Network Working Group S. Bortzmeyer Internet-Draft AFNIC Obsoletes: 7816 (if approved) R. Dolmans Intended status: Standards Track NLnet Labs Expires: 1 April 2021 P. Hoffman ICANN 28 September 2020 DNS Query Name Minimisation to Improve Privacy draft-ietf-dnsop-rfc7816bis-06 Abstract This document describes techniques called "QNAME minimisation" to improve DNS privacy, where the DNS resolver no longer always sends the full original QNAME to the upstream name server. This document obsoletes RFC 7816. This document is part of the IETF DNSOP (DNS Operations) Working Group. The source of the document, as well as a list of open issues, is at Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 1 April 2021. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Bortzmeyer, et al. Expires 1 April 2021 [Page 1] Internet-Draft QNAME Minimisation September 2020 Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction and Background . . . . . . . . . . . . . . . . . 2 1.1. Experience From RFC 7816 . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Description of QNAME Minimisation . . . . . . . . . . . . . . 3 3. Algorithm to Perform Aggressive Method QNAME Minimisation . . 5 4. QNAME Minimisation Examples . . . . . . . . . . . . . . . . . 6 5. Limit Number of Queries . . . . . . . . . . . . . . . . . . . 7 6. Performance Considerations . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 8.1. Normative References . . . . . . . . . . . . . . . . . . 9 8.2. Informative References . . . . . . . . . . . . . . . . . 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11 Changes from RFC 7816 . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction and Background The problem statement for this document is described in [RFC7626]. 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. Before QNAME minimisation, when a resolver received the query "What is the AAAA record for www.example.com?", it sent to the root (assuming a resolver whose cache is empty) the very same question. Sending the full QNAME to the authoritative name server was 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. Bortzmeyer, et al. Expires 1 April 2021 [Page 2] Internet-Draft QNAME Minimisation September 2020 QNAME minimisation is compatible with the current DNS system and therefore can easily be deployed. Because it is only a change to the way that the resolver operates, it does not change the DNS protocol itself. The behaviour suggested here (minimising the amount of data sent in QNAMEs from the resolver) is allowed by Section 5.3.3 of [RFC1034] and Section 7.2 of [RFC1035]. 1.1. Experience From RFC 7816 This document obsoletes [RFC7816]. RFC 7816 was labelled "experimental", but ideas from it were widely deployed since its publicaiton. Many resolver implementations now support QNAME minimisation. The lessons learned from implementing QNAME minimisation were used to create this new revision. Data from DNSThought [dnsthought-qnamemin] and Verisign [verisign-qnamemin] shows that a large percentage of the resolvers deployed on the Internet already support QNAME minimisation in some way. Academic research has been performed on QNAME minimisation [devries-qnamemin]. This work shows that QNAME minimisation in relaxed mode causes almost no problems. The paper recommends using the A QTYPE, and limiting the number of queries in some way. 1.2. Terminology The terminology used in this document is defined in [RFC8499]. In this document, a "cold" cache is one that is empty, having literally no entries in it. A "warm" cache is one that has some entries in it. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. 2. Description of QNAME Minimisation The idea behind QNAME minimisation is to minimise the amount of privacy sensitive data sent from the DNS resolver to the authoritative name server. This section describes the RECOMMENDED way to do QNAME minimisation -- the way that maximises privacy benefits. That algorithm is summarised in Section 3. Bortzmeyer, et al. Expires 1 April 2021 [Page 3] Internet-Draft QNAME Minimisation September 2020 When a resolver is not able to answer a query from cache it has to send a query to an authoritative nameserver. Traditionally these queries would contain the full QNAME and the original QTYPE as received in the client query. The full QNAME and original QTYPE are only needed at the nameserver that is authoritative for the record requested by the client. All other nameservers queried while resolving the query only need to receive enough of the QNAME to be able to answer with a delegation. The QTYPE in these queries is not relevant, as the