Internet Engineering Task Force (IETF) S. Bortzmeyer Request for Comments: 7816 AFNIC Category: Experimental March 2016 ISSN: 2070-1721
DNS Query Name Minimisation to Improve Privacy
This document describes a technique to improve DNS privacy, a technique called "QNAME minimisation", where the DNS resolver no longer sends the full original QNAME to the upstream name server.
Status of This Memo
This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.
This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.
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Table of Contents
1. Introduction and Background .....................................2 2. QNAME Minimisation ..............................................3 3. Possible Issues .................................................4 4. Protocol and Compatibility Discussion ...........................5 5. Operational Considerations ......................................5 6. Performance Considerations ......................................6 7. On the Experimentation ..........................................6 8. Security Considerations .........................................7 9. References ......................................................7 9.1. Normative References .......................................7 9.2. Informative References .....................................8 Appendix A. An Algorithm to Perform QNAME Minimisation .............9 Appendix B. Alternatives .........................................10 Acknowledgments ...................................................11 Author's Address ..................................................11
The problem statement is described in [RFC7626]. The terminology ("QNAME", "resolver", etc.) is also defined in this companion document. This specific solution is not intended to fully solve the DNS privacy problem; instead, it should be viewed as one tool amongst many.
QNAME minimisation follows the principle explained in Section 6.1 of [RFC6973]: the less data you send out, the fewer privacy problems you have.
Currently, 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 the full QNAME to the authoritative name server is a tradition, not a protocol requirement. In a conversation with the author in January 2015, Paul Mockapetris explained that this tradition comes from a desire to optimise the number of requests, when the same name server is authoritative for many zones in a given name (something that was more common in the old days, where the same name servers served .com and the root) or when the same name server is both recursive and authoritative (something that is strongly discouraged now). Whatever the merits of this choice at this time, the DNS is quite different now.
The idea is to minimise the amount of data sent from the DNS resolver to the authoritative name server. In the example in the previous section, sending "What are the NS records for .com?" would have been sufficient (since it will be the answer from the root anyway). The rest of this section describes the recommended way to do QNAME minimisation -- the way that maximises privacy benefits (other alternatives are discussed in the appendices).
Instead of sending the full QNAME and the original QTYPE upstream, a resolver that implements QNAME minimisation and does not already have the answer in its cache sends a request to the name server authoritative for the closest known ancestor of the original QNAME. The request is done with:
o the QTYPE NS
o the QNAME that is the original QNAME, stripped to just one label more than the zone for which the server is authoritative
For example, a resolver receives a request to resolve foo.bar.baz.example. Let's assume that it already knows that ns1.nic.example is authoritative for .example and the resolver does not know a more specific authoritative name server. It will send the query QTYPE=NS,QNAME=baz.example to ns1.nic.example.
The minimising resolver works perfectly when it knows the zone cut (zone cuts are described in Section 6 of [RFC2181]). But zone cuts do not necessarily exist 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 that 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 where the zone cut will be. To find the zone cut, it will query the .example name servers for the NS records for bar.example. It will get a NODATA response, indicating that there is no zone cut at that point, so it has to query the .example name servers again with one more label, and so on. (Appendix A describes this algorithm in deeper detail.)
Since the information about the zone cuts will be stored in the resolver's cache, the performance cost is probably reasonable. Section 6 discusses this performance discrepancy further.
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Note that DNSSEC-validating resolvers already have access to this information, since they have to know the zone cut (the DNSKEY record set is just below; the DS record set is just above).
QNAME minimisation is legal, since the original DNS RFCs do not mandate sending the full QNAME. So, in theory, it should work without any problems. However, in practice, some problems may occur (see [Huque-QNAME-Min] for an analysis and [Huque-QNAME-storify] for an interesting discussion on this topic).
Some broken name servers do not react properly to QTYPE=NS requests. For instance, some authoritative name servers embedded in load balancers reply properly to A queries but send REFUSED to NS queries. This behaviour is a protocol violation, and there is no need to stop improving the DNS because of such behaviour. However, QNAME minimisation may still work with such domains, since they are only leaf domains (no need to send them NS requests). Such a setup breaks more than just QNAME minimisation. It breaks negative answers, since the servers don't return the correct SOA, and it also breaks anything dependent upon NS and SOA records existing at the top of the zone.
