Understanding Linux Network Internals

Chapter 32. Routing: Li nux Implementation

Chapter 30 provided an overview of the main tasks of the routing subsystem, and Chapter 31 introduced the features such as Policy Routing and Multipath that IP implements on top of the basic routing functionality. In this chapter, I introduce the main data structures used by the routing code. I then show:

• How scopes are defined for routes and IP addresses

• How the routing subsystem is initialized

• What kind of external events the routing subsystem needs to be notified of to keep its routing information updated

Later chapters will go into detail on the routing cache, routing tables, and routing lookups.

Kernel Options

As we will see in the rest of this chapter, routing does not involve just receiving a packet on one interface, consulting the routing table, and forwarding the packet out of the right outgoing interface. There are a number of additional tasks to take care of at the same time. Quite a few interesting routing-related features have been implemented in the Linux kernel. In addition to those we will see later in this chapter, many others are waiting for the green light from Linus or owners of other subsystems to be integrated into the kernel.

Here I briefly introduce the features of the Linux kernel that influence the behavior of the routing code so that you will not suffer confusion when you peruse the source code. Each feature is further described in dedicated sections in this and later chapters.

Routing options can be classified into two ...