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gensio(5) File Formats Manual gensio(5)

NAME

gensio - How to specify a gensio

SYNOPSIS

<type>[(options)][,gensio|terminaloptions]

DESCRIPTION

gensio stands for GENeral Stream Input Output. It provides an abstraction for all kinds of stream I/O, and even makes some packet I/O look like stream I/O (like UDP). In particular, gensio makes it easy to create encrypted and authenticated connections.

The gensio library specifies a connection (gensio) using a string format. This consists of a gensio type, optional options in parenthesis. For a terminal gensio (one that is at the bottom of the stack), it may take more options separated by a comma. For filter gensios (ones not on the bottom of the stack) another gensio must be specified after the comma. For instance:

serialdev,/dev/ttyS0

specifies a serial port gensio. Or:

tcp(readbuf=100),localhost,4000

specifies a TCP connection with a 100 byte read buffer, to connect to port 4000 on localhost. Or:

telnet,tcp,localhost,4000

specifies a telnet filter on top of a TCP connection.

When specifying a gensio, you can add quotes (single or double) to remove the special meaning for some characters, so you can have commas in options and such. They may also be escaped with a "\". For instance, if you are specifying a laddr in an sctp address, you need to do this. The following address:

sctp(laddr=localhost,4001),localhost,3023

will result in a failure because the option splitting code will split at the commas. Instead, do:

sctp(laddr="localhost,4001"),localhost,3023

and it will work.

Accepter gensios, gensios used for accepting connections, as opposed to connecting gensios, are specified in the same way. Each individual type can vary, and some gensios are only connecting gensios. The options can vary from the accepter and connecting gensios of the same type. For instance, an accepting TCP gensio does not have to have a hostname, but a connecting one does.

When an accepter gensio receives a connection, it will create an accepted gensio. This works mostly like a connecting gensio, except some functions may be limited. You may not be able to close and then open an accepted gensio.

The gensio library has a concept of client and server. The accepted gensios are generally considered servers. Connecting gensios are generally considered clients. Some gensios may allow this to be overridden.

A gensio may be reliable or not. A reliable gensio will reliably deliver all data in sequence, like TCP. An gensio that is not reliable may drop data or deliver data out of sequence, like UDP. This can be queried with gensio_is_reliable().

A gensio may be packet or not. A packet gensio will exactly match up writes and reads. So if you write 15 bytes into one side, a 15 byte read for that data will appear on the other side. A gensio that is not packet will not respect write boundaries, that 15 byte write may result in multiple reads or it may be combined with another write into more than 15 bytes. Packet I/O requires careful use to make it work correctly. Particularly, you must set the max read and write size to the same value, and you must accept all data on receipt and write complete packets all at once.

A gensio may be message oriented. This implementation is stolen from SCTP (even though it's not really supported on Linux at the moment). It basically means you can explicitly mark message boundaries when sending data, and that explicit mark will be set on the read side. You do this by adding an "eom" auxdata on the write; the end of that write it assumed to be the end of a message. If the write does not accept all the data, the "eom" is ignored, you must write the remaning data again with "eom" set. You may also do partial write of messages and set "eom" at the end. On the receive side, "eom" will be set when the end of a message is delivered. The data delivered in the receive callback will be only the data for that message. If the user does not accept all the data, the data left in the message is again presented to the user with "eom" set.

The options vary greatly between the different gensios. Each gensio type will be covered in a separate section. Also note that gensio types can be dynamically added by the user, so there may be gensios available that are not described here.

Unless othersize noted, every gensio takes a:

readbuf=<n>
option to specify the read buffer size.

DEFAULTS

Every option to a gensio (including the serialdev and ipmisol options), unless othersize stated, is available as a default for the gensios. You can use gensio_set_default() to set the default value used by all gensios allocated after that point. If you leave the class NULL, it will set the base default, which will affect all gensios unless they have an override. If you set the class, it will only affect gensios with that class name.

Be very careful with some defaults. Setting "mode" default, for instance, could really screw things up.

For string defaults, setting the default value to NULL causes the gensio to use it's backup default.

Serial gensios

Some gensio types support serial port setting options. Standard serial ports, IPMI Serial Over LAN, and telnet with RFC2217 enabled.

A client serial gensio can set and get serial port options using the sergensio_xxx() functions. Server serial gensios receive requests from the client via GENSIO_EVENT_SER_xxx events in the callback.

Streams and Channels

Some gensios support the concept of a stream and/or a channel.

A stream is delivered as part of the normal data stream of a gensio. The "default" stream will be treated normally. All other streams will have "stream=<x>" given in the auxdata to specify which stream to write on or which stream was read from. Streams cannot be individually flow controlled.

A channel is a flow of data like a stream, but it can be individually flow controlled. It appears as a new gensio in the GENSIO_EVENT_NEW_CHANNEL callback. You can create a channel with gensio_alloc_channel() and then open it with gensio_open(). Once open, a channel works like a normal gensio. If you close the main channel for a gensio, the other channels will stay open; the resources for the main channel will still be kept around until all channels are closed.

See the indvidual gensio description for more information on streams and channels.

PUBLIC KEY CRYPTOGRAPHY

The ssl and certauth gensios use public key cryptography. This section gives a little overview of how that works. You can safely skip this section if you already understand these concepts.

Public key cryptography is used to authenticate and encrypt information at the beginning of a session. It is a fairly expensive operation and is not generally used to encrypt information after the beginning.

In public key cryptography, you have three basic things: A private key, a certificate (or public key), and a certificate authority (CA).

The private key is just that: private. You don't even send your private key to a certificate authority for signing of your certificate. Keep it private, non-readable by everyone else. Protect it, if it becomes known your certificate becomes useless to you, anyone can impersonate you.

The certificate is the public key, and is mathematically associated with a single private key. It's just that, public, anyone can use it to test that you have the private key by asking you to sign some data. The data in the certificate (like the Common Name) is part of the certificate. If that data is modified, the certificate validation will fail.

