message on failure. The following Lua code is an example of a simple handler calling image:copy2file():

--- Lua Handler.
--- @param  image  img_type  Lua equivalent of `struct img_type`
--- @return number           # 0 on success, 1 on error
function lua_handler(image)

local err, msg = image:copy2file("/tmp/destination.path")
if err ~= 0 then
swupdate.error(string.format("Error calling copy2file: %s", msg))
return 1
return 0 end

Beyond using image:copy2file() or chain-calling C handlers, the image table passed as parameter to a Lua handler has a image:read(<callback()>) method that reads from the input stream and calls the Lua callback function <callback()> for every chunk read, passing this chunk as parameter. On success, 0 is returned by image:read(). On error, -1 plus an error message is returned. The following Lua code is an example of a simple handler printing the artifact's content:

--- Lua Handler.
--- @param  image  img_type  Lua equivalent of `struct img_type`
--- @return number           # 0 on success, 1 on error
function lua_handler(image)

err, msg = image:read(function(data) print(data) end)
if err ~= 0 then
swupdate.error(string.format("Error reading image: %s", msg))
return 1
return 0 end

Using the image:read() method, an artifact's contents may be (post-)processed in and leveraging the power of Lua without relying on preexisting C handlers for the purpose intended.

Just as C handlers, a Lua handler must consume the artifact described in its image parameter so that SWUpdate can continue with the next artifact in the stream after the Lua handler returns. Chaining handlers, calling image:copy2file(), or using image:read() satisfies this requirement.

The swupdate Lua module interface specification that details what functionality is made available to Lua handlers by SWUpdate's corelib/lua_interface.c is found in handlers/swupdate.lua. It serves as reference, for mocking purposes, and type checking thanks to the EmmyLua-inspired annotations.

Note that although the dynamic nature of Lua handlers would technically allow one to embed them into a to be processed .swu image, this is not implemented as it carries some security implications since the behavior of SWUpdate is changed dynamically.

Remote handler

Remote handlers are thought for binding legacy installers without having the necessity to rewrite them in Lua. The remote handler forward the image to be installed to another process, waiting for an acknowledge to be sure that the image is installed correctly. The remote handler makes use of the zeromq library - this is to simplify the IPC with Unix Domain Socket. The remote handler is quite general, describing in sw-description with the "data" attribute how to communicate with the external process. The remote handler always acts as client, and try a connect() using the socket identified by the "data" attribute. For example, a possible setup using a remote handler could be:

images: (

filename = "myimage"";
type = "remote";
data = "test_remote";
} )

The connection is instantiated using the socket test_remote (according to the "data" field's value) in the directory pointed to by the environment variable TMPDIR with /tmp as fall-back if TMPDIR is not set. If connect() fails, the remote handler signals that the update is not successful. Each zeromq message from SWUpdate is a multi-part message split into two frames:

  • first frame contains a string with a command.
  • second frame contains data and can be of 0 bytes.

There are currently just two possible commands: INIT and DATA. After a successful connect, SWUpdate sends the initialization string in the format:

INIT:<size of image to be installed>

The external installer is informed about the size of the image to be installed, and it can assign resources if it needs. It will answer with the string ACK or NACK. The first NACK received by SWUpdate will interrupt the update. After sending the INIT command, the remote handler will send a sequence of DATA commands, where the second frame in message will contain chunks of the image to be installed. It is duty of the external process to take care of the amount of data transferred and to release resources when the last chunk is received. For each DATA message, the external process answers with a ACK or NACK message.

SWU forwarder

The SWU forwarder handler can be used to update other systems where SWUpdate is running. It can be used in case of master / slaves systems, where the master is connected to the network and the "slaves" are hidden to the external world. The master is then the only interface to the world. A general SWU can contain embedded SWU images as single artifacts, and the SWU handler will forward it to the devices listed in the description of the artifact. The handler can have a single "url" properties entry with an array of urls. Each url is the address of a secondary board where SWUpdate is running with webserver activated. The SWU handler expects to talk with SWUpdate's embedded webserver. This helps to update systems where an old version of SWUpdate is running, because the embedded webserver is a common feature present in all versions. The handler will send the embedded SWU to all URLs at the same time, and setting installed-directly is supported by this handler. [image]

The following example shows how to set a SWU as artifact and enables the SWU forwarder:

images: (

filename = "image.swu";
type = "swuforward";
properties: {
url = ["", ""];

rdiff handler

The rdiff handler adds support for applying binary delta patches generated by librsync's rdiff tool.

Naturally, the smaller the difference between the diff's source and target, the more effective is using this handler rather than shipping the full target, e.g., via the image handler. Hence, the most prominent use case for the rdiff handler is when having a read-only root filesystem and applying a small update like security fixes or feature additions. If the sweet spot is crossed, an rdiff patch may even exceed the full target's size due to necessary patch metadata. Also note that in order to be most effective, an image to be processed with rdiff should be built deterministic (see

The rdiff algorithm requires no resources whatsoever on the device as the patch is fully computed in the backend. Consequently, the backend has to have knowledge of the current software running on the device in order to compute a sensible patch. Alike, the patch has to be applied on the device to an unmodified source as used in the backend for patch computation. This property is in particular useful for resource-constrained devices as there's no need for the device to, e.g., aid in the difference computation.

First, create the signature of the original (base) file via rdiff signature <basefile> <signaturefile>. Then, create the delta file (i.e., patch) from the original base file to the target file via rdiff delta <signaturefile> <targetfile> <deltafile>. The <deltafile> is the artifact to be applied via this handler on the device. Essentially, it mimics running rdiff patch <basefile> <deltafile> <targetfile> on the device. Naturally for patches, the very same <basefile> has to be used for creating as well as for applying the patch to.

This handler registers itself for handling files and images. An exemplary sw-description fragment for the files section is

files: (

type = "rdiff_file"
filename = "";
path = "/usr/bin/file";
} );

Note that the file referenced to by path serves as <basefile> and gets replaced by a temporary file serving as <targetfile> while the rdiff patch processing.

An exemplary sw-description fragment for the images section is

images: (

type = "rdiff_image";
filename = "";
device = "/dev/mmcblk0p2";
properties: {
rdiffbase = ["/dev/mmcblk0p1"];
} );

Here, the property rdiffbase qualifies the <basefile> while the device attribute designates the <targetfile>. Note that there's no support for the optional offset attribute in the rdiff_image handler as there's currently no apparent use case for it and skipping over unchanged content is handled well by the rdiff algorithm.

ucfw handler

This handler allows one to update the firmware on a microcontroller connected to the main controller via UART. Parameters for setup are passed via sw-description file. Its behavior can be extended to be more general. The protocol is ASCII based. There is a sequence to be done to put the microcontroller in programming mode, after that the handler sends the data and waits for an ACK from the microcontroller.

The programming of the firmware shall be:

Enter firmware update mode (bootloader)
Set "reset line" to logical "low"
Set "update line" to logical "low"
Set "reset line" to logical "high"

Send programming message


to the microcontroller. (microcontroller will remain in programming state)

microcontroller confirms with


Data transmissions package based from mainboard to microcontroller package definition:
  • within a package the records are sent one after another without the end of line marker <CR><LF>
  • the package is completed with <CR><LF>

The microcontroller requests the next package with $READY;<<CS>><CR><LF>
Repeat step 4 and 5 until the complete firmware is transmitted.
The keypad confirms the firmware completion with $COMPLETED;<<CS>><CR><LF>
Set "Update line" to logical "high"
Perform a reset over the "reset line"

<<CS>> : checksum. The checksum is calculated as the two's complement of the modulo-256 sum over all bytes of the message string except for the start marker "$". The handler expects to get in the properties the setup for the reset and prog gpios. They should be in this format:

properties = {

reset = "<gpiodevice>:<gpionumber>:<activelow>";
prog = "<gpiodevice>:<gpionumber>:<activelow>"; }


images: (

filename = "microcontroller-image";
type = "ucfw";
device = "/dev/ttymxc5";
properties: {
reset = "/dev/gpiochip0:38:false";
prog = "/dev/gpiochip0:39:false";
} );

SSBL Handler

This implements a way to switch two software sets using a duplicated structure saved on the flash (currently, only NOR flash is supported). Each of the two structures contains address and size of the image to be loaded by a first loader. A field contain the "age", and it is incremented after each switch to show which is the active set.

Structure of SSBL Admin

SSBL Magic Number (29 bit)Name Age (3 bit)
Image Address Offset
Image Size

The handler implements a post install script. First, it checks for consistency the two structures and find the active reading the first 32 bit value with a magic number and the age. It increments the age and saves the new structure in the inactive copy. After a reboot, the loader will check it and switch the software set.

scripts: (

type = "ssblswitch";
properties: {
device = ["mtdX", "mtdY"];
offset = ["0", "0"];
imageoffs = ["0x780000", "0xA40000"];
imagesize = ["0x800000", "0x800000"];
} }

Properties in sw-description are all mandatory. They define where the SSBL Administration data are stored for both sets. Each properties is an array of two entries, containing values for each of the two SSBL administration.

Properties for SSBL handler

Name Type Description
device string MTD device where the SSBL Admin Header is stored
offset hex Offset of SSBL header inside the MTD device
imageoffset hex Offset of the image to be loaded by a bootloader when this SSBL is set.
imagesize hex Size of the image to be loaded by a bootloader when this SSBL is set.

Readback Handler

To verify that an image was written properly, this readback handler calculates the sha256 hash of a partition (or part of it) and compares it against a given hash value.

The following example explains how to use this handler:

scripts: (

device = "/dev/mmcblk2p1";
type = "readback";
properties: {
sha256 = "e7afc9bd98afd4eb7d8325196d21f1ecc0c8864d6342bfc6b6b6c84eac86eb42";
size = "184728576";
offset = "0";
}; } );

Properties size and offset are optional, all the other properties are mandatory.

Properties for readback handler

Name Type Description
device string The partition which shall be verified.
type string Identifier for the handler.
sha256 string Expected sha256 hash of the partition.
size string Data size (in bytes) to be verified. If 0 or not set, the handler will get the partition size from the device.
offset string Offset (in bytes) to the start of the partition. If not set, default value 0 will be used.

Rawcopy handler

The rawcopy handler copies one source to a destination. It is a script handler, and no artifact in the SWU is associated with the handler. It can be used to copy configuration data, or parts that should be taken by the current installation. It requires the mandatory property (copyfrom), while device contains the destination path. The handler performs a byte copy, and it does not matter which is the source - it can be a file or a partition. An optional type field can set if the handler is active as pre or postinstall script. If not set, the handler is called twice.

scripts : (

device = "/dev/mmcblk2p1";
type = "rawcopy";
properties : {
copyfrom = "/dev/mmcblk2p2";
type = "postinstall";
} }

Bootloader handler

The bootloader handler allows to set bootloader's environment with a file. The file shold have the format:

# Comments are allowed using the hash char

Empty lines are skipped. This simplifies the update of the whole environment instead of setting each variable inside the "bootenv" section in sw-description. The property nooverride allows to skip variables that are already set in sw-description. If not set, variables set in bootenv are overwritten.

images: (

filename = "uEnv.txt";
type = "bootloader";
properties: {
nooverride = "true";
} ); bootenv: ( {
name = "bootenv01key";
value = "SOME VALUE"; });

In the example above, bootenv01key is not overwritten by a value in uEnv.txt because the flag "nooverride" is set.

Archive handler

The archive handler extracts an archive to a destination path. It supports whatever format libarchive has been compiled to support, for example even if swupdate itself has no direct support for xz it can be possible to extract tar.xz files with it.

The attribute preserve-attributes must be set to preserve timestamps. uid/gid (numeric), permissions (except +x, always preserved) and extended attributes.

The property create-destination can be set to the string true to have swupdate create the destination path before extraction.

files: (

filename = "examples.tar.zst";
type = "archive";
path = "/extract/here";
preserve-attributes = true;
installed-directly = true;
properties: {
create-destination = "true";
} );

Disk partitioner

This handler creates or modifies partitions using the library libfdisk. Handler must be put into the partitions section of sw-description. Setup for each partition is put into the properties field of sw-description. After writing the partition table it may create a file system on selected partitions. (Available only if CONFIG_DISKFORMAT is set.)

