libcurl does not recognize [::] as a valid IPv6 address

[volpav]

I did this

1. Started a local server, providing the IPv6 binding of [::]:12345
2. Attempted to use libcurl to connect to the server

I expected the following

libcurl successfully connecting to the server.

instead, I got this

Received URL using bad/illegal format or missing URL (code 3) error from libcurl.

curl/libcurl version

7.69.1 (with IPv6 enabled)

operating system

Debian-based corporate build.

notes

Changing [::] to [::1] fixes the problem.

Looks like the bug was introduced in 4706603. To comment on this, [::] (and even ::) seems to be a valid IPv6 address according to https://en.wikipedia.org/wiki/0.0.0.0.

This works as expected in 7.66.0.

[jay]

It's a non-routable address used for binding, how is it a valid destination?

[bagder]

:: is indeed a valid IPv6 address (consisting of all zeroes), I just didn't think about that case when I added that extra check in the URL parser. Should we accept it?

[volpav]

For posterity, the [::] (0.0.0.0), even though not being a valid destination, essentially means to listen to all interfaces when you perform a binding.

While [::], theoretically-speakig, is not a valid destination (since it means "any destination"), it seem that there's a number of network stack implementations that would treat it as a loopback address. For Linux, this looks like a relevant piece of code for IPv4 (still trying to find the similar example for IPv6 although I would expect the behavior to be the same):

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

(source: net/ipv4/route.c)

[jay]

Should we accept it?

I vote no

[bagder]

I vote no

With what motivation? Why should our URL parser restrict what IP address the user can specify? It seems entirely arbitrary. We already accept 0.0.0.0 which also is a perfectly valid address. I think we should accept ::.

[emilengler]

I agree with @bagder.
I don't think it is our task to tamper that much with the IP and OSI-Level 3 protocols in general as this is a task of the operating system or other lower level software

[jay]

I changed my mind. Those are all good points.