Linux Loadbalancing with two adsl modems

My Client, a cybercafe in Tondano, North Celebes, Indonesia, using two speedy adsl modems for the Internet and other connections and they want to use both of the connection together to share the load.

After search the Internet for the example with no luck, I finally manage to made a working script based on all the example I’ve got.

I put the simple and short script (I’ve made a long one with transparent proxy and more complex iptables rule) here so anyone looking for same configuration or script won’t stumble on internet looking for the working script (like myself).

Save the script below as rc.firewall or other named according to your like, don’t forget to #chmod +x rc.firewall

I use Slackware Linux for the server/gateway so I saved it as /etc/rc.d/rc.firewall.

#!/bin/sh

# EXT1 = network interface #1 that connect to adsl modem #1
# EXT2 = network interface #2 that connect to adsl modem #2
# INT  = network interface for internal network
# IPEXT1 = ip address for network interface #1
# IPEXT2 = ip address for network interface #2
# GWEXT1 = gateway ip address for network interface #1
# GWEXT2 = gateway ip address for network interface #2

IPT=”/usr/sbin/iptables”
IP=”/sbin/ip”
EXT1=”eth0″
EXT2=”eth1″
INT=”eth2″
IPEXT1=”192.168.1.2″
IPEXT2=”192.168.0.2″
GWEXT1=”192.168.1.1″
GWEXT2=”192.168.0.1″

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

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

# initialise chains that will do the work and log the packets
$IPT -t mangle -N CONNMARK1
$IPT -t mangle -A CONNMARK1 -j MARK –set-mark 1
$IPT -t mangle -A CONNMARK1 -j CONNMARK –save-mark
$IPT -t mangle -A CONNMARK1 -j LOG –log-prefix ‘IPT-mark1: ‘ –log-level info

$IPT -t mangle -N CONNMARK2
$IPT -t mangle -A CONNMARK2 -j MARK –set-mark 2
$IPT -t mangle -A CONNMARK2 -j CONNMARK –save-mark
$IPT -t mangle -A CONNMARK2 -j LOG –log-prefix ‘IPT-mark2: ‘ –log-level info

$IPT -t mangle -N RESTOREMARK
$IPT -t mangle -A RESTOREMARK -j CONNMARK –restore-mark
$IPT -t mangle -A RESTOREMARK -j LOG –log-prefix ‘restore-mark: ‘ –log-level info

$IPT -t nat -N SNAT1
$IPT -t nat -A SNAT1 -j LOG –log-prefix ’snat-to-192.168.1.2: ‘ –log-level info
$IPT -t nat -A SNAT1 -j SNAT –to-source $IPEXT1

$IPT -t nat -N SNAT2
$IPT -t nat -A SNAT2 -j LOG –log-prefix ’snat-to-192.168.0.2: ‘ –log-level info
$IPT -t nat -A SNAT2 -j SNAT –to-source $IPEXT2

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

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

$IPT -t nat -A POSTROUTING -o $EXT1 -j SNAT1
$IPT -t nat -A POSTROUTING -o $EXT2 -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 via $GWEXT1 dev $EXT1

$IP route flush table rt_link2 2>/dev/null
$IP route add table rt_link2 default via $GWEXT2 dev $EXT2

$IP rule del from all fwmark 0×1 lookup rt_link1 2>/dev/null
$IP rule del from all fwmark 0×2 lookup rt_link2 2>/dev/null
$IP rule del from all fwmark 0×2 2>/dev/null
$IP rule del from all fwmark 0×1 2>/dev/null
$IP rule add fwmark 1 table rt_link1
$IP rule add fwmark 2 table rt_link2

$IP route flush cache

BTW … I set the MTU for both EXT1 and EXT2 card to 1454 according to this Optimal MTU for PPPOE ADSL

Have fun!