Properties for diskpart handler

Name Type Description
labeltype string "gpt" or "dos"
nolock string "true" or "false" (default=false) This is like a force. If it is set, a lock failure will be ignored(lock will still be attempted).
noinuse string "true" or "false" (default=false) If set, it does not require the device to be not in use (mounted, etc.)
partition-X array Array of values belonging to the partition number X

For each partition, an array of couples key=value must be given. The following keys are supported:

Setup for a disk partition

Name Type Description
size string Size of partition. K, M and G can be used for Kilobytes, Megabytes and Gigabytes.
start integer First sector for the partition
name string Name of the partition
type string Type of partition, it has two different meanings. It is the hex code for DOS (MBR) partition table or it is the string identifier in case of GPT.
dostype string Type of DOS (MBR) partition entry when using a table with a "gpt" labeltype. Using this option will create a hybrid MBR table. It is the hex code for DOS (MBR) partition table. This would typically be used when one wants to use a GPT formatted disk with a board that requires a dos table entry for initial bootstrapping. Note: A maximum of 3 partitions can have a dostype specified, this limit only applies to dos table entries and does not affect partitions without a dostype specified.
fstype string Optional filesystem type to be created on the partition. If no fstype key is given, no file will be created on the corresponding partition. vfat / ext2 / ext3 /ext4 file system is supported
partuuid string The partition UUID (GPT only). If omitted, a UUID will be generated automatically.
flag string The following flags are supported: Dos Partition : "boot" set bootflag

GPT example:

partitions: (

type = "diskpart";
device = "/dev/sde";
properties: {
labeltype = "gpt";
partition-1 = [ "size=64M", "start=2048",
"name=bigrootfs", "type=C12A7328-F81F-11D2-BA4B-00A0C93EC93B"];
partition-2 = ["size=256M", "start=133120",
"name=ldata", "type=EBD0A0A2-B9E5-4433-87C0-68B6B72699C7",
partition-3 = ["size=512M", "start=657408",
"name=log", "fstype =ext4", 63DAF-8483-4772-8E79-3D69D8477DE4"];
partition-4 = ["size=4G", "start=1705984",
"name=system", "type=0FC63DAF-8483-4772-8E79-3D69D8477DE4"];
partition-5 = ["size=512M", "start=10094592",
"name=part5", "type=0FC63DAF-8483-4772-8E79-3D69D8477DE4"];
} }

MBR Example:

partitions: (

type = "diskpart";
device = "/dev/sde";
properties: {
labeltype = "dos";
partition-1 = [ "size=64M", "start=2048", "name=bigrootfs", "type=0x83"];
partition-2 = ["size=256M", "start=133120", "name=ldata", "type=0x83"];
partition-3 = ["size=256M", "start=657408", "name=log", "type=0x83"];
partition-4 = ["size=6G", "start=1181696", "name=system", "type=0x5"];
partition-5 = ["size=512M", "start=1183744", "name=part5", "type=0x83"];
partition-6 = ["size=512M", "start=2234368", "name=part6", "type=0x83"];
partition-7 = ["size=16M", "start=3284992", "name=part7", "type=0x6",
} }

Toggleboot Handler

This handler is a script handler. It turns on the bootflag for one of a disk partition if the partition table is DOS. It reports an error if the table is GPT.

script: (

type = "toggleboot";
device = "/dev/sde";
properties: {
partition = "1";
} }

gpt partition installer

There is a handler gptpart that allows writing an image into a gpt partition selected by the name. This handler do not modify the gpt partition (type, size, ...), it just writes the image in the GPT partition.

images: (

filename = "u-boot.bin";
type = "gptpart";
device = "/dev/vdb";
volume = "u-boot-1";
offset = "1024";
filename = "kernel.bin";
type = "gptpart";
device = "/dev/vdb";
volume = "kernel-1";
}, );

gpt partition swap

There is a handler gptswap that allow to swap gpt partitions after all the images were flashed. This handler only swap the name of the partition. It coud be usefull for a dual bank strategy. This handler is a script for the point of view of swupdate, so the node that provide it should be added in the section scripts.

Simple example:

scripts: (

type = "gptswap";
device = "/dev/vdb";
properties =
swap-0 = [ "u-boot-0" , "u-boot-1" ];
swap-1 = [ "kernel-0" , "kernel-1" ];
}, );

Diskformat Handler

This handler checks if the device already has a file system of the specified type. (Available only if CONFIG_DISKFORMAT is set.) If the file system does not yet exist, it will be created. In case an existing file system shall be overwitten, this can be achieved by setting the property force to true.

partitions: (

type = "diskformat";
device = "/dev/loop0p1";
properties: {
fstype = "vfat";
force = "true";
} })

Unique UUID Handler

This handler checks if the device already has a filesystems with a provide UUID. This is helpful in case the bootloader chooses the device to boot from the UUID and not from the partition number. One use case is with the GRUB bootloader when GRUB_DISABLE_LINUX_UUID is not set, as usual on Linux Distro as Debian or Ubuntu.

The handler iterates all UUIDs given in sw-description and raises error if one of them is found on the device. It is a partition handler and it runs before any image is installed.

partitions: (

type = "uniqueuuid";
properties: {
fs-uuid = ["21f16cae-612f-4bc6-8ef5-e68cc9dc4380",
} });

Delta Update Handler

The handler processes a ZCHUNK header and finds which chunks should be downloaded after generating the corresponding header of the running artifact to be updated. The handler uses just a couple of attributes from the main setup, and gets more information from the properties. The attributes are then passed to a secondary handler that will install the artefact after the delta handler has assembled it. The handler requires ZST because this is the compression format for Zchunk.

The SWU must just contain the ZCK's header, while the ZCK file is put as it is on the server. The utilities in Zchunk project are used to build the zck file.

zck -u -h sha256 <artifact>

This will generates a file <arifact>.zck. To extract the header, use the zck_read_header utility:

HSIZE=`zck_read_header -v <artifact>.zck | grep "Header size" | cut -d':' -f2`
dd if=<artifact>.zck of=<artifact>.header bs=1 count=$((HSIZE))

The resulting header file must be packed inside the SWU.

Properties for delta update handler

Name Type Description
url string This is the URL from where the handler will download the missing chunks. The server must support byte range header.
source string name of the device or file to be used for the comparison.
chain string this is the name (type) of the handler that is called after reassembling the artifact.
max-ranges string Max number of ranges that a server can accept. Default value (150) should be ok for most servers.
zckloglevel string this sets the log level of the zcklib. Logs are intercepted by SWupdate and appear in SWUpdate's log. Value is one of debug,info warn,error,none
debug-chunks string "true", default is not set. This activates more verbose debugging output and the list of all chunks is printed, and it reports if a chunk is downloaded or copied from the source.
source-size string This limits the index of the source It is helpful in case of filesystem in much bigger partition. It has the value for the size or it can be set to "detect" and the handler will try to find the effective size of fs.



filename = "software.header";
type = "delta";
device = "/dev/mmcblk0p2";
properties: {
url = "";
chain = "raw";
source = "/dev/mmcblk0p3";
zckloglevel = "error";
/* debug-chunks = "true"; */
}; }



Mongoose is a daemon mode of SWUpdate that provides a web server, web interface and web application.

The web application in web-app uses the Node.js package manager and gulp as build tool. It depends on Bootstrap 4, Font Awesome 5 and Dropzone.js.


After having configured and compiled SWUpdate with enabled mongoose web server:

./swupdate --help

lists the mandatory and optional arguments to be provided to mongoose. As an example,

./swupdate -l 5 -w '-r ./examples/www/v2 -p 8080' -p 'reboot'

runs SWUpdate in mongoose daemon mode with log-level TRACE and a web server at http://localhost:8080.


The ready to use example of the web application in the examples/www/v2 directory uses a Public Domain background.jpg image from pixabay with is released under the Creative Commons CC0 license. The used favicon.png and logo.png images are made from the SWUpdate logo and therefore subject to the GNU General Public License version 2. You must comply to this license or replace the images with your own files.


You could customize the web application inside the web-app directory. Beside the replace of the favicon.png, logo.png and background.jpg images inside the images directory you could customize the Bootstrap colors and settings inside the scss/bootstrap.scss style sheet. The style sheet changes need a rebuild of the web application source code.


The development requires Node.js version 6 or greater and a prebuilt SWUpdate project with enabled mongoose web server and web application interface version 2 support.

Enter the web application directory:

cd ./web-app

Install the dependencies:

npm install

Build the web application:

npm run build

Start the web application:

../swupdate -w '-r ./dist -p 8080' -p 'echo reboot'

Test the web application:

Pack the web application (optional):

npm run package -- --output swupdate-www.tar.gz


Please run the linter before any commit

npm run lint



Suricatta is -- like mongoose -- a daemon mode of SWUpdate, hence the name suricatta (engl. meerkat) as it belongs to the mongoose family.

Suricatta regularly polls a remote server for updates, downloads, and installs them. Thereafter, it reboots the system and reports the update status to the server, based on an update state variable currently stored in bootloader's environment ensuring persistent storage across reboots. Some U-Boot script logics or U-Boot's bootcount feature may be utilized to alter this update state variable, e.g., by setting it to reflect failure in case booting the newly flashed root file system has failed and a switchback had to be performed.

Suricatta is designed to be extensible in terms of the servers supported as described in Section The Suricatta Interface. Currently, support for the hawkBit server is implemented via the hawkBit Direct Device Integration API alongside a simple general purpose HTTP server. The support for suricatta modules written in Lua is not a particular server support implementation but rather an option for writing such in Lua instead of C.

Running suricatta

After having configured and compiled SWUpdate with enabled suricatta support,

./swupdate --help

lists the mandatory and optional arguments to be provided to suricatta when using hawkBit as server. As an example,

./swupdate -l 5 -u '-t default -u -i 25'

runs SWUpdate in suricatta daemon mode with log-level TRACE, polling a hawkBit instance at with tenant default and device ID 25.

Note that on startup when having installed an update, suricatta tries to report the update status to its upstream server, e.g., hawkBit, prior to entering the main loop awaiting further updates. If this initial report fails, e.g., because of a not (yet) configured network or a currently unavailable hawkBit server, SWUpdate may exit with an according error code. This behavior allows one to, for example, try several upstream servers sequentially. If suricatta should keep retrying until the update status is reported to its upstream server irrespective of the error conditions, this has to be realized externally in terms of restarting SWUpdate on exit.

After an update has been performed, an agent listening on the progress interface may execute post-update actions, e.g., a reboot, on receiving DONE. Additionally, a post-update command specified in the configuration file or given by the -p command line option can be executed.

Note that at least a restart of SWUpdate has to be performed as post-update action since only then suricatta tries to report the update status to its upstream server. Otherwise, succinct update actions announced by the upstream server are skipped with an according message until a restart of SWUpdate has happened in order to not install the same update again.

The Suricatta Interface

Support for servers other than hawkBit or the general purpose HTTP server can be realized by implementing the "interfaces" described in include/channel.h and include/suricatta/server.h, the latter either in C or in Lua. The channel interface abstracts a particular connection to the server, e.g., HTTP-based in case of hawkBit. The server interface defines the logics to poll and install updates. See corelib/channel_curl.c / include/channel_curl.h and suricatta/server_hawkbit.{c,h} for an example implementation in C targeted towards hawkBit.

include/channel.h describes the functionality a channel has to implement:

typedef struct channel channel_t;
struct channel {

... }; channel_t *channel_new(void);

which sets up and returns a channel_t struct with pointers to functions for opening, closing, fetching, and sending data over the channel.

include/suricatta/server.h describes the functionality a server has to implement:

server_op_res_t server_has_pending_action(int *action_id);
server_op_res_t server_install_update(void);
server_op_res_t server_send_target_data(void);
unsigned int server_get_polling_interval(void);
server_op_res_t server_start(const char *cfgfname, int argc, char *argv[]);
server_op_res_t server_stop(void);
server_op_res_t server_ipc(int fd);

The type server_op_res_t is defined in include/suricatta/suricatta.h. It represents the valid function return codes for a server's implementation.

In addition to implementing the particular channel and server, the suricatta/ file has to be adapted to include a new option so that the new implementation becomes selectable in SWUpdate's configuration. In the simplest case, adding an option like the following one for hawkBit into the menu "Server" section is sufficient.


bool "hawkBit support"
depends on HAVE_LIBCURL
depends on HAVE_JSON_C
select JSON
select CURL
Support for hawkBit server.

