Understanding Linux Network Internals

fib_lookup Function

As mentioned in the section "Special Routes" in Chapter 30, the kernel uses two routing tables by default when there is no support for Policy Routing. A routing table lookup simply consists of two table lookups (two calls to fn_hash_lookup), so the fib lookup function defined in include/net/ip_fib.h is quite brief:

static inline int fib_lookup(const struct flowi *flp, struct fib_result *res)
{
        if (ip_fib_local_table->tb_lookup(ip_fib_local_table, flp, res) &&
            ip_fib_main_table->tb_lookup(ip_fib_main_table, flp, res))
                return --ENETUNREACH;
        return 0;
}

The search key is flp. The function first checks the ip_fib_local_table routing table, and if that fails, it checks the ip_fib_main_table routing table. If neither table manages to find a match, fib_loopkup returns —ENETUNREACH (unreachable destination network).