This is one of the reasons why I appreciate the geekiness of Cloudflare with their DNS service IP addresses, particularly:

1.1 which to me is the shortest useful IP address I am aware of.

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

Would you be further humbled if the ipad accepted http://CXXVII.I also?

I'm never writing anything that positively accepts 127.1, or 0127.000.000.0001 as a valid address no matter what garbage implementations do.

The issue we have with this are situations when we have to accept only inputs that are domain names which are sure not to be treated as an IP address by some software downstream of us.

All my regex (are now) a lie

... now you've got two^h^h^hthree problems.