nameserver is not able to authoritatively answer the records the client is looking for. Sending the full QNAME and original QTYPE to these nameservers therefore exposes more privacy sensitive data than necessary to resolve the client's request. A resolver that implements QNAME minimisation changes the QNAME and QTYPE in queries to an authoritative nameserver that is not known to be responsible for the original QNAME. These queries contain: * a QTYPE selected by the resolver to hide the original QTYPE * the QNAME that is the original QNAME, stripped to just one label more than the longest matching domain name for which the nameserver is known to be authoritative This method is called the "aggressive method" in this document because the resolver won't expose the original QTYPE to nameservers that are not known to be responsible for the desired name. This method is the safest from a privacy point of view, and is thus the RECOMMENDED method for this document. Note that this document relaxes the recommendation in RFC 7816 to use the NS QTYPE to hide the original QTYPE. Using the NS QTYPE is still allowed. The authority of NS records lies at the child side. The parent side of the delegation will answer using a referral, like it will do for queries with other QTYPEs. Using the NS QTYPE therefore has no added value over other QTYPEs. The QTYPE to use while minimising queries can be any possible data type (as defined in [RFC6895] Section 3.1) for which the authority always lies below the zone cut (i.e. not DS, NSEC, NSEC3, OPT, TSIG, TKEY, ANY, MAILA, MAILB, AXFR, and IXFR), as long as there is no relation between the incoming QTYPE and the selection of the QTYPE to use while minimising. A good candidate is to always use the "A" QTYPE because this is the least likely to raise issues in DNS software and middleboxes that do not properly support all QTYPEs. The QTYPE=A queries will also blend into traffic from non-minimising resolvers, making it in some cases harder to observe that the Bortzmeyer, et al. Expires 1 April 2021 [Page 4] Internet-Draft QNAME Minimisation September 2020 resolver is using QNAME minimisation. Using the QTYPE that occurs most in incoming queries will slightly reduce the number of queries, as there is no extra check needed for delegations on non-apex records. Another potential benefit of using QTYPE=A is that [RFC8305] clients that need answers for both the A and AAAA types will send the AAAA query first. When minimising using QTYPE=A the minimised query might be useful, and now already in the cache, for the happy eyeballs query for the A QTYPE. 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. In 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 a record for bar.example. It will get a non-referral answer, it has to query the example name servers again with one more label, and so on. (Section 3 describes this algorithm in deeper detail.) Stub and forwarding resolvers MAY implement QNAME minimisation. Minimising queries that will be sent to an upstream resolver does not help in hiding data from the upstream resolver because all information will end up there anyway. It might, however, limit the data exposure between the upstream resolver and the authoritative nameserver in the situation where the upstream resolver does not support QNAME minimisation. Using QNAME minimisation in a stub or forwarding resolvers that does not have a mechanism to find and cache zone cuts will drastically increase the number of outgoing queries. 3. Algorithm to Perform Aggressive Method QNAME Minimisation This algorithm performs name resolution with aggressive method 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 zone cuts. (0) If the query can be answered from the cache, do so; otherwise, iterate as follows: (1) Get the closest delegation point that can be used for the original QNAME/QTYPE combination from the cache. (1a) For queries with QTYPE=DS this is the NS RRset with the Bortzmeyer, et al. Expires 1 April 2021 [Page 5] Internet-Draft QNAME Minimisation September 2020 owner matching the most labels with the QNAME stripped by one label. The QNAME will be a subdomain of (but not equal to) this NS RRset. Call this ANCESTOR. (1b) For queries with other original QTYPEs this is the NS RRset with the owner matching the most labels with the QNAME. The QNAME will be equal to or a subdomain of this NS RRset. Call this ANCESTOR. (2) Initialise CHILD to the same as ANCESTOR. (3) If CHILD is the same as the QNAME, or if the CHILD is one label shorter than the QNAME and the original QTYPE is DS, 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) Look