Table of Contents
Routing and understanding routing in an IP network is one of the fundamentals you will need to grasp the flexibility of IP networking, and services which run on IP networks. It is not enough to address the machines and mix yourself a dirty martini. You'll need to verify that the machine has a route to any network with which it needs to exchange IP packets.
One key element to remember when designing networks, viewing routing tables, debugging networking problems, and viewing network traffic on the wire is that IP routing is stateless . This means that every time a new packet hits the routing stage, the router makes an independent decision about where to send this packet.
In this section, we'll look at the tools available to manipulate and view the routing table(s). We'll start with the well known route command, and move on to the increasingly used ip route and ip rule tools which are part of the iproute2 package.
 For those who have some doubt, netfilter provides a connection tracking mechanism for packets passing through a linux router. This connection tracking, however, is independent of routing. It is important to not conflate the packet filtering connection tracking statefulness with the statelessness of IP routing. For an example of a complex networking setup where netfilter's statefulness and the statelessness of IP routing collide, see Section 10.4, “Multiple Connections to the Internet”.