Re: [spfbis] Proposed spf TXT record change

"Murray S. Kucherawy" <superuser@gmail.com> Thu, 11 February 2016 05:26 UTC

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Date: Wed, 10 Feb 2016 21:26:43 -0800
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From: "Murray S. Kucherawy" <superuser@gmail.com>
To: "Roy A. Gilmore" <rag@ragged-software.com>
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Subject: Re: [spfbis] Proposed spf TXT record change
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On Tue, Feb 9, 2016 at 7:24 PM, Roy A. Gilmore <rag@ragged-software.com>;
wrote:

> Double query? What if a single domain query returns multiple TXT
> records.


Section 4.5 of RFC7208 covers that.


> How long will it take to process multiple free form (i.e. RFC
> 1035: "The semantics of the text depends on the domain where it is
> found.") TXT records to be sure you select the right one?


I don't think processing time is the concern here.


> Can you be
> sure you selected the right one?


Yes, because after discarding things that aren't SPF records, you evaluate
what's left: if there's none, you get a "none" result; if one, you get the
real result; if more, you get an error.


> If you query for a _spf TXT record, you
> know you got the right one (well, I guess in the degenerate case where
> DNS has been misconfigured, you could get multiple TXT records).


I don't know that wildcard TXT records constitute a "degenerate" case.


> I
> suppose someone could say that only one TXT record should be returned.
> RFC 1035 doesn't say whether TXT is a singleton resource record or not.
> But, RFC 1035 has been updated by so many RFC's and BCP's that I'm not
> sure (I'll have to look into that). I know bind allows multiple TXT
> records per key (I'm not sure if "key" is the correct terminology here).
> Anyway, assume for a minute that only one TXT record should to be
> returned, should spfbis hijack that TXT record? I don't think so. What
> about all the other services that query for a service specific TXT
> record, are their workgroups "wrong", and the spfbis workgroup "right"?
> I'd willing to bet the other workgroups have some pretty smart people
> too (probably some of the same smart people).
>

The rules for handling the DNS reply in RFC7208 cover all of these
possibilities, I believe.  They have to, given how much random stuff one
finds in the domain-level TXT reply sometimes.  Query for the TXT of "ut.edu",
for example.  And that's a bunch cleaner than what I found when I did the
RFC6686 surveys.  I found some that had valid SPF records mixed in with
junk like that.

SPF isn't making a claim to the domain level TXT record.  That's just where
it landed (again, see RFC6686 for the history), and it can easily coexist
with other things.

The spfbis archive, going back to early 2013 at least, documents the
extensive discussions we had on how we got here and why we should never do
it again, but fixing it now is simply not pragmatic.

As to spurious interoperablity arguments, that's easy, set a transition
> period where either record is acceptable with a preference for the _spf
> TXT record (i.e. query for the _spf TXT record first, and fall back to
> the domain TXT record if the _spf TXT record doesn't exist). Maybe, even
> a "double" transition period where in the first transition period the
> domain TXT record was the preferred record. POP3 didn't replace POP2
> overnight. And IMAP4 still hasn't replaced POP3 even though it has many
> advantages.
>

Sure, transition periods could work if everyone is on board with making the
change.  Selling that is the challenge.

What problem are you really trying to solve here?

-MSK