spf-discuss
[Top] [All Lists]

Re: MX and TXT

2004-01-12 01:25:17
--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.

I think he's talking about glue records.


Yes. In that case the answer is, it is the DNS server that serves the answer which is providing the "extra information" - because it thinks it knows what your next request is likely to be. For example if you ask for MX, it would be legal to give you just a name, but since we expect the next query to be "what number goes with that name?" then it's more efficient to send them as additional records in the first response. In other words, it is hardcoded into the DNS server, and doesn't have any relationship to how you arrange the lines in the file. (the "dig" command shows this better - the answer to your question is in the ANSWER section and the extra glue is in the ADDITIONAL section)

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>

-------
Sender Permitted From: http://spf.pobox.com/
Archives at http://archives.listbox.com/spf-discuss/current/
Latest draft at http://spf.pobox.com/draft-mengwong-spf-02.9.4.txt
To unsubscribe, change your address, or temporarily deactivate your subscription, please go to http://v2.listbox.com/member/?listname(_at_)©#«Mo\¯HÝÜîU;±¤Ö¤Íµø?¡


<Prev in Thread] Current Thread [Next in Thread>