docker-veth-lookup.sh

#!/bin/bash

CONTAINER_NAME=${1:?container name is empty}
NETWORK_MODE=$(docker inspect -f "{{.HostConfig.NetworkMode}}" $CONTAINER_NAME)
STATE_PID=$(docker inspect --format '{{.State.Pid}}' $CONTAINER_NAME)
VETH_NAME=

if [[ $NETWORK_MODE = "host" ]]; then
  VETH_NAME="host"
else
  IF_INDEX=$(nsenter -t $STATE_PID -n ip addr | awk -F: '/eth0@/{print $1}')
  if [ -z "$IF_INDEX" ]; then
    VETH_NAME="not found"
  else
    VETH_NAME=$(ip addr | awk -F '[: @]' -vIF_NAME="if$IF_INDEX" '$4==IF_NAME{print $3}')
  fi
fi

echo "$VETH_NAME"

IF_INDEX=$(docker exec $CONTAINER_NAME cat /sys/class/net/eth0/iflink)