Internet DRAFT - draft-ietf-dnsop-refuse-any
draft-ietf-dnsop-refuse-any
Network Working Group J. Abley
Internet-Draft Afilias
Updates: 1034, 1035 (if approved) O. Gudmundsson
Intended status: Standards Track M. Majkowski
Expires: February 15, 2019 Cloudflare Inc.
E. Hunt
ISC
August 14, 2018
Providing Minimal-Sized Responses to DNS Queries that have QTYPE=ANY
draft-ietf-dnsop-refuse-any-07
Abstract
The Domain Name System (DNS) specifies a query type (QTYPE) "ANY".
The operator of an authoritative DNS server might choose not to
respond to such queries for reasons of local policy, motivated by
security, performance or other reasons.
The DNS specification does not include specific guidance for the
behaviour of DNS servers or clients in this situation. This document
aims to provide such guidance.
This document updates RFC 1034 and RFC 1035.
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 February 15, 2019.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivations for Use of ANY Queries . . . . . . . . . . . . . 3
3. General Approach . . . . . . . . . . . . . . . . . . . . . . 4
4. Behaviour of DNS Responders . . . . . . . . . . . . . . . . . 4
4.1. Answer with a Subset of Available RRSets . . . . . . . . 5
4.2. Answer with a Synthesised HINFO RRSet . . . . . . . . . . 5
4.3. Answer with Best Guess as to Intention . . . . . . . . . 6
4.4. Behaviour with TCP Transport . . . . . . . . . . . . . . 6
5. Behaviour of DNS Initiators . . . . . . . . . . . . . . . . . 6
6. HINFO Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Updates to RFC 1034 and RFC 1035 . . . . . . . . . . . . . . 7
8. Implementation Experience . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 9
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Editorial Notes . . . . . . . . . . . . . . . . . . 10
A.1. Change History . . . . . . . . . . . . . . . . . . . . . 10
A.1.1. draft-ietf-dnsop-refuse-any-07 . . . . . . . . . . . 10
A.1.2. draft-ietf-dnsop-refuse-any-06 . . . . . . . . . . . 10
A.1.3. draft-ietf-dnsop-refuse-any-05 . . . . . . . . . . . 10
A.1.4. draft-ietf-dnsop-refuse-any-04 . . . . . . . . . . . 10
A.1.5. draft-ietf-dnsop-refuse-any-03 . . . . . . . . . . . 10
A.1.6. draft-ietf-dnsop-refuse-any-02 . . . . . . . . . . . 10
A.1.7. draft-ietf-dnsop-refuse-any-01 . . . . . . . . . . . 11
A.1.8. draft-ietf-dnsop-refuse-any-00 . . . . . . . . . . . 11
A.1.9. draft-jabley-dnsop-refuse-any-01 . . . . . . . . . . 11
A.1.10. draft-jabley-dnsop-refuse-any-00 . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
The Domain Name System (DNS) specifies a query type (QTYPE) "ANY".
The operator of an authoritative DNS server might choose not to
respond to such queries for reasons of local policy, motivated by
security, performance or other reasons.
The DNS specification [RFC1034] [RFC1035] does not include specific
guidance for the behaviour of DNS servers or clients in this
situation. This document aims to provide such guidance.
1.1. Terminology
This document uses terminology specific to the Domain Name System
(DNS), descriptions of which can be found in [RFC7719].
In this document, "ANY Query" refers to a DNS meta-query with
QTYPE=ANY. An "ANY Response" is a response to such a query.
In this document, "conventional ANY response" means an ANY response
that is constructed in accordance with the algorithm documented in
section 4.3.2 of [RFC1034] and specifically without implementing any
of the mechanisms described in this document.
In an exchange of DNS messages between two hosts, this document
refers to the host sending a DNS request as the initiator, and the
host sending a DNS response as the responder.
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. Motivations for Use of ANY Queries
ANY queries are legitimately used for debugging and checking the
state of a DNS server for a particular name.