for a cache entry for the RRset at CHILD with hidden QTYPE. If this entry is for an NXDOMAIN and the resolver has support for RFC8020 the NXDOMAIN can be used in response to the original query, and stop. If the entry is for a NOERROR answer go back to step 3. If the entry is for an NXDOMAIN answer and the resolver does not support RFC8020, go back to step 3. (6) Query for CHILD with the minimised QTYPE 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) A NOERROR answer. Cache this answer, and go back to step 3. (6c) An NXDOMAIN answer. Return an NXDOMAIN answer in response to the original query, and stop. (6d) An answer with another RCODE, or no answer. Try another name server at the same delegation point. Stop if none of them are able to return a valid answer. 4. QNAME Minimisation Examples Assume that a resolver receives a request to resolve foo.bar.baz.example. Assume that the resolver already knows that ns1.nic.example is authoritative for .example, and that the resolver does not know a more specific authoritative name server. It will send the query with QNAME=baz.example and the QTYPE selected to hide the original QTYPE to ns1.nic.example. Bortzmeyer, et al. Expires 1 April 2021 [Page 6] Internet-Draft QNAME Minimisation September 2020 Here are more detailed examples of queries with the aggressive method of QNAME minimisation: Cold cache, traditional resolution algorithm without QNAME minimisation, request for MX record of a.b.example.org: QTYPE QNAME TARGET NOTE MX a.b.example.org root nameserver MX a.b.example.org org nameserver MX a.b.example.org example.org nameserver Cold cache, aggressive QNAME minimisation method, request for MX record of a.b.example.org, using the A QTYPE to hide the original QTYPE: QTYPE QNAME TARGET NOTE A org root nameserver A example.org org nameserver A b.example.org example.org nameserver A a.b.example.org example.org nameserver "a" may be delegated MX a.b.example.org example.org nameserver Note that in above example one query would have been saved if the incoming QTYPE would have been the same as the QTYPE selected by the resolver to hide the original QTYPE. Only one query needed with as QTYPE a.b.example.org would have been needed if the original QTYPE would have been A. Using the most used QTYPE to hide the original QTYPE therefore slightly reduces the number of outgoing queries. Warm cache with only org delegation known, (example.org's NS RRset is not known), aggressive QNAME minimisation method, request for MX record of a.b.example.org, using A QTYPE to hide the original QTYPE: QTYPE QNAME TARGET NOTE A example.org org nameserver A b.example.org example.org nameserver A a.b.example.org example.org nameserver "a" may be delegated MX a.b.example.org example.org nameserver 5. Limit Number of Queries When using QNAME minimisation, the number of labels in the received QNAME can influence the number of queries sent from the resolver. This opens an attack vector and can decrease performance. Resolvers supporting QNAME minimisation MUST implement a mechanism to limit the number of outgoing queries per user request. Bortzmeyer, et al. Expires 1 April 2021 [Page 7] Internet-Draft QNAME Minimisation September 2020 Take for example an incoming QNAME with many labels, like www.host.group.department.example.com, where host.group.department.example.com is hosted on example.com's name servers. Assume a resolver that knows only the name servers of example.com. Without QNAME minimisation, it would send these example.com 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 behaviour can be exploited by sending queries with a large number of labels in the QNAME that will be answered using a wildcard record. Take for example a record for *.example.com, hosted on example.com's name servers. An incoming query containing a QNAME with more than 100 labels, ending in example.com, will result in a query per label. By using random labels the attacker can bypass the cache and always require the resolver to send many queries upstream. Note that [RFC8198] can limit this attack in some cases. One mechanism to reduce this attack vector is by appending more than one label per iteration for QNAMEs with a large number of labels. To do this a maximum number of QNAME minimisation iterations has to be selected (MAX_MINIMISE_COUNT), a good value is 10. Optionally a value for the number of queries that should only have one label appended can be selected (MINIMISE_ONE_LAB), a good value is 4. The assumption here is that the number of labels on delegations higher in the hierarchy are rather small, therefore not exposing too