rfc9156v1.txt   rfc9156.txt 
Internet Engineering Task Force (IETF) S. Bortzmeyer Internet Engineering Task Force (IETF) S. Bortzmeyer
Request for Comments: 9156 AFNIC Request for Comments: 9156 AFNIC
Obsoletes: 7816 R. Dolmans Obsoletes: 7816 R. Dolmans
Category: Standards Track NLnet Labs Category: Standards Track NLnet Labs
ISSN: 2070-1721 P. Hoffman ISSN: 2070-1721 P. Hoffman
ICANN ICANN
October 2021 November 2021
DNS Query Name Minimisation to Improve Privacy DNS Query Name Minimisation to Improve Privacy
Abstract Abstract
This document describes a technique called "QNAME minimisation" to This document describes a technique called "QNAME minimisation" to
improve DNS privacy, where the DNS resolver no longer always sends improve DNS privacy, where the DNS resolver no longer always sends
the full original QNAME and original QTYPE to the upstream name the full original QNAME and original QTYPE to the upstream name
server. This document obsoletes RFC 7816. server. This document obsoletes RFC 7816.
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Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of (https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of include Revised BSD License text as described in Section 4.e of the
the Trust Legal Provisions and are provided without warranty as Trust Legal Provisions and are provided without warranty as described
described in the Simplified BSD License. in the Revised BSD License.
Table of Contents Table of Contents
1. Introduction and Background 1. Introduction and Background
1.1. Experience from RFC 7816 1.1. Experience from RFC 7816
1.2. Terminology 1.2. Terminology
2. Description of QNAME Minimisation 2. Description of QNAME Minimisation
2.1. QTYPE Selection 2.1. QTYPE Selection
2.2. QNAME Selection 2.2. QNAME Selection
2.3. Limit Number of Queries 2.3. Limitation of the Number of Queries
2.4. Stub and Forwarding Resolvers 2.4. Implementation by Stub and Forwarding Resolvers
3. Algorithm to Perform QNAME Minimisation 3. Algorithm to Perform QNAME Minimisation
4. QNAME Minimisation Examples 4. QNAME Minimisation Examples
5. Performance Considerations 5. Performance Considerations
6. Security Considerations 6. Security Considerations
7. References 7. References
7.1. Normative References 7.1. Normative References
7.2. Informative References 7.2. Informative References
Acknowledgments Acknowledgments
Authors' Addresses Authors' Addresses
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still allowed. The authority of NS records lies at the child side. still allowed. The authority of NS records lies at the child side.
The parent side of the delegation will answer using a referral, like The parent side of the delegation will answer using a referral, like
it will do for queries with other QTYPEs. Using the NS QTYPE it will do for queries with other QTYPEs. Using the NS QTYPE
therefore has no added value over other QTYPEs. therefore has no added value over other QTYPEs.
The QTYPE to use while minimising queries can be any possible data 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 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, 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 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 relation between the incoming QTYPE and the selection of the QTYPE to
use while minimising. Good candidates are to always use the A or use while minimising. The A or AAAA QTYPEs are always good
AAAA QTYPEs because these are the least likely to raise issues in DNS candidates to use because these are the least likely to raise issues
software and middleboxes that do not properly support all QTYPEs. in DNS software and middleboxes that do not properly support all
QTYPE=A or QTYPE=AAAA queries will also blend into traffic from QTYPEs. QTYPE=A or QTYPE=AAAA queries will also blend into traffic
nonminimising resolvers, making it in some cases harder to observe from nonminimising resolvers, making it in some cases harder to
that the resolver is using QNAME minimisation. Using a QTYPE that observe that the resolver is using QNAME minimisation. Using a QTYPE
occurs most in incoming queries will slightly reduce the number of that occurs most in incoming queries will slightly reduce the number
queries, as there is no extra check needed for delegations on non- of queries, as there is no extra check needed for delegations on non-
apex records. apex records.
2.2. QNAME Selection 2.2. QNAME Selection
The minimising resolver works perfectly when it knows the zone cut The minimising resolver works perfectly when it knows the zone cut
(zone cuts are described in Section 6 of [RFC2181]). But zone cuts (zone cuts are described in Section 6 of [RFC2181]). But zone cuts
do not necessarily exist at every label boundary. In the name 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 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 "foo" and "bar" but not between "bar" and "example". So, assuming
that the resolver already knows the name servers of example, when it 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?", 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 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 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 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. the example name servers again with one more label, and so on.