The CA is generally a trusted third-party that validates you and signs your certificate (generally for a fee). CAs issue their own public certificates that are well-known and generally available on your system. The CA certificates are used to prove that your certificate is valid.

SIGNING

The process if signing has been mentioned already, but not described. Basically, you use your private key to generate a value over some given data that proves you have the private key. The certificate is ised to mathematically verify the signature.

Two things are normally done with this:

In a public key exchange, the entity wishing to be authorized sends a certificate. The authorizing entity will look through it's CA for a certificate that has signed the sent certificate. If the authorizing entity finds a certificate that can be used to validate the sent certificate, the sent certificate is valid.

After that, the authorizing entity sends some generally random data to the other entity. The other entity will take that data, perhaps some other important data that it want to make sure is not modified in the transfer, and signs that data. It sends the signature back to the authorizing entity. The authorizing entity can then use the data and the signature to validate that the sending entity has the private key associated with the certificate.

This is basically how https works. Note it is the web client that authenticates the web server, not the other way around. This proves that you are connecting to the entity you say you are connecting to. The authentication of a web client to the web server is generally done via a different mechanism (though SSL/TLS used by the ssl gensio has a way to do it, it is not generally used for that purpose).

In the web server scenario, data in the certificate (specifically the Common Name and Subject Alternate Name) must match the name of the web page to which you are connecting. The ssl and certauth gensios do not do this authentication, that is up to the user if it is necessary.

ENCRYPTING

The certificate can be used to encrypt data that can only be decrypted with the private key. When establishing an encrypted connection, this is generally used to transfer a symmetric cryptography key from one entity to another (the authorizing entity to the requesting entity in the case above). You could encrypt all the data with the public key, but that is very expensive and in our example above would require certificates and private keys on both ends.

SELF-SIGNED CERTIFICATES

It is possible to create a certificate that can act as its own certificate authority. This is how ssh works. You create a public and private key. You put the public key in the .ssh/authorized_keys directory on systems where you want to log in. The certificate acts as both the public key (as part of the initial transfer) and the CA (in the authorized_key directory) for authorizing you use of the system you are logging in to.

ssh also stores places it has connected to in .ssh/known_hosts, which is the CA in the opposite direction. This is why it asks you if you have never connected to a system before, it doesn't have the key in its CA. Or why, if you connect to a system you have connected to before and the certificates don't match or fail validation, it complains about it.

So if you are using self-signed certificates, you need to be careful to put only ones you trust in the CA. This is obviously not possible in a large system like the world wide web, thus the creation of third-party trusted CAs.

TRUST AND CRYPTOGRAPHY

The above discussions mention trust several times. Cryptography does not remove the need for trust. It just makes trust more convenient. If someone sends you a certificate, you need to validate that it was actually them that sent you the certificate, and that it was not modified in transit. If you add that certificate to your CA, you are trusting the certificate, and you better make sure (with fingerprints, generally, see the openssl docs for details) that it came from a trusted entity.

The hardest part of cryptography in general is key management. Breaking cryptographic algorithms is really hard. Getting people to divulge private keys or use the wrong keys is a lot easier.

For more on cryptography in general, and cryptography and trust, Bruce Schneier has some excellent books on the subject.

IPv6, IPV4, and host names

Note that a single hostname may result in more than one address. For instance, it may have both an IPv4 and IPv6 address. These are treated just like multiple hostnames. The hostname may be prefixed with ipv4 or ipv6, which will force the connections to those protocols. Specifying ipv6n4 will create a socket that is IPv6 but will handle IPv4 connections. This is the default.

In general IPv6 addresses are preferred if both are available.

If you do not specify a hostname on an accepting gensio (like sctp,1234) it will only create an IPv6 socket that is IPv4 mapped. Even though getaddrinfo would normally return two addresses, only the IPv6 one is used unless there are no IPv6 addresses configured where it will return an IPv4 address.

In general, for connecting gensios only the first address that is found will be used. SCTP is the exception, it will do multi-homing on all the addresses that come up. Do you may need to be fairly specific with addresses.

TCP

tcp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
hostname = [ipv4|ipv6|ipv6n4,]<name>

A TCP connecting gensio must have the hostname specified. Mulitiple hostname/port pairs may be specified. For a connecting TCP gensio, each one will be tried in sequence until a connection is established. For acceptor gensios, every specified hostname/port pair will be listened to.

Dynamic Ports

For accepters, if the port is specified as zero, a random port in the dynamic port range specified by IANA will be chosen. If more than one address is present, the same port will be chosen for all addresses. You can fetch the port using the gensio_acc_control() function with the option GENSIO_ACC_CONTROL_LPORT.

Out Of Band Data

TCP supports out of band (oob) data, which is data that will be delivered out of order as soon as possible. This comes in a normal read, but with "oob" in the auxdata. You can send oob data by adding "oob" to the write auxdata, but oob writes are limited to 1 byte and writing any more than this results in undefined behavior. Note that "oobtcp" is also delivered and accepted in auxdata, so you can tell TCP oob data from other oob data.

Options

In addition to readbuf, the tcp gensio takes the following options:
nodelay[=true|false]
Sets nodelay on the socket.
laddr=<addr>
An address specification to bind to on the local socket to set the local address.
reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for accepting gensios only. Defaults to true.
tcpdname=<name>
Accepter only, sets the name to use for tcpd access control. This defaults to "gensio", and the default can be overriden with gensio_set_progname(). This option allows you to override it on a per-gensio accepter basis.

Remote Address String

The remote address will be in the format "[ipv4|ipv6],<addr>,<port>" where the address is in numeric format, IPv4, or IPv6.

Remote Address

TCP returns a standard struct sockaddr for GENSIO_CONTROL_RADDR_BIN control.

UDP

udp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
hostname = [ipv4|ipv6|ipv6n4,]<name>

A UDP gensio creates a UDP socket, but it makes it look like an unrealiable stream of data. The specification is the same as a TCP socket, except that a UDP socket is created, obviously.

The semantics of a UDP socket are a little bit strange. A connecting UDP socket is fairly straightforward, it opens a local socket and sends data to the remote socket.

