Internet DRAFT - draft-ietf-dnsop-rfc7816bis
draft-ietf-dnsop-rfc7816bis
Network Working Group S. Bortzmeyer
Internet-Draft AFNIC
Obsoletes: 7816 (if approved) R. Dolmans
Intended status: Standards Track NLnet Labs
Expires: 5 March 2022 P. Hoffman
ICANN
1 September 2021
DNS Query Name Minimisation to Improve Privacy
draft-ietf-dnsop-rfc7816bis-11
Abstract
This document describes a technique called "QNAME minimisation" to
improve DNS privacy, where the DNS resolver no longer always sends
the full original QNAME and original QTYPE to the upstream name
server. This document obsoletes RFC 7816.
Status of This Memo
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Copyright Notice
Copyright (c) 2021 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
1.1. Experience From RFC 7816 . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Description of QNAME Minimisation . . . . . . . . . . . . . . 4
2.1. QTYPE Selection . . . . . . . . . . . . . . . . . . . . . 4
2.2. QNAME Selection . . . . . . . . . . . . . . . . . . . . . 5
2.3. Limit Number of Queries . . . . . . . . . . . . . . . . . 5
2.4. Stub and Forwarding Resolvers . . . . . . . . . . . . . . 7
3. Algorithm to Perform QNAME Minimisation . . . . . . . . . . . 7
4. QNAME Minimisation Examples . . . . . . . . . . . . . . . . . 8
5. Performance Considerations . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 11
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Changes from RFC 7816 . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction and Background
The problem statement for this document is described in [RFC9076].
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 one of
the authors 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
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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.
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
publication. 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], Verisign
[verisign-qnamemin] and APNIC [apnic-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. Some of
the issues that the paper found are covered in Section 5.
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.
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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 how to do QNAME
minimisation. The algorithm is summarised in Section 3.
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 obscures the QNAME and
QTYPE in queries directed 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 possibly obscure 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
2.1. QTYPE Selection
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
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use while minimising. Good candidates are to always use the "A" or
"AAAA" QTYPE because these are the least likely to raise issues in
DNS software and middleboxes that do not properly support all QTYPEs.
QTYPE=A or QTYPE=AAAA queries will also blend into traffic from non-
minimising resolvers, making it in some cases harder to observe that
the resolver is using QNAME minimisation. Using a 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.
2.2. QNAME Selection
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.)
2.3. 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.
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. (Such deep domains are especially common under
ip6.arpa.) 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 send more queries, one per label. Once the cache
is warm, there will be less difference with a traditional resolver.
Testing of this is described in [Huque-QNAME-Min].
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The behaviour of sending multiple queries 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 that MAY be used 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 MUST be selected (MAX_MINIMISE_COUNT); a RECOMMENDED value
is 10. Optionally, a value for the number of queries that should
only have one label appended MAY 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 many labels early on has the most privacy benefit.
Another potential, optional mechanism for limiting the number of
queries is to assume that labels that begin with an underscore (_)
character do not represent privacy-relevant administrative
boundaries. For example, if the QNAME is "_25._tcp.mail.example.org"
and the algorithm has already searched for "mail.example.org", the
next query can be for all the underscore-prefixed names together,
namely "_25._tcp.mail.example.org".
When a resolver needs to send out a query, it will look for the
closest known delegation point in its cache. The number of not yet
exposed labels is the difference between this closest nameserver and
the incoming QNAME. The first MINIMISE_ONE_LAB labels will be
handled as described in Section 2. The number of labels that are
still not exposed now need to be divided proportionally over the
remaining iterations (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB). If the
not yet exposed labels can not be equally divided over the remaining
iterations, the remainder of the division should be added to the last
iterations. For example, when resolving a QNAME with 18 labels with
MAX_MINIMISE_COUNT set to 10 and MINIMISE_ONE_LAB set to 4, the
number of labels added per iteration are: 1,1,1,1,2,2,2,2,3,3.
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2.4. Stub and Forwarding Resolvers
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 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 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 from the cache.