(Section 3 describes this algorithm in deeper detail.) (Section 3 describes this algorithm in deeper detail.)
2.3. Limit Number of Queries 2.3. Limitation of the Number of Queries
When using QNAME minimisation, the number of labels in the received When using QNAME minimisation, the number of labels in the received
QNAME can influence the number of queries sent from the resolver. QNAME can influence the number of queries sent from the resolver.
This opens an attack vector and can decrease performance. Resolvers This opens an attack vector and can decrease performance. Resolvers
supporting QNAME minimisation MUST implement a mechanism to limit the supporting QNAME minimisation MUST implement a mechanism to limit the
number of outgoing queries per user request. number of outgoing queries per user request.
Take for example an incoming QNAME with many labels, like Take for example an incoming QNAME with many labels, like
www.host.group.department.example.com, where www.host.group.department.example.com, where
host.group.department.example.com is hosted on example.com's name host.group.department.example.com is hosted on example.com's name
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the incoming QNAME. The first MINIMISE_ONE_LAB labels will be the incoming QNAME. The first MINIMISE_ONE_LAB labels will be
handled as described in Section 2. The number of labels that are handled as described in Section 2. The number of labels that are
still not exposed now need to be divided proportionally over the still not exposed now need to be divided proportionally over the
remaining iterations (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB). If the remaining iterations (MAX_MINIMISE_COUNT - MINIMISE_ONE_LAB). If the
not-yet-exposed labels cannot be equally divided over the remaining not-yet-exposed labels cannot be equally divided over the remaining
iterations, the remainder of the division should be added to the last iterations, the remainder of the division should be added to the last
iterations. For example, when resolving a QNAME with 18 labels with 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 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. number of labels added per iteration are: 1,1,1,1,2,2,2,2,3,3.
2.4. Stub and Forwarding Resolvers 2.4. Implementation by Stub and Forwarding Resolvers
Stub and forwarding resolvers MAY implement QNAME minimisation. Stub and forwarding resolvers MAY implement QNAME minimisation.
Minimising queries that will be sent to an upstream resolver do not Minimising queries that will be sent to an upstream resolver does not
help in hiding data from the upstream resolver because all help in hiding data from the upstream resolver because all
information will end up there anyway. It might however limit the information will end up there anyway. It might however limit the
data exposure between the upstream resolver and the authoritative data exposure between the upstream resolver and the authoritative
name server in the situation where the upstream resolver does not name server in the situation where the upstream resolver does not
support QNAME minimisation. Using QNAME minimisation in a stub or support QNAME minimisation. Using QNAME minimisation in a stub or
forwarding resolver that does not have a mechanism to find and cache forwarding resolver that does not have a mechanism to find and cache
zone cuts will drastically increase the number of outgoing queries. zone cuts will drastically increase the number of outgoing queries.
3. Algorithm to Perform QNAME Minimisation 3. Algorithm to Perform QNAME Minimisation
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4. QNAME Minimisation Examples 4. QNAME Minimisation Examples
As a first example, assume that a resolver receives a request to As a first example, assume that a resolver receives a request to
resolve foo.bar.baz.example. Assume that the resolver already knows resolve foo.bar.baz.example. Assume that the resolver already knows
that ns1.nic.example is authoritative for .example and that the that ns1.nic.example is authoritative for .example and that the
resolver does not know a more specific authoritative name server. It resolver does not know a more specific authoritative name server. It
will send the query with QNAME=baz.example and the QTYPE selected to will send the query with QNAME=baz.example and the QTYPE selected to
hide the original QTYPE to ns1.nic.example. 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:
+=======+=================+=========================+======+ +=======+=================+=========================+======+
| QTYPE | QNAME | TARGET | NOTE | | QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+======+ +=======+=================+=========================+======+
| MX | a.b.example.org | root name server | | | MX | a.b.example.org | root name server | |
+-------+-----------------+-------------------------+------+ +-------+-----------------+-------------------------+------+
| MX | a.b.example.org | org name server | | | MX | a.b.example.org | org name server | |
+-------+-----------------+-------------------------+------+ +-------+-----------------+-------------------------+------+
| MX | a.b.example.org | example.org name server | | | MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------+ +-------+-----------------+-------------------------+------+
Table 1 Table 1: Cold Cache, Traditional Resolution Algorithm
without QNAME Minimisation, Request for MX Record of
a.b.example.org
Cold cache, with QNAME minimisation, request for MX record of The following are more detailed examples of requests for an MX record
