Load balancing & NAT-ing multiple ISP connections on Linux

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I have done load balancing using both lartc.org and iptables methods, and I find that the iptables method is easier to understand and implement. The only downside is that you need a fairly recent iptables version to be able to use statistic module

Let’s suppose a few things:

LAN: eth0: 192.168.0.1/24

ISP1: eth1: 192.168.1.1/24, gateway: 192.168.1.2/24

ISP2: eth2: 192.168.2.1/24, gateway: 192.168.2.2/24

So here is how I would do by using iptables method:

Route tables

First edit the /etc/iproute2/rt_tables to add a map between route table numbers and ISP names

...
10 ISP1
20 ISP2
...

So table 10 and 20 is for ISP1 and ISP2, respectively. I need to populate these tables with routes from main table with this code snippet (which I have taken from hxxp://linux-ip.net/html/adv-multi-internet.html)

ip route show table main | grep -Ev '^default' \
   | while read ROUTE ; do
     ip route add table ISP1 $ROUTE
done

And add default gateway to ISP1 through that ISP1’s gateway:

ip route add default via 192.168.1.2 table ISP1

Do the same for ISP2

So now I have 2 route tables, 1 for each ISP.

Iptables

OK now I use iptables to evenly distribute packets to each route tables. More info on how this work can be found here (http://www.diegolima.org/wordpress/?p=36) and here (http://home.regit.org/?page_id=7)

# iptables -t mangle -A PREROUTING -j CONNMARK --restore-mark
# iptables -t mangle -A PREROUTING -m mark ! --mark 0 -j ACCEPT
# iptables -t mangle -A PREROUTING -j MARK --set-mark 10
# iptables -t mangle -A PREROUTING -m statistic --mode random --probability 0.5 -j MARK --set-mark 20
# iptables -t mangle -A PREROUTING -j CONNMARK --save-mark

NAT

Well NAT is easy:

# iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
# iptables -t nat -A POSTROUTING -o eth2 -j MASQUERADE

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