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vnetbuild.conf(5) 3.1.7 vnetbuild.conf(5)


vnetbuild.conf - VNetBuild configuration file


host *ID*

dev *DEVICE* [ *ID*/*PAIRDEV* ] [ *IP*/*MASK*... ]
bridgedev *BRIDGE* [ *DEVICE*... ] [ *IP*/*MASK*... ]
route *ROUTECMD*
... ... switch *ID*
dev *DEVICE* [ *ID*/*PAIRDEV* ]
... ...


There is no default configuration file for vnetbuild(1); one must always be specified on the command line.

The configuration file defines a set of namespaces that will be operated on.

VNetBuild defines two types of namespace, a host and a switch. Any number of each may be specified, with any number of configuration statements in each.

The Linux kernel does not see any difference between a host and a switch namespace. VNetBuild provides the distinction to make it easy build full virtual networks.


Namespace definitions come in two types, host and switch. Simply provide a simple unique alphanumeric ID. Any subsequent statements apply to this namespace until the next host or switch statement.

A host definition is designed to work like a physical machine. It allows you to specify any number of dev entries for network interfaces, with their IP addresses. You can also define any number of Linux bridges with bridgedev to add your defined interfaces to.

A host also allows any number of custom exec commands for extensibility and provides a route statement to deal with the common case of wanting to add network routes to the host.

A switch definition is designed to work like a physical network switch. It allows you to add any number of dev entries (and also custom exec commands for extensibility) but nothing else.

In addition, dev entries in a switch may only specify device names, they cannot have an IP address associated. A switch has a bridge automatically created in it and all dev entries are automatically added to it.


Define a virtual ethernet device, DEVICE in a host or switch.

Devices must exist in pairs. A dev must first be defined unpaired in a namespace, then some subsequent dev must define the pair:

host a

dev veth0 host b
dev vppp0 a/veth0

Any DEVICE name which is acceptable to the Linux kernel may be used. We recommend sticking to e.g. veth0, vppp0 etc. to make it clear that they are virtual and also how you are thinking of the device in terms of your setup. Devices will be created as type veth, irrespective of what you call them.

Hosts may optionally specify one or more IP/MASK values which will be applied (along with the calculated broadcast address) automatically, e.g.:

host a

dev veth0 host b
dev vppp0 a/veth0

A dev may not specify an IP address if it is in a switch. Switches exist just to tie together multiple devices in hosts, just like a physical network switch.

Define an ethernet bridge, BRIDGE in a host. These are setup automatically using ip(8) and shown with bridge(8).

A bridge can specify network devices from its own namespace to be automatically added, as well as its own IP address(es).

host a

dev veth0
dev veth1 otherns/vdev0
bridgedev vbr0 veth0 veth1

Devices included in a bridge generally do not need their own IP address (although that is permitted).

Bridges cannot have a pair themselves, but any devices added to a bridge need a pair as usual.

Specify an additional network route for a host.

Most commonly to add a default route from hosts on a “LAN” to the machine that acts as a gateway, e.g.:

route default via

The syntax of ROUTECMD is anything that can fit this pattern:

ip route add ROUTECMD

See ip(8) and ip-route(8) for help adding routes. If you want to do anything more complex than simply adding routes, use the exec configuration statement.

Execute custom commands in a host or switch just before bringing up the specified device. All of the pre_up statements for a device are combined and executed in the namespace.

In addition to any explicitly defined interfaces, switches have an implicit bridge device called switch which can also be used in pre_up commands.

Bridges always start after other devices, so to run a command after all everything has been created but before any interfaces are up, you can make use of pre_up on the first defined dev.

See below for some common uses for custom pre_up and exec commands.

Execute a custom command in a host or switch once the rest of the namespace setup is complete.

Once all the namespaces are created, the final step in setting each one up is to have its exec statements combined and executed.

It is roughly the equivalent to writing your own script and executing it after vnetbuild start has finished:

sudo ip netns exec myns ./

See below for some common uses for custom pre_up and exec commands.


For the most part it doesn’t matter whether these commands are used in pre_up or exec operations - the only difference is when they will execute, so e.g. if you want a firewall in place before any interfaces come up then start it from the pre_up of the first device. If you only want the firewall after all devices are up, put it in exec, e.g.:

host myfirewall

exec firehol myfirewall.conf start

Forwarding is not enabled by the Linux kernel when a namespace is first created. This can be easily done for any hosts that need to forward traffic:

host mygateway

exec echo 1 > /proc/sys/net/ipv4/ip_forward

The exec operates in the mygateway namespace so your host is not affected.

Bridges are created without STP being enabled. To enable STP to ensure loops are not created, the following can be done:

host myhost

bridgedev vbr0 ...
pre_up vbr0 echo 2 > /sys/class/net/vbr0/bridge/stp_state switch myswitch
pre_up switch echo 2 > /sys/class/net/vbr0/bridge/stp_state

You could also use brctl stp vbr0 on and brctl stp switch on instead of setting the values directly. To disable multicast snooping you can use exactly the same method e.g.:

switch myswitch

pre_up switch echo 0 > /sys/class/net/switch/bridge/multicast_snooping

It is possible to run firehol within a namespace to set up custom

Logs from network namespaces are not included in the normal system logs. To enable iptables logging you must start an instance of ulogd(8) in the namespace and use ULOG or NFLOG logging. For FireHOL, that means set FIREHOL_LOG_MODE=ULOG or FIREHOL_LOG_MODE=NFLOG. Note that NFLOG only works with ulogd version 2.

The default configuration for ulogd(8) is /etc/ulogd.conf. Assuming the default place it will write iptables logs to is /var/log/ulog/syslogemu.log (otherwise change the sed command as required), it is simple to set up per-namespace logging:

host mygateway

exec sed 's:/var/log/ulog/syslogemu.log:/var/log/ulog/mygateway.log:' /etc/ulogd.conf > $NSTMP/ulogd.conf
exec /usr/sbin/ulogd -d -c $NSTMP/ulogd.conf

The -d flag to ulogd(8) makes it become a daemon; when vnetbuild stop executes it will automatically kill any programs running in the namespaces is is stopping, which includes the logging daemon.

The configuration file will get cleaned as soon as vnetbuild start is finished. To be able to access such files you need to write them to a location not under $NSTMP or create them outside the vnetbuild configuration altogether.


A simple LAN arrangement with two hosts, one of which is a gateway to third host:

host host01

dev veth0
dev vppp0
exec echo 1 > /proc/sys/net/ipv4/ip_forward
route default via host host02
dev veth0
route default via switch lan
dev d01 host01/veth0
dev d02 host02/veth0 host extern01
dev veth0 host01/vppp0
route default via
exec echo 1 > /proc/sys/net/ipv4/ip_forward


When created, the namespaces setup by vnetbuild are completely disconnected from any real network. There is no way of defining such a connection in the vnetbuild configuration as allowing it would lead to conflicts with the normal network setup tools and configuration files in most distributions.

It is possible to arrange your network so you can connect real devices into one or more network namespaces. For the general approach see this mailing list post (


vnetbuild(1) - VNetBuild program
FireHOL Website (
VNetBuild Online PDF Manual (
VNetBuild Online Documentation (
ip(8) ( - show/manipulate network devices
ip-route(8) ( - routing table management
bridge(8) ( - routing table management
ulogd(8) ( - netfilter/iptables logging daemon


FireHOL Team.

Built 15 Oct 2022 VNetBuild Reference