An accepter gensio is not so straightforward. The accepter gensio will create a new accepted gensio for any packet it receives from a new remote host. If you disable read on any of the accepted gensio or disable accepts on the accepting gensio, it will stop all reads on all gensios associated with that accepting gensio.

Note that UDP accepter gensios are not really required for using UDP, the are primarily there for handling ser2net accepter semantics. You can create two connecting UDP gensios and communicate between them.

UDP gensios are not reliable, but are obviously packet-oriented.

Port 0 is supported just like TCP for accepters, see Dynamic Ports in the TCP section above for details.

The destination address defaults to the one specified on the gensio specifier (for connecting gensios) or the remote address that initiated the connection (for accepting gensios), but may be overridden using "addr:<addr>" in the write auxdata.

Options

In addition to readbuf, the udp gensio takes the following options:
laddr=<addr>
An address specification to bind to on the local socket to set the local address.
nocon[=true|false]
Don't be connection oriented, just receive all packets and deliver them without establishing connections. Only valid for the client gensio. The receive address is passed into the auxdata prefixed by "addr:", this is the address formatted by gensio_addr_to_str().
mloop[=true|false]
If false, multicast packets transmitted will not be received on the local host. If true, they will.
mcast=<addr>
Add an address to receive multicast packets on. There is no port number, this is just addresses. You can specify multiple addresses in a single multicast option and/or the multicast option can be used multiple times to add multiple multicast addresses.
reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for connecting and accepting gensios. Defaults to false.

Remote Address String

The remote address will be in the format "[ipv4|ipv6],<addr>,<port>" where the address is in numeric format, IPv4, or IPv6.

Remote Address

UDP returns a standard struct sockaddr for GENSIO_CONTROL_RADDR_BIN control.

UDP Multicast

Multicast can be used on UDP gensios with the nocon, maddr and laddr options. To set up a multicast, create a client UDP gensio and set the laddr for the receive port and the destination address to the multicast and enable nocon, like:
"udp(mcast='ff02::1',laddr='ipv6,3000',nocon),ff02::1,3000"

and you will receive and send data on the multicast address. The laddr option is required to set the port to receive on. It means you will have a local address, too, and will receive packets on that, too.

SCTP

sctp[(<options>)][,<hostname>],<port>[[,<hostname>],<port>[...]]
hostname = [ipv4|ipv6|ipv6n4,]<name>

An SCTP gensio is specified like a UDP or TCP one. However, the semantics are different. For a connecting gensio, it will attempt to create a multi-homed connect with all the specified hostnames and ports. All the ports must be the same.

For an accepter gensio, it will create a single socket with all the specified addresses as possible destinations. Again, all the ports must be the same.

SCTP gensios are reliable. They are not, at the moment, packet oriented. There is currently no support of SCTP_EXPLICIT_EOR in the Linux implementation of SCTP, and without that it would be hard to make it packet oriented.

When specifying IPv6 addresses that might map to IPv4, you must be careful. If one side can do IPv4 and the other side can only do IPv6, the connection may come up, but will disconnect quickly because it cannot communicate on the IPv4 side. For instance, the following accepter:

tools/gensiot -a "sctp,ipv6,::,1234"

and the following connector:

tools/gensiot "sctp,::1,1234"

will fail this way because the connector will support IPv4 add but the accepter will not.

Nagle and SCTP

SCTP implements the Nagle algorithm by default, which can interact badly if sack_freq is set to more than one. At least Linux defaults sack_freq to 2, but the gensio overrides this to avoid surprising behaviour. What happens is in some situations you can get an outstanding packet that is unacked, since sack_freq is greater than one. The Nagle algorithm will not send any new data until any already sent data is acked. So one end is waiting for a new packet to send a sack, and the other end is holding data until it gets a sack. So you get stuck waiting for the sack_delay where the sack will go out and kick things back off again.

You need to be aware of this if you modify sack_freq.

Options

In addition to readbuf, the sctp gensio takes the following options:
instreams=<n>
ostreams=<n>
These specify the number of incoming and outgoing streams for the connection. The default is one. The stream is given in the auxdata for read and write in the format "stream=<n>".
sack_freq=<n>
sack_delay=<n>
These specify the handling of selective acknowledgements (sacks). sack_freq sets the number of outstanding packets that must be received before sending a sack. The default is 1, meaning it doesn't wait at all. sack_delay sets the maximum time before a sack is sent if outstanding packets are present, in milliseconds. The default is 10, but this is disabled if sack_freq is set to 1. Setting either of these to 0 enables the system defaults.
nodelay[=true|false]
Sets nodelay on the socket.
laddr=<addr>
An address specification to bind to on the local socket to set the local address.
reuseaddr[=true|false]
Set SO_REUSEADDR on the socket, good for accepting gensios only. Defaults to true.

Port 0 is supported just like TCP for accepters, see Dynamic Ports in the TCP section above for details.

SCTP support out of band (oob) data, which is data that will be delivered out of order as soon as possible. This comes in a normal read, but with "oob" in the auxdata. You can send oob data by adding "oob" to the write auxdata.

See documentation on SCTP for more details.

Remote Address String

The remote address will be in the format "[ipv4|ipv6],<addr>,<port>[;[ipv4|ipv6],<addr>,<port>[...]]" where the address is in numeric format, IPv4, or IPv6. Each remote address for the SCTP connection is listed.

Remote Address

SCTP returns a packed struct sockaddr for GENSIO_CONTROL_RADDR_BIN control, per SCTP semantics.

UNIX

unix[(<options>)],<socket_path>

Create a unix domain socket as an accepter, or connect to a unix domain socket as a connecter.

A file will be created with the given socket path, you must have permissions to create a writeable file in that location. If the file already exists, an error will be returned on an accepter socket unless you specify delsock which will cause the file to be deleted.

You should read the unix(7) man page for details on the semantics of these sockets, especially permissions. The options below allow setting various permission and ownership of the file, but this may not have any effect on who can open socket depending on the particular operating system. Portable programs should not rely on these permissions for security. Also note that Linux remote credentials are not currently implemented.

