tungate.sh

#!/bin/bash

short_route() {
	ip route | sed -n "s/$1 \\(.* dev [^ ]\\+\\).*/\\1/p" | head -n 1
}

resolve_ip() {
	local filter='/^[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}$/{p;q;}'
	[[ $(sed -n "$filter" <<<"$1") == "$1" ]] && echo "$1" ||
		host -t a "$1" | cut -d ' ' -f 4 | sed -n "$filter"
}

set_route() {
	local default_route="$(short_route default)"
	[[ -n $default_route ]] || { echo "[-] Could not determine default route"; return; }
	echo "[+] Adding route for $1 to be $default_route"
	ip route del "$1/32" 2>/dev/null
	ip route add "$1/32" $default_route
}

check_loop() {
	local ip="$(resolve_ip "$1")"
	[[ -n $ip ]] || { echo "[-] Could not determine IP of '$1'" && return 1; }
	echo "[+] Making sure $ip goes over the default (non-0/1,128/1) route"

	set_route "$ip"

	while read; do
		[[ $(short_route "$ip") != "$(short_route default)" ]] && set_route "$ip"
	done < <(exec ip monitor route)
}

[[ $# -ne 1 ]] && { echo "Usage: $0 IP|HOST" >&2; exit 1; }
[[ $UID -ne 0 ]] && exec sudo "$(readlink -f "$0")" "$@"

check_loop "$1"
exit $?