Another way to deal with such incorrect name servers would be to try with QTYPE=A requests (A being chosen because it is the most common and hence a QTYPE that will always be accepted, while a QTYPE NS may ruffle the feathers of some middleboxes). Instead of querying name servers with a query "NS example.com", we could use "A _.example.com" and see if we get a referral.
A problem can also appear when a name server does not react properly to ENTs (Empty Non-Terminals). If ent.example.com has no resource records but foobar.ent.example.com does, then ent.example.com is an ENT. Whatever the QTYPE, a query for ent.example.com must return NODATA (NOERROR / ANSWER: 0). However, some name servers incorrectly return NXDOMAIN for ENTs. If a resolver queries only foobar.ent.example.com, everything will be OK, but if it implements QNAME minimisation, it may query ent.example.com and get an NXDOMAIN. See also Section 3 of [DNS-Res-Improve] for the other bad consequences of this bad behaviour.
A possible solution, currently implemented in Knot, is to retry with the full query when you receive an NXDOMAIN. It works, but it is not ideal for privacy.
Other practices that do not conform to the DNS protocol standards may pose a problem: there is a common DNS trick used by some web hosters that also do DNS hosting that exploits the fact that the DNS protocol
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(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 for each TLD, like:
*.example. 60 IN A 192.0.2.6
(They could just wildcard all of "*.", which would be sufficient. We don't know why they don't do it.)
This lets them have many web-hosting customers without having to configure thousands of individual zones on their name servers. They just tell the prospective customer to point their NS records at the hoster's name servers, and the web hoster doesn't have to provision anything in order to make the customer's domain resolve. NS queries to the hoster will therefore not give the right result, which may endanger QNAME minimisation (it will be a problem for DNSSEC, too).
QNAME minimisation is compatible with the current DNS system and therefore can easily be deployed; since it is a unilateral change to the resolver, it does not change the protocol. (Because it is a unilateral change, resolver implementers may do QNAME minimisation in slightly different ways; see the appendices for examples.)
One should note that the behaviour suggested here (minimising the amount of data sent in QNAMEs from the resolver) is NOT forbidden by Section 5.3.3 of [RFC1034] or Section 7.2 of [RFC1035]. As stated in Section 1, the current method, sending the full QNAME, is not mandated by the DNS protocol.
One may notice that many documents that explain the DNS and that are intended for a wide audience incorrectly describe the resolution process as using QNAME minimisation (e.g., by showing a request going to the root, with just the TLD in the query). As a result, these documents may confuse readers that use them for privacy analysis.
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) [Kaliski-Minimum].
DNS administrators are reminded that the data on DNS requests that they store may have legal consequences, depending on your jurisdiction (check with your local lawyer).
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.example followed by B.example followed by C.example, the result will be three NXDOMAINs, since .example does not exist in the root zone. Under query name minimisation, the root name servers would hear only one question (for .example 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.example nor C.example could exist. Thus, in this common case the total number of upstream queries under QNAME minimisation would be counterintuitively less than the number of queries under the traditional iteration (as described in the DNS standard).
QNAME minimisation may also improve lookup performance for TLD operators. For a typical TLD, delegation-only, and with delegations just under the TLD, a two-label QNAME query is optimal for finding the delegation owner name.
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). Let's assume a resolver that knows only the name servers of .example. Without QNAME minimisation, it would send these .example name servers a query for www.host.group.department.example.com and immediately get a specific referral or an answer, without the need for more queries to probe for the zone cut. For such a name, a cold resolver with QNAME minimisation will, depending on how QNAME minimisation is implemented, send more queries, one per label. Once the cache is warm, there will be no difference with a traditional resolver. Actual testing is described in [Huque-QNAME-Min]. Such deep domains are especially common under ip6.arpa.
This document has status "Experimental". Since the beginning of time (or DNS), the fully qualified host name was always sent to the authoritative name servers. There was a concern that changing this behaviour may engage the Law of Unintended Consequences -- hence this status.
