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Versions: (draft-bortzmeyer-rfc7816bis) 00 01
02 03 04 05 06
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 <https://framagit.org/bortzmeyer/rfc7816-bis>
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.
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Please review these documents carefully, as they describe your rights
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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.
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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.
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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
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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
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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.
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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.
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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.
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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,
<https://www.rfc-editor.org/info/rfc1034>.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
<https://www.rfc-editor.org/info/rfc7816>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[devries-qnamemin]
"A First Look at QNAME Minimization in the Domain Name
System", March 2019,
<https://nlnetlabs.nl/downloads/publications/
devries2019.pdf>.
[dnsthought-qnamemin]
"DNSThought QNAME minimisation results. Using Atlas
probes", March 2020,
<https://dnsthought.nlnetlabs.nl/#qnamemin>.
[Huque-QNAME-Min]
Huque, S., "Query name minimization and authoritative
server behavior", May 2015,
<https://indico.dns-oarc.net/event/21/contribution/9>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
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[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<https://www.rfc-editor.org/info/rfc7626>.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <https://www.rfc-editor.org/info/rfc8020>.
[RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
July 2017, <https://www.rfc-editor.org/info/rfc8198>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[verisign-qnamemin]
Thomas, M., "Maximizing Qname Minimization: A New Chapter
in DNS Protocol Evolution", September 2020,
<https://blog.verisign.com/security/maximizing-qname-
minimization-a-new-chapter-in-dns-protocol-evolution/>.
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
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* 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
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