Options

In addition to readbuf, the unix gensio takes the following options:
delsock[=true|false]
If the socket path already exists, delete it before opening the socket.
umode=[0-7|[rwx]*]
Set the user file mode for the unix socket file. This is the usual read(4)/write(2)/execute(2) bitmask per chmod, but only for the user portion. If a mode is specified, all other modes default to "6" (rw) +unless they are specified, and the final mode is modified by the umask +per standard *nix semantics. If no mode is specified, it is set to +the default and not modified. Note that the perm option below is +probably a better way to set this.
gmode=[0-7|[rwx]*]
Set the group file mode for the unix socket file, see umode for details.
omode=[0-7|[rwx]*]
Set the other file mode for the uix socket file, see umode for details.
perm=[0-7][0-7][0-7]
Set the full mode for the unix socket file per standard *nix semantics, modified by umask as the above mode operations are.
owner=<name>
Set the owner of the unix socket file to the given user.
group=<name>
Set the group of the unix socket file to the given group.

Remote Address String

The remote address will be: "unix,<socket path>".

Remote Address

UNIX returns a standard struct sockaddr_un for GENSIO_CONTROL_RADDR_BIN control.

serialdev

serialdev[(<options>)],<device>[,<serialoption>[,<serialoption>]]

A serialdev connection is a local serial port. The device is a /dev/xxx type, and should be real stream device of some type that normal termios work on (except for wronly).

This is, no surprise, a serial gensio.

One problem with serialdev and UUCP locking is that if you fork() a process while one is open, the forked process will have the serialdev but the value in the UUCP lockfile will be incorrect. There's not much that can be done about this, so be careful.

Options

In addition to readbuf, the serialdev gensio takes the following options:
nouucplock[=true|false]
disables UUCP locking on the device. Useful for /dev/tty, which shouldn't use locking. This is not available as a default.

There are a plethora of serialoptions, available as defaults:

[speed=]<speed><parity><databits><stopbits>
This is a normal serial port configuration specification, like "9600N81". The "speed=" at the beginning is optional, but "speed" is a default type for this.
wronly[=true|false]
Set the device to write only. No termios definition is done on the port. This can be done to talk to a line printer port, for instance. False by default.
nobreak[=true|false]
Clear the break line at start (or don't clear it). Default it to not clear it (false).
rs485=off|<delay rts before send>:<delay rts after send>[:<conf>[:<conf>]]
Set up RS-485 for the serial port. The first two parameters set the RTS delay (in milliseconds) of RTS before and after sending. The conf values can be: "rts_on_send" - RTS set when sending, "rts_after_send" - RTS set after sending, "rx_during_tx" - can receive and transmit at the same time, and "terminate_bus" - enable bus termination.

Using "off" will disable rs485, that is useful for overriding a default. default is "off".

xonxoff[=true|false]
Enable/disable xon/xoff flow control. Default is false.
rtscts[=true|false]
Enable/disable rts/cts flow control. Default is false.
local[=true|false]
Ignore/don't ignore the modem control lines. The default it to not ignore them (false). However, if you don't ignore the modem control lines, it can result in long shutdown delays.
hangup-when-done[=true|false]
Lower/don't lower the modem control lines when the gensio is closed. The default is to not lower the modem control lines on close (false). custspeed[=true|false] Allow/don't allow setting of custom baud rates. Ignored if custom baud rates are not supported. Normally only standard baud rates are supported (1200, 2400, etc). If supported by the hardware, this allows any arbitrary value to be set.

Remote Address String

The remote address string is the device and serial parms, in a format like "9600N81[,<option>[,<option>[...]]]". Options are one of: XONXOFF, RTSCTS, CLOCAL, HANGUP_WHEN_DONE, RTSHI, RTSLO, DTRHI, DRLO, offline.

Remote ID

The remote ID for the serial dev is the file descriptor returned as an integer.

stdio

accepter = stdio[(options)]
connecting = stdio[(options)],<program>

The stdio gensio is a fairly strange one, but it's fairly useful.

A connecting stdio gensio runs the given program (unless self is set as an option) and connects its standard input and output to the gensio's main channel. So it's easy to run a program and interact with it. If you want to get stderr from the gensio, open a channel on the main channel gensio, see below.

NOTE: Though epoll() doesn't work with normal files, the stdio gensio has special handling for normal files so they mostly work. This can have surprising side effects if you use stdin as a normal file. When you hit the end of stdin, the stdio gensio will return GE_REMCLOSE and you may shut down the gensio and possibly lose any output going to stdout. Use the file gensio if you can.

For connecting gensios, in the forked process, the code will set the uid and guid to the current set effective uid and guid if the effective and real uids are different. This way a user can set the effective uid and gid to what they want the program to run under, but keep the uid and gid to the (probably root) values so they can be restored to those values after opening the pty. The group list is also set to the groups the effective userid is in. Note that nothing is done if the effective and real userids are the same.

If you have both stdin/stdout and stderr opened, you must close both of them to completely close the gensio. You cannot re-open the gensio until both are closed.

The connecting gensio support the GENSIO_CONTROL_ENVIRONMENT control to allow the environment to be set for the new process.

An accepting gensio immediately does a connection when started and connection stdin and stdout of the running program to the gensio.

Options

In addition to readbuf, a connecting stdio takes the following options:
self[=true|false]
Instead of starting a program, connect to the running program's stdio. This is the same as an accepting stdio gensio, except you don't have to go through the accept process.
stderr-to-stdout
Send standard error output to standard out instead of having a separate channel for it.

Channels

The stdio connecting gensio that start another program does not provide stderr as part of the main gensio. You must create a channel with gensio_alloc_channnel() and then open it get stderr output. The args for gensio_alloc_channel() may be an optional "readbuf=<num>" and sets the size of the input buffer.

Remote Address String

The remote address string is either "stdio,<args>" for the main channel or "stderr,<args>" for the error channel. The args will be a set of quoted strings with a space between each string, one for each argument, with '"' around each argument, each '"' in the string converted to '\"' and each '\' in the string converted to '\\'.

