[Bug network/16826] New: getaddrinfo() doesn't get scope ID for IPv6-only look-ups

["pub at craig dot mcqueen.id.au" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16826

            Bug ID: 16826
           Summary: getaddrinfo() doesn't get scope ID for IPv6-only
                    look-ups
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: pub at craig dot mcqueen.id.au

I'm trying to follow up on Ubuntu Launchpad bug #369008, in which mDNS IPv6
look-ups don't get the scope ID. There is a patch for nss-mdns which makes it
work by implementing _nss_gethostbyname4_r(), which is the only way to pass
through the IPv6 scope ID. However, while it works for lookups with family
AF_UNSPEC, it doesn't work for family AF_INET6.

It turns out that getaddrinfo() is calling _nss_gethostbyname4_r() only for
lookups with family AF_UNSPEC. But for family AF_INET6, it is calling
_nss_gethostbyname2_r() instead. It really should always call
_nss_gethostbyname4_r() for any IPv6 look-ups, in order to get the IPv6 scope
ID.

It used to work, however, I've traced the problem to glibc git commit
8479f23aa1.
http://repo.or.cz/w/glibc.git/commit/8479f23aa1d5e5477a37f46823856bdafaedfa46

In that commit, it was changed to only use _nss_gethostbyname4_r() for
AF_UNSPEC, and not AF_INET6. That breaks IPv6 scope ID retrieval. Can we revert
that change?

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