Having included the new server implementation into the configuration, edit suricatta/Makefile to specify the implementation's linkage into the SWUpdate binary, e.g., for the hawkBit example implementation, the following lines add server_hawkbit.o to the resulting SWUpdate binary if SURICATTA_HAWKBIT was selected while configuring SWUpdate.

lib-$(CONFIG_SURICATTA) += server_hawkbit.o

Support for general purpose HTTP server

This is a very simple backend that uses standard HTTP response codes to signal if an update is available. There are closed source backends implementing this interface, but because the interface is very simple interface, this server type is also suitable for implementing an own backend server. For inspiration, there's a simple (mock) server implementation available in examples/suricatta/

The API consists of a GET with Query parameters to inform the server about the installed version. The query string has the format:

http(s)://<base URL>?param1=val1&param2=value2...

As examples for parameters, the device can send its serial number, MAC address and the running version of the software. It is duty of the backend to interpret this - SWUpdate just takes them from the "identify" section of the configuration file and encodes the URL.

The server answers with the following return codes:

HTTP Code Text Description
302 Found A new software is available at URL in the Location header
400 Bad Request Some query parameters are missing or in wrong format
403 Forbidden Client certificate not valid
404 Not found No update is available for this device
503 Unavailable An update is available but server can't handle another update process now.

Server's answer can contain the following headers:

Header's name Codes Description
Retry-after 503 Contains a number which tells the device how long to wait until ask the next time for updates. (Seconds)
Content-MD5 302 Contains the checksum of the update file which is available under the url of location header
Location 302 URL where the update file can be downloaded.

The device can send logging data to the server. Any information is transmitted in a HTTP PUT request with the data as plain string in the message body. The Content-Type Header need to be set to text/plain.

The URL for the logging can be set as separate URL in the configuration file or via --logurl command line parameter:

The device sends data in a CSV format (Comma Separated Values). The format is:


The format can be specified in the configuration file. A format For each event can be set. The supported events are:

Event Description
check dummy. It could send an event each time the server is polled.
started A new software is found and SWUpdate starts to install it
success A new software was successfully installed
fail Failure by installing the new software

The general server has an own section inside the configuration file. As example:

gservice =

url = ....;
logurl = ;
logevent : (
{event = "check"; format="#2,date,fw,hw,sp"},
{event = "started"; format="#12,date,fw,hw,sp"},
{event = "success"; format="#13,date,fw,hw,sp"},
{event = "fail"; format="#14,date,fw,hw,sp"}
); }

date is a special field and it is interpreted as localtime in RFC 2822 format. Each Comma Separated field is looked up inside the identify section in the configuration file, and if a match is found the substitution occurs. In case of no match, the field is sent as it is. For example, if the identify section has the following values:

identify : (

{ name = "sp"; value = "333"; },
{ name = "hw"; value = "ipse"; },
{ name = "fw"; value = "1.0"; } );

with the events set as above, the formatted text in case of "success" will be:

Formatted log: #13,Mon, 17 Sep 2018 10:55:18 CEST,1.0,ipse,333

Support for Suricatta Modules in Lua

The server_lua.c C-to-Lua bridge enables writing suricatta modules in Lua. It provides the infrastructure in terms of the interface to SWUpdate "core" to the Lua realm, enabling the "business logic" such as handling update flows and communicating with backend server APIs to be modeled in Lua. To the Lua realm, the server_lua.c C-to-Lua bridge provides the same functionality as the other suricatta modules written in C have, realizing a separation of means and control. Effectively, it lifts the interface outlined in Section The Suricatta Interface to the Lua realm.

As an example server implementation, see examples/suricatta/ for a simple (mock) server of a backend that's modeled after the "General Purpose HTTP Server" (cf. Section Support for general purpose HTTP server). The matching Lua suricatta module is found in examples/suricatta/swupdate_suricatta.lua. Place it in Lua's path so that a require("swupdate_suricatta") can load it or embed it into the SWUpdate binary by enabling CONFIG_EMBEDDED_SURICATTA_LUA and setting CONFIG_EMBEDDED_SURICATTA_LUA_SOURCE accordingly.

The interface specification in terms of a Lua (suricatta) module is found in suricatta/suricatta.lua.


The suricatta table is the module's main table housing the exposed functions and definitions via the sub-tables described below. In addition, the main functions suricatta.install() and as well as the convenience functions suricatta.getversion(), suricatta.sleep(), and suricatta.get_tmpdir() are exposed:

The function suricatta.install(install_channel) installs an update artifact from a remote server or a local file. The install_channel table parameter designates the channel to be used for accessing the artifact plus channel options diverging from the defaults set at channel creation time. For example, an install_channel table may look like this:

{ channel = chn, url = "" }

where chn is the return value of a call to The other table attributes, like url in this example, are channel options diverging from or omitted while channel creation time, see For installing a local file, an install_channel table may look like this:

{ channel = chn, url = "file:///path/to/file.swu" }

The function, localpath) just downloads an update artifact. The parameter download_channel is as for suricatta.install(). The parameter localpath designates the output path for the artifact. The suricatta.get_tmpdir() function (see below) is in particular useful for this case to supply a temporary download location as localpath. A just downloaded artifact may be installed later using suricata.install() with an appropriate file:// URL, realizing a deferred installation.

Both, suricatta.install() and return true, or, in case of error, nil, a suricatta.status value, and a table with messages in case of errors, else an empty table.

The function suricatta.getversion() returns a table with SWUpdate's version and patchlevel fields. This information can be used to determine API (in-)compatibility of the Lua suricatta module with the SWUpdate version running it.

The function suricatta.sleep(seconds) is a wrapper around SLEEP(3) for, e.g., implementing a REST API call retry mechanism after a number of given seconds have elapsed.

The function suricatta.get_tmpdir() returns the path to SWUpdate's temporary working directory where, e.g., the function may place the downloaded artifacts.


The suricatta.status table exposes the server_op_res_t enum values defined in include/util.h to the Lua realm.


The suricatta.notify table provides the usual logging functions to the Lua suricatta module matching their uppercase-named pendants available in the C realm.

One notable exception is suricatta.notify.progress(message) which dispatches the message to the progress interface (see progress). Custom progress client implementations listening and acting on custom progress messages can be realized using this function.

All notify functions return nil.


The suricatta.pstate table provides a binding to SWUpdate's (persistent) state handling functions defined in include/state.h, however, limited to the bootloader environment variable STATE_KEY defined by CONFIG_UPDATE_STATE_BOOTLOADER and defaulting to ustate. In addition, it captures the update_state_t enum values.

The function requires one of suricatta.pstate's "enum" values as parameter and returns true, or, in case of error, nil. The function suricatta.pstate.get() returns true, or, in case of error, nil, plus one of suricatta.pstate's "enum" values in the former case.


The suricatta.server table provides the sole function suricatta.server.register(function_p, purpose). It registers a Lua function "pointed" to by function_p for the purpose purpose which is defined by suricatta.server's "enum" values. Those enum values correspond to the functions defined in the interface outlined in the Section on The Suricatta Interface.

In addition to these functions, the two callback functions CALLBACK_PROGRESS and CALLBACK_CHECK_CANCEL can be registered optionally: The former can be used to upload progress information to the server while the latter serves as dwlwrdata function (see include/channel_curl.h) to decide on whether an installation should be aborted while the download phase.

For details on the (callback) functions and their signatures, see the interface specification suricatta/suricatta.lua and the documented example Lua suricatta module found in examples/suricatta/swupdate_suricatta.lua.

The suricatta.server.register() function returns true, or, in case of error, nil.

The table captures channel handling for suricatta Lua modules. The single function creates and opens a channel to a server. Its single parameter options is a table specifying the channel's default options such as proxy, retries, usessl, strictssl, or headers_to_send. For convenience, options that may change per request such as url, content-type, or headers_to_send may be set as defaults on channel creation time while being selectively overruled on a per request basis. The channel options currently supported to be set are listed in the table. In essence, the options parameter table is the Lua table equivalent of include/channel_curl.h's channel_data_t.

The function returns a channel table which is either passed to the suricatta.install() and functions or used directly for communication with a server. More specifically, it has the three functions

  • get(options) for retrieving information from the server,
  • put(options) for sending information to the server, and
  • close() for closing the channel.

The get() and put() functions' single parameter options is a per-request channel option table as described above.

The functions get() and put() return true, or, in case of error, nil, a suricatta.status value, and an operation result table. The latter contains the fields:

  • http_response_code carrying the HTTP error code,
  • format as one of's options,
  • raw_reply if options contained format =,
  • json_reply if options contained format =, and
  • the HTTP headers received in the received_headers table, if any.

The "enum" table defines the "format", i.e., the response body content type and whether to parse it or not:

  • NONE means the response body is discarded.
  • RAW means the raw server's reply is available as raw_reply.
  • JSON means the server's JSON reply is parsed into a Lua table and available as json_reply.

The "enum" table defines the HTTP method to use for a request issued with the put(options) function, i.e., POST, PATCH, or PUT as specified in the options parameter table via the method attribute. In addition to the HTTP method, the request body's content is set with the request_body attribute in the options parameter table.

As a contrived example, consider the following call to a channel's put() function

local res, _, data = channel.put({

url = string.format("%s/%s", base_url, device_id),
content_type = "application/json",
method =,
format =,
request_body = "{ ... }"
}) ...

that issues a HTTP PATCH to some URL with a JSON content without having interest in the response body.

More examples of how to use a channel can be found in the example suricatta Lua module examples/suricatta/swupdate_suricatta.lua.


A user-contributed recipe based on hawkBit (0.2.0-SNAPSHOT) + swupdate (v2018.03)


Use HTTPS on a hawkBit server to avoid server spoofing. Anonymous client connections are authorized.


On the PKI:

  • Create a pkcs#12 (.p12) file, rolling server key, server, private CA, sub CA certs into a single file.
  • Use a password on the server key you won't be ashamed of.
  • Also create a single .pem file for the private CA + sub-CA

On the hawkBit host:

  • hawkBit uses the Java KeyStore to access credentials, but a JKS is not designed apparently to hold CA certs, which is a problem for private CAs. The workaround is to make it gulp an entire pkcs#12 file.
  • It looks like a JKS like this cannot have a password different from the one protecting the .p12. Keytool also seems to have a little tendency to destruct the .jks if you change your mind and want to change the password... Basically do everything you need with openssl and use only keytool for generating the .jks file.

The following command imports a .p12 into a "pkcs12 Java keystore", keeping the same password:

keytool -importkeystore -srckeystore hb-pass.p12 -srcstoretype pkcs12 \

-destkeystore hb-pass.jks -deststoretype pkcs12 \
-alias 1 -deststorepass <password_of_p12>

Then you need to adapt of the hawkBit server to make use of the keystore. There are extra requirements to make hawkBit send artifacts via HTTPS.

This is the relevant part of <hawkBit dir>/hawkbit-runtime/hawkbit-update-server/src/main/resources/

# HTTPS mode working w/ swupdate
# See also
# Need to run as root to use port 443
# Overriding some of is required
# Upgrades http:8443 to https:8443
# Would redirect + upgrade http:80 to https:443
# Server cert+key w/ private CA + subCA
# See also
# (2008, still relevant!?)
# File .jks is a .p12 imported via keytool. Only one password supported, set from openssl.

On the swupdate client host(s):

  • The client needs the private CA certificate(s) to authenticate the server.
  • There is a setting in swupdate to specify the path to a single CA cert, not a directory. Beyond that, libcurl looks into /etc/ssl/certs. So we're using a compound "CA chain" .pem file to hold both private CA and sub-CA in our preferred location.

This is the relevant part of /etc/swupdate/swupdate.conf:

suricatta :

tenant = "default";
id = "machineID";
url = "https://hb.domain:8443";
nocheckcert = false;
cafile = "/etc/swupdate/priv-cachain.pem"; /* CA + sub CA in one file */
/* sslkey = anon client: do not set; */
/* sslcert = anon client: do not set; */ ...



SWUpdate contains an integrated web-server to allow remote updating. However, which protocols are involved during an update is project specific and differs significantly. Some projects can decide to use FTP to load an image from an external server, or using even a proprietary protocol. The integrated web-server uses this interface.

SWUpdate has a simple interface to let external programs to communicate with the installer. Clients can start an upgrade and stream an image to the installer, querying then for the status and the final result. The API is at the moment very simple, but it can easy be extended in the future if new use cases will arise.