Remote ID

The remote ID is the pid for the remote device, only for connecting gensios that start a program, not for any other type.

echo

connecting = echo[(options)]

The echo gensio just echos back all the data written to it. Useful for testing.

Options

In addition to readbuf, a connecting echo takes the following options:
noecho[=true|false]
Instead of echoing the data, just become a data sink.

Remote Address String

The remote address string is "echo".

file

connecting = file[(options)]

The file gensio opens an (optional) input file and (optional) output file for the gensio. If you need to read/write a file in gensio, you must use this. Semantics on files for stdio do not alway work correctly.

If an input file is specified, data will be read from the input file and delivered on the read interface of the gensio until end of file, when the GE_REMCLOSE error will be returned.

If an output file is specified, data will be written to the output file on writes. The output file is always ready to receive data.

Options

In addition to readbuf, a connecting file takes the following options:
infile=filename
Set the input filename.
outfile=filename
Set the output filename.
create[=true|false]
Create the output file if it does not exist.
umode=[0-7|[rwx]*]
Set the user file mode for the file if the file is created. This is the usual read(4)/write(2)/execute(2) bitmask per chmod, but only for the user portion. If a mode is specified, all other modes default to "6" (rw) +unless they are specified, and the final mode is modified by the umask +per standard *nix semantics. If no mode is specified, it is set to +the default and not modified. Note that the perm option below is +probably a better way to set this.
gmode=[0-7|[rwx]*]
Set the group file mode for the file if the file is created, see umode for details.
omode=[0-7|[rwx]*]
Set the other file mode for the file if the file is created, see umode for details.
perm=[0-7][0-7][0-7]
Set the full mode for the file per standard *nix semantics, modified by umask as the above mode operations are.

Remote Address String

The remote address string is "file([infile=<filename][,][outfile=<filename>])".

dummy

accepter = dummy

The dummy gensio accepter is useful for place holding, where you don't want a real working accepter but need to put something.

As you might guess, it doesn't do anything.

ipmisol

ipmisol[(options)],<openipmi arguments>[,ipmisol option[,...]]

An ipmisol gensio creates an IPMI Serial Over LAN connection to an IPMI server. See the OpenIPMI documentation for information on the arguments.

This is a serial gensio, but the baud rate settings are fairly limited.

Options

In addition to readbuf, the ipmisol gensio takes the following options:
writebuf=<n>
to set the size of the write buffer.

It also takes the following ipmisol options:

9600, 19200, 38400, 57600, 115200
Baud rate, 9600 by default. Anything after the number is ignored, for compatibility with things like "N81". You cannot set those values for ipmisol, they are fixed at "N81".
nobreak[=true|false]
Disable break processing. False by default.
authenticated[=true|false]
Enable or disable authentication. True by default.
encrypted[=true|false]
Enable or disable encrypted. True by default.
deassert-CTS-DCD-DSR-on-connect[=true|false]
False by default
shared-serial-alert=fail|deferred|succeed
Behavior of handling serial alerts. fail by default.
ack-timeout=<number>
The time (in microseconds) when an ack times out. 1000000 by default.
ack-retries=<number>
The number of times (ack-timeouts) an ack is re-sent before giving up. 10 by default.

telnet

accepter = telnet[(options)]
connecting = telnet[(options)]

A telnet gensio is a filter that sits on top of another gensio. It runs the standard telnet protocol andn support RFC2217.

Options

In addition to readbuf, the telnet gensio takes the following options:
writebuf=<n>
set the size of the write buffer.
rfc2217[=true|false]
enable or disable RFC2217 mode for this gensio. If this is enabled and the remote end of the connection supports RFC2217 also, the gensio will be set up as a serial gensio and you can do normal serial gensio handling on it.
mode=client|server
Set the telnet mode to client or server. This lets you run a telnet server on a connecting gensio, or a telnet client on an accepter gensio.

Remote info

telnet passes remote id, remote address, and remote string to the child gensio.

ssl

accepter = ssl[(options)]
connecting = ssl[(options)]

An SSL gensio runs the SSL/TLS protocol on top of another gensio. That gensio must be reliable.

Use is pretty straightforward. The hardest part about using the SSL gensio is the certificates. A server SSL gensio must be given a certificate and a key. A client SSL gensio must be given a certificate authority. A client will user the certificate authority to verify that the server has the proper certificate and keys.

The gensio has options to have the server request the certificate from the client and validate it.

SSL gensios are reliable. They are also packet-oriented.

Options

In addition to readbuf, the SSL gensio takes the following options:
writebuf=<n>
set the size of the write buffer.
CA=<filepath>
Set a place to look for certificates for authorization. If this ends in a "/", then this is expected to be a directory that contains files with certificates that must be hashed per OpenSSL (see the "openssl rehash" command for details. Otherwise it is a single file that contains one or more certificates. The default CA path is used if not specified. Note that setting this to an empty string disables it, so you can override a default value if necessary.
key=<filename>
Specify the file to get the private key for the server. This is required for servers. It may be specified for clients, and is required for the client if the server requires a certificate (it has CA set).
cert=<filename>
Specify the file that contains the certificate used for the connection. If this is not specified, the certificate is expected to be in the key file. Note that setting this to an empty string disables it, so you can override a default value if necessary.
mode=client|server
Normally an accepter gensio is in server mode and a connecting gensio is in client mode. This can be used to switch the roles and run in SSL server mode on a connecting gensio, or vice versa.
clientauth[=true|false]
Normally a client is not authorized by the server. This requires that the client provide a certificate and authorizes that certificate. Ignored for client mode.
allow-authfail[=true|false]
Normally if the remote end certificate is not valid, the SSL gensio will close the connection. This open allows the open to succeed with an invalid or missing certificate. Note that the user should verify that authentication is set using gensio_is_authenticated().

Verification of the common name is not done. The application should do this, it can fetch the common name and other certificate data through a control interface.

