spf-discuss
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Re: MX and TXT

2004-01-12 17:19:48
Thanks Greg

In the end this boils done to a personal preference
(not of course the way to run a protocol!). Since we
have (as yet) no type=SPF and have decided to use TXT
and not SRV or whatever, it becomes a sort of hierarchy
question whether to lookup TXT for the domain or lookup
MX. To my eye the latter would have been better, since
it means less lookup, and less parsing (programmer's
perspective!). And it is less subversive, because the TXT
record is implicitly tagged as related to email by placing it
in the MX record. This assumes (correctly?) that an MX
record must exist for dispatching domains. I suppose that this
is not necessarily the case actually, though there couldn't
be many legitimate domains which only send mail, since
even smtp servers which only send seem rare (v=spf mx ...)

For all this, the answer to my original question is clear.
Admins are constructing TXT records for TXT lookup, and
patches are doing TXT lookups, not parsing MX for TXT
records. Not the way I would have done it perhaps, but
it's too late to argue otherwise.

geoffj

--Meng Weng Wong <mengwong(_at_)dumbo(_dot_)pobox(_dot_)com> wrote:

On Mon, Jan 12, 2004 at 05:09:45PM +1100, geoffj wrote:
| mx.example.com. A    10.9.8.7
|               TXT "v=spf mx -all"
|
| Try any dns client seeking an MX record
| on 210.8.17.100 to see this. I know that MX lookups
| don't usually return TXT records, but there is no reason
| I suspect why they shouldn't, just as they usually return
| NS and A records. This is of course a redundancy which
| is intended to reduce lookups I take it. And the same
| might go for the spf TXT record perhaps.




Right now there isn't a strong correlation between MX and TXT. If SPF catches on, there might be, but this is probably better to save until SPF has its own record type and is not piggy-backing on TXT.

Then again, SPF is used by people getting mail from you, and MX is used by people who want to send mail back to you, so not everyone who asks for MX will want SPF. It makes a lot of sense to send additional info when you ask for SPF though (like A or MX).. but since that requires rewriting the dns servers it may not happen. In the larger scheme of things, sending an extra couple of queries (UDP) is still less overhead than the back and forth handshaking of a TCP connection like SMTP.

--
Greg Connor <gconnor(_at_)nekodojo(_dot_)org>



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