ANY queries are sometimes used as a attempt to reduce the number of
queries needed to get information, e.g. to obtain MX, A and AAAA
RRSets for a mail domain in a single query. There is no documented
guidance available for this use case, however, and some
implementations have been observed not to function as perhaps their
developers expected. Implementers that assume that an ANY query will
ultimately be received by an authoritative server and will fetch all
existing RRSets, should include a fallback mechanism to use when that
does not happen.
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ANY queries are frequently used to exploit the amplification
potential of DNS servers/resolvers using spoofed source addresses and
UDP transport (see [RFC5358]). Having the ability to return small
responses to such queries makes DNS servers less attractive
amplifiers.
ANY queries are sometimes used to help mine authoritative-only DNS
servers for zone data, since they are expected to return all RRSets
for a particular query name. If a DNS operator prefers to reduce the
potential for information leaks, they might choose not to send large
ANY responses.
Some authoritative-only DNS server implementations require additional
processing in order to send a conventional ANY response, and avoiding
that processing expense might be desirable.
3. General Approach
This proposal provides a mechanism for an authority server to signal
that conventional ANY queries are not supported for a particular
QNAME, and to do so in such a way that is both compatible with and
triggers desirable behaviour by unmodified clients (e.g. DNS
resolvers).
Alternative proposals for dealing with ANY queries have been
discussed. One approach proposed using a new RCODE to signal that an
authoritative server did not answer ANY queries in the standard way.
This approach was found to have an undesirable effect on both
resolvers and authoritative-only servers; resolvers receiving an
unknown RCODE would re-send the same query to all available
authoritative servers, rather than suppress future such ANY queries
for the same QNAME.
This proposal avoids that outcome by returning a non-empty RRSet in
the ANY response, providing resolvers with something to cache and
effectively suppressing repeat queries to the same or different
authority servers.
4. Behaviour of DNS Responders
Below are the three different modes of behaviour by DNS responders
when processing queries with QNAMEs that exist, QCLASS=IN and
QTYPE=ANY. Operators/Implementers are free to choose whichever
mechanism best suits their environment.
1. A DNS responder can choose to select one or a larger subset of
the available RRSets at the QNAME.
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2. A DNS responder can return a synthesised HINFO resource record.
See Section 6 for discussion of the use of HINFO.
3. Resolver can try to give out the most likely records the
requester wants. This is not always possible and the result
might well be a large response.
Except as described below in this section, the DNS responder MUST
follow the standard algorithms when constructing a response.
4.1. Answer with a Subset of Available RRSets
A DNS responder which receives an ANY query MAY decline to provide a
conventional ANY response, or MAY instead send a response with a
single RRSet (or a larger subset of available RRSets) in the answer
section.
The RRSets returned in the answer section of the response MAY consist
of a single RRSet owned by the name specified in the QNAME. Where
multiple RRSets exist, the responder SHOULD choose a small subset of
those avialable to reduce the amplification potential of the
response.
If the zone is signed, appropriate RRSIG records MUST be included in
the answer.
Note that this mechanism does not provide any signalling to indicate
to a client that an incomplete subset of the available RRSets has
been returned.
4.2. Answer with a Synthesised HINFO RRSet
If there is no CNAME present at the owner name matching the QNAME,
the resource record returned in the response MAY instead be
synthesised, in which case a single HINFO resource record SHOULD be
returned. The CPU field of the HINFO RDATA SHOULD be set to RFCXXXX
[note to RFC Editor, replace with RFC number assigned to this
document]. The OS field of the HINFO RDATA SHOULD be set to the null
string to minimize the size of the response.
The TTL encoded for the synthesised HINFO RR SHOULD be chosen by the
operator of the DNS responder to be large enough to suppress frequent
subsequent ANY queries from the same initiator with the same QNAME,
understanding that a TTL that is too long might make policy changes
relating to ANY queries difficult to change in the future. The
specific value used is hence a familiar balance when choosing TTL for
any RR in any zone, and be specified according to local policy.
