Hi there,
Last week I got load balancing working between our 2 lans and 2 adsl connections. This script here is how I set it up. A tad excessive stipulating the eth setup at the beginning, but I wanted it all in one script. The firewall config was more taxing, but it is perfectly possible to allow incoming services on either ISP connection and route it back over the correct one, just need to write lots of firewall rules, matching on the outgoing interface is how I managed the source nat'ing for both.

Cheers,
Daniel.

#!/bin/bash

# ISP Variables
ISP1DEV="eth0"
ISP1IP="1.1.1.54"
ISP1MASK="255.255.255.248"
ISP1NET="1.1.1.48/29"
ISP1BCAST="1.1.1.55"
ISP1GW="1.1.1.49"

ISP2DEV="eth3"
ISP2IP="2.2.2.210"
ISP2MASK="255.255.255.248"
ISP2NET="2.2.2.208/29"
ISP2BCAST="2.2.2.215"
ISP2GW="2.2.2.209"

# LAN Variables
# DMZ
LAN1DEV="eth1"
LAN1IP="10.0.0.1"
LAN1MASK="255.255.255.0"
LAN1NET="10.0.0.0/24"
LAN1BCAST="10.0.0.255"

# LAN
LAN2DEV="eth2"
LAN2IP="192.168.42.250"
LAN2MASK="255.255.255.0"
LAN2NET="192.168.42.0/24"
LAN2BCAST="192.168.42.255"

# Setup interfaces with correct IPs/Netmasks

ifconfig $ISP1DEV down
ifconfig $ISP2DEV down
ifconfig $LAN1DEV down
ifconfig $LAN2DEV down

ifconfig $ISP1DEV $ISP1IP netmask $ISP1MASK broadcast $ISP1BCAST 
ifconfig $ISP2DEV $ISP2IP netmask $ISP2MASK broadcast $ISP2BCAST 
ifconfig $LAN1DEV $LAN1IP netmask $LAN1MASK broadcast $LAN1BCAST  
ifconfig $LAN2DEV $LAN2IP netmask $LAN2MASK broadcast $LAN2BCAST 

# Clear current routing tables
ip route del

# Setup routing tables
ip route add $ISP1NET dev $ISP1DEV src $ISP1IP table T1
ip route add $ISP2NET dev $ISP2DEV src $ISP2IP table T1
ip route add default via $ISP1GW table T1
ip route add default via $ISP2GW table T2

# Setup main routing table
ip route add $ISP1NET dev $ISP1DEV src $ISP1IP
ip route add $ISP2NET dev $ISP2DEV src $ISP2IP

# Setup matching to tables based on connection
ip rule add from $ISP1IP table T1
ip rule add from $ISP2IP table T2

ip route add $LAN1NET dev $LAN1DEV table T1
ip route add $LAN2NET dev $LAN2DEV table T1
ip route add $ISP2NET dev $ISP2DEV table T1
ip route add 127.0.0.0/8 dev lo   table T1
ip route add $LAN1NET dev $LAN1DEV table T2
ip route add $LAN2NET dev $LAN2DEV table T2
ip route add $ISP1NET dev $ISP1DEV table T2
ip route add 127.0.0.0/8 dev lo   table T2

# Set weighting for load balancing
ip route add default scope global nexthop via $ISP1GW dev $ISP1DEV weight 5 nexthop via $ISP2GW dev $ISP2DEV weight 5