The idea behind the experiment is to observe QNAME minimisation in action with multiple resolvers, various authoritative name servers, etc.
QNAME minimisation's benefits are clear in the case where you want to decrease exposure to the authoritative name server. But minimising the amount of data sent also, in part, addresses the case of a wire sniffer as well as the case of privacy invasion by the servers. (Encryption is of course a better defense against wire sniffers, but, unlike QNAME minimisation, it changes the protocol and cannot be deployed unilaterally. Also, the effect of QNAME minimisation on wire sniffers depends on whether the sniffer is on the DNS path.)
QNAME minimisation offers zero protection against the recursive resolver, which still sees the full request coming from the stub resolver.
All the alternatives mentioned in Appendix B decrease privacy in the hope of improving performance. They must not be used if you want maximum privacy.
[Kaliski-Minimum] Kaliski, B., "Minimum Disclosure: What Information Does a Name Server Need to Do Its Job?", March 2015, <http://blogs.verisigninc.com/blog/entry/ minimum_disclosure_what_information_does>.
Appendix A. An Algorithm to Perform QNAME Minimisation
This algorithm performs name resolution with QNAME minimisation in the presence of zone cuts that are not yet known.
Although a validating resolver already has the logic to find the zone cuts, implementers of other resolvers may want to use this algorithm to locate the cuts. This is just a possible aid for implementers; it is not intended to be normative:
(0) If the query can be answered from the cache, do so; otherwise, iterate as follows:
(1) Find the 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 ANCESTOR.
(2) Initialise CHILD to the same as ANCESTOR.
(3) If CHILD is the same as the QNAME, resolve the original query using ANCESTOR'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 ANCESTOR'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.
Remember that QNAME minimisation is unilateral, so a resolver is not forced to implement it exactly as described here.
There are several ways to perform QNAME minimisation. See Section 2 for the suggested way. It can be called the aggressive algorithm, since the resolver only sends NS queries as long as it does not know the zone cuts. This is the safest, from a privacy point of view. Another possible algorithm, not fully studied at this time, could be to "piggyback" on the traditional resolution code. At startup, it sends traditional full QNAMEs and learns the zone cuts from the referrals received, then switches to NS queries asking only for the minimum domain name. This leaks more data but could require fewer changes in the existing resolver codebase.
In the above specification, the original QTYPE is replaced by NS (or may be A, if too many servers react incorrectly to NS requests); this is the best approach to preserve privacy. But this erases information about the relative use of the various QTYPEs, which may be interesting for researchers (for instance, if they try to follow IPv6 deployment by counting the percentage of AAAA vs. A queries). A variant of QNAME minimisation would be to keep the original QTYPE.
Another useful optimisation may be, in the spirit of the HAMMER idea [HAMMER], to probe in advance for the introduction of zone cuts where none previously existed (i.e., confirm their continued absence, or discover them).
To address the "number of queries" issue described in Section 6, a possible solution is to always use the traditional algorithm when the cache is cold and then to move to QNAME minimisation (precisely defining what is "hot" or "cold" is left to the implementer). This will decrease the privacy but will guarantee no degradation of performance.
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Thanks to Olaf Kolkman for the original idea during a KLM flight from Amsterdam to Vancouver, although the concept is probably much older (e.g., <https://lists.dns-oarc.net/pipermail/dns-operations/ 2010-February/005003.html>). Thanks to Shumon Huque and Marek Vavrusa for implementation and testing. Thanks to Mark Andrews and Francis Dupont for the interesting discussions. Thanks to Brian Dickson, Warren Kumari, Evan Hunt, and David Conrad for remarks and suggestions. Thanks to Mohsen Souissi for proofreading. Thanks to Tony Finch for the zone cut algorithm in Appendix A and for discussion of the algorithm. Thanks to Paul Vixie for pointing out that there are practical advantages (besides privacy) to QNAME minimisation. 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.
Stephane Bortzmeyer AFNIC 1, rue Stephenson Montigny-le-Bretonneux 78180 France