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 ip_route_input_slow and ip_route_output_slow 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 fib_lookup

• How policy routing lookups differ from normal lookups

• When and how multipath routing is handled

• How the selection of a default gateway works