Verify running docker containers run with the same MTU as the Uplink

docker_mtu_connectivity.sh

#!/bin/bash

set -o nounset
set -o errexit
set -o pipefail

function get_route_device() {
    ip route get "${1}" | grep -Po '(?<=dev\s)[\w-]+' | cut -f1 -d ' '
}

function get_network_mtu() {
    ip link show "${1}" | head -n 1 | awk '{print $5}'
}

function get_docker_ip() {
    docker inspect "${1}" | jq -r '.[].NetworkSettings.Networks | .[].IPAddress'
}

# get name of uplink lan interface
LANIFACE=$(get_route_device 1.1.1.1)
# get currently used MTU for uplink
MTU_LAN=$(get_network_mtu "${LANIFACE}")

echo "Uplink \"${LANIFACE}\" MTU ${MTU_LAN}"

RUNNING_CONTAINERS=$(docker ps --format '{{.Names}}')
for container in ${RUNNING_CONTAINERS}; do
    container_ip=$(get_docker_ip "${container}")
    container_network=$(get_route_device "${container_ip}")
    network_mtu=$(get_network_mtu "${container_network}")

   if [[ ${MTU_LAN} -ne ${network_mtu} ]]; then
        echo "Container \"${container}\" is running in network \"${container_network}\" with MTU ${network_mtu}"
   fi
done