Subject: [spfbis] New Issue: #32 DNS data limits

From: Scott Kitterman <[email-masked]>
Date: Tue, 03 Jul 2012 19:39:22 -0400

The RFC 4408 processing limits suggest the following:

          SPF implementations SHOULD limit the total amount of data obtained
          from the DNS queries.  For example, when DNS over TCP or EDNS0 are
          available, there might need to be an explicit limit to how much data
          will be accepted to prevent excessive bandwidth usage or memory
          usage and DoS attacks.

Given the limits in record lookups and time (see my last new issue) do we also 
need a data limit?  My problem is that this requirement is entirely 
unquantifiable.  How much is too much?

I'm not aware of any implementations that actually do data volume limits, just 
number of lookups and time.  Can/should this be deleted as unused?  Is anyone 
aware of its use?

Scott K

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From: Andrew Sullivan <[email-masked]>
Date: Tue, 3 Jul 2012 21:51:56 -0400

No hat.

>           will be accepted to prevent excessive bandwidth usage or memory

I confess, I always found that passage mystifying.  If you issue a
query and fall back to TCP, there's no mechanism for you to "turn off"
a big answer.  Same thing with a large EDNS0 buffer (indeed, this
problem is what makes reflector attacks so devastating in the DNS).
 
> number of lookups and time.  Can/should this be deleted as unused?  Is anyone 

Given the above, there's no practical difference between "limit number
of queries" and "data volume limit", I suspect.  At the same time (and
I emphasise, no hat), since the passage doesn't seem to be operative
and removing it would be a change, perhaps it'd be better to leave it
until a later update?

Best,

A

-- 
Andrew Sullivan
[email-masked]

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From: Scott Kitterman <[email-masked]>
Date: Tue, 03 Jul 2012 22:18:06 -0400

On Tuesday, July 03, 2012 09:51:56 PM Andrew Sullivan wrote:
> > I'm not aware of any implementations that actually do data volume limits,
> > just number of lookups and time.  Can/should this be deleted as unused? 
> > Is anyone aware of its use?
> 

The charter says we can remove things that are unused.  Since this particular 
SHOULD is unimplementable, it's unused by definition.  I think we can remove 
it.  Is there a reason we should not?  I agree that there's not a practical 
difference to leave/remove it, but removing it makes the document clearer and 
more concise.

Scott K

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From: "Murray S. Kucherawy" <[email-masked]>
Date: Tue, 3 Jul 2012 20:59:35 -0700

> difference to leave/remove it, but removing it makes the document clearer
I agree that it's both removable (in terms of the charter) and that it
should be removed.

-MSK