API Description

The communication runs via UDS (Unix Domain Socket). The socket is created at startup by SWUpdate in /tmp/sockinstctrl as per default configuration. This socket should, however, not be used directly but instead by the Client Library explained below.

The exchanged packets are described in network_ipc.h

typedef struct {

int magic;
int type;
msgdata data; } ipc_message;

Where the fields have the meaning:

  • magic : a magic number as simple proof of the packet
  • msgdata : a buffer used by the client to send the image or by SWUpdate to report back notifications and status.

The client sends a REQ_INSTALL packet and waits for an answer. SWUpdate sends back ACK or NACK, if for example an update is already in progress.

After the ACK, the client sends the whole image as a stream. SWUpdate expects that all bytes after the ACK are part of the image to be installed. SWUpdate recognizes the size of the image from the CPIO header. Any error lets SWUpdate to leave the update state, and further packets will be ignored until a new REQ_INSTALL will be received. [image]

It is recommended to use the client library to communicate with SWUpdate. On the lower level with direct socket communication, it cannot be guaranteed that the structures will remain compatible in the future. The client library was affected by this issue, too, and it is changed to accept an opaque interface that will survive API changes. Compatibility layers could be added on-demand in the future due to API changes.

Client Library

Functions to start an update

A library simplifies the usage of the IPC making available a way to start asynchronously an update.

The library consists of one function and several call-backs.

int swupdate_async_start(writedata wr_func, getstatus status_func,

terminated end_func, void *req, ssize_t size) typedef int (*writedata)(char **buf, int *size); typedef int (*getstatus)(ipc_message *msg); typedef int (*terminated)(RECOVERY_STATUS status);

swupdate_async_start creates a new thread and start the communication with SWUpdate, triggering for a new update. The wr_func is called to get the image to be installed. It is responsibility of the callback to provide the buffer and the size of the chunk of data.

The getstatus call-back is called after the stream was downloaded to check how upgrade is going on. It can be omitted if only the result is required.

The terminated call-back is called when SWUpdate has finished with the result of the upgrade.

Example about using this library is in the examples/client directory.

The req structure is casted to void to ensure API compatibility. Am user should instantiate it as struct swupdate_request. This contains fields that can control the update process:

struct swupdate_request {

unsigned int apiversion;
sourcetype source;
int dry_run;
size_t len;
char info[512];
char software_set[256];
char running_mode[256]; };

A user should first call swupdate_prepare_req()

void swupdate_prepare_req(struct swupdate_request *req);

This fills the request structure with default values. After that, the user can fill the other fields as:

  • dry_run : one of RUN_DEFAULT (set from command line), RUN_DRYRUN, RUN_INSTALL.
  • info, len : a variable length data that can be forwarded to the progress interface. The installer in SWUpdate does not evaluate it.
  • software_set and running_mode : this allows one to set the selection for the update.

Functions to set AES keys

The key for decryption can be set with command line parameter (see -K), but it is possible to set it via IPC. In this way, each update could have a different key.

int swupdate_set_aes(char *key, char *ivt)

The key is for AES-256. The length for key and ivt are then defined by the algorithm amd they are passed as ASCII string, so the length must be 64 bytes for key and 32 bytes for IVT.

Functions to control SWUpdate

int ipc_send_cmd(ipc_message *msg);

ipc_send_cmd is used to send a command to a SWUpdate subprocess (as suricatta). The function is synchron, that means it clocks until the subprocess has answered with ACK or NACK. This function sets type to SWUPDATE_SUBPROCESS. The caller must then set the other fields in message according to the destination. The msgdata field is a structure as:

struct {

sourcetype source; /* Who triggered the update */
int cmd; /* Optional encoded command */
int timeout; /* timeout in seconds if an aswer is expected */
unsigned int len; /* Len of data valid in buf */
char buf[2048]; /*
* Buffer that each source can fill
* with additional information

The caller fills source with the subprocess that acceps the command. Values of cmd are in network_ipc.h.

Messages for suricatta

suricatta accepts messages in JSON format. The message must be formatted in the buf field of the message data.

Setting the polling time

{ "polling" : <value in seconds, range 0..X>}

Setting it to 0 has the special meaning that the polling time is retrieved from the Backend (if this is supported by the server).

Enable / disable Suricatta daemon

{ "enable" : true }
{ "enable" : false }

Set custom device attributes for Suricatta (for Hawkbit implementation)

{ "identify" : [

"name" : "customizableAttributeOne",
"value" : "valueOne"
"name" : "customizableAttributeTwo",
"value" : "valueTwo"
} ]}

New attributes can be added at runtime, and existing attributes can be modified in the same way. Changes will be reflected on the server in the next poll iteration.

Trigger a check on the server

This is useful in case the device is mostly offline, and when it is online, it should check immediately if an update exists and run it. In fact, after enabling the suricatta daemon, the update follows the usual states, and the daemon waits for a polling time before loading the new software. This command forces an update (if available) without changing the polling time.

{ "trigger" : true }

Activate an already installed Software

After a software was installed, the new software boots and if everything runs fine, an acknowledge should be sent to the hawkBit server. If this feature is used, for example to let the end user decide if the new software is accepted, the parameters used by the installation should be stored during the update process.

{ "id" : <action id>,

"finished" : "success", "failure", "none",
"execution" : ["closed", "proceeding", canceled", "rejected", "resumed"]
"details" : [ ] }

Get hawkBit Server Status

To provide the hawkBit server status to other processes, it can be requested by sending an empty message with message type CMD_GET_STATUS.

The response is a JSON object containing the hawkBit server status <status>. <status> is a number representing the value of the channel_op_res_t enum from channel_op_res.h. As the hawkBit server is polled, its status can only be updated when it has been polled. Therefore the response also contains the time <time>, when the hawkBit server has been polled the last time. It is provided as ISO 8601 date and time string. (2021-10-14T13:42:37.000+00)

{ "server" : {

"status" : <status>
"time" : <time>
} }

An example application can be found under tools/swupdate-gethawkbitstatus.c

API to the integrated Webserver

The integrated Webserver provides REST resources to push a SWU package and to get inform about the update process. This API is based on HTTP standards. There are to kind of interface:

  • Install API to push a SWU and to restart the device after update.
  • A WebSocket interface to send the status of the update process.

Install API

POST /upload

This initiates an update: the initiator sends the request and start to stream the SWU in the same way as described in API Description.

Restart API

POST /restart

If configured (see post update command), this request will restart the device.

WebSocket API

The integrated Webserver exposes a WebSocket API. The WebSocket protocol specification defines ws (WebSocket) and wss (WebSocket Secure) as two new uniform resource identifier (URI) schemes that are used for unencrypted and encrypted con nections, respectively and both of them are supported by SWUpdate. A WebSocket provides full-duplex communication but it is used in SWUpdate to send events to an external host after each change in the update process. The Webserver sends JSON formatted responses as results of internal events.

The response contains the field type, that defines which event is sent.

Event Type

type Description of event
status Event sent when SWUpdate's internal state changes
source Event to inform from which interface an update is received
info Event with custom message to be passed to an external process
message Event that contains the error message in case of error
step Event to inform about the running update

Status Change Event

This event is sent when the internal SWUpdate status change. Following status are supported:




"type": "status",
"status": "SUCCESS" }

Source Event

This event informs from which interface a SWU is loaded.


"type": "source",
"source": "WEBSERVER" }

The field source can have one of the following values:


Info Event

This event forwards all internal logs sent with level=INFO.


"type": "info",
"source": < text message > }

Message Event

This event contains the error message in case of failure.

Fields for message event

name Description
status "message"
level "3" in case of error, "6" as info
text Message associated to the event



"type": "message",
"level": "3",
"text" : "[ERROR] : SWUPDATE failed [0] ERROR core/cpio_utils.c : ", }

Step event

This event contains which is the current step running and which percentage of this step is currently installed.

Fields for step event

name Description
number total number of steps N for this update
step running step in range [1..N]
name filename of artefact to be installed
percent percentage of the running step



"type": "step",
"number": "7",
"step": "2",
"name": "rootfs.ext4.gz",
"percent": "18" }


It is often required to inform the operator about the status of the running update and not just to return if the update was successful or not. For example, if the target has a display or a remote interface, it can be forwarded which is reached percentage of the update to let estimate how much the update will still run. SWUpdate has an interface for this ("progress API"). An external process can register itself with SWUpdate, and it will receive notifications when something in the update was changed. This is different from the IPC API, because the last one is mainly used to transfer the SWU image, and it is only possible to poll the interface to know if the update is still running.

API Description

An external process registers itself to SWUpdate with a connect() request to the domain socket "/tmp/swupdateprog" as per default configuration of SWUpdate. There is no information to send, and SWUpdate simply inserts the new connection into the list of processes to be informed. SWUpdate will send a frame back after any change in the update process with the following data (see include/progress_ipc.h):

struct progress_msg {

unsigned int magic; /* Magic Number */
unsigned int status; /* Update Status (Running, Failure) */
unsigned int dwl_percent; /* % downloaded data */
unsigned int nsteps; /* No. total of steps */
unsigned int cur_step; /* Current step index */
unsigned int cur_percent; /* % in current step */
char cur_image[256]; /* Name of image to be installed */
char hnd_name[64]; /* Name of running handler */
sourcetype source; /* Interface that triggered the update */
unsigned int infolen; /* Len of data valid in info */
char info[2048]; /* additional information about install */ };

The single fields have the following meaning:

  • magic is not yet used, it could be added for simply verification of the frame.
  • status is one of the values in swupdate_status.h (START, RUN, SUCCESS, FAILURE, DOWNLOAD, DONE).
  • dwl_percent is the percentage of downloaded data when status = DOWNLOAD.
  • nsteps is the total number of installers (handlers) to be run.
  • cur_step is the index of the running handler. cur_step is in the range 1..nsteps
  • cur_percent is the percentage of work done inside the current handler. This is useful when updating a slow interface, such as a slow flash, and signals which is the percentage of image already copied into the destination.
  • cur_image is the name of the image in sw-description that is currently being installed.
  • hnd_name reports the name of the running handler.
  • source is the interface that triggered the update.
  • infolen length of data in the following info field.
  • info additional information about installation.

As an example for a progress client, tools/swupdate-progress.c prints the status on the console and drives "psplash" to draw a progress bar on a display.



In general, SWUpdate is agnostic to a particular language it is operated from, thanks to SWUpdate's socket-based control and progress APIs for external programs. As long as the language of choice has proper socket (library) support, SWUpdate can be operated with it.

However, for convenience, a Lua language binding in terms of a shared library, currently, is provided.

Lua Language Binding

The Lua language binding is realized in terms of the lua_swupdate module that defines three bindings, namely for the control interface, the progress interface, and a convenience function yielding a table holding all local network interfaces including their IP addresses and submasks.

The lua_swupdate Lua module interface specification that details what functionality is made available by bindings/lua_swupdate.c is found in bindings/lua_swupdate.lua. It serves as reference, for mocking purposes, and type checking thanks to the EmmyLua-inspired annotations.

Note that, depending on the filesystem location of the Lua binding's shared library, Lua's package.cpath may have to be adapted by setting the environment variable LUA_CPATH, modifying package.cpath prior to a require('lua_swupdate'), or , as last resort, using package.loadlib() instead of require('lua_swupdate').

Control Interface

The lua_swupdate module's control interface binding conveniently makes SWUpdate's socket-based control API available to pure Lua.

The binding is captured in the swupdate_control object that is returned by a call to swupdate.control(). This object offers the three methods connect(), write(<chunkdata>), and close():

The connect() method initializes the connection to SWUpdate's control socket, sends REQ_INSTALL, and waits for ACK or NACK, returning the socket connection file descriptor, mostly for information purposes, or, in case of an error, nil plus an error message.

The artifact's data can then be sent to SWUpdate via the write(<chunkdata>) method, returning true, or, in case of errors, nil plus an error message.

Finally, the close() method closes the connection to SWUpdate's control socket after which it waits for SWUpdate to complete the update transaction and executes the post-install command, if given.

