Understanding Linux Network Internals

Adding and Removing Routes

In Chapter 36, we will see how routes are added, deleted, and modified by user commands and routing daemons. Both are satisfied through a single set of routines in the kernel's routing subsystem. In this section, we will see what the kernel has to do when asked to add or remove a route from one of its routing tables. As shown in Table 34-1, fn_hash_insert and fn_hash_delete are the routines used to insert and delete routes, and we will analyze them in the sections "Adding a Route" and "Deleting a Route." fn_hash_insert has several related uses, all involving changes of routes.

Here are a few operations common to the two routines:

• Given a route to add or remove, derive the search key and use it to make a fib_node lookup and a fib_alias lookup. These lookups are similar to the ones done to route data packets, but are done for a different purpose: to check whether a route being added is a duplicate of an existing route, or whether a route being removed really exists.

• Populate (in case of insert) and clean up (in case of delete) the right hash tables.

• Flush the routing cache if necessary.

• Generate a Netlink broadcast notification to tell the interested listeners that a route has been added to or removed from a routing table (see the section "Netlink Notifications" in Chapter 32).

Adding a Route

The insertion of a new route is taken care of by fn_hash_insert, whose logic is described in Figures 34-3(a) and 34-3(b).^[*] This routine is actually called for many operations: ...