Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
Shane Kerr <shane@time-travellers.org> Wed, 24 October 2018 09:22 UTC
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From: Shane Kerr <shane@time-travellers.org>
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Date: Wed, 24 Oct 2018 11:22:10 +0200
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Subject: Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
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Viktor, On 24/10/2018 11.01, Viktor Dukhovni wrote: > > My reading of RFC 1035 is that DNS name "compression" > via "pointers" is restricted to name strictly earlier > in the DNS message: > > 4.1.4. Message compression > > In order to reduce the size of messages, the domain system utilizes a > compression scheme which eliminates the repetition of domain names in a > message. In this scheme, an entire domain name or a list of labels at > the end of a domain name is replaced with a pointer to a prior occurance > --------------- > of the same name. > > 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. I think the concern is something like this: offset octet 100 3 101 'f' 102 'o' 103 'o' 104 192 105 100 104 and 105 together point to offset 100, which will cause a loop even though they point to an earlier label. > Is there a later RFC that relaxes the constraint and allows pointers to names > later in the message? I'm having a bit of trouble finding the later text... > > Secondarily, can the pointer point to some odd-ball location earlier in the > message that is not semantically a label in its original context, but just > happens to carry data that decodes as the desired label? Or, are pointers > only valid to prior locations that are corresponding labels in their original > context? I have seen code which does this (there was an early version of the Knot authoritative server which used non-label offsets if it happened to have the correct data). Certainly it is easily possible that RDATA contains valid labels which could be used to by compression. I think that for performance reasons most DNS software producing packets will maintain some data structure of prior labels & names in a packet and only reference those in compression. However, anything consuming DNS packets should probably be prepared for arbitrary pointers to previous locations in the packet - and also handle loops somehow (the usual options apply: either explicitly recording locations visited in traversal or just maintaining a counter and failing at some point). Cheers, -- Shane
- [DNSOP] Clarification question: compression point… Viktor Dukhovni
- Re: [DNSOP] Clarification question: compression p… bert hubert
- Re: [DNSOP] Clarification question: compression p… Shane Kerr
- Re: [DNSOP] Clarification question: compression p… Joe Abley
- Re: [DNSOP] Clarification question: compression p… Martin Hoffmann
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Martin Hoffmann
- Re: [DNSOP] Clarification question: compression p… John Dickinson
- Re: [DNSOP] Clarification question: compression p… Ray Bellis
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Viktor Dukhovni
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Ted Lemon
- Re: [DNSOP] Clarification question: compression p… Mark Andrews
- Re: [DNSOP] Clarification question: compression p… Shane Kerr
- Re: [DNSOP] Clarification question: compression p… Ted Lemon
- Re: [DNSOP] Clarification question: compression p… Mukund Sivaraman
- Re: [DNSOP] Clarification question: compression p… George Michaelson