(1a) For queries with a QTYPE for which the authority only lies
at the parent side (like QTYPE=DS) this is the NS RRset with
the owner matching the most labels with QNAME stripped by
one label. 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 QNAME. 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 QNAME, or if CHILD is one label shorter
than QNAME and the original QTYPE can only be at the parent side
(like QTYPE=DS), resolve the original query as normal starting
from ANCESTOR's name servers. Start over from step 0 if new
names need to be resolved as result of this answer, for example
when the answer contains a CNAME or DNAME [RFC6672] record.
(4) Otherwise, update the value of CHILD by adding the next relevant
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label or labels from QNAME to the start of CHILD. The number of
labels to add is discussed in Section 2.3.
(5) Look for a cache entry for the RRset at CHILD with the original
QTYPE. If the cached response code is NXDOMAIN and the resolver
has support for [RFC8020], the NXDOMAIN can be used in response
to the original query, and stop. If the cached response code is
NOERROR (including NODATA), go back to step 3. If the cached
response code is NXDOMAIN and the resolver does not support RFC
8020, go back to step 3.
(6) Query for CHILD with the selected QTYPE using one of 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 DNAME response. Proceed as if a DNAME is received for
the original query. Start over from step 0 to resolve the
new name based on the DNAME target.
(6c) All other NOERROR answers (including NODATA). Cache this
answer. Regardless of the answered RRset type, including
CNAMEs, continue with the algorithm from step 3 by building
the original QNAME.
(6d) An NXDOMAIN response. If the resolver supports RFC8020,
return an NXDOMAIN response to the original query and stop.
If the resolver does not support RFC8020, go to step 3.
(6e) A timeout or response with another RCODE. The
implementation may choose to retry step (6) with a different
ANCESTOR name server.
4. QNAME Minimisation Examples
As a first example, 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.
The following are more detailed examples of other queries with QNAME
minimisation, using other names and authoritative servers:
Cold cache, traditional resolution algorithm without QNAME
minimisation, request for MX record of a.b.example.org:
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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, with QNAME minimisation, 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 was the same as the QTYPE selected by the resolver to
hide the original QTYPE. Only one query for 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 compared to using any other QTYPE to
hide the original QTYPE.
Warm cache with only org delegation known, (example.org's NS RRset is
not known), 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. 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).
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QNAME minimisation can increase the number of queries based on the
incoming QNAME. This is described in Section 2.3. As described in
[devries-qnamemin], QNAME minimisation both increases the number of
DNS lookups by up to 26% and leads to up to 5% more failed lookups.
Filling the cache in a production resolver will soften that overhead.
6. Security Considerations
QNAME minimisation's benefits are clear in the case where you want to
decrease exposure of the queried name 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 authoritative name 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.
A resolver using QNAME minimisation can possibly be used to cause a
query storm to be sent to servers when resolving queries containing a
QNAME with a large number of labels, as described in Section 2.3.
That section proposes methods to significantly dampen the effects of
such attacks.
7. References
7.1. 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>.
[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>.
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[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>.
[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>.
[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>.
7.2. Informative References
[apnic-qnamemin]
Huston, G. and J. Damas, "Measuring Query Name
Minimization", September 2020, <https://indico.dns-
oarc.net/event/34/contributions/787/
attachments/777/1326/2020-09-28-oarc33-qname-
minimisation.pdf>.
[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>.
[RFC6672] Rose, S. and W. Wijngaards, "DNAME Redirection in the
DNS", RFC 6672, DOI 10.17487/RFC6672, June 2012,
<https://www.rfc-editor.org/info/rfc6672>.
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[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>.
[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>.
[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>.
[RFC9076] Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
DOI 10.17487/RFC9076, July 2021,
<https://www.rfc-editor.org/info/rfc9076>.
[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
The acknowledgements from RFC 7816 apply here. In addition, many
participants from the DNSOP Working Group helped with proposals for
simplification, clarification, and general editorial help.
Changes from RFC 7816
Changed in -07
* Stopped using the term "aggressive" for the method described
* Clarified some terminology
* More reorganization
Changed in -06
* Removed lots of text from when this was experimental
* Lots of reorganization
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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
* 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
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Email: paul.hoffman@icann.org
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