[Bug 279618] getnameinfo - IPv6 problem

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=279618

            Bug ID: 279618
           Summary: getnameinfo - IPv6 problem
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Keywords: ipv6
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: bz@FreeBSD.org

6cb9418289f90 introduced a special check on the first nibble of an in6_addr:

+               switch (sin6->sin6_addr.s6_addr[0]) {
+               case 0x00:

If it doesn't fall into any of the two categories it will not attempt to do
name resolution and lookup the PTR because it sets NI_NUMERICHOST.

I cannot see why this "else" case was ever added (and someone should dig into
the KAME archives to understand).  Also someone should check if POSIX says
anything beyond the RFC and fully understand what is supposed to go on.

Addresses with the IPv6 DNS64 default prefix 64:ff9b::/96 such as
64:ff9b::4007:8d09 (zoo) will not be resolved and that does indeed break
software (e.g. postfix in an ipv6-only setup behind a NAT64).

Manually calling
hp = getipnodebyaddr(&sin6.sin6_addr, sizeof(struct in6_addr), AF_INET6,
&h_error);
will result in the correct answer from an accordingly setup resolver.

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