[knorker]

Not quite. Yes, that may be true for ping, but ping is very much special. It builds its raw packets.

But do a "strace -econnect nc 0 22" and you'll see that yes, actually, a connect() syscall to "0.0.0.0" does connect to localhost.

[firethief]

Interesting. That doesn't seem to be specified in the RFC1122 standard or the Linux ip(7) docs, but it's an explicit special case in the kernel (ip_route_output_key_hash_rcu):

        if (!fl4->daddr) {
                fl4->daddr = fl4->saddr;
                if (!fl4->daddr)
                        fl4->daddr = fl4->saddr = htonl(INADDR_LOOPBACK);
                ...