Linux Networking 1: server work load balancing with multiple NICs

We'll cover some advanced use of linux networking capabilities in this mini series (including load balancings, 2 nic with same IP--bonding, bridge, and router). We'll begin with load balancing of server workload. Although this discussion does not cover subjects such as routing, packet manipulation (through iptables), it illustrates from an application level, how to use a linux box with multiple NICs to do useful things. A linux box with multiple NICs will be a common theme of this mini series.

The concept of server work load balancing is to allow a linux box to use multiple NIC to service network traffic. A simple setup is to have a single external IP exposed to clients. Behind the external IP are multiple NICs that can form a dispatch table based on traffic and workload behind each NIC.

Let's use 2 NIC load balancing to illustrate the idea:

ext_nic(ip)-----load-balancer-demon-----NIC0-----SERVER0
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>+----NIC1-----SERVER1

Whenever a client request comes in, the load-balancer-demon queries the SERVER behind a NIC to figure out which NIC the traffic/request should be forwarded to depending on the current workload on each server.

Thus the load-balancer-demon is a user space application that proxies client requests.