The following example snippet illustrates how to use the control interface binding:

local artifact ="/some/path/to/artifact.swu", "rb" )
swupdate = require('lua_swupdate')
local ctrl = swupdate.control()
if not ctrl:connect() then

-- Deliberately neglecting error message.
io.stderr:write("Error connecting to SWUpdate control socket.\n")
return end while true do
local chunk = artifact:read(1024)
if not chunk then break end
if not ctrl:write(chunk) then
-- Deliberately neglecting error message.
io.stderr:write("Error writing to SWUpdate control socket.\n")
end end local res, msg = ctrl:close() if not res then
io.stderr:write(string.format("Error finalizing update: %s\n", msg)) end

Progress Interface

The lua_swupdate module's progress interface binding conveniently makes SWUpdate's socket-based progress API available to pure Lua.

The binding is captured in the swupdate_progress object that is returned by a call to swupdate.progress(). This object offers the three methods connect(), receive(), and close():

The connect() method connects to SWUpdate's progress socket, waiting until the connection has been established. Note that it is only really required to explicitly call connect() to reestablish a broken connection as the swupdate_progress object's instantiation already initiates the connection.

The receive() method returns a table representation of the struct progress_msg described in the progress interface's API description.

The close() method deliberately closes the connection to SWUpdate's progress socket.

IPv4 Interface

For convenience, the lua_swupdate module provides the ipv4() method returning a table holding the local network interfaces as the table's keys and their space-separated IP addresses plus subnet masks as respective values.



SWUpdate has bindings to various bootloaders in order to store persistent state information across reboots. Currently, the following bootloaders are supported:

  • A fake bootloader called "Environment in RAM",
  • EFI Boot Guard,
  • U-Boot, and
  • GRUB.

The actual (sub)set of bootloaders supported is a compile-time choice. At run-time, the compile-time set default bootloader interface implementation is used unless overruled to use another bootloader interface implementation via the -B command line switch or a configuration file (via the bootloader setting in the globals section, see examples/configuration/swupdate.cfg).

Note that the run-time support for some bootloaders, currently U-Boot and EFI Boot Guard, relies on loading the respective bootloader's environment modification shared library at run-time. Hence, even if support for a particular bootloader is compiled-in, the according shared library must be present and loadable on the target system at run-time for using this bootloader interface implementation. This allows, e.g., distributions to ship a generic SWUpdate package and downstream integrators to combine this generic package with the appropriate bootloader by just providing its environment modification shared library.

Bootloader Interface Description

The bootloader interface implementations are located in bootloader/. Each bootloader has to implement the interface functions as defined in include/bootloader.h, more precisely

char *env_get(const char *name);
int env_set(const char *name, const char *value);
int env_unset(const char *name);
int apply_list(const char *filename);

which retrieve a key's value from the bootloader environment, set a key to a value in the bootloader environment, delete a key-value pair from the bootloader environment, and apply the key=value pairs found in a file.

Then, each bootloader interface implementation has to register itself to SWUpdate at run-time by calling the register_bootloader(const char *name, bootloader *bl) function that takes the bootloader's name and a pointer to struct bootloader as in include/bootloader.h which is filled with pointers to the respective above mentioned interface functions. If the bootloader setup fails and hence it cannot be successfully registered, e.g., because the required shared library for environment modification cannot be loaded, NULL is to be returned as pointer to struct bootloader.

For example, assuming a bootloader named "trunk" and (static) interface functions implementations do_env_{get,set,unset}() as well as do_apply_list() in a bootloader/trunk.c file, the following snippet registers this bootloader to SWUpdate at run-time:

static bootloader trunk = {

.env_get = &do_env_get,
.env_set = &do_env_set,
.env_unset = &do_env_unset,
.apply_list = &do_apply_list }; __attribute__((constructor)) static void trunk_probe(void) {
(void)register_bootloader(BOOTLOADER_TRUNK, &trunk); }


#define BOOTLOADER_TRUNK "trunk"

added to include/bootloader.h as a single central "trunk" bootloader name definition aiding in maintaining the uniqueness of bootloader names. This new "trunk" bootloader should also be added to the Suricatta Lua Module interface specification's bootloader Table suricatta.bootloader.bootloaders = { ... } in suricatta/suricatta.lua.


Take care to uniquely name the bootloader.

See, e.g., bootloader/{uboot,ebg}.c for examples of a bootloader using a shared environment modification library and bootloader/{grub,none}.c for a simpler bootloader support example.

Bootloader Build System Integration

A bootloader support implementation needs to be registered to the kconfig build system.

First, the bootloader support implementation, named "trunk" and implemented in bootloader/trunk.c for example, needs to be added to bootloader/ in the Bootloader Interfaces menu as follows:

menu "Bootloader"
menu "Bootloader Interfaces"

bool "TrUnK Bootloader"
Support for the TrUnK Bootloader

Then, in order to enable the compile-time selection of the "trunk" bootloader as default, add a section to the Default Bootloader Interface choice submenu of the Bootloader menu as follows:


prompt "Default Bootloader Interface"
Default bootloader interface to use if not explicitly
overridden via configuration or command-line option
at run-time. ... config BOOTLOADER_DEFAULT_TRUNK
bool "TrUnK"
Use TrUnK as default bootloader interface.

Finally, bootloader/Makefile needs to be adapted to build the "trunk" bootloader support code, given BOOTLOADER_TRUNK was enabled:


If the "trunk" bootloader, for example, requires loading a shared environment modification library, then Makefile.flags needs to be adapted as well, e.g., as follows:

LDLIBS += dl



The Yocto-Project is a community project under the umbrella of the Linux Foundation that provides tools and template to create the own custom Linux-based software for embedded systems.

Add-on features can be added using layers. meta-swupdate is the layer to cross-compile the SWUpdate application and to generate the compound SWU images containing the release of the product. It contains the required changes for mtd-utils and for generating Lua. Using meta-SWUpdate is a straightforward process. As described in Yocto's documentation about layers, you should include it in your bblayers.conf file to use it.

Add meta-SWUpdate as usual to your bblayers.conf. You have also to add meta-oe to the list.

In meta-SWUpdate there is a recipe to generate an initrd with a rescue system with SWUpdate. Use:

MACHINE=<your machine> bitbake swupdate-image

You will find the result in your tmp/deploy/<your machine> directory. How to install and start an initrd is very target specific - please check in the documentation of your bootloader.

What about libubootenv ?

This is a common issue when SWUpdate is built. SWUpdate depends on this library, that is generated from the U-Boot's sources. This library allows one to safe modify the U-Boot environment. It is not required if U-Boot is not used as bootloader. If SWUpdate cannot be linked, you are using an old version of U-Boot (you need at least 2016.05). If this is the case, you can add your own recipe for the package u-boot-fw-utils, adding the code for the library.

It is important that the package u-boot-fw-utils is built with the same sources of the bootloader and for the same machine. In fact, the target can have a default environment linked together with U-Boot's code, and it is not (yet) stored into a storage. SWUpdate should be aware of it, because it cannot read it: the default environment must be linked as well to SWUpdate's code. This is done inside the libubootenv.

If you build for a different machine, SWUpdate will destroy the environment when it tries to change it the first time. In fact, a wrong default environment is taken, and your board won't boot again.

To avoid possible mismatch, a new library was developed to be hardware independent. A strict match with the bootloader is not required anymore. The meta-swupdate layer contains recipes to build the new library (libubootenv) and adjust SWUpdate to be linked against it. To use it as replacement for u-boot-fw-utils:

  • set PREFERRED_PROVIDER_u-boot-fw-utils = "libubootenv"
  • add to SWUpdate config:


With this library, you can simply pass the default environment as file (u-boot-initial-env). It is recommended for new project to switch to the new library to become independent from the bootloader.

The swupdate class

meta-swupdate contains a class specific for SWUpdate. It helps to generate the SWU image starting from images built inside the Yocto. It requires that all components, that means the artifacts that are part of the SWU image, are present in the Yocto's deploy directory. This class should be inherited by recipes generating the SWU. The class defines new variables, all of them have the prefix SWUPDATE_ in the name.

SWUPDATE_IMAGES : this is a list of the artifacts to be packaged together. The list contains the name of images without any extension for MACHINE or filetype, that are added automatically. Example :

SWUPDATE_IMAGES = "core-image-full-cmdline uImage"

SWUPDATE_IMAGES_FSTYPES : extension of the artifact. Each artifact can have multiple extension according to the IMAGE_FSTYPES variable. For example, an image can be generated as tarball and as UBIFS for target. Setting the variable for each artifact tells the class which file must be packed into the SWU image.

SWUPDATE_IMAGES_FSTYPES[core-image-full-cmdline] = ".ubifs"

SWUPDATE_IMAGES_NOAPPEND_MACHINE : flag to use drop the machine name from the artifact file. Most images in deploy have the name of the Yocto's machine in the filename. The class adds automatically the name of the MACHINE to the file, but some artifacts can be deployed without it.


  • SWUPDATE_SIGNING : if set, the SWU is signed. There are 3 allowed values: RSA, CMS, CUSTOM. This value determines used signing mechanism.
  • SWUPDATE_SIGN_TOOL : instead of using openssl, use SWUPDATE_SIGN_TOOL to sign the image. A typical use case is together with a hardware key. It is available if SWUPDATE_SIGNING is set to CUSTOM
  • SWUPDATE_PRIVATE_KEY : this is the file with the private key used to sign the image using RSA mechanism. Is available if SWUPDATE_SIGNING is set to RSA.
  • SWUPDATE_PASSWORD_FILE : an optional file containing the password for the private key. It is available if SWUPDATE_SIGNING is set to RSA.
  • SWUPDATE_CMS_KEY : this is the file with the private key used in signing process using CMS mechanism. It is available if SWUPDATE_SIGNING is set to CMS.
  • SWUPDATE_CMS_CERT : this is the file with the certificate used in signing process using CMS method. It is available if SWUPDATE_SIGNING is set to CMS.
  • SWUPDATE_AES_FILE : this is the file with the AES password to encrypt artifact. A new fstype is supported by the class (type: enc). SWUPDATE_AES_FILE is generated as output from openssl to create a new key with

openssl enc -aes-256-cbc -k <PASSPHRASE> -P -md sha1 -nosalt > $SWUPDATE_AES_FILE

To use it, it is enough to add IMAGE_FSTYPES += "enc" to the artifact. SWUpdate supports decryption of compressed artifact, such as

IMAGE_FSTYPES += ".ext4.gz.enc"

Automatic sha256 in sw-description

The swupdate class takes care of computing and inserting sha256 hashes in the sw-description file. The attribute sha256 must be set in case the image is signed. Each artifact must have the attribute:

sha256 = "$swupdate_get_sha256(artifact-file-name)"

For example, to add sha256 to the standard Yocto core-image-full-cmdline:

sha256 = "$swupdate_get_sha256(core-image-full-cmdline-machine.ubifs)";

The name of the file must be the same as in deploy directory.

BitBake variable expansion in sw-description

To insert the value of a BitBake variable into the update file, pre- and postfix the variable name with "@@". For example, to automatically set the version tag:

version = "@@DISTRO_VERSION@@";

Automatic versions in sw-description

By setting the version tag in the update file to @SWU_AUTO_VERSION it is automatically replaced with PV from BitBake's package-data-file for the package matching the name of the provided filename tag. For example, to set the version tag to PV of package u-boot:

filename = "u-boot";
version = "@SWU_AUTO_VERSION";

Since the filename can differ from package name (deployed with another name or the file is a container for the real package) you can append the correct package name to the tag: @SWU_AUTO_VERSION:<package-name>. For example, to set the version tag of the file packed-bootloader to PV of package u-boot:

filename = "packed-bootloader";
version = "@SWU_AUTO_VERSION:u-boot";

To automatically insert the value of a variable from BitBake's package-data-file different to PV (e.g. PKGV) you can append the variable name to the tag: @SWU_AUTO_VERSION@<package-data-variable>. For example, to set the version tag to PKGV of package u-boot:

filename = "u-boot";

Or combined with a different package name:

filename = "packed-bootloader";
version = "@SWU_AUTO_VERSION:u-boot@PKGV";

Using checksum for version

It is possible to use the hash of an artifact as the version in order to use "install-if-different". This allows versionless artifacts to be skipped if the artifact in the update matches the currently installed artifact.

In order to use the hash as the version, the sha256 hash file placeholder described above in Automatic sha256 in sw-description must be used for version.

Each artifact must have the attribute:

version = "@artifact-file-name"

The name of the file must be the same as in deploy directory.