a.b.example.org, using the A QTYPE to hide the original QTYPE: 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 | | QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+============+ +=======+=================+=========================+============+
| A | org | root name server | | | A | org | root name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| A | example.org | org name server | | | A | example.org | org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| A | b.example.org | example.org name server | | | A | b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| A | a.b.example.org | example.org name server | "a" may be | | A | a.b.example.org | example.org name server | "a" may be |
| | | | delegated | | | | | delegated |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| MX | a.b.example.org | example.org name server | | | MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
Table 2 Table 2: Cold Cache with QNAME Minimisation
Note that, in the above example, one query would have been saved if 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 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 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 have been needed if the original QTYPE would have been A. Using the
most-used QTYPE to hide the original QTYPE therefore slightly reduces most-used QTYPE to hide the original QTYPE therefore slightly reduces
the number of outgoing queries compared to using any other QTYPE to the number of outgoing queries compared to using any other QTYPE to
hide the original QTYPE. 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 | | QTYPE | QNAME | TARGET | NOTE |
+=======+=================+=========================+============+ +=======+=================+=========================+============+
| A | example.org | org name server | | | A | example.org | org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| A | b.example.org | example.org name server | | | A | b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| A | a.b.example.org | example.org name server | "a" may be | | A | a.b.example.org | example.org name server | "a" may be |
| | | | delegated | | | | | delegated |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
| MX | a.b.example.org | example.org name server | | | MX | a.b.example.org | example.org name server | |
+-------+-----------------+-------------------------+------------+ +-------+-----------------+-------------------------+------------+
Table 3 Table 3: Warm Cache with QNAME Minimisation
5. Performance Considerations 5. Performance Considerations
The main goal of QNAME minimisation is to improve privacy by sending The main goal of QNAME minimisation is to improve privacy by sending
less data. However, it may have other advantages. For instance, if less data. However, it may have other advantages. For instance, if
a resolver sends a root name server queries for A.example followed by 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, B.example followed by C.example, the result will be three NXDOMAINs,
since .example does not exist in the root zone. When using QNAME since .example does not exist in the root zone. When using QNAME
minimisation, the resolver would send only one question (for .example minimisation, the resolver would send only one question (for .example
itself) to which they could answer NXDOMAIN. The resolver can cache itself) to which they could answer NXDOMAIN. The resolver can cache
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[devries-qnamemin], QNAME minimisation both increases the number of [devries-qnamemin], QNAME minimisation both increases the number of
DNS lookups by up to 26% and leads to up to 5% more failed lookups. 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. Filling the cache in a production resolver will soften that overhead.
6. Security Considerations 6. Security Considerations
QNAME minimisation's benefits are clear in the case where you want to QNAME minimisation's benefits are clear in the case where you want to
decrease exposure of the queried name to the authoritative name decrease exposure of the queried name to the authoritative name
server. But minimising the amount of data sent also, in part, 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 addresses the case of a wire sniffer as well as the case of privacy
invasion by the authoritative name servers. (Encryption is of course invasion by the authoritative name servers. Encryption is of course
a better defense against wire sniffers, but, unlike QNAME a better defense against wire sniffers, but, unlike QNAME
minimisation, it changes the protocol and cannot be deployed minimisation, it changes the protocol and cannot be deployed
unilaterally. Also, the effect of QNAME minimisation on wire unilaterally. Also, the effect of QNAME minimisation on wire
sniffers depends on whether the sniffer is on the DNS path.) sniffers depends on whether the sniffer is on the DNS path.
QNAME minimisation offers no protection against the recursive QNAME minimisation offers no protection against the recursive
resolver, which still sees the full request coming from the stub resolver, which still sees the full request coming from the stub
resolver. resolver.
A resolver using QNAME minimisation can possibly be used to cause a A resolver using QNAME minimisation can possibly be used to cause a
query storm to be sent to servers when resolving queries containing 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. QNAME with a large number of labels, as described in Section 2.3.
That section proposes methods to significantly dampen the effects of That section proposes methods to significantly dampen the effects of
such attacks. such attacks.
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