Find out which network interface belongs to docker container

To locate interface
In my case getting value from container was like (check eth0 to):

$ docker exec -it my-container cat /sys/class/net/eth1/iflink
123

And then:

$ ip ad | grep 123
123: vethd3234u4@if122: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master docker_gwbridge state UP group default

Check with tcpdump -i vethd3234u4

Reference about mysterious iflink from http://lxr.free-electrons.com/source/Documentation/ABI/testing/sysfs-class-net:

150 What:           /sys/class/net/<iface>/iflink
151 Date:           April 2005
152 KernelVersion:  2.6.12
153 Contact:        netdev@vger.kernel.org
154 Description:
155                 Indicates the system-wide interface unique index identifier a
156                 the interface is linked to. Format is decimal. This attribute is
157                 used to resolve interfaces chaining, linking and stacking.
158                 Physical interfaces have the same 'ifindex' and 'iflink' values.