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IPv4 / IPv6 interaction

2008-04-16 11:38:25

Tony Finch <dot(_at_)dotat(_dot_)at> wrote:
On Wed, 16 Apr 2008, Ned Freed wrote:
On Tue, 15 Apr 2008, Arnt Gulbrandsen wrote:

I doubt that it makes sense to accept email from 
on a system that can only communicate with IPv4 addresses.

   (It would help to be clear whether "from" refers to HELO, MailFrom,
or some miscellaneous header.)

In a single system case with fully symmetric routing, perhaps not.
But as I mentioned previously, there are many cases where multiple
systems with highly asymmetric routing are used to provide a mail

   Absolutely (for any of the cases I listed).

In such a setup it is entirely possible that a system hosting an
inbound MTA would have DNS query access but no outbound IPv6 access
even if such access exists somewhere else in the setup.

   We would be ill-advised to assume even that IPv4-only systems have
no reason to query (and perhaps even use in some fashion) AAAA RRs.

Of course, but in that case the postmaster would know that it is
possible to communicate indirectly with IPv6 addresses and would
therefore not disable IPv6 DNS lookup support.

   (We're really talking about an implementation detail outside the
spec here.)

   In a legacy IPv4 system _right_now_ suppressing AAAA RRs might
make sense. I do not agree it makes sense next year. Even if the
postmaster _today_ does not know of a host speaking both IPv4 and
IPv6 that will agree to relay his/her email, that doesn't mean that
software like Exim shouldn't give an error message describing the
problem as lacking a path to reach an IPv6 address.

John Leslie <john(_at_)jlc(_dot_)net>