Chapter 35. Routing: Lookups
In Chapter 33, we saw how lookups are triggered by both ingress
and egress traffic. The cache is always searched first, and when there is a miss, the
routing tables are consulted through the
functions. In this chapter, we will analyze these functions; in particular, we will cover:
How ingress and egress routing differ
How a routing table is searched with
How policy routing lookups differ from normal lookups
When and how multipath routing is handled
How the selection of a default gateway works
High-Level View of Lookup Functions
the direction of the traffic, a routing table lookup is made with
fib_lookup. However, as mentioned in the section "Double Definitions for Functions" in
Chapter 34, there are two versions of
fib_lookup, one used when the kernel has support for
Policy Routing (net/ipv4/fib_rules.c) and one when
that support is not included (include/net/ip_fib.h).
The selection of the right routine is made at compile time, so when
transparently invoke the right one.
fib_lookup routine is a wrapper around the lookup function provided by each routing table. The version provided when there is no policy routing simply runs the lookup function for the local and main tables, ...