Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?

Martin Hoffmann <martin@opennetlabs.com> Wed, 24 October 2018 10:06 UTC

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Date: Wed, 24 Oct 2018 12:06:38 +0200
From: Martin Hoffmann <martin@opennetlabs.com>
To: bert hubert <bert.hubert@powerdns.com>
Cc: dnsop <dnsop@ietf.org>
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References: <BC2CDF40-4FF0-4111-88B7-04969491D2E0@dukhovni.org> <20181024092148.GA24150@server.ds9a.nl>
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Subject: Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
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bert hubert wrote:
> On Wed, Oct 24, 2018 at 05:01:53AM -0400, Viktor Dukhovni wrote:
> > And yet, here and there I see mention of having to take care to
> > avoid "loops", but loops are impossible in a monotone strictly
> > decreasing sequence.  
> 
> Yes. This is one of the best ways of preventing such loops. Some
> libraries accidentally allowed a jump to the same place though, if I
> recall correctly.
> 
> PowerDNS, for reasons I'm trying to find out, not only checks if a
> jump went backwards but also refuses to go backwards more than 100
> times. It may be that someone made a packet that burned a lot of CPU
> time.

Alternatively, you could track and enforce the 255 octet limit of
domain names while chasing the compression pointers.

Kind regards,
Martin