You can use self-signed certificates in this interface. Just be aware of the security ramifications. This gensio is fairly flexible, but you must use it carefully to have a secure interface.

The SSL gensio will call the gensio event callback (client) or the gensio acceptor event callback (server) after the certificate is received but before it is validated with the GENSIO_EVENT_PRECERT_VERIFY or GENSIO_ACC_EVENT_PRECERT_VERIFY events. This allows the user to validate data from the certificate (like common name) with GENSIO_CONTROL_GET_PEER_CERT_NAME or set a certificate authority for the validation with GENSIO_CONTROL_CERT_AUTH.

Remote info

ssl passes remote id, remote address, and remote string to the child gensio.

certauth

accepter = certauth[(options)]
connecting = certauth[(options)]

A certauth gensio runs an authentication protocol to on top of another gensio. That gensio must be reliable and encrypted.

This is like the reverse of SSL. The client has a key and certificate, the server has a certificate authority (CA). This also supports password authentication.

Once authentication occurs, this gensio acts as a passthrough. The readbuf option is not available in the gensio.

Options

The certauth gensio takes the following options:
CA=<filepath>
Set a place to look for certificates for authorization. If this ends in a "/", then this is expected to be a directory that contains files with certificates that must be hashed per OpenSSL, see the gensio-keygen(1) in the rehash section for details. Otherwise it is a single file that contains one or more certificates. The default CA path for openssl is used if not specified. This is used on the server only, it is ignored on clients. Note that setting this to an empty string disables it, so you can override a default value if necessary.
key=<filename>
Specify the file to get the private key for the client. This is required for clients. It is ignored on server.
cert=<filename>
Specify the file to get the private key for the client. This is required for clients. It is ignored on server. If this is not specified, the certificate is expected to be in the key file. Note that setting this to an empty string disables it, so you can override a default value if necessary.
username=<name>
Specify a username to authenticate with on the remote end. This is required for the client. It is ignored on the server.
service=<string>
Set the remote service requested by the client. Optional, but the other end may reject the connection if it is not supplied. Ignored on the server.
password=<string>
Specify the password to use for password authentication. This is not recommended, the callback is more secure to use.
mode=client|server
Normally an accepter gensio is in server mode and a connecting gensio is in client mode. This can be used to switch the roles and run in server mode on a connecting gensio, or vice versa.
allow-authfail[=true|false]
Normally if the remote end certificate is not valid, the certauth gensio will close the connection. This open allows the open to succeed with an invalid or missing certificate. Note that the user should verify that authentication is set using gensio_is_authenticated().
use-child-auth[=true|false]
If the child gensio is authenticated, then do not run the protocol, just go straight into passthrough mode and don't do any authentication.
enable-password[=true|false]
On the server, allow passwords for login. On the client, send a password if asked for one. By default passwords are disabled. Use of passwords is much less secure than certificates, so this is discouraged.

Verification of the common name is not done. The application should do this, it can fetch the common name and other certificate data through a control interface. It may also use the username fetched through the control interface.

You can use self-signed certificates in this interface. Just be aware of the security ramifications. This gensio is fairly flexible, but you must use it carefully to have secure authentication.

The certauth gensio has 4 major callbacks dealing with authentication of the user. They may or may not be called depending on the circumstances. The normal events come in if you have allocated a gensio and are doing an open. The _ACC_ events come in if it is coming in from an accept and there is no gensio reported yet. In the _ACC_ case, be careful, do not use the given gensio for anything but checking certificate and username parameters, and do not save it.

All these calls should return 0 if they want the authentication to immediately succeed, EKEYREJECTED if they reject the authentication, ENOTSUP if they want certauth to ignore that part of the authentication, or any other errno will result in the connetion being shut down.

The callbacks are:

GENSIO_EVENT_AUTH_BEGIN / GENSIO_ACC_EVENT_AUTH_BEGIN
On the server side, this is called when to authentication is requested buy the client. The username will be available if the user provided it via GENSIO_CONTROL_USERNAME.
GENSIO_EVENT_PRECERT_VERIFY / GENSIO_ACC_EVENT_PRECERT_VERIFY
On the server side, thi is called after the certificate has been received but before it is verified. The user can use this to query the certificate and update the certificate authority based on username or certificate information.
GENSIO_EVENT_VERIFY_PASSWORD / GENSIO_ACC_EVENT_VERIFY_PASSWORD
On the server side, this is called if the certificate verification has failed and after the password has been requested from the remote end. The password is passed in, it is cleared immediately after this call.
GENSIO_EVENT_REQUEST_PASSWORD / GENSIO_ACC_EVENT_REQUEST_PASSWORD
On the client side, this is called if the server requests that a password be sent and the password is not already set for the gensio. The requested password is immediately cleared after being sent to the server.

Remote info

certauth passes remote id, remote address, and remote string to the child gensio.

mux

accepter = mux[(options)]
connecting = mux[(options)]

A mux gensio is a gensio filter that allows one or more channels to be created on top of a single gensio, multiplexing the connection. Each channel has full individual flow-control. The channels are message oriented as described above, and use can use a mux without additional channels to just do message demarcation. They also support out-of-bounds messages.

Options

A mux gensio takes the following options:
max_channels=<n>
Allow at most <n> channels to be created in the mux. The default is 1000. The minimum value of <n> is 1, the maximum is 65536.
service=<string>
Set the remote service requested by the client. Optional, but the other end may reject the connection if it is not supplied. Ignored on the server.
mode=client|server
By default a mux accepter is a server and a mux connecter is a client. The protocol is mostly symmetric, but it's hard to kick things off properly if both sides try to start things. This option lets you override the default mode in case you have some special need to do so.

When the open is complete on the mux gensio, it will work just like a transparent filter with message demarcation. In effect, you have one channel open.

Creating Channels

To create a channel, call the gensio_alloc_channel() function on the mux gensio. This will return a new gensio that is a channel on the mux gensio. You can pass in arguments, which is an array of strings, currently readbuf, writebuf, and service are accepted. The service you set here will be set on the remote channel so the other end can fetch it. The new channel needs to be opened with gensio_open() before it can be used.

