Understanding Linux Network Internals

Setting Functions for Reception and Transmission

Both received packets and locally generated packets need to be routed: in the case of received packets, to find out whether they should be locally delivered or forwarded, and in the case of locally generated packets, to find out whether they should be delivered locally or transmitted out.

In both cases, given a packet to route skb, the result of the routing lookup is saved in skb->dst. This is a data structure of type dst_entry, described in detail in the section "dst_entry Structure" in Chapter 36. This data structure includes several fields; two of them are function pointers named input and output that process the packet in accordance with the result of the routing lookup. The next section goes into detail on the initializations of these function pointers.

Then the sections "Input Routing" and "Output Routing" describe in detail the routines ip_route_input_slow and ip_route_output_slow, used respectively to find routes for ingress and egress packets when the cache lookup fails. These two functions can be a bit scary because of their size and the extent to which they apply macros, conditional code (such as #ifdef), and special feature handling (e.g., Multipath), but they actually are simpler than they look. In addition, programmers who do not like the use of goto statements in the source code may be disappointed by their heavy use. I have included a flowchart for each function, which you might want to look at for high-level descriptions ...