debug @20210222

#!/bin/bash

INFINCOMING=wlp4s0
INFOUTGOING1=enp0s25
INFOUTGOING2=wlp0s20u9u4u1

echo 1 >| /proc/sys/net/ipv4/ip_forward
echo 0 >| /proc/sys/net/ipv4/conf/all/rp_filter

# flush all iptables entries
iptables -t filter -F
iptables -t filter -X
iptables -t nat -F
iptables -t nat -X
iptables -t mangle -F
iptables -t mangle -X
iptables -t filter -P INPUT ACCEPT
iptables -t filter -P OUTPUT ACCEPT
iptables -t filter -P FORWARD ACCEPT

if [ "$1" == "stop" ]; then
	# iptables -t nat -A POSTROUTING -o
	echo "iptables stoped."
	exit
fi

# initialise chains that will do the work and log the packets
iptables -t mangle -N CONNMARK1
iptables -t mangle -A CONNMARK1 -j MARK --set-mark 1
iptables -t mangle -A CONNMARK1 -j CONNMARK --save-mark
iptables -t mangle -A CONNMARK1 -j LOG --log-prefix 'iptables-mark1: ' --log-level info

iptables -t mangle -N CONNMARK2
iptables -t mangle -A CONNMARK2 -j MARK --set-mark 2
iptables -t mangle -A CONNMARK2 -j CONNMARK --save-mark
iptables -t mangle -A CONNMARK2 -j LOG --log-prefix 'iptables-mark2: ' --log-level info

iptables -t mangle -N RESTOREMARK
iptables -t mangle -A RESTOREMARK -j CONNMARK --restore-mark
iptables -t mangle -A RESTOREMARK -j LOG --log-prefix 'restore-mark: ' --log-level info

# restore the fwmark on packets that belong to an existing connection
iptables -t mangle -A PREROUTING -i ${INFINCOMING} -p tcp \
		 -m state --state ESTABLISHED,RELATED -j RESTOREMARK

# if the mark is zero it means the packet does not belong to an existing connection
iptables -t mangle -A PREROUTING -p tcp -m state --state NEW \
         -m statistic --mode nth --every 2 --packet 0 -j CONNMARK1
iptables -t mangle -A PREROUTING -p tcp -m state --state NEW \
         -m statistic --mode nth --every 2 --packet 1 -j CONNMARK2

iptables -t nat -N SNAT1
iptables -t nat -A SNAT1 -j LOG --log-prefix 'snat-to-10.37.1.253: ' --log-level info
iptables -t nat -A SNAT1 -j SNAT --to-source 10.37.1.253

iptables -t nat -N SNAT2
iptables -t nat -A SNAT2 -j LOG --log-prefix 'snat-to-10.37.2.253: ' --log-level info
iptables -t nat -A SNAT2 -j SNAT --to-source 10.37.2.253

iptables -t nat -A POSTROUTING -o ${INFOUTGOING1} -j SNAT1
iptables -t nat -A POSTROUTING -o ${INFOUTGOING2} -j SNAT2

if ! cat /etc/iproute2/rt_tables | grep -q '^251'
then
	echo '251     rt_link1' >> /etc/iproute2/rt_tables
fi
if ! cat /etc/iproute2/rt_tables | grep -q '^252'
then
	echo '252     rt_link2' >> /etc/iproute2/rt_tables
fi

ip route flush table rt_link1 2>/dev/null
ip route add table rt_link1 default dev ${INFOUTGOING1}
ip route flush table rt_link2 2>/dev/null
ip route add table rt_link2 default dev ${INFOUTGOING2}

ip rule del from all fwmark 0x1 lookup rt_link1 2>/dev/null
ip rule del from all fwmark 0x2 lookup rt_link2 2>/dev/null
ip rule del from all fwmark 0x2 2>/dev/null
ip rule del from all fwmark 0x1 2>/dev/null
ip rule add fwmark 1 table rt_link1
ip rule add fwmark 2 table rt_link2

ip route flush cache