DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-00

Abstract

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 take place on the DNSOP working group mailing list [dnsop].

Status of This Memo

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This Internet-Draft will expire on April 25, 2015.

Copyright Notice

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Table of Contents

• 1. Introduction and background
• 2. Qname minimisation
• 3. Operational considerations
• 4. Other advantages
• 5. Security considerations
• 6. Acknowledgments
• 7. References
• 7.1. Normative References
• 7.2. Informative References
• Appendix A. An algorithm to find the zone cut
• Author's Address

1. Introduction and background

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.

2. Qname minimisation

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.

3. Operational considerations

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 of today, 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). Anyway, such setup breaks many things (besides qname minimisation), it breaks negative answers as the servers don't return the correct SOA. It also breaks anything that depends on NS and SOA records existing at the top of the zone.

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.

;; ANSWER SECTION:
*.com.          60  IN  A   74.220.199.6

; and:

;; ANSWER SECTION:
*.uk.           60  IN  A   74.220.199.6

; etc.

Other strange and illegal practice may pose a problem: for instance, there is a common DNS anti-pattern used by low-end web hosters that also do DNS hosting that exploits the fact that the DNS protocol (pre-DNSSEC) allows certain serious misconfigurations, such as parent and child zones disagreeing on the location of a zone cut. Basically, they have a single zone with wildcards like:

This lets them turn up tons of web hosting customers without having to configure thousands of individual zones on their nameservers. They just tell the prospective customer to point their NS records at their nameservers, and the Web hoster doesn't have to provision anything in order to make the customer's domain resolve.

Qname minimisation can decrease performance in some cases, for instance for a deep domain name (like www.host.group.department.example.com where host.group.department.example.com is hosted on example.com's name servers). For such a name, a cold resolver will, depending how qname minimisation is implemented, send more queries. Once warm, there will be no difference with a traditional resolver. A possible solution is to always use the traditional algorithm when the cache is cold and then to move to qname minimisation. This will decrease the privacy a bit but will guarantee no degradation of performance.

4. Other advantages

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).

5. Security considerations

[CREF1]Consequences of qname minimisation for phantom domains (domains deleted on the parent but still served by original name servers)?

No security consequence (besides privacy improvment) is known at this time.

6. Acknowledgments

Thanks to Olaf Kolkman for the original idea. Thanks to Mark Andrews and Francis Dupont for the interesting discussions. 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. Thanks to Robert Edmonds for an interesting anti-pattern.

7. References

7.1. Normative References

7.2. Informative References

Appendix A. An algorithm to find the zone cut

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:

• (0) If the query can be answered from the cache, do so, otherwise iterate as follows:
• (1) Find closest enclosing NS RRset in your cache. The owner of this NS RRset will be a suffix of the QNAME - the longest suffix of any NS RRset in the cache. Call this PARENT.
• (2) Initialize CHILD to the same as PARENT.
• (3) If CHILD is the same as the QNAME, resolve the original query using PARENT's name servers, and finish.
• (4) Otherwise, add a label from the QNAME to the start of CHILD.
• (5) If you have a negative cache entry for the NS RRset at CHILD, go back to step 3.
• (6) Query for CHILD IN NS using PARENT's name servers. The response can be:
  • (6a) A referral. Cache the NS RRset from the authority section and go back to step 1.
  • (6b) An authoritative answer. Cache the NS RRset from the answer section and go back to step 1.
  • (6c) An NXDOMAIN answer. Return an NXDOMAIN answer in response to the original query and stop.
  • (6d) A NOERROR/NODATA answer. Cache this negative answer and go back to step 3.

Author's Address