Understanding Linux Network Internals

Generic Helper Routines and Macros

The routing code uses quite a few small routines and macros that make the code more readable. This section lists some of the generic ones; more-specialized ones will be introduced later in the section "Helper Routines." It is important to keep in mind that the same function or macro can have different definitions, depending on such factors as:

• Support for policy routing in the kernel

• Support for multipath routing in the kernel

• L3 protocol (e.g., IPv4 versus DECnet)

The generic routines follow:

FIB_RES_ XXX

Given a fib_result structure, these macros extract specific fields. For example, FIB_RES_DEV extracts the nh_dev field. These macros are defined in include/net/ip_fib.h.

change_nexthops

for_nexthops

endfor_nexthops

Used to browse all the fib_nh structures of a given fib_info instance. change_nexthops starts a loop over the structures, designating the local variable nh to represent each structure; as the name of the macro suggests, it can be used to alter the structures. for_nexthops is very similar and ends with the same endfor_nexthops macro. The only difference is that for_nexthop defines the local variable nh as a pointer to a constant and therefore the code inside the loop cannot change the content of any of the fib_nh instances browsed.

For IPv4, these macros are defined in net/ipv4/fib_semantics.c. Note that for each macro there are two versions: one used when there is Policy Routing support in the kernel and one when there is no Policy Routing. ...