December 2005
Intermediate to advanced
1066 pages
33h 38m
English
Content preview from Understanding Linux Network Internals
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Source Routing
We saw in Chapter 18 that IP packets can be source routed. Because this is taken care of by the IP protocol directly without involving the routing subsystem, it is covered in the part of the book about IP. Here we are interested just in the implications of source routing on the routing lookups.
Let's use Figure 18-1 in Chapter 18 as a reference. When an ingress packet
reaches ip_rcv_finish, it triggers the first routing
lookup. In the absence of source routing, this is the only routing lookup needed. However,
before ip_rcv_finish calls dst_input, it checks whether the IP header specifies source routing and, if
so, takes care of it.
Source routing here is handled by ip_options_rcv_srr. It extracts the next hop to use from the IP header and
makes a second routing lookup with ip_route_input. This
second lookup replaces the existing skb->dst with a
newer one. See the sequence of calls in Figure
35-13.
When locally generated traffic carries the Source Routing IP option, it triggers only
one routing lookup because the correct next hop is selected before the lookup (see
ip_queue_xmit for an example).
Figure 35-12. Policy Routing version of fib_lookup function
Figure 35-13. Source routing for ingress traffic
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