Template for recipe using the class

DESCRIPTION = "Example recipe generating SWU image"
# Add all local files to be added to the SWU
# sw-description must always be in the list.
# You can extend with scripts or wahtever you need
SRC_URI = " \

file://sw-description \
" # images to build before building swupdate image IMAGE_DEPENDS = "core-image-full-cmdline virtual/kernel" # images and files that will be included in the .swu image SWUPDATE_IMAGES = "core-image-full-cmdline uImage" # a deployable image can have multiple format, choose one SWUPDATE_IMAGES_FSTYPES[core-image-full-cmdline] = ".ubifs" SWUPDATE_IMAGES_FSTYPES[uImage] = ".bin" inherit swupdate

Simplified version for just image

In many cases there is a single image in the SWU. This is for example when just rootfs is updated. The generic case described above required an additional recipe that must be written and maintained. For this reason, a simplified version of the class is introduced that allowed to build the SWU from the image recipe.

Users just need to import the swupdate-image class. This already sets some variables. A sw-description must still be added into a files directory, that is automatically searched by the class. User still needs to set SWUPDATE_IMAGE_FSTYPES[your image] to the fstype that should be packed into the SWU - an error is raised if the flag is not set.

In the simple way, your recipe looks like

<your original recipe code>

SWUPDATE_IMAGES_FSTYPES[<name of your image>] = <fstype to be put into SWU> inherit swupdate-image


This is intended as general rule to integrate SWUpdate into a custom project.

SWUpdate is an update agent and it is thought to be a framework. This means it is highly configurable and SWUpdate should be configured to fit into a project, not vice versa. SWUpdate makes just a few requirements on the system and it has no fixed update schema. There is no restriction on how many partitions or which storage you are using. In some more complex cases, the update depends on a lot of conditions, and SWUpdate can run differently according to the mode a device is started in. Think about SWUpdate not being a ready-to-use updater but a framework, and hence you should first write a meaningful:

Update Concept

Take your time and write first an update concept for your device. It is not wasted time. You have to imagine conditions when an update is not working, and try to write down the use cases when an update can fail and how the device can be restored. SWUpdate installs new software, but a successful update means that the new software is started and runs flawlessly. The interface with the bootloader (or the one that starts the software) must be checked in details. A successful update means:

  • SWUpdate runs successfully,
  • the device reboots,
  • the bootloader can start the new software, and
  • the new software runs, makes some consistency checks, and declares that the transaction (that is from old to new software) is terminated.

This means that some coordination between the bootloader and the update agent is necessary. In most cases, this is done via persistent variables that are available to both SWUpdate and the bootloader. SWUpdate has two built-in variables:

  • recovery_status: this is set when SWUpdate starts to write to the device, and it is unset after the installation completed with success or it is set to failed in case of error. A bootloader can use it to check if an update was interrupted. This is a must in case a single copy of the software is on the device.
  • ustate: this triggers a state machine. SWUpdate sets it to 1 (INSTALL) after an update. The bootloader can use it to check whether a new software must be tested. The bootloader should implement a counter mechanism to check how many times it tried to start a new software. When a threshold is reached, the bootloader should declare the new software as buggy, and proceed with a fallback in case the old software is available.

A fallback is always initiated by the bootloader, because it knows if the new software is running. It should toggle the copies and start the old software set. To communicate this to user space and to SWUpdate, the bootloader sets the ustate variable to 3 (FAILED). SWUpdate uses this information in case the result must be forwarded to an external server (like a backend). There is a time window when a fallback can take place. In fact, after a reboot and some attempts, the update transaction is declared successful or failed, and later a new update can be executed. When a new update runs, the status of the stand-by copy is unknown, because it could be the result of an interrupted update. Running an incomplete software can lead to unpredictable results and must be strongly avoided. A common pattern for a toggling in the bootloader is:

  • ustate is not set or set to 0 (no update pending). The bootloader runs the configured copy and won't ever toggle. In case of failure, a rescue system can be started if available.
  • ustate is set to 1 (INSTALLED): the new software is under test. The bootloader initiates a fallback if the new software is not running and sets ustate to 3 (FAILED). If the new software runs, it is the duty of user space (often SWUpdate or the init script for SWUpdate) to reset ustate to 0. Note that resetting the variable is project specific, and it could be set as last action after having sufficiently checked that the new software is running. This includes performing in the application a database migration, starting communicating with peers, whatever.

A possible diagram is shown in next picture - it is not supposed to work with any project, but it gives an idea how fallback is working together with the bootloaders. [image]

Check in advance which security topics are relevant for your project. This includes:

  • signed images (SWU is verified before installing), and then which crypto mechanism is used (RSA keys, certificates, PKI)
  • encryption for artifacts
  • under which user and group SWUpdate and other components are allowed to run. Set user id and group id if not root in swupdate.cfg.
  • if any version can be installed or if you forbid a downgrade, and then be sure to pass the range of versions you allow via --M, -R and --max-version.
  • hardware-software compatibility check and how your device knows which hardware revision is running.

Reduce dependencies to minimum

An update should be possible in any condition. Even if the system is degraded or in a bad shape, if an update can work, the device can be functional again without returning it back to the factory. SWUpdate is thought to be self contained: that means it does not make use of external tools. If your system is degraded and filesystems get corrupted, there are less chances to restore it if the update calls external tools. SWUpdate is started at boot time and there are good chances it succeeds even if your system has some (software) flaws. Be careful to make an update depending on your application or try to reduce the dependencies. In fact, the application is updated often and an introduction of new bugs can make the device no longer updatable. Take the dependencies under control, and if you have any, be sure that the update is still working. You can fix any bugs if the update works, but not anymore if the device cannot be updated.

Make a risk analysis

A more accurate analysis brings less surprises in the field. Think twice about what you want to update, which components should be updated, and the risks of updating a single point of failure. Very often, this means the bootloader. Compare risks and benefits: it happens in many projects that having the possibility (with some risk) to update the bootloader is better that returning the devices back to service. A cost / benefits analysis should be part of the integration of the update agent.

SWUpdate builtin configuration

SWUpdate has a compile time configuration. The default configuration delivered with meta-swupdate is not suitable for most projects. The easy way to check configuration in Yocto is to run:

bitbake -c menuconfig swupdate

Outside Yocto, just run in SWUpdate's sources:

make menuconfig

Check security, bootloader, and which handlers should be installed. They depend strongly on your project. If you build with OE, add a swupdate_%.bbappend to one of your layers, and put the resulting configuration file as defconfig that can be fetched. Please review the following configuration:

  • Security settings
  • Interfaces required (where the software is coming from). Disable the interface you do not need.
  • Handlers required for your project. Disable what you do not need, but consider if you could need some of them in future. As example, you can safely disable ubivol if you do not use raw NAND, but you can let archive enabled if you plan to install artifacts from tarballs in future.
  • It is highly recommended to enable Lua to extend runtime behavior.

SWUpdate startup

An easy way to start SWUpdate is provided only with meta-swupdate and Yocto. A generic SystemV init script or a systemd unit for SWUpdate are executing a script, that is delivered together with the SWUpdate binaries. The script goes through /etc/swupdate/conf.d/ and sources all found files. The integrator can use a set of predefined variables to configure SWUpdate's command line parameters.

  • SWUPDATE_WEBSERVER_ARGS : This string is passed if the webserver must be started. It consists of the webserver specific parameters. If this variable is set, the script will add -w to the list of parameters. Note: meta-swupdate contains a default configuration for SWUPDATE_WEBSERVER_ARGS, that uses /www as document root for the Website and default port 8080.
  • SWUPDATE_SURICATTA_ARGS : Suricatta (backend) specific parameters. There is no default.
  • SWUPDATE_ARGS : Parameters not belonging to Webserver or Suricatta.

Note that sources the files in sorted order, so it is possible to override the variables with a configuration file whose filename is loaded at the end. Preferred style is to use SystemV like names, for example 10-webserver, 11-suricatta, and so on.

Write sw-description

sw-description is the central file that describes a new software release and how a release must be installed. It should be a consequence of the update concept. There is not a single right way. SWUpdate heavily uses 'selections' and links to extract just one part of the whole sw-description, that can be used for different situations and different ways to run the device. One use case for selections is to implement the dual-copy (often referred to as A/B) mode: one selection contains instructions for one copy, the other for the second copy. Which copy is the stand-by must be detected before running SWUpdate and passed via the -e <selection,mode> switch. Other methods set up a link to the standby storage (like /dev/standby) during boot. Or the standby device can be detected at runtime with an embedded-script, as part of sw-description, with Lua code. Please note that for the last case, SWUpdate is extended with functions exported to the Lua context that simplify the detection. SWUpdate exports a getroot() function that returns type and value for the device used as rootfs. See SWUpdate documentation for a complete list of functions exported by SWUpdate that can be used in Lua. An embedded Lua script must just start with

require ('swupdate')

to make use of them.

Use OE variables as much as possible

meta-swupdate replaces a special construct in sw-description with the values of build variables. The recognized construct in sw-description is delimited by @@, that is @@VARIABLE-NAME@@. The exception (for compatibility reasons) is the automatic generation of sha256. The syntax in that case is :

sha256 = "$swupdate_get_sha256(<name of artifact>)"

You can again use variable substitution for artifact names. Example:

sha256 = "$swupdate_get_sha256(@@SYSTEM_IMAGE@@-@@MACHINE@@@@SWUPDATE_IMAGES_FSTYPES[@@SYSTEM_IMAGE@@]@@)";

Please note that each variable is double delimited (at the beginning and at the end) by @@.

Deliver your scripts instead of relying on them being installed

You have the freedom to call any tools during an update. However, take care if you are using some tools from the running rootfs / current software. This implies that the current software is running flawlessly, as well as the tools you are calling. And this may not always be the case.

Prefer Lua to shell scripts

Shell scripts are very popular, and they are often used even when they are not strictly required. They can raise security issues. In fact, take as example a simple shell script. Goal of rootkits is often the shell, because taking control of the shell means to control the whole device. If the shell is compromised, the whole system is compromised. Running a shell script means that SWUpdate should call "fork" followed by an "exec". This means also that many resources are duplicated in the child process, and it could cause a further problem if system is getting rid of resources. A better approach is to use Lua and to deliver the scripts inside the SWU. In fact, the Lua interpreter is linked to SWUpdate and runs in context of the SWUpdate process without forking a child process. Shell is not involved at all. Of course, Lua scripts should be written to be self-contained, too, and executing external tools should be done only if unavoidable.

Use installed-directly when possible

SWUpdate can be enabled for zero-copy (or streaming mode), that is the incoming SWU is analyzed on the fly and it is installed by the associated handler without any temporary copy. If this is not set, SWUpdate creates a temporary copy in $TMPDIR before passing it to the handlers. Note that $TMPDIR generally points to a RAMDISK and storing files there reduces the amount of memory available for the application. It makes sense to disable the flag in case the artifact is a single point of failure. A typical example could be the bootloader (not duplicated on the devices), and if the SWU is corrupted or the connection gets broken, the board is left in a bricked state. It makes sense then to download the whole artifact before installing.

Always enable sha256 verification

The SWU image is a CPIO archive with CRC (new ASCII format), but the check in CPIO is very weak. Do not trust it, but enable sha256 for each artifact.

Always set the type attribute

SWUpdate sets some default handler if the type is not set. Do not use it, but set explicitly the type (that is, which handler should install the artifact) in sw-description.

Do not rely on install order

SWUpdate does not require that artifacts are put into the CPIO in a specific order. The exception is sw-description, that must be the first file in a SWU. Avoid dependencies inside the SWU, that is an artifact that can be installed only after another one was installed before. If you really need it, for example if you want to install a file into a filesystem provided as image, disable installed-directy for the file and enable it for the filesystem image.

Do not drop atomicity !

SWUpdate guarantees atomicity as long as you don't do something that simply breaks it. As example, think about the bootloader's environment. In an sw-description, there is a specific section where the environment can be set, adding / modifying / deleting variables. SWUpdate does not change single variables, but generates the resulting new environment for the supported bootloader and this is written in one shot in a way (for U-Boot / EFIBootguard, not for GRUB) that is power-cut safe. You can of course change the environment in a postinstall script, like in the following way (for U-Boot):

fw_setenv var1 val1
fw_setenv var2 val2
fw_setenv var3 val3
fw_setenv var4 val4
fw_setenv var5 val5

If a power cut happens during two calls of fw_setenv, the environment is in an intermediate state and this can brick the device.

