[Bug libc/5873] New: RFC3484 IPv4 scoping rules conflicts with RFC4380

["aurelien at aurel32 dot net" <sourceware-bugzilla at sourceware dot org>]

Rule 2 of the Destination Address Selection algorithm in RFC3484
specifies:

| Rule 2:  Prefer matching scope.
| If Scope(DA) = Scope(Source(DA)) and Scope(DB) <> Scope(Source(DB)),
| then prefer DA.  Similarly, if Scope(DA) <> Scope(Source(DA)) and
| Scope(DB) = Scope(Source(DB)), then prefer DB.

When it comes to private RFC1918 IPv4 addresses, they are given a different 
scope from public IPv6 addresses. This does not make much sense, especially 
with the widespread adoption of Network Address Translation.

Because of this, a host with a (NATed) private IPv4 address and a 6to4
(2002::/16, global scope) or Teredo (2001:0::/32, global scope) IPv6
address will first try to use 6to4 or Teredo to reach any "native" IPv6
destination. It violates the RFC4380 requirement that Teredo be used as a last 
resort - i.e. after IPv4.

IPv4 should be preferred in this case.  Also note that this make the separate 
6to4 and Teredo _labels_ from /etc/gai.conf pretty useless.

-- 
           Summary: RFC3484 IPv4 scoping rules conflicts with RFC4380
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: aurelien at aurel32 dot net
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=5873

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