Understanding Linux Network Internals

Chapter 31. Routing: Advanced

The previous chapter gave an introduction to basic routing. This chapter introduces routing features such as policy routing and multipath that can be used to configure routing in more complicated scenarios. It also shows how routing interacts with the Traffic Control subsystem in charge of QoS, and the firewall code (Netfilter). The chapter concludes with two smaller features: ICMP redirects and reverse path filtering.

Concepts Behind Policy Routing

We saw in the section "Special Routes" in Chapter 30 that the Linux kernel uses two routing tables by default, one for local routes and one configurable by the administrator. When the kernel is compiled with support for policy routing, you can have up to 255 distinct and independent routing tables. In this chapter, we will see what policy routing can be used for, and in Chapter 35 we will see its implications on the design of the routing subsystem.

The main idea behind policy routing is to allow the user to configure routing based on more parameters than just the destination IP addresses.

The Internet thrived for years with most routers configured just to route packets based on the destination IP address. (For the sake of simplicity, I’ll leave out particular factors such as crossing ISP or country boundaries.) And basing the route on only the destination address can (with the help of some external configuration parameters) lead to pretty optimal routing tables for a surprisingly wide range of situations.