#!/usr/bin/env bash

# Title               : isp-switch
# Last modified date  : 30.05.2023
# Author              : Martin Tonusoo
# Description         : Script pings anycast DNS servers IPv4 addresses
#                       using a primary connection(wan0; Telia fiber) and
#                       if none of those replies, then switches v4 default
#                       route to backup connection(wwan0; Telia mobile
#                       broadband). Once the primary connection restores,
#                       the v4 default route is switched back to primary
#                       connection.
# Options             :
# Notes               : "ignore_routes_with_linkdown" settings in
#                       /etc/sysctl.conf ensure that the route is
#                       instantly ignored if its link is down.


ping_check() {

	ping_success=0

	for ip in 1.1.1.1 8.8.8.8 9.9.9.9; do
		# Even if one ping out of four succeeds, then
		# the exit code is 0.
		ping -I "$1" -W 1 -c 4 -q "$ip" &>/dev/null
		ping_success=$(( ping_success + $? ))
	done

	if (( ping_success >= 3 )); then
		# All 12 pings failed.
		return 1
	else
		return 0
	fi
}


read -r ip_regex << EOF
^((25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)\.){3}\
(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9][0-9]?|0)$
EOF


while true; do

	sleep 1

	read -r _ _ wan0_gw _ wan0_metric _ <<-EOF
		$(ip -4 -o route show default dev wan0 2>/dev/null | grep -v " linkdown")
	EOF

	read -r _ _ wwan0_gw _ wwan0_metric _ <<-EOF
		$(ip -4 -o route show default dev wwan0 2>/dev/null | grep -v " linkdown")
	EOF


	# There is no point to proceed if both or one of the default
	# routes is missing. In addition, sanity check the next hop
	# address.
	if ! { [[ "$wan0_gw" =~ $ip_regex ]] || [[ "$wwan0_gw" =~ $ip_regex ]]; }; then

		echo "No default route found."
		continue

	elif ! [[ "$wan0_gw" =~ $ip_regex ]]; then

		echo "Primary default route via wan0 is missing."
		continue

	elif ! [[ "$wwan0_gw" =~ $ip_regex ]]; then

		echo "Failover default route via wwan0 is missing."
		continue

	fi


	# Default IPv4 route metric is 0.
	if (( ${wan0_metric:-0} < ${wwan0_metric:-0} )); then

		if ! ping_check wan0; then

			# Ping check on primary connection failed.
			echo "Switching to backup via $wwan0_gw dev wwan0"

			ip -4 route flush default
			ip -4 route add default via "$wwan0_gw" dev wwan0 metric 100
			ip -4 route add default via "$wan0_gw" dev wan0 metric 200
		fi

	else
		# Backup connection is in use. Check if it is possible
		# to switch back to primary connection.
		ip -4 route add default via "$wan0_gw" table 100
		# Specifying the "priority" ensures that there are
		# no duplicate rules created.
		ip rule add oif wan0 lookup 100 priority 100

		if ping_check wan0; then

			# Ping check on primary connection succeeded.
			echo "Switching to primary via $wan0_gw dev wan0"

			ip -4 route flush default
			ip -4 route add default via "$wan0_gw" dev wan0 metric 100
			ip -4 route add default via "$wwan0_gw" dev wwan0 metric 200
		fi

		ip -4 route flush table 100
		ip rule delete oif wan0 lookup 100 priority 100

	fi
done