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Re: Why implicit MX is a bad idea for IPv6

2008-04-05 11:30:59
On 2008-04-05 14:26:50 +0200, Alex van den Bogaerdt wrote:

Here's why implicit MX for IPv6 is a bad idea.

a domain with no MX _and_ no A record, is not a maildomain.
a domain with no A record in its MX set, is in error.

Interoperability in combination with the least surprise principle
dictates that IPv6 AAAA RR cannot function as implicit MX.

To me following the  least surprise principle means that an AAAA record
must be treated exactly like an A record. Treating it differently is
highly surprising to me.

(only MX, A and AAAA RR are relevant to this discussion)

Consider a sender $sender which is IPv4 only, has no clue about IPv6:

When asking for an MX RR, it gets zero answers. It will then ask for
an A record and still get zero answers. It ends here.

1) no MX record, no A, no AAAA
2) no MX record, no A, only AAAA

According to $sender, these scenario's are the same. $sender will
ignore AAAA records:
1) no MX record, no A
2) no MX record, no A

Apparently this domain is not a maildomain. It doesn't matter if the
AAAA record is present or not.

IMPORTANT: a domain with no MX _and_ no A record, is not a maildomain.

This means an AAAA record is not an implicit MX record.

No, that doesn't follow at all. If all the MX records for a domain
contain hostnames with only AAAA records, the domain is just as
unreachable for an IPv4-only host as if it contained only an AAAA
record. And for a IPv6-only host it is just the other way around. 

Many people arguing for the "only A records are implicit MX records"
side seem to ignore the fact that an MX record doesn't contain an
address on the right side. It contains a host name, which still needs to
be resolved to an address (via an A or AAAA lookup). The implicit MX
record isn't "synthesized from an A record", it is synthesized from the
domain name. There is no difference between the A lookup on the
hostnames in the real MX records and the one in the implicit MX records,
and neither should there be a difference in the AAAA lookups on those
hostnames, unless you want to follow the principle of most surprise.

Similar reasoning but now with MX records:

One or more MX records are found, thus a list of hostnames are built
from it/them.  All AAAA records will be ignored, leaving only a set
of A records and priorities.

If that set is empty, this is an error condition.

IMPORTANT: a domain with no A record in its MX set, is in error.

No. It is merely unreachable from IPv4. This is *probably* an error, but
may be intentional.

This means the only valid setup for IPv6 hosts which want to receive
mail is to have one or more MX records, with at least one A record in
them.  That A record will point to a gateway.
Any different setup will destroy email reliability.

Yes. Any IPv6-only host currently has very limited
internet-connectivity, and will need to use relays, proxies, or NAT to
use many services. But that's no different from many IPv4 hosts which
are behind firewalls, in RFC1918 networks, etc.


   _  | Peter J. Holzer    | It took a genius to create [TeX],
|_|_) | Sysadmin WSR       | and it takes a genius to maintain it.
| |   | hjp(_at_)hjp(_dot_)at         | That's not engineering, that's art.
__/   | |    -- David Kastrup in comp.text.tex

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