December 2005
Intermediate to advanced
1066 pages
33h 38m
English
Content preview from Understanding Linux Network InternalsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.
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_lookupHow 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
Regardless of
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 ip_route_input_slow and ip_route_output_slow call fib_lookup, they
transparently invoke the right one.
Let's briefly see the key functions used to make a route lookup. You will find it helpful to refer to Figure 34-1 in Chapter 34 during this discussion.
The 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, ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
and much more.
Read now
Unlock full access
More than 5,000 organizations count on O’Reilly
O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.