User:Jchdel

to be run periodically (I do it every minutes for a 3 ISP setup)

#!/bin/bash
# This script configure the router to use fastest available gateway
# as default one. It uses some bashism...
[ -f /tmp/router.order ] && logger "Already running" && exit 1 
NICS=($(ip -br link|awk '{print $1}')) # index 0 is loopback (lo)
NNICS=$((${#NICS[@]}-1))
GWS=()
IPS=()
iptables -t mangle -F
iptables -t nat -F
# Did we lost all of our gateways?
ip route | grep -q default || \
for ((i=1; i<=$NNICS; i++))
do 
	ifdown ${NICS[$i]}
	ifup ${NICS[$i]} && \
	logger "${NICS[$i]} restarted." || \
	logger "Unable to restart ${NICS[$i]}."
done
# Prepare routing tables for each gateway
for ((i=1; i<=$NNICS; i++))
do
	IPS[$i]="$(ip a s ${NICS[$i]}|awk '/inet/{print $2}'|cut -d/ -f1)"
	[ -z "${IPS[$i]}" ] && GWS[$i]="" && continue
	# disable kernel filtering of "unwanted" packets
	echo 0 > /proc/sys/net/ipv4/conf/${NICS[$i]}/rp_filter
	GWS[$i]="$(ip r | grep default | grep -v dead | grep ${NICS[$i]} | cut -d\  -f3)"
	[ -z "${GWS[$i]}" ] && continue
	# duplicate main routing table for each live default gateway
	ip route flush table $i
	ip route show table main | grep -Ev ^default | while read ROUTE ; do
		ip route add table $i $ROUTE
	done
	# add default gateway
	ip route add table $i default via ${GWS[$i]} dev ${NICS[$i]}
	# mark packets from localhost
	iptables -t mangle -A OUTPUT -s ${IPS[$i]} -j MARK --set-mark $i
	# activate alternate routing
	while $(ip rule del fwmark $i table $i);do .;done 2>/dev/null
	ip rule add fwmark $i table $i prio ${i}0
	# MASQUERADE or SNAT ?
	iptables -t nat -I POSTROUTING -o ${NICS[$i]} -j MASQUERADE #SNAT --to-source ${IPS[$i]}
done
# activate changes in policy routing
ip route flush cache
# ping same target via each gateway
for ((i=1; i<=$NNICS; i++))
do
	[ -z "${GWS[$i]}" ] && continue
	( ping -n -c 5 -q -I ${IPS[$i]} 1.1 >/dev/null \
		&& logger "1.1 responded for ${IPS[$i]} dev ${NICS[$i]}" \
		|| logger "No response from 1.1 for ${IPS[$i]} dev ${NICS[$i]}" ; \
		echo $i >> /tmp/router.order ) &
done
wait -n # first gateway to complete
# meanwhile, main routing table is still active
INDEX=$(head -1 /tmp/router.order)
METRIC=1
for ((i=1; i<=$NNICS; i++))
do
	[ -z "${GWS[$i]}" ] && continue
	ip route del default via ${GWS[$i]} dev ${NICS[$i]}
done
ip route add default via ${GWS[$INDEX]} dev ${NICS[$INDEX]} metric $METRIC
echo 1 > /proc/sys/net/ipv4/ip_forward
logger "Default route set via ${GWS[$INDEX]} dev ${NICS[$INDEX]}."
wait # other gateways
cat /tmp/router.order | while read i
do
	[ $i -eq $INDEX ] && continue
	METRIC=$(($METRIC+1))
	ip r a default via ${GWS[$i]} dev ${NICS[$i]} metric $METRIC
done
rm -f /tmp/router.order
exit 0