Understanding Linux Network Internals

Routing Subsystem Initialization

The initialization of the IPv4 routing code starts in net/ipv4/route.c with ip_rt_init, which is called by the IP subsystem when it is initialized with ip_init in net/ipv4/ip_output.c at boot time. ip_init is described in Chapter 19; here we will see ip_rt_init. ^[*] Figure 32-4 shows how the main routing initialization routines are invoked.

Figure 32-4. Sequence of calls for the main routing initialization functions

In ip_rt_init, the IPv4 routing code initializes its data structures and global variables. Among other things, the function:

• Defines the size of the routing cache based on the available RAM.

• Creates the memory pool that will be used to allocate elements of the routing cache.

• Initializes the routing cache

• Defines the gc_thresh threshold used by the garbage collection algorithm (see the section "rt_garbage_collect Function" in Chapter 33).

• Starts the timer rt_periodic_timer (see the section "Garbage Collection" in Chapter 33).

• Starts the rt_secret_timer timer (see the section "Flushing the Routing Cache" in Chapter 33).

• Adds a few files to the /proc filesystem (see the section "Tuning via /proc Filesystem" in Chapter 36).

Two routines are of particular interest:

ip_fib_init

Initializes the default routing tables and registers two handlers with the two notification chains^[*] netdev_chain and inetaddr_chain (see the section "External Events").

devinet_init ...