Created attachment 14951 [details]
Reproducer code

While writing a procedure that validates IP addresses by using getaddrinfo I
noticed the following inconsistencies when it comes to addresses with a zone index:

An existing interface …
… as a numeric value:
2001:db8::1%2: getaddrinfo: OK
fe80::1%2: getaddrinfo: OK

… as an interface name:
2001:db8::1%enp4s0: getaddrinfo: Name or service not known
fe80::1%enp4s0: getaddrinfo: OK

An absent interface…
… as a numeric value:
2001:db8::1%9999: getaddrinfo: OK
fe80::1%9999: getaddrinfo: OK

… as an interface name:
2001:db8::1%foobar: getaddrinfo: Name or service not known
fe80::1%foobar: getaddrinfo: Name or service not known


I've included a small reproducer example below.
(regarding the absent interface cases, I've no opinion on what behavior to expect here.)

It strikes me as odd that the "2001:db8::1%enp4s0" case is treated differently on the
basis of its prefix (compared to "fe80:…").
Although at the moment only link-local and multicast scopes have defined meaning
[RFC4007], within the Introduction of the same RFC it states:
  … the IPv6 working group decided to … and is now investigating other
  forms of local IPv6 addressing.

If I understood correctly this means that zone indexes should not be
preferentially handled on basis of prefix since other scopes might be
introduced later.


In any case, I think that getaddrinfo should handle "2001:db8::1%enp4s0"
in the same way it handles "2001:db8::1%2". Allowing %2 but not %enp4s0
is just massive hair-splitting. (especially when %2 happens to match %enp4s0)