As you might imaging, the other end of a mux needs to know about the new channel. If one end (either end, doesn't matter) calls gensio_alloc_channel() and then opens the new channel, the other end will get a GENSIO_EVENT_NEW_CHANNEL event as described in the Streams and Channels section. You can call it using any mux channel. The first element in the auxdata is the service.

You can modify the service value after you allocate the channel but before you open it.

Out Of Band Messages

mux support out of band (oob) data, which is data that will be delivered normally. This comes in a normal read, but with "oob" in the auxdata. You can send oob data by adding "oob" to the write auxdata. You should normally use the "eom" flag so the end of the out of band messages ismarked.

This is so you can send special data outside of the normal processing of data.

Mux Events

As you might imaging, normal data events come through the gensio channel they are associated with. Close events will, too. If a mux gensio closes abnormally (like the underlying connection fails) you will get a read error on each channel.

New channel events (and other global events coming from lower gensios, like if you are running over ssl or telnet) come in through the original gensio normally. However, you can close that, another channel will be chosen to receive those event. In general, it's best to handle the global events on all channels and not assume.

Closing and Freeing a Mux

To close a mux gensio, just close all channels associated with it. There is no global close mechanism (you would not believe the complexity that adds). Once you have closed a mux gensio, you can re-open it with gensio_open(). It will not recreate all the channels for you, you will have one channel, and the channel you use to call gensio_open() will become the first channel.

You cannot re-open individual channels.

To free a mux gensio, you must free all the channels on it.

pty

connecting = pty[(options)][,<program>]

Create a pty and optionally run a program on the other end of it. With a program specified, this is sort of like stdio, but the program is running on a terminal. Only connection gensios are supported.

pty has some unusual handling to allow execution of login shells of users from root.

If the first character of the program is '-', it is removed from the execution of the command but left in argv[0]. This will cause a shell to act as a login shell.

In the forked process, the pty code will set the uid and guid to the current set effective uid and guid if the effective and real uids are different. This way a user can set the effective uid and gid to what the want to pty to run under, but keep the real uid and gid to the (probably root) values so they can be restored to those values after opening the pty. The group list is also set to the groups the effective userid is in. Note that nothing is done if the effective and real userids are the same.

The pty gensio supports the GENSIO_CONTROL_ENVIRONMENT control to allow the environment to be set for the new process.

If no program is specified, when the pty gensio is opened it will just create the pty but won't attach anything to it. Another program can open the pty. You can get the slave pty device by getting the local address string.

Options

In addition to readbuf, the pty gensio takes the following options. These options are only allowed if the pty is unattached. ptys with programs run on them need to follow the standard semantics.
link=<filename>
create a symbolic link from filename to the pty name. This way you can have a fixed reference to refer to the pty. Standard permissions apply. Without forcelink the open will file if filename already exists.
forcelink[=true|false]
if link is specified, if a file already exists where the symbolic link is, replace it. Be careful, it deletes whatever is there indiscriminately.
raw[=true|false]
causes the pty to be set in raw mode at startup. This is important for anything that will start writing immediately to the pty. If you don't set this, echo will be on and writes on the master will be echoed back. In general, this is useful for unattached ptys. It was added for testing, but will be usedful in many situations.
umode=[0-7|[rwx]*]
Set the user file mode for the pty slave. This is the usual read(4)/write(2)/execute(2) bitmask per chmod, but only for the user portion. If a mode is specified, all other modes default to "6" (rw) +unless they are specified, and the final mode is modified by the umask +per standard *nix semantics. If no mode is specified, it is set to +the default and not modified. Note that the perm option below is +probably a better way to set this.
gmode=[0-7|[rwx]*]
Set the group file mode for the pty slave, see umode for details.
omode=[0-7|[rwx]*]
Set the other file mode for the pty slave, see umode for details.
perm=[0-7][0-7][0-7]
Set the full mode for the pty per standard *nix semantics, modified by umask as the above mode operations are.
owner=<name>
Set the owner of the slave pty to the given user.
group=<name>
Set the group of the slave pty to the given group.

Remote Address String

The remote address string the program and arguments run on the pty. The argiuments will be a set of quoted strings with a space between each string, one for each argument, with '"' around each argument, each '"' in the string converted to '\"' and each '\' in the string converted to '\\'.

Remote Address

The address is a pointer to an integer, the ptym file descriptor is returned. addrlen must be set to sizeof(int) when passed in.

Local Address String

This returns the device of the pty, generally /dev/pts/<n>. This is useful for pty gensios with no program, it allows you to get the value for the other end to connect to. Note that if you close and re-open a pty gensio, you may be a different local address string.

msgdelim

accepter = msgdelim[(options)]
connecting = msgdelim[(options)]

A message delimiter converts an unreliable stream (like a serial port) into an unreliable packet interface. This is primarily to allow a reliable packet interface like relpkt to run on top of a serial port. It does not support streaming of data, so it's not very useful by itself.

Messages are delimited with a start of message and end of message and have a CRC16 on the end of them.

This is primarily for use on serial ports and other streams that can munge up the data. Use the mux gensio for TCP and such.

THe default buffer size is 128 bytes, which is ok for a reasonably fast serial port interface.

Options

In addition to readbuf, the msgdelim gensio takes the following options:
writebuf=<n>
set the size of the write buffer.
crc[=on|off]
Enable/disable the CRC at the end of the packet. Useful if you are running over a reliable protocol, and especially for testing relpkt so you can fuzz it and bypass the crc errors.

relpkt

accepter = relpkt[(options)]
connecting = relpkt[(options)]

Converts an unreliable packet interface (currently only UDP and msgdelim) into a reliable packet interface. This lets you have a reliable connection over a serial port (with msgdelim) or UDP.

