Relationship between interface "vethxxxxx" and container?

Docker Engine General
22

From host:

cat /sys/class/net/veth45562ed/iflink

768

ethtool -S veth45562ed

NIC statistics:
peer_ifindex: 767

From Container

6b63d46e1ac7# ethtool -S eth0
NIC statistics:
peer_ifindex: 768
6b63d46e1ac7:/# cat /sys/class/net/eth0/iflink
767
6b63d46e1ac7:/#

23

@khatribharat is correct; however, to be able to cat the files in question, you need cat available inside your container. Iā€™m often working with containers that are simple compiled binaries and donā€™t have access to normal utilities. In that case, you can still get at the needed information, but it requires a little more work. So I made a script to correlate containers with their veth interfaces: https://github.com/micahculpepper/dockerveth

Example output:

[root@dockervisor-1 ~]# dockerveth
CONTAINER ID	VETH       	NAMES
60d27ce962ff	vethe353e93	hopeful_bhaskara
d07a2979e69a	vethe4c3cee	silly_meitner
1e8656e195ba	veth1ce04be	thirsty_meitner
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24

Another way, albeit not perfectly simple, is to first do a ā€œdocker psā€ and see what ports a container is using.
Then do a ā€œiptables -S -t natā€ to see which bridge that port is DNAT:et to.

25

In order to identify the relation between the veth on the host and eth0 interface on the container, we can check the interfaces on the container using the following command:

[root@docker js]# docker exec -it dc ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
94: eth0@if95: <BROADCAST,MULTICAST,UP,LOWER_UP,M-DOWN> mtu 1500 qdisc noqueue state UP
link/ether 02:42:ac:11:00:03 brd ff:ff:ff:ff:ff:ff
inet 172.17.0.3/16 brd 172.17.255.255 scope global eth0
valid_lft forever preferred_lft forever

We can see that the eth0 is suffixed by if[number]. This number also prefixed to corresponding the host virtual interface (veth). To validate this, check the network interfaces on the host:

[root@docker js]# ip a | egrep veth
93: veth5e8ceb7@if92: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default
95: veth3de0e04@if94: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker0 state UP group default

26

great work! but output looks a bit distorted on Linux but it due to the fact that the scrip was created on Windows what required translation into the Linux world anyway, otherwise you get /bin/bash^M: bad interpreter as well descripbed in command line - Not able to execute a .sh file: /bin/bash^M: bad interpreter - Ask Ubuntu and Bash script and /bin/bash^M: bad interpreter: No such file or directory - Stack Overflow

27

If you find the previous script not convenient enough, I wrote another simple script to do this which I use in many cases.

https://github.com/cslev/find_veth_docker

It is as simple as it can be :wink:

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