Virtual Ethernet device

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<translate> Virtual Ethernet device is an Ethernet-like device that can be used inside a container. Unlike a venet network device, a veth device has a MAC address. Therefore, it can be used in more configurations. When veth is bridged to a CT0 network interface (e.g., eth0), the container can act as an independent host on the network. The container's user can set up all of the networking himself, including IPs, gateways, etc.

A virtual Ethernet device consists of two Ethernet devices, one in CT0 (e.g., vethN.0) and a corresponding one in CT (e.g., eth0) that are connected to each other. If a packet is sent to one device it will come out the other device.

Virtual Ethernet device usage[edit]

Kernel module[edit]

The vzethdev module should be loaded. You can check it with the following commands.

# lsmod | grep vzeth
vzethdev                8224  0
vzmon                  35164  5 vzethdev,vznetdev,vzrst,vzcpt
vzdev                   3080  4 vzethdev,vznetdev,vzmon,vzdquota

In case it is not loaded, load it:

# modprobe vzethdev

MAC addresses[edit]

The following steps to generate a MAC address are not necessary, since newer versions of vzctl will automatically generate a MAC address for you. These steps are provided in case you want to set a MAC address manually.

You should use a random MAC address when adding a network interface to a container. Do not use MAC addresses of real eth devices, because this can lead to collisions.

MAC addresses must be entered in XX:XX:XX:XX:XX:XX format.

There is a utility script available for generating MAC addresses: It is used like this:

chmod +x

./ -R

Adding veth to a CT[edit]

vzctl set <CTID> --netif_add <ifname>[,<mac>,<host_ifname>,<host_mac>,<bridge>]


  • ifname is the Ethernet device name in the CT
  • mac is its MAC address in the CT
  • host_ifname is the Ethernet device name on the host (CT0)
  • host_mac is its MAC address on the host (CT0), if you want independent communication with the Container through the bridge, you should explicitly specify multicast MAC address here (FE:FF:FF:FF:FF:FF).
  • bridge is an optional parameter which can be used in custom network start scripts to automatically add the interface to a bridge. (See the reference to the vznetaddbr script below and persistent bridge configurations.)
Yellowpin.svg Note: All parameters except ifname are optional. Missing parameters, except for bridge, are automatically generated, if not specified.


vzctl set 101 --netif_add eth0 --save

If you want to specify everything:

vzctl set 101 --netif_add eth0,00:12:34:56:78:9A,veth101.0,00:12:34:56:78:9B --save

If you want to use independent communication through the bridge:

vzctl set 101 --netif_add eth0,00:12:34:56:78:9A,veth101.0,FE:FF:FF:FF:FF:FF,vzbr0 --save

If you want to specify the bridge and autogenerate the other values:

vzctl set 101 --netif_add eth0,,,,vzbr0 --save

Removing veth from a CT[edit]

vzctl set <CTID> --netif_del <dev_name>|all


  • dev_name is the Ethernet device name in the CT.
Yellowpin.svg Note: If you want to remove all Ethernet devices in CT, use all.


vzctl set 101 --netif_del eth0 --save

Common configurations with virtual Ethernet devices[edit]

Module vzethdev must be loaded to operate with veth devices.

Simple configuration with virtual Ethernet device[edit]

Assuming that is being used on your LAN, the following sections show how to configure a container for the LAN using veth.

Start a CT[edit]

[host-node]# vzctl start 101

Add veth device to CT[edit]

[host-node]# vzctl set 101 --netif_add eth0 --save

This allocates a MAC address and associates it with the host eth0 port.

Configure devices in CT0[edit]

The following steps are needed when the CT is not bridged to a CT0 network interface. That is because the CT is connected to a virtual network that is "behind" CT0. CT0 must forward packets between its physical network interface and the virtual network interface where CT is located. The first step below to configure the interface is not necessary if the container has been started, since the device will have been initialized.

[host-node]# ifconfig veth101.0 0
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/veth101.0/forwarding
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/veth101.0/proxy_arp
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/eth0/forwarding
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp

Configure device in CT[edit]

The following steps show an example of a quick manual configuration of the CT network interface. Typically, you would configure the network settings in /etc/network/interfaces (Debian, see below) or however it is normally configured on your distribution. You can also comment or remove the configuration for venet0, if it exists, because that device will not be used.

[host-node]# vzctl enter 101
[ve-101]# /sbin/ifconfig eth0 0
[ve-101]# /sbin/ip addr add dev eth0
[ve-101]# /sbin/ip route add default dev eth0


Add route in CT0[edit]

Since CT0 is acting as a router between its physical network interface and the virtual network interface of the CT, we need to add a route to the CT to direct traffic to the right destination.

[host-node]# ip route add dev veth101.0

Using a directly routed IPv4 with virtual Ethernet device[edit]


Hardware Node (HN/CT0) has with router

We also know that IPv4 is directly routed to (this is called a fail-over IP).

We want to give this directly routed IPv4 address to a container (CT).

Start container[edit]

[host-node]# vzctl start 101

Add veth device to CT[edit]

[host-node]# vzctl set 101 --netif_add eth0 --save

This allocates a MAC address and associates it with the host eth0 port.

Configure device and add route in CT0[edit]

[host-node]# ifconfig veth101.0 0
[host-node]# ip route add dev veth101.0

You can automatize this at VPS creation by using a mount script $VEID.mount.

