| Internet-Draft | Qname minimisation | May 2014 |
| Bortzmeyer | Expires 2 December 2014 | [Page] |
This document describes one of the techniques that could be used to improve DNS privacy (see [I-D.bortzmeyer-dnsop-dns-privacy]), a technique called "qname minimisation".¶
Discussions of the document should currently take place on the dns-privacy mailing list [dns-privacy].¶
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The problem statement is exposed in [I-D.bortzmeyer-dnsop-dns-privacy]. The terminology ("qname", "resolver", etc) is also defined in this companion document. This specific solution is not intended to completely solve the problem, far from it. It is better to see it as one tool among a toolbox.¶
It follows the principle explained in section 6.1 of [RFC6973]: the less data you send out, the less privacy problems you'll get.¶
The idea is to minimise the amount of data sent from the DNS resolver. When a resolver receives the query "What is the AAAA record for www.example.com?", it sends to the root (assuming a cold resolver, whose cache is empty) the very same question. Sending "What are the NS records for .com?" would be sufficient (since it will be the answer from the root anyway). To do so would be compatible with the current DNS system and therefore could be easily deployable, since it is an unilateral change to the resolvers.¶
If "minimisation" is too long, you can write it "m12n".¶
To do such minimisation, the resolver needs to know the zone cut [RFC2181]. There is not a zone cut at every label boundary. If we take the name www.foo.bar.example, it is possible that there is a zone cut between "foo" and "bar" but not between "bar" and "example". So, assuming the resolver already knows the name servers of .example, when it receives the query "What is the AAAA record of www.foo.bar.example", it does not always know if the request should be sent to the name servers of bar.example or to those of example. [RFC2181] suggests a method to find the zone cut (section 6), so resolvers may try it.¶
Note that DNSSEC-validating resolvers already have access to this information, since they have to find the zone cut (the DNSKEY record set is just below, the DS record set just above).¶
It can be noted that minimising the amount of data sent also partially addresses the case of a wire sniffer, not just the case of privacy invasion by the servers.¶
One should note that the behaviour suggested here (minimising the amount of data sent in qnames) is NOT forbidden by the [RFC1034] (section 5.3.3) or [RFC1035] (section 7.2). Sending the full qname to the authoritative name server is a tradition, not a protocol requirment.¶
The administrators of the forwarders, and of the authoritative name servers, will get less data, which will reduce the utility of the statistics they can produce (such as the percentage of the various qtypes). On the other hand, it will decrease their legal responsability, in many cases.¶
Some broken name servers do not react properly to qtype=NS requests. As an example, look at www.ratp.fr (not ratp.fr), which is delegated to two name servers that reply properly to "A www.ratp.fr" queries but send REFUSED to queries "NS www.ratp.fr". This behaviour is a gross protocol violation and there is no need to stop improving the DNS because of such brokenness. However, qname minimisation may still work with such domains since they are only leaf domains (no need to send them NS requests).¶
Another way to deal with such broken name servers would be to try with A requests (A being choosen because it is the most common and hence the least revealing qtype). Instead of querying name servers with a query "NS example.com", we could use "A _.example.com" and see if we get a referral.¶
The main goal of qname minimisation is to improve privacy, by sending less data. However, it may have other advantages. For instance, if a root name server receives a query from some resolver for A.CORP followed by B.CORP followed by C.CORP, the result will be three NXDOMAINs, since .CORP does not exist in the root zone. Under query minimization, the root name servers would hear only one question (for .CORP itself) to which they could answer NXDOMAIN, thus opening up a negative caching opportunity in which the full resolver could know a priori that neither B.CORP or C.CORP could exist. Thus in this common case the total number of upstream queries under query minimisation would be counter-intuitively less than the number of queries under the traditional iteration (as described in the DNS standard).¶
Under study. TODO: better handling of phantom domains?¶
Thanks to Olaf Kolkman, Mark Andrews and Francis Dupont for the interesting discussions on this qname minimisation. Thanks to Mohsen Souissi for proofreading. Thanks to Tony Finch for the zone cut algorithm in Appendix A. Thanks to Paul Vixie for pointing out that there are practical advantages (besides privacy) to qname m12n. Thanks to Phillip Hallam-Baker for the fallback on A queries, to deal with broken servers.¶
Although a validating resolver already has the logic to find the zone cut, other resolvers may be interested by this algorithm to follow in order to locate this cut:¶
(6) Query for CHILD IN NS using PARENT's name servers. The response can be:¶