On Fri, Jan 04, 2008 at 07:57:29PM -0500, John C Klensin wrote:
(3) As Keith Moore has pointed out repeatedly for the general
case and as I and others have pointed out for more specific ones
(including today's mail-and-DNS case), dual stack is a nice
thing to do if one is developing operating systems and maybe if
one is developing servers. But any form of "the application has
a choice of multiple addresses or interfaces and must choose"
puts the application into the routing business.
:) welcome to the end2end model and the rise of the
stupid network... :)
Even the trivial example that Jeroen and Phillip used may be
problematic, especially if there are multiple IPv4 and multiple
IPv6 addresses. There are no MX rules at all about which
address must be tried first and some handwaving in RFC 2821
about how many addresses at a given preference level need to be
tried at all. With most SMTP senders, such things can be
configured, but the IETF has published approximately zero advice
as to how to configure them (there actually is some reasonable
advice in RFC 3974, but that isn't an IETF document and it bears
one of the stronger forms of a "you really shouldn't pay any
attention to this" disclaimer from the IESG.
well the failures fo the MX record for both DNS and
specific application use are well known and there was
a well designed "charge" to replace that capability
in the DNS with a more general purpose tool that does
help w/ the "which interface to use" problem.. which
is in fact, very old indeed.. I point you to the SRV RR.
--bill
john
--
--bill
Opinions expressed may not even be mine by the time you read them, and
certainly don't reflect those of any other entity (legal or otherwise).
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