Load balancing using iptables

load_balance.sh

#!/bin/bash

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

iptables -F
iptables -t mangle -F
iptables -t mangle -X
iptables -t nat -F
iptables -t nat -X

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 -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 PREROUTING -p tcp -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m conntrack --ctstate ESTABLISHED,RELATED -j CONNMARK --restore-mark
iptables -t mangle -A PREROUTING -p tcp -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 0 -j CONNMARK1
iptables -t mangle -A PREROUTING -p tcp -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m conntrack --ctstate NEW -m statistic --mode nth --every 2 --packet 1 -j CONNMARK2

iptables -t mangle -A PREROUTING ! -p tcp -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m statistic --mode random --probability 0.5 -j MARK --set-mark 1
iptables -t mangle -A PREROUTING ! -p tcp -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m mark --mark 0x0 -j MARK --set-mark 2

iptables -t nat -A POSTROUTING -o wlp7s0 -j MASQUERADE
iptables -t nat -A POSTROUTING -o enp8s0 -j MASQUERADE

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 via 192.168.48.1 dev wlp7s0

ip route flush table rt_link2 2>/dev/null
ip route add table rt_link2 default via 192.168.1.1 dev enp8s0

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

https://www.system-rescue.org/networking/Load-balancing-using-iptables-with-connmark/