Set docker dns options dynamically on a Ubuntu 16.04 (systemd) host

fix-docker-dns.sh

#!/usr/bin/env bash
#
# Set docker dns options dynamically on a Ubuntu 16.04 host
#
# See:
# https://github.com/docker/docker/issues/541
# https://docs.docker.com/engine/admin/systemd/
# http://stackoverflow.com/questions/33784295/setting-dns-for-docker-daemon-on-os-with-systemd
# 
set -eui

if [ ! -d "/etc/systemd/system" ]; then
	echo "Directory /etc/systemd/system not found. This may not be a systemd system. Cowardly exiting .."
	exit 1
fi

DOCKERD_CONF_DIR="/etc/systemd/system/docker.service.d"
DOCKERD_DNS_CONF_FILE="$DOCKERD_CONF_DIR/dockerd-dns.conf"

if [ ! -f "$DOCKERD_DNS_CONF_FILE" ]; then
	echo "$DOCKERD_DNS_CONF_FILE not found, creating .."
	sudo mkdir -p "$DOCKERD_CONF_DIR"
	sudo touch "$DOCKERD_DNS_CONF_FILE"
fi

NEW_DNS_OPTS=$(nmcli device show | grep IP4.DNS | egrep -o "([0-9]{1,3}\.){3}[0-9]{1,3}" | sed -e 's/^/--dns /' | paste -s -d ' ')

NEW_CONF="[Service]
ExecStart=
ExecStart=/usr/bin/dockerd ${NEW_DNS_OPTS} -H fd://"

CURRENT_CONF=$(cat "$DOCKERD_DNS_CONF_FILE")

if [ "$CURRENT_CONF" == "$NEW_CONF" ]; then
	echo "Docker DNS options untouched, exiting .."
	exit 0
else
	echo "Updating $DOCKERD_DNS_CONF_FILE .."
	echo "$NEW_CONF" | sudo tee "$DOCKERD_DNS_CONF_FILE"

	echo "Reloading dockerd .."
	sudo systemctl stop docker
	sudo systemctl daemon-reload
	sudo systemctl start docker
fi