may labels early on has the most privacy benefit. When a resolver needs to send out a query if will look for the closest known delegation point in its cache. The number of QNAME minimisation iterations is the difference between this closest nameserver and the incoming QNAME. The first MINIMISE_ONE_LAB iterations will be handles as described in Section 2. The number of labels that are not exposed yet now need to be divided over the iterations that are left (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB). The remainder of the division should be added to the last iterations. For example, when resolving a QNAME with 18 labels, the number of labels added per iteration are: 1,1,1,1,2,2,2,2,3,3. Bortzmeyer, et al. Expires 1 April 2021 [Page 8] Internet-Draft QNAME Minimisation September 2020 6. Performance Considerations The main goal of QNAME minimisation is to improve privacy by sending less data. However, it may have other advantages. For instance, if a resolver sends a root name server queries 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. When using QNAME minimisation, the resolver would send only one question (for .example itself) to which they could answer NXDOMAIN. The resolver can cache this answer and use it as to prove that nothing below .example exists ([RFC8020]). A resolver now knows a priori that neither B.example nor C.example exist. Thus, in this common case, the total number of upstream queries under QNAME minimisation could counterintuitively be 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 TLD that is delegation-only, a two-label QNAME query may be optimal for finding the delegation owner name, depending on the way domain matching is implemented. QNAME minimisation can increase the number of queries based on the incoming QNAME. This is described in Section 5. 7. Security Considerations 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 no protection against the recursive resolver, which still sees the full request coming from the stub resolver. 8. References 8.1. Normative References [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, . Bortzmeyer, et al. Expires 1 April 2021 [Page 9] Internet-Draft QNAME Minimisation September 2020 [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013, . [RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . 8.2. Informative References [devries-qnamemin] "A First Look at QNAME Minimization in the Domain Name System", March 2019, . [dnsthought-qnamemin] "DNSThought QNAME minimisation results. Using Atlas probes", March 2020, . [Huque-QNAME-Min] Huque, S., "Query name minimization and authoritative server behavior", May 2015, . [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, . [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, April 2013, . Bortzmeyer, et al. Expires 1 April 2021 [Page 10] Internet-Draft QNAME Minimisation September 2020 [RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626, DOI 10.17487/RFC7626, August 2015, . [RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, November 2016, . [RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198, July 2017, . [RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2: Better Connectivity Using Concurrency", RFC 8305, DOI 10.17487/RFC8305, December 2017, . [RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499, January 2019, . [verisign-qnamemin] Thomas, M., "Maximizing Qname Minimization: A New Chapter in DNS Protocol Evolution", September 2020, . Acknowledgments TODO (refer to 7816) Changes from RFC 7816 Changed in -06 * Removed lots of text from when this was experimental * Lots of reorganization Changed in -04 * Start structure for implementation section * Add clarification why the used QTYPE does not matter * Make algorithm DS QTYPE compatible Changed in -03 Bortzmeyer, et al. Expires 1 April 2021 [Page 11] Internet-Draft QNAME Minimisation September 2020 * Drop recommendation to use the NS QTYPE to hide the incoming QTYPE * Describe DoS attach vector for QNAME with large number of labels, and propose a mitigation. * Simplify examples and change qname to a.b.example.com to show the change in number of queries. Changed in -00, -01, and -02 * Made changes to deal with errata #4644 * Changed status to be on standards track * Major reorganization Authors' Addresses Stephane Bortzmeyer AFNIC 1, rue Stephenson 78180 Montigny-le-Bretonneux France Phone: +33 1 39 30 83 46 Email: bortzmeyer+ietf@nic.fr URI: https://www.afnic.fr/ Ralph Dolmans NLnet Labs Email: ralph@nlnetlabs.nl Paul Hoffman ICANN Email: paul.hoffman@icann.org Bortzmeyer, et al. Expires 1 April 2021 [Page 12]