On Fri, 4 Jan 2008, John C Klensin wrote: > On Friday, 04 January, 2008 12:01 -0800 Bill Manning <bmanning@xxxxxxx> wrote: > > > > The general answer when needing to communicate between similar > > applications that run on different address families has traditionally > > been the application layer gateway (ALG) ... MTAs *are* ALGs. > [...] If the MX resolution doesn't work smoothly for IPv6, then the DNS > isn't IPv6-ready no matter how many AAAA records are defined and spread > around. [...] I hope the additional information rules have been adjusted > if needed to encourage return of relevant AAAA records if they exist > [...] Yes AAAA records are returned in the additional section just like A records. However this behaviour is less useful than one would hope, because of the RFC 2181 rules about the TC bit. In particular, it is not set when RR sets have been dropped from the additional section of the reply. This means an MTA can't immediately tell the difference between an IPv4-only MX record and a dual-stack MX record where the IPv6 addresses have been truncated. It must do extra DNS lookups to resolve the ambiguity, which (at the present time) is usually a waste of effort. The extra lookups are not optional because you can also get dual-stack MX replies in which the A records have been dropped, which can (and does) lead a naive IPv4-only MTA to think erroneously that it cannot reach the other server. Tony. -- f.a.n.finch <dot@xxxxxxxx> http://dotat.at/ NORTH FITZROY: SOUTHWESTERLY 6 TO GALE 8, PERHAPS SEVERE GALE 9 LATER. VERY ROUGH. RAIN. MODERATE. _______________________________________________ Ietf@xxxxxxxx https://www1.ietf.org/mailman/listinfo/ietf