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Some notes on veth interfaces

Since there's not much documentation, here are the results of some experimentation.

veth interfaces are virtual ethernet interfaces that always exist in pairs. Whatever enters on one interface, exits from the other one, and viceversa. A simple test to check:

# ip link add type veth
# ip link show
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 52:54:00:5a:d2:86 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether 52:54:00:d7:26:a6 brd ff:ff:ff:ff:ff:ff
23: veth0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether ee:c0:0e:d6:ae:09 brd ff:ff:ff:ff:ff:ff
24: veth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether 4e:e8:84:bd:01:f0 brd ff:ff:ff:ff:ff:ff
# ip addr add 10.0.0.1/24 dev veth0
# ip link set veth0 up
# ip link set veth1 up
# force sending packets out veth0
# ping 10.0.0.2

While the ping is running, tcpdump on veth1 will show traffic (most likely ARP).

One thing to note is that for an interface to be up, the other one must be up too.

The main (or perhaps only?) use for veth interfaces seems to be in the context of container virtualization, especially LXC. Once a veth pair is created, one end is assigned to the container and one end is assigned to the main host. Communication can then happen either using the pair as a point-to-point direct link (assigning IPs to both ends and doing routing on the host) or via bridging (the host-side interface is added to a bridge, perhaps where other interfaces are already connected, and the guest-side interface is assigned an IP inside the container).

Since containers are possible because of namespaces (two very good introductory articles can be found here and here), this is also the method used to "assign" the guest-side interface to the container. Note that doing so makes the interface deisappear from the host.

Let's try a simple example. First we start a container without network (lxc.network.type = empty):

container# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

Then, on the host, we create a pair of veth:

host# ip link add vHOST type veth peer name vGUEST
host# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 52:54:00:5a:d2:86 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether 52:54:00:d7:26:a6 brd ff:ff:ff:ff:ff:ff
25: vGUEST: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether be:86:db:5b:ec:a5 brd ff:ff:ff:ff:ff:ff
26: vHOST: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether d6:c9:65:e3:bb:e9 brd ff:ff:ff:ff:ff:ff

Bring the links up:

host# ip link set vHOST up
host# ip link set vGUEST up

Now we have to find out the PID of the container's main process; there are a few ways to do this, pstree is probably visually easier:

host# pstree -Apc
...
        |-lxc-start(3615)---init(3619)-+-getty(4236)
        |                              |-getty(4238)
        |                              |-getty(4239)
        |                              |-getty(4240)
        |                              |-login(4237)---bash(4447)
        |                              `-sshd(4223)
...

so we want PID 3619, and thus we do

host# ip link set vGUEST netns 3619

Doing this makes the interface disappear from the host:

host# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether 52:54:00:5a:d2:86 brd ff:ff:ff:ff:ff:ff
3: eth1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT qlen 1000
    link/ether 52:54:00:d7:26:a6 brd ff:ff:ff:ff:ff:ff
26: vHOST: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT qlen 1000
    link/ether d6:c9:65:e3:bb:e9 brd ff:ff:ff:ff:ff:ff

and appear in the guest container:

container# ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
25: vGUEST: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether be:86:db:5b:ec:a5 brd ff:ff:ff:ff:ff:ff

Now we can configure IPs and we're set:

host# ip addr add 10.0.0.1/24 dev vHOST
container# ip addr add 10.0.0.2/24 dev vGUEST
container# ping 10.0.0.1
PING 10.0.0.1 (10.0.0.1) 56(84) bytes of data.
64 bytes from 10.0.0.1: icmp_req=1 ttl=64 time=0.103 ms
...

As mentioned, another possibility would be adding vHOST to a bridge.

Of course, all this doesn't have to be done manually, since common LXC tool can automate things, but it's just for explanation purposes. In fact, when lxc.network.type = veth is used, what happens is that a veth pair is created, one end assigned to the container's namespace, and the other end (on the host) added to an existing bridge (specified with lxc.network.link)

As said, veth interfaces always exist in pair, so when the guest is destroyed (eg shut down), both interfaces disappear. In the same way, if we delete one end on the host, the other end disappears from the guest.

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4 Comments

  1. Ashwin says:

    Hi,

    I have the following questions:

    1. Is there a way to find the name of a peer for a given interface. For example, if I have the name vGUEST, is there a command that responds with vHOST, the name of the interface which is the peer for vGUEST?

    2. Is there a command that dumps all the peers created. For example, ip link list dumps the interfaces but there is no way of knowing the pairs.

    3. Where can I find information about a given interface in /proc? For example, cat /proc/net/dev just gives the stats on the transmit and received packets/frames, but is there a way to get additional information such as the IPv* addresses and the peers.

    Thanks for this crisp and clear article.

    • waldner says:

      I'm not aware of any means to get the information you mention in 1. and 2. (which doesn't mean that there isn't one; more information is always welcome). For 3., the file /proc/net/if_inet6 gives IPv6 addresses for the interfaces. I'm not aware of an equivalent file for IPv4 addresses. In any case, no information about veth pairs is given.

      • Ashwin says:

        Hi,

        Thanks for the suggestion. You can get the peer name by using the ip and ethtool commands. First, you use ethtool to get the index (used by the OS) of the interface and its peer.
        peerid=$(ethtool -S | grep "peer" | awk '{print $2}')

        Then you can extract the name of the interface from the index using the ip command
        peername=$(ip link list | grep "^${peerid}" | awk '{print $2}' | cut -d ':' -f 1 | tr -d ' ')

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