Internet Engineering Task Force (IETF) S. Bortzmeyer

Internet Engineering Task Force (IETF) S. Bortzmeyer
Request for Comments: 9156 AFNIC
Obsoletes: 7816 R. Dolmans
Category: Standards Track NLnet Labs
ISSN: 2070-1721 P. Hoffman
ICANN
November 2021

DNS Query Name Minimisation to Improve Privacy

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

This is an Internet Standards Track document.

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). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9156.

Copyright Notice

Copyright (c) 2021 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. 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction and Background
1.1. Experience from RFC 7816
1.2. Terminology
2. Description of QNAME Minimisation
2.1. QTYPE Selection
2.2. QNAME Selection
2.3. Limitation of the Number of Queries
2.4. Implementation by Stub and Forwarding Resolvers
3. Algorithm to Perform QNAME Minimisation
4. QNAME Minimisation Examples
5. Performance Considerations
6. Security Considerations
7. References
7.1. Normative References
7.2. Informative References
Acknowledgments
Authors' Addresses

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
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]. [RFC7816] was categorised
"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.

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 name server. 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 name server
that is authoritative for the record requested by the client. All
other name servers 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 name server is not
able to authoritatively answer the records the client is looking for.
Sending the full QNAME and original QTYPE to these name servers
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 name server 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 name
server is known to be authoritative

2.1. QTYPE Selection

Note that this document relaxes the recommendation in [RFC7816] 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 Section 3.1 of [RFC6895]) 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. The A or AAAA QTYPEs are always good
candidates to use 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 nonminimising 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, so 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. Limitation of the 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].

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 name server 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 cannot 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.

2.4. Implementation by 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
name server in the situation where the upstream resolver does not
support QNAME minimisation. Using QNAME minimisation in a stub or
forwarding resolver 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 a 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
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
[RFC8020], 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.

+=======+=================+=========================+======+
| QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+======+
| MX | a.b.example.org | root name server | |
+-------+-----------------+-------------------------+------+
| MX | a.b.example.org | org name server | |
+-------+-----------------+-------------------------+------+
| MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------+

Table 1: Cold Cache, Traditional Resolution Algorithm
without QNAME Minimisation, Request for MX Record of
a.b.example.org

The following are more detailed examples of requests for an MX record
of a.b.example.org with QNAME minimisation, using A QTYPE to hide the
original QTYPE and using other names and authoritative servers:

+=======+=================+=========================+============+
| QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+============+
| A | org | root name server | |
+-------+-----------------+-------------------------+------------+
| A | example.org | org name server | |
+-------+-----------------+-------------------------+------------+
| A | b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+
| A | a.b.example.org | example.org name server | "a" may be |
| | | | delegated |
+-------+-----------------+-------------------------+------------+
| MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+

Table 2: Cold Cache with QNAME Minimisation

Note that, in the 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.

+=======+=================+=========================+============+
| QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+============+
| A | example.org | org name server | |
+-------+-----------------+-------------------------+------------+
| A | b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+
| A | a.b.example.org | example.org name server | "a" may be |
| | | | delegated |
+-------+-----------------+-------------------------+------------+
| MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+

Table 3: Warm Cache with QNAME Minimisation

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 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 be counterintuitively
less than the number of queries under the traditional iteration (as
described in the DNS standard).

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>.

[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]
de Vries, W., Scheitle, Q., Müller, M., Toorop, W.,
Dolmans, R., and R. van Rijswijk-Deij, "A First Look at
QNAME Minimization in the Domain Name System", March 2019,
<https://nlnetlabs.nl/downloads/publications/
devries2019.pdf>.

[dnsthought-qnamemin]
"Qname Minimisation", October 2021,
<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>.

[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 acknowledgments from RFC 7816 apply here. In addition, many
participants from the DNSOP Working Group helped with proposals for
simplification, clarification, and general editorial help.

Authors' Addresses

Stephane Bortzmeyer
AFNIC
1, rue Stephenson
78180 Montigny-le-Bretonneux
France

Phone: +33 1 39 30 83 46
Email: [email protected]
URI: https://www.afnic.fr/

Ralph Dolmans
NLnet Labs

Email: [email protected]

Paul Hoffman
ICANN

Email: [email protected]