On 2008-04-05 14:26:50 +0200, Alex van den Bogaerdt wrote: > > Here's why implicit MX for IPv6 is a bad idea. > > Summary: > 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
> or
> 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.
hp
--
_ | Peter J. Holzer | It took a genius to create [TeX],
|_|_) | Sysadmin WSR | and it takes a genius to maintain it.
| | | [EMAIL PROTECTED] | That's not engineering, that's art.
__/ | http://www.hjp.at/ | -- David Kastrup in comp.text.tex
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