Script to create static routes for individual host names and network interfaces

host-interface-route.sh

#!/usr/bin/env bash

# Create a static route for a given host/domain over a specific network interface.
# Put this script in /etc/network/if-up.d/host-interface-route so it runs when the interface comes up
# Script was developer for Ubuntu 16.04 LTS running Network Manager (Gnome 3)

# I use this script to prevent my VPN connections from interfering with automatic reverse SSH tunnles
#  to my jump box running in the cloud.

# network interface to target
TARGET_IFACE="eth0"
# hostname to target
TARGET_HOST="jump.your-host-here.io"
# metric (lower number is higher priority)
# useful if you have more than one interface such as a wired Ethernet and a wifi radio
METRIC=50

if [ "${IFACE}" == "${TARGET_IFACE}" ]; then
	# resolve the hostname to an IP address
	DST_IP=$(dig +short "${TARGET_HOST}")
	# determine the gateway for our interface and IP
	GATEWAY=$(ip route get "${DST_IP}" oif "${IFACE}" | awk "/${IFACE}/ { print \$3 }")
	# add the static route
	ip route add "${DST_IP}" via "${GATEWAY}" dev "${IFACE}" metric "${METRIC}"
fi

Some resources that were helpful when I was writing this script:

• http://askubuntu.com/questions/417868/network-manager-breaks-manual-routes
• http://serverfault.com/questions/648276/routing-selection-specificity-vs-metric
• https://www.cyberciti.biz/faq/howto-linux-configuring-default-route-with-ipcommand/
• https://www.digitalocean.com/community/tutorials/how-to-use-the-awk-language-to-manipulate-text-in-linux