Understanding Linux Network Internals

Policy Routing

A routing lookup in a kernel that has support for Policy Routing has to take into account the possible presence of multiple tables. The next two sections show how the Policy Routing versions of fib_lookup and fib_select_default differ from the basic versions we saw earlier in this chapter.

fib_lookup with Policy Routing

When Policy Routing is configured, this function contains an extra step: it needs to find out what routing table to use based on the configured policies.

We saw in the section "Main Data Structures" in Chapter 32 that routing policies are defined with fib_rule data structures. All the fib_rule instances are linked together with the global list fib_rules. The list is kept sorted in increasing order as indicated by the priority field. This allows the configuration to define the order in which the rules should be checked, therefore reducing lookup time. The more commonly matched rules or most important rules (as defined by the administrator, depending on the context) are closer to the head of the list. The priority is a 32-bit field, which means a host can theoretically have up to 2³² policies. Of course, because policies are stored in a sorted, flat list, a high number of policies can decrease routing performance significantly.

Without any user configuration, fib_rules includes the three default instances defined in net/ipv4/fib_rules.c, as shown in Figure 35-11:

local_rule

This is the highest-priority rule and is therefore at the head of the list. It always ...