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Comment by knorker | Hacker News Reader

0 connects to localhost. It's shorter.

0 is 0.0.0.0, which is not a valid address for most purposes. Some programs, like iputils ping, have special handling for that case (i.e. using it as an alias for the unroutable host address); some programs, like FreeBSD's ping, do not [1]. Unlike most of these address tricks, it's not standardized, except that treating it as a normal address is technically disallowed.

1: https://unix.stackexchange.com/questions/99336/how-does-ping...

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.

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);
                ...

wow this is shortest way to localhost

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