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If the DNS query includes DO=1 and the QNAME corresponds to a zone
that is known by the responder to be signed, a valid RRSIG for the
RRSets in the answer (or authority if answer is empty) section MUST
be returned. In the case of DO=0, the RRSIG SHOULD be omitted.
A system that receives an HINFO response SHOULD NOT infer that the
response was generated according to this specification and apply any
special processing of the response, since in general it is not
possible to tell with certainty whether the HINFO RRSet received was
synthesised. In particular, systems SHOULD NOT rely upon the HINFO
RDATA described in this seection to distinguish between synthesised
and non-synthesised HINFO RRSets.
4.3. Answer with Best Guess as to Intention
In some cases it is possible to guess what the initiator wants in the
answer (but not always). Some implementations have implemented the
spirit of this document by returning all RRSets of RRType CNAME, MX,
A and AAAA that are present at the owner name but suppressing others.
This heuristic seems to work well in practice, satisfying the needs
of some applications whilst suppressing other RRSets such as TXT and
DNSKEY that can often contribute to large responses. Whilst some
applications may be satisfied by this behaviour, the resulting
responses in the general case are larger than the approaches
described in Section 4.1 and Section 4.2.
As before, if the zone is signed and the DO bit is set on the
corresponding query, an RRSIG RRSet MUST be included in the response.
4.4. Behaviour with TCP Transport
A DNS responder MAY behave differently when processing ANY queries
received over different transport, e.g. by providing a conventional
ANY response over TCP whilst using one of the other mechanisms
specified in this document in the case where a query was received
using UDP.
Implementers SHOULD provide configuration options to allow operators
to specify different behaviour over UDP and TCP.
5. Behaviour of DNS Initiators
A DNS initiator which sends a query with QTYPE=ANY and receives a
response containing an HINFO resource record or a single RRset, as
described in Section 4, MAY cache the response in the normal way.
Such cached resource records SHOULD be retained in the cache
following normal caching semantics, as it would with any other
response received from a DNS responder.
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A DNS initiator MAY suppress queries with QTYPE=ANY in the event that
the local cache contains a matching HINFO resource record with
RDATA.CPU field, as described in Section 4. A DNS initiator MAY
instead respond to such queries with the contents of the local cache
in the usual way.
6. HINFO Considerations
It is possible that the synthesised HINFO RRSet in an ANY response,
once cached by the initiator, might suppress subsequent queries from
the same initiator with QTYPE=HINFO. Thus the use of HINFO in this
proposal would hence have effectively mask the HINFO RRSet present in
the zone.
Authority-server operators who serve zones that rely upon
conventional use of the HINFO RRTYPE SHOULD sensibly choose the
"single RRset" method described in this document or select another
type.
The HINFO RRTYPE is believed to be rarely used in the DNS at the time
of writing, based on observations made at recursive servers,
authority servers and in passive DNS.
7. Updates to RFC 1034 and RFC 1035
This document extends the specification for processing ANY queries
described in section 4.3.2 of [RFC1034].
It is important to note that returning a subset of available RRSets
when processing an ANY query is legitimate and consistent with
[RFC1035]; it can be argued that ANY does not always mean ALL, as
used in section 3.2.3 of [RFC1035]. The main difference here is that
the TC bit SHOULD NOT be set on the response indicating that this is
not a complete answer.
This document describes optional behaviour for both DNS initiators
and responders, and implementation of the guidance provided by this
document is OPTIONAL.
RRSIG queries (i.e. queries with QTYPE=RRSIG) are similar to ANY
queries in the sense that they have the potential to generate large
responses as well as extra work for the responders that process them,
e.g. in the case where signatures are generated on-the-fly. RRSIG
RRSets are not usually obtained using such explicit queries, but are
rather included in the responses for other RRSets that the RRSIGs
cover. This document does not specify appropriate behaviour for
RRSIG queries, but note that future such advice might well benefit
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from consistency with and experience of the approaches for ANY
queries described here.
8. Implementation Experience
In October 2015 Cloudflare Authoritative Name server implementation
implemented the HINFO response. A few minor problems were reported
and have since been resolved.