The problem here is that the veth interface appears in CT0 after VPS has started, therefore we cannot directly use the commands in the mount script. We launch a shell script (enclosed by { }) in background (operator &) that waits for the interface to be ready and then adds the IP route.

Contents of the mount script /etc/vz/conf/101.mount:

# This script source VPS configuration files in the same order as vzctl does

# if one of these files does not exist then something is really broken
[ -f /etc/vz/vz.conf ] || exit 1
[ -f $VE_CONFFILE ] || exit 1

# source both files. Note the order, it is important
. /etc/vz/vz.conf

# Configure veth with IP after VPS has started
  while sleep 1; do
    /sbin/ifconfig $DEV 0 >/dev/null 2>&1
    if [ $? -eq 0 ]; then
      /sbin/ip route add $IP dev $DEV
} &

Make sure IPv4 forwarding is enabled in CT0[edit]

[host-node]# echo 1 > /proc/sys/net/ipv4/ip_forward
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/eth0/forwarding
[host-node]# echo 1 > /proc/sys/net/ipv4/conf/veth101.0/forwarding

You can permanently set this by using /etc/sysctl.conf.

Configure device in CT[edit]

1. Configure IP address

2. Add gateway

3. Add default route

[ve-101]# /sbin/ifconfig eth0 netmask
[ve-101]# /sbin/ip route add dev eth0
[ve-101]# /sbin/ip route add default via

In a Debian container, you can configure this permanently by using /etc/network/interfaces:

auto eth0
iface eth0 inet static
        up /sbin/ip route add dev eth0
        up /sbin/ip route add default via

Virtual Ethernet device with IPv6[edit]

See the VEs and HNs in same subnets article.

Independent Virtual Ethernet communication through the bridge[edit]

Bridging a CT interface to a CT0 interface is the magic that allows the CT to be an independent host on the network with its own IP address, gateway, etc. CT0 does not need any configuration for forwarding packets to the CT or performing proxy arp for the CT or event the routing.

To manually configure a bridge and add devices to it, perform steps 1 - 4 from Simple configuration chapter for several containers and/or veth devices using FE:FF:FF:FF:FF:FF as a CT0 veth side MAC address and then follow these steps.

Create bridge device[edit]

[host-node]# brctl addbr vzbr0

Add veth devices to bridge[edit]

[host-node]# brctl addif vzbr0 veth101.0
[host-node]# brctl addif vzbr0 veth101.n
[host-node]# brctl addif vzbr0 veth102.0
[host-node]# brctl addif vzbr0 vethXXX.N

Configure bridge device[edit]

[host-node]# ifconfig vzbr0 0

Automating the bridge[edit]

The most convenient method is to automatically create the bridge at boot as a network interface, add the physical interface from CT0 and then add the interface from each CT as it starts. All devices are connected to a virtual switch, and containers directly access the network just as any other host without additional configuration on CT0.

In Debian, configure the network interface on CT0 to plug into a bridge in /etc/network/interfaces. The CT0 physical device is added to the bridge as the "uplink" port to the physical network. You need to have bridge-utils installed for this to work.

The bridge forwarding delay is set to 0 seconds so that forwarding begins immediately when a new interface is added to a bridge. The default delay is 30 seconds, during which the bridge pauses all traffic to listen and figure out where devices are. This can interrupt services when a container is added to the bridge. If you aren't running the spanning tree protocol (off by default) and the bridge does not create a loop in your network, then there is no need for a forwarding delay.

iface eth0 inet manual

auto vzbr0
iface vzbr0 inet static
        bridge_ports eth0
        bridge_fd 0

Follow the steps below for making a veth bridge persistent with the included script. That will automatically add each container to the bridge when it is started. Finally, specify vzbr0 as the bridge when adding the network interface to a container, as describe above. No configuration is needed on CT0 for forwarding packets, proxy arp or additional routes. The interface in each CT can be configured as desired. Everything "just works" according to normal network interface configuration and default routing rules. Note that as discussed in the troubleshooting section below, bridged packets by default pass through the FORWARD iptables chain. Take care when adding rules to that table that bridged packets are not mistakenly blocked. This behavior can be disabled, if desired (sysctl: net.bridge.bridge-nf-call-iptables).

Making a veth-device persistent[edit]

These steps are no longer necessary, as the veth device is automatically created when the container is started. They remain here as a reference.

According to , a bug that stopped the veth device persistent was "Obsoleted now when --veth_add/--veth_del are introduced"

See for a workaround that used to be described in this section.

That's it! At this point, when you restart the CT you should see a new line in the output, indicating that the interface is being configured and a new route being added. And you should be able to ping the host, and to enter the CT and use the network.

Making a bridged veth-device persistent[edit]

Like the above example, here it is how to add the veth device to a bridge in a persistent way.

vzctl includes a 'vznetaddbr' script, which makes use of the bridge parameter of the --netif_add switch.

Just create /etc/vz/vznet.conf containing the following.


Or just run command

echo 'EXTERNAL_SCRIPT="/usr/sbin/vznetaddbr"' > /etc/vz/vznet.conf

The script uses 'vmbr0' as default bridge name when no bridge is specified.

Virtual Ethernet devices + VLAN[edit]

This configuration can be done by adding vlan device to the previous configuration.

See also[edit]

External links[edit]