Curl local host names on Mac OS X Yosemite

I just got it to work by commenting out one of the IPv6 loopback lines from my /etc/hosts file:

#fe80::1%lo0    localhost

Now all of my loopback hostnames work, not just localhost. I wonder what's up with this?

Alternative (does not require sudo or modifying /etc/hosts) - always use ipv4 until curl gets smarter.

$ echo '--ipv4' >> ~/.curlrc

(then everything will work as desired)

First of all, 0.0.0.0 is a special address meaning "any IPv4 address".

A socket can be bound to either IPv4 or IPv6 protocol. If a socket is bound to 0.0.0.0, that means it will listen to any IPv4 trying to connect to it, and will be represented as it follows:

$ nc -l 0.0.0.0 8085
$ lsof -i4 -Pnl | grep 8085
  nc        23994 [xxx]    3u  IPv4 [xxx]      0t0  TCP *:8085 (LISTEN)

The * sign is equivalent to 0.0.0.0 on IPv4.

For IPv6:

$ nc -l :: 8085
$ lsof -i6 -Pnl | grep 8085
  nc        24145 [xxx]    3u  IPv6 [xxx]      0t0  TCP *:8085 (LISTEN)

The * sign is equivalent to :: on IPv6, as in the official specification.

The reason is that curl tries to resolve to a random localhost entry in /etc/hosts, and as @NickRetallack mentioned, that entry is the chosen one by curl when resolving localhost in its default mode (assumedly IPv6 or IPv4, whatever resolves first).

Forcing it in --ipv4 mode, as @CharlesHebdough suggested, will make curl resolve localhost to 127.0.0.1 (assuming there are no other IPv4 entries for localhost in /etc/hosts).

Each implementation will resolve localhost as they wish, hence why you had intermittent success with different tools.

To be the most precise possible, use 127.0.0.1 instead of localhost, but it will bound you to IPv4. localhost gives you the flexibility to work in both IPv6 and IPv4 protocols, however in some implementations you might have trouble, as in that specific version of curl.