Note that UDP is not recommended, it doesn't handle flow control and such in a friendly manner.

relpkt is unusual in dealing with clients and servers. The protocol is symmetric, for the most part, you can start two clients and they will connect to each other, if they are started relatively close in time to avoid one timing out. A relpkt server will simply wait forever for an incoming connection on an open.

relpkt does not support readbuf. It supports the following:

max_pktsize=<n>
Sets the maximum size of a packet. This may be reduced by the remote end, but will never be exceeded. This must be at least 5 bytes shorter than the maximum packet size of the interface below it. This defaults to 123 (msgdelim max packet size - 5). If run over UDP, this should probably be increased for performance.
max_packets=<n>
Sets the maximum number of outstanding packets. This may be reduced by the remote end, but will never be exceeded.
mode=client|server
By default a relpkt is a server on an accepter and a client on a connecter. See the discussion above on clients and servers.

trace

accepter = trace[(options)]
connecting = trace[(options)]

A transparent gensio that allows the data passing through it to be sent to a file. Useful for debugging.

Note that the trace gensio only prints data that is accepted by the other end. So, for instance, if the trace gensio receives 100 bytes of read data, it will deliver it immediately to the gensio above it. If that only accepts 40 bytes, trace will only print 40 bytes and will only accept 40 bytes from the gensio below it. Same for sent data.

Options

trace does not support readbuf. It supports the following options:
dir=none|read|write|both
Sets what data is traced. "none" means no data, "read" means data flowing up the gensio stack (read by the parent), write means data flowing down the gensio stack (written by the parent), and "both" traces reads and writes.
raw[=yes|no]
If set, data will be written as raw bytes. If not set, data will be written in human-readable form.
file=<filename>
The filename to write to. If not supplied, tracing is disabled. Note that unless delold is specified, the file is opened append, so it will add new trace data onto the end of an existing file.
stderr[=yes|no]
Send the output to standard error. Overrides file.
stdout[=yes|no]
Send the output to standard output. Overrides file and stderr.
delold[=yes|no]
Delete the old data in the file instead of appending to the file.

perf

accepter = perf[(options)]
connecting = perf[(options)]

A gensio for measuring throughput for a connection. This allows the performance of a connection to be measured. It does not pass any data through. Instead, it writes raw data to the lower layer (if write_len is set) and reads data from the lower layer, counting the bytes and measuring the time.

To the upper layer, perf prints out statistics about the data transfer. It prints out the number of bytes written and read each second, and at the end it prints a total.

If write_len and/or expect_len is non-zero, then the filter will return GE_REMCLOSE when it runs out of write data and has received all expected data. If both are zero, the connection will not be closed by the gensio.

Options

perf does not support readbuf. It supports the following options:
writebuf=<n>
Sets the size of the buffer to write to the child gensio. Each write will be this size. This defaults to zero.
write_len=<n>
The number of bytes to write.
expect_len=<n>
The number of bytes to expect from the other end.

conacc

accepter = conacc,<gensio string>

conacc is a gensio accepter that takes a gensio as a child. When the accepter is started, it opens the child gensio. When the child gensio finishes the open, it reports a new child for the accepter. The reported gensio can be used normally.

When the gensio is closed, when the close completes the accepter will attempt to re-open the gensio. This means that if the gensio has some sort of random address (like a pty or a tcp address with the port set to zero) you can get a different address for the re-opened gensio. So you must refetch the local address or local port in this case.

mdns

connecting = mdns[(options)][,<name>]

This gensio uses mDNS to find a service and then attempts to connect to it. This can be convenient, it finds the connection type andport for you, automatically adds telnet and rfc2217 if it's available. The mDNS name can be set as an option or as the string after the comma show above.

Options

The readbuf option is accepted, but if it is not specified the default value for readbuf will be taken for the sub-gensio is taken. In addition to readbuf, the mdns gensio takes the following options:
nostack[=true|false]
By default the mdns gensio will attempt to use the "gensiostack" text string from the mDNS service. If you don't want it to do this, setting this option will turn it off.
name=<mdnsstr>
Set the mdns name to search for. You generally want to set this, otherwise you will get the first thing that comes up from the search.
type=<mdnsstr>
Set the mdns type to search for.
domain=<mdnsstr>
Set the mdns domain to search for. You generally don't use this.
host=<mdnsstr>
Set the mdns host to search for. You generally don't use this.
interface=<num>
Set the network interface number to search on. The default is -1, which means all interfaces.
nettype=unspec|ipv4|ipv6
The network type to search for. unspec means any type, otherwise you can choose to limit it to ipv4 and ipv6.
nodelay[=true|false]
Sets nodelay on the socket. This will be ignored for udp. Note that the default value for mdns is ignored, if you don't set it here it will take the default value for the sub-gensio that gets chosen. laddr=<addr> An address specification to bind to on the local socket to set the local address.

Forking and gensios

Unlike normal file descriptors, when you fork with a gensio, you now have two unassociated copies of the gensios. So if you do operations on one, it might mess up the other. Even a close might cause issues, if you close an SSL connection, it sends data to the other end to close the connection, even if the other fork doesn't want that.

To avoid issues with this, you should generally first make sure that no thread is in a wait, service call, or any type of thing that would examine file descriptors or timers and act on them. This is very important, and you must do it before you fork.

Then after you fork, you should call:

gensio_disable(io)

on all the gensios that fork is not using, then free the gensios. Don't use close, use free. Then you should call:

gensio_acc_disable(acc)

on every gensio accepter that fork is not using, then free them. If a connection is in progress and has not been reported to the user, it will be disabled then closed.

You cannot share a gensio between two different processes. If a gensio is used in one fork, it must be disabled and closed in the other fork.

Another issue with forking on Linux is epoll. An epoll fd is not duplicated on a fork, both forks get the same epoll fd. If you close the epoll fd in one for, it will close it for the other. To avoid this issue, the os handler has a handle_fork() function that you must call after a fork in the new fork (not the old one). It will handle any cleanup required after the fork. Other systems may require other cleanups after a fork, so you should always call this after a fork.

SEE ALSO

gensiot(1), sctp(7), udp(7), tcp(7), unix(7)

KNOWN PROBLEMS

None.

AUTHOR

Corey Minyard <minyard@acm.org>
01/02/19 Specifying a gensio