Understanding Linux Network Internals

Chapter 30. Routing: Concepts

Figure 30-1 shows where the routing subsystem (the gray box) fits into the network stack. The figure does not include all the details (Netfilter, bridging, etc.) but shows the other major kernel subsystems that are traversed before and after routing.

Figure 30-1. Relationship between the routing subsystem and the other main network subsystems

To explain some of the features or the details of their implementation, I’ll often show snapshots of user-space configurations. You are encouraged to use Chapter 36 as a reference if you need to learn more about the user-space tools I employ in the examples.

The discussion on routing will focus on IPv4 networks. However, I will point out the aspects of IPv6 that differ significantly.

Routers, Routes, and Routing Tables

In its simplest form, a router can be defined as a network device that is equipped with more than one network interface card (NIC), and that uses its knowledge of the network to forward ingress traffic appropriately.^[*]

The information required to decide whether an ingress packet is addressed to the local host or should be forwarded, together with the information needed to correctly forward the packets in the latter case, is stored in a database called the Forwarding Information Base (FIB). It is often referred to simply as the routing table .

Figure 30-2 shows a simple scenario with a LAN whose hosts are ...