Plan to have a rescue system

Even if you have a double-copy setup, something can go wrong. Plan to have a rescue system (swupdate-image in meta-swupdate) and to install it on a separate storage than the main system, if it is possible. This helps when the main storage is corrupted, and the device can be restored in the field without returning it back to the factory. Plan to update the rescue system as well: it is software, too, and its bugs should be fixed, too.



The size of update packages is steadily increasing. While once the whole software was just a bunch of megabytes, it is not unusual now that OS and application on devices running Linux as OS reach huge size of Gigabytes.

Several mechanisms can be used to reduce the size of downloaded data. The resulting images can be compressed. However, this is not enough when bandwidth is important and not cheap. It is very common that a device will be upgraded to a version that is similar to the running one but add new features and solves some bugs. Specially in case of just fixes, the new version is pretty much equal as the original one. This asks to find methods to download just the differences with the current software without downloading a full image. In case an update is performed from a known base, we talk about delta updates. In the following chapter some well known algorithms are considered and verified if they can be integrated into SWUpdate. The following criteria are important to find a suitable algorithm:

  • license must be compatible with GPLv2
  • good performance for smaller downloads, but not necessarily the best one.
  • SWUpdate remains with the concept to deliver one package (SWU), the same independently from the source where the SWU is stored (USB, OTA, etc.)
  • It must comply to SWUpdate's security requirements (signed images, privilege separation, etc.)

Specific ad-hoc delta updates mechanisms can be realized when the nature of the updated files is the same. It is always possible with SWUpdate to install single files, but coherency and compatibility with the runningsoftware must be guaranteed by the integratot / manufacturer. This is not covered here: the scope is to get an efficient and content unaware delta mechanism, that can upgrade in differential mode two arbitrary images, without any previous knowledge about what they content.

FOSS projects for delta encoding

There are several algorithms for delta encoding, that is to find the difference between files, generally in binary format. Only algorithms available under a compatible FOSS license (GPLv2) are considered for SWUpdate. One of the goals in SWUpdate is that it should work independently which is the format of the artifacts. Very specialized algorithm and libraries like Google's Courgette used in Chromium will give much better results, but it works on programs (ELF files) and take advantages of the structure of compiled code. In case of OTA update, not only software, but any kind of artifact can be delivered, and this includes configuration data, databases, videos, docs, etc.


librsync is an independent implementation for rsync and does not use the rsync protocol. It is well suited to generate offline differential update and it is already integrated into SWUpdate. However, librsync takes the whole artifact and generates a differential image that is applied on the whole image. It gives the best results in terms of reduced size when differences are very small, but the differential output tends to be very large as soon as the differences are meaningful. Differential images created for SWUpdate show that, as soon as the difference larger is, the resulting delta image can even become larger as the original one.

SWUpdate supports librsync as delta encoder via the rdiff handler.


xdelta uses the VCDIFF algorithm to compute differences between binaries. It is often used to deliver smaller images for CD and DVD. The resulting images are created from an installed image that should be loaded entirely in main memory. For this reason, it does not scale well when the images are becoming larger and it is unsuitable for embedded systems and SWUpdate.


casync is, according to his author. a tool for distributing images. It has several interesting aspects that can be helpful with OTA update. Files itself are grouped together in chunks and casync creates a "Chunk storage" where each chunk is stored on a separate file. The chunk storage is part of the delivery, and it must be stored on a server. casync checks if the chunk is already present on the target, and if not download it. If this seems to be what is required, there are some drawbacks if casync should be integrated in SWUpdate:

  • because of the nature of casync, each chunk is a separate file. This cause a huge number of new connections, because each file is a separate GET on the server. The overhead caused to re-instantiate connection is high on small devices, where SSL connections are also increasing CPU load. There are downloads of hundreds or thousands of small files just to recreate the original metadata file.
  • casync has no authentication and verification and the index (.caidx or .caibx) are not signed. This is known, but casync goals and scopes are outside the ones on embedded devices.
  • it is difficult to deliver a whole chunk storage. The common usage for OTA is to deliver artifacts, and they should be just a few. Thousands of files to be delivered to let casync to compute the new image is not practical for companies: they have a new "firmware" or "software" and they need an easy way to deliver this file (the output from their build system) to the devices. In some cases, they are even not responsible for that, and the firmware is given to another authority that groups all packages from vendors and realizes a sort of OTA service.
  • casync is quite a huge project - even if it was stated that it will be converted into a library, this never happened. This makes difficult to interface to SWUpdate, and using it as external process is a no way in SWUpdate for security reason. It breaks privilege separation, and adds a lot of code that is difficult to maintain.

For all these reasons, even if the idea of a chunk storage is good for an OTA updater, casync is not a candidate for SWUpdate. A out-of-the-box solution cannot be found, and it is required to implement an own solution that better suits for SWUpdate.

Zchunk - compression format

zchunk seems to combine the usage of a chunk storage without having to deliver it on a server. zchunk is a FOSS project released under BSD by its author. The goal of this project is something else: zchunk creates a new compression format that adds the ability to download the differences between new and old file. This matches very well with SWUpdate. A zchunk file contains a header that has metadata for all chunks, and according to the header, it is known which chunks must be downloaded and which ones can be reused. zchunk has utilities to download itself the missing chunks, but it could be just used to find which part of an artifact must be downloading, and SWUpdate can go on with its own way to do this.

One big advantage on this approach is that metadata and compressed chunks are still bound into a single file, that can be built by the buildsystem and delivered as it is used to. The updater needs first the metadata, that is the header in zchunk file, and processes it to detect which chunks need to be downloaded. Each chunk has its own hash, and the chunks already available on the device are verified against the hash to be sure they are not corrupted.

Zchunk supports multiple sha algorithms - to be compatible with SWUpdate, zchunk should be informed to generate sha256 hashes.

Design Delta Update in SWUpdate

For all reasons stated before, zchunk is chosen as format to deliver delta update in SWUpdate. An artifact can be generated in ZCK format and then the ZCK's header (as described in format) can be extracted and added to the SWU. In this way, a ZCK file is signed (and if requested compressed and/or encrypted) as part of the SWU, and loading chunks from an external URL can be verified as well because the corresponding hashes are already verified as part of the header.

Changes in ZCHUNK project

Zchunk has an API that hides most of its internal, and provides a set of tools for creating and downloading itself a file in ZCK format. Nevertheless, Zchunk relies on hashes for the compressed (ZST) chunks, and it was missing for support for uncompressed data. To combine SWUpdate and zchunk, it is required that a comparison can be done between uncompressed data, because it is unwanted that a device is obliged to compress big amount of data just to perform a comparisons. A short list of changes in the Zchunk project is:

  • create hashes for uncompressed data and extend format to support it. The header must be extended to include both size and hash of uncompressed data.
  • make the library embedded friendly, that means reports errors in case of failure instead of exiting and find a suitable way to integrate the log output for the caller.
  • allow to use sha256 (already foreseen in zchunk) as this is the only hash type used in SWUpdate.
  • add API to allow an external caller to take itself the decision if a chunk must be downloaded or reused.

These changes were merged into Zchunk project - be sure to get a recent version of Zchunk, at least with commit 1b36f8b5e0ecb, that means newer as 1.1.16.

Most of missing features in Zchunk listed in TODO for the project have no relevance here: SWUpdate already verifies the downloaded data, and there is no need to add signatures to Zchunk itself.

Integration in sw-description

The most important part in a Zchunk file is the header: this contains all metadata and hashes to perform comparisons. The zck tool splits a file in chunks and creates the header. Size of the header are know, and the header itself can be extracted from the ZCK file. The header will be part of sw-description: this is the header for the file that must be installed. Because the header is very small compared to the size of the whole file (quite 1 %), this header can be delivered into the SWU.

Integration in SWUpdate: the delta handler

The delta handler is responsible to compute the differences and to download the missing parts. It is not responsible to install the artifact, because this breaks the module design in SWUpdate and will constrain to have just one artifact type, for example installing as raw or rawfile. But what about if the artifact should be installed by a different handler, for example UBI, or a custom handler ? The best way is that the delta handler does not install, but it creates the stream itself so that this stream can be passed to another (chained) handler, that is responsible for installing. All current SWUpdate's handlers can be reused: each handler does not know that the artifact is coming with separate chunks and it sees just a stream as before. The delta handler has in short the following duties:

  • parse and understanf the ZCK header
  • create a ZCK header from the file / partition used as source for the comparison
  • detect which chunks are missing and which one must be copied.
  • build a mixer that copies and downloads all chunks and generates a stream for the following handler.
  • detect any error coming form the chained handler.

Because the delta handler requires to download more data, it must start a connection to the storage where the original ZCK is stored. This can lead to security issues, because handlers run with high priviliges because they write into the hardware. In fact, this breaks privilege separation that is part of SWUpdate design. To avoid this, the delta handler does not download itself. A separate process, that can runs with different userid and groupid, is responsible for this. The handler sends a request to this process with a list of ranges that should be downloaded (see HTTP Range request). The delta handler does not know how the chunks are downlaoded, and even if using HTTP Range Request is the most frequent choice, it is open to further implementations. The downloader process prepares the connection and asks the server for ranges. If the server is not able to provide ranges, the update aborts. It is in fact a requirement for delta update that the server storing the ZCK file is able to answer to HTTP Range Request, and there is no fallback to download the full file. An easy IPC is implemented between the delta handler and the downloader process. This allows to exchange messages, and the downloader can inform the handler if any error occurs so that the update can be stopped. The downloader will send a termination message when all chunks will be downloaded. Because the number of missing chunks can be very high, the delta handler must sends and organize several requests to the downloader, and tracking each of them. The downloader is thought as dummy servant: it starts the connection, retrieves HTTP headers and data, and sends them back to the caller. The delta handler is then responsible to parse the answer, and to retrieve the missing chunks from the multipart HTTP body.

Creation of ZCK Header and ZCK file for SWUpdate

Zchunk supports more SHA algorithms and it sets as default SHA512/128. This is not compatible with SWUpdate that just support SHA256. Be sure to generate header and chunks with SHA256 support. You have to enable the generation of hashes for uncompressed chunk, too. A possible usage of the tool is:

zck --output <output file> -u --chunk-hash-type sha256 <artifact, like rootfs>

The output is the ZCK file with all chunks. This file should be put on a Webserver accessible to the target, and that supports Range Request (RFC 7233). All modern Webserver support it.

The SWU must just contain the header. This can be easy extracted from the ZCK file with:

HSIZE=`zck_read_header -v <ZCK file> | grep "Header size" | cut -d':' -f2`
dd if=<ZCK FILE> of=<ZCK HEADER file> bs=1 count=$((HSIZE))

Using ZCK tools to foresee download size

There are tools that can be used at build time to know how many chunks should be downloaded when a device is upgrading from a known version. You can use zck_cmp_uncomp from the test directory:

../build/test/zck_cmp_uncomp --verbose <uncompressed old version file> <ZCK file>

This prints a list with all chunks, marking them with SRC if they are the same in the old version and they should not retrieved and with DST if they are new and must be downloaded. The tool show at the end a summary with the total number of bytes of the new release (uncompressed) and how many bytes must be downloaded for the upgrade. Please remmeber that these value are just payload. SWUpdate reports a summary, too, but it takes into account also the HTTP overhead (headers, etc.), so that values are not the same and the ones from SWUpdate are slightly bigger.


swupdate-client is a small tool that sends a SWU image to a running instance of SWUpdate. It can be used if the update package (SWU) is downloaded by another application external to SWUpdate. It is an example how to use the IPC to forward an image to SWUpdate.


swupdate-client [OPTIONS] <image.swu to be installed>...


print help and exit
ask the server to only perform a dry-run
<software>,<mode> select software image set and source (for example: stable,main)
go quiet, resets verbosity
go verbose, essentially print upgrade status messages from server
ask the server to run post-update commands if upgrade succeeds


swupdate-progress tries to connect to a running instance of SWUpdate to get the status of a running update.


swupdate-progress [option]


swupdate-progress is an example how to connect to SWUpdate via the progress interface. It shows on stdout a simple bar with the percent indication of the current update and reports the result of the update. It can optionally drive "psplash" or execute a script after an update.

Use colors to show results on stdout
Command to be execute after an update
send percentage to psplash
optionally reboot the target after a successful update
path to progress IPC socket in case the default is not taken
waits for a SWUpdate connection instead of exit with error
print a help


swupdate-ipc sends a specific command to SWUpdate. The following commands are enabled:


send a new aes key to SWUpdate


sends a range of versions that can be accepted.


return status of the connection to Hawkbit.


send data to the Hawkbit Server. A typical use case is acknowledgement for activation by an application or operator after a new software has been installed. The tool can forward the result for the activation to the hawkBit server.


send a restart command after a network update


swupdate-ipc <cmd> [option]

Where cmd is one of the listed above.


aes <key> <ivt>
AES key to be used for decryption
setversion <min> <max> <current>
configure the accepted range of versions
configuration for Hawkbit Module

allows one to set the polling time (in seconds)
enable suricatta mode
disable suricatta mode

Send Acknolwedge to Hawkbit server after an update if SWUpdate is set to wait for.
Used with SWU handler, allow to perform a network restart

optionally reboot the target after a successful update
waits for a SWUpdate connection instead of exit with error
path to progress IPC socket


Mailing List

There is a mailing list for this project:

Issue related to the project or to the documentation are discussed here.

SWUpdate Flyer

A short description about the project and the features (in English and German) can be found in the flyer

Workshop and SWUpdate integration in project

For quick integration of SWUpdate in your project, you could be interested in the Training

Commercial support and board integration

Please check for services <> if you need professional support or you need help to get SWUpdate on your device.

Talks about SWUpdate

  • Software Update in Embedded Systems by Stefano Babic
  • Updating Embedded Linux devices in field by Chris Simmonds
  • OpenEmbedded in the Real World by Scott Murray
  • [RFC] Device-side support for software update in AGL by Matt Porter
  • Open Source secure software updates for Linux-based IVI systems by Arthur Taylor
  • How do you update your embedded Linux devices? by Daniel Sangorrin / Keijiro Yano
  • Comparison of Linux Software Update Technologies by Matt Porter
  • Software update for IoT: the current state of play by Chris Simmonds, ELCE 2016, Slides, Video
  • OSS Remote Firmware Updates for IoT-like Projects by Silvano Cirujano Cuesta, ELCE 2016, Slides ELCE 2016, Video ELCE 2016
  • System Upgrade with SWUpdate by Gabriel Huau, ELC 2017, Slides ELC 2017, Video ELC 2017
  • BoF: Secure OTA Collaboration, by Ricardo Salveti and Alan Bennett, ELCE 2017
  • Orchestrated Android-Style System Upgrades for Embedded Linux by Diego Rondini, ELCE 2017, Slides Android-Style, Video Android-Style
  • Updating an Embedded System with SWUpdate Framework by Stefano Babic, ELCE 2017, Slides ELCE 2017, Video ELCE 2017
  • Upgrading buildroot based devices with SWUpdate by Angelo Compagnucci, LinuxLab 2018, Slides LinuxLab 2018, Video LinuxLab 2018,
  • Evolution of (OTA) Update in the IoT world by Stefano Babic, ELC 2019, Slides ELC 2019, Video ELC 2019,
  • Introduction of CIP Software Updates Working Group by Akihiro Suzuki, CIP Mini Summit 2019, Slides CIP 2019,
  • There is No Store For Self-Driving Car Parts by Stephen Segal and Matt Fornero (Cruise LLC), ELC 2020, Slides Cruise ELC 2020, Video Cruise
  • Secure Boot and Over-the-Air Updates - That's simple, no ? by Jan Kiszka (Siemens AG), ELC 2020, Slides Secure OTA ELC 2020, Video Secure OTA
  • Diving into SWUpdate: adding new platform support with Yocto/OE! by Pierre-Jean Texier, LiveEmbedded 2020, Slides Diving into SWUpdate, Video Diving into SWUpdate
  • Implementing UEFI-based Secure Boot + OTA Update for Embedded ARM Devices by Jan Kiszka & Christian Storm, ELCE 2022 Slides Implementing UEFI Secure Boot + OTA Update, Video Implementing UEFI-based Secure Boot + OTA Update for Embedded ARM Devices
  • Delta OTA Update with SWUpdate by Stefano Babic, ELCE 2022 Slides Delta OTA Update with SWUpdate, Video Delta OTA Update with SWUpdate

Useful references

  • Boundary Devices, Using SWUpdate to upgrade your system
  • Présentation de Software Update (French)
  • Easy OS upgrades with SWUpdate
  • SWUpdate for feature-rich IoT applications
  • Implement swupdate - replacing opkg based updating, VictronEnergy
  • Variscite, SWUpdate
  • Updating Embedded Linux Devices: SWUpdate
  • Approach to Software Update Management, Pelux
  • SOTA System, Pelux
  • Building a Linux system for the STM32MP1: remote firmware updates, Bootlin


Contributions are welcome ! Please follow the following guideline for contributions.

Contribution Checklist

These are mostly general recommendations and are common practice in a lot of FOSS projects.

  • use git to manage your changes [recomended]
  • follow as much as possible kernel codestyle [recomended] Nevertheless, some rules are not so strict as in kernel. The maximum line length can be extended over 80 chars if this increase code readability.
  • add the required copyright header to each new file introduced [required]
  • to certify the "Developer's Certificate of Origin", see below
  • check with your employer when not working on your own!

  • Add a "Change from Vx" description under the commit message to take track of the history of the patch.
  • It is suggested to use excellent "patman" tool to manage patches series. This is part of U-Boot's project (tools/patman), but it can be used in other projects, too.

check that your patches do not break build [required]
There is a set of configuration files in the configs/ directory. Please run a build for all files in the directory to ensure that SWUpdate is still buildable from configurations different as yours.

  • use git format-patch to generate your patches.
  • use git send-email if possible. This avoid corruptions due to the mailers
  • add a prefix [meta-swupdate] if patches are intended to the Yocto's meta layer.
  • send patches inline, do not append them
  • no HTML emails!

do not use github Pull Request. github facilities are not used for this project. The review is done in a single place : the Mailing List. PR from github are ignored.

Patches are tracked by patchwork (see You can see the status of your patches at

Developer's Certificate of Origin 1.1

When signing-off a patch for this project like this

Signed-off-by: Random J Developer <>

using your real name (no pseudonyms or anonymous contributions), you declare the following:

By making a contribution to this project, I certify that:
The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or
The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or
The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.
I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.


Please take into account that most of the items here are proposals. I get some ideas talking with customers, some ideas are my own thoughts. There is no plan when these features will be implemented - this depends if there will be contribution to the project in terms of patches or financial contributions to develop a feature.

Thanks again to all companies that have supported my work up now and to everybody who has contributed to the project, let me bring SWUpdate to the current status !

Main goal

First goal is to reach a quite big audience, making SWUpdate suitable for a large number of products. This will help to build a community around the project itself.

Core features

Support for further compressors

SWUpdate supports image compressed with following formats: zlib, zstd. This is a compromise between compression rate and speed to decompress the single artifact. To reduce bandwidth or for big images, a stronger compressor could help. Adding a new compressor must be careful done because it changes the core of handling an image.

Support for OpenWRT

OpenWRT is used on many routers and has its own way for updating that is not power-cut safe.

Selective downloading

Bandwidth can be saved not only via delta, but identifying which part of the SWu must be loaded and skipping the rest. For example, SWUpdate can detect the versions for artifact before downloading them and ask the servers to send just the relevant artifacts.

Software-Software compatibility

SWUpdate has from the early stage a hardware to software compatibility check. In case software is split in several components (like OS and application), it is desirable to have a sort of software compatibility check. For example, SWUpdate verifies if a component (like an application) is compatible with a runningOS and reject the update in case of mismatch.


SWUpdate supports two parsers : libconfig and JSON. It would be nice if tools can be used to convert from one format to the other one. Currently, due to some specialties in libconfig, a manual conversion is still required.

Fetcher and interfaces


The downloader is a one-shot command: when -d is set, SWUpdate loads the SWU from the provided URL. This behavior is high requested and must be even supported in future, but another use case is to run the downloader as daemon (like suricatta) and checks if a new SWU is available at the specified URL. It should be as an alternative server for suricatta and this allows to control it via IPC (enable/disable/trigger).

Tools and utilities

Self contained tool to generate Update Packages (SWU)

Generation of SWUs is fully supported inside OE via meta-swupdate, but there is no support at all with other buildsystems (Buildroot, Debian). The user have a not preordered bunch of programs and scripts to generate the SWU, and mostly they are not generic enough. It will be interesting to create a buildswu tool, running on host system, that can create form a configuration a SWU. The tool must support all features, that means it should be able to pack artfact, generate sw-description from templates, sign the SWU, encrypt the artifact, etc.


  • API between SWUpdate and Lua is poorly documented.
  • Extend Lua to load modules at startup with functions that are globally visible and can be used by own Lua scripts or by the embedded-script in sw-description.
  • Store in SWUpdate's repo Lua libraries and common functions to be reused by projects.


New Handlers

Users develop own custom handlers - I just enforce and encourage everyone to send them and discuss how to integrate custom handler in mainline.

  • FPGA updater for FPGA with Flash
  • Package handler to install packages (ipk, deb) Packages can be inserted into the SWU and the atomicity is guaranteed by SWUpdate.
  • Lua handlers should be added if possible to the project to show how to solve custom install.

Handlers installable as plugin at runtime

The project supports Lua as script language for pre- and postinstall script. It will be easy to add a way for installing a handler at run-time written in Lua, allowing to expand SWUpdate to the cases not covered in the design phase of a product.

Of course, this issue is related to the security features: it must be ensured that only verified handlers can be added to the system to avoid that malware can get the control of the target.

Current release supports verified images. That means that a handler written in Lua could be now be part of the compound image, because a unauthenticated handler cannot run.

Support for BTRFS snapshot

BTRFS supports subvolume and delta backup for volumes - supporting subvolumes is a way to move the delta approach to filesystems, while SWUpdate should apply the deltas generated by BTRFS utilities.


add support for asymmetryc decryption

Support for evaluation boards

meta-swupdate-boards contains examples with evaluation boards. Currently, there are examples using Beaglebone Black, Raspberri PI 3 and Wandboard. The repo is a community driven project: patches welcome.

Back-end support (suricatta mode)

Back-end: responsiveness for IPC

Suricatta is implemented as process that launches functions for the selected module. This means that the IPC does not answer if Suricatta is doing something, specially if it is downloading and upgrading the system. This can be enhanced adding a separate thread for IPC and of course all required synchronization with the main modules.

Back-end: check before installing

In some cases (for example, where bandwidth is important), it is better to check if an update must be installed instead of installing and performs checks later. If SWUpdate provides a way to inform a checker if an update can be accepted before downloading, a download is only done when it is really necessary.

Back-end: hawkBit Offline support

There are several discussions on hawkBit's ML about how to synchronize an offline update (done locally or via the internal Web-server) with the hawkBit's server. Currently, hawkBit thinks to be the only one deploying software. hawkBit DDI API should be extended, and afterwards changes must be implemented in SWUpdate.

Back-end: support for generic down-loader

SWUpdate in down-loader mode works as one-shot: it simply try to download a SWU from a URL. For simple applications, it could be moved into suricatta to detect if a new version is available before downloading and installing.

Back-end: support for Mender

There was several discussion how to make a stronger collaboration between different update solution and a proposal discussed previously is to use SWUpdate as client to upgrade from a Mender server, see BOF at ELCE 2017

Support for multiple Servers simultaneously

Currently, suricatta's server backends are a mutually exclusive compile-time choice. There is no interest to have multiple OTA at the same time. This feature won't be implemented and I will remove this from roadmap if no interest will be waked up.

Test and Continuous Integration

The number of configurations and features in SWUpdate is steadily increasing and it becomes urgent to find a way to test all incoming patch to fix regression issues. One step in this direction is the support for Travis build - a set of configuration files is stored with the project and should help to find fast breakages in the build. More in this direction must be done to perform test on targets. A suitable test framework should be found. Scope is to have a "SWUpdate factory" where patches are fast integrated and tested on real hardware.


Documentation is a central point in SWUpdate - maintaining it up to date is a must in this project. Help from any user fixing wrong sentence, bad english, adding missing topics is high appreciated.


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Stefano Babic


2013-2023, Stefano Babic

April 29, 2023 2022.12