Load balancing is a method to distribute workload across multiple computer over a network. The purpose of load balance in web server is to avoid one web server from being overwhelmed by requests which eventually leads the machine to come down to a crawling halt.
Assuming that you have 3 web server to assign the load to each with this IP Address:
10.20.20.1
10.20.20.2
10.20.20.3
You can drive the traffic to each of this on every third packet with this iptables rules:
iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 0 -j DNAT --to-destination 10.20.20.1:80
iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 1 -j DNAT --to-destination 10.20.20.2:80
iptables -A PREROUTING -i eth0 -p tcp --dport 80 -m state --state NEW -m nth --counter 0 --every 3 --packet 2 -j DNAT --to-destination 10.20.20.3:80
This will ensure that every 3rd packet of the request will be properly distributed among the three servers to balance the load. Note that this is only useful in simple website which serves static content or for a download servers that serve large files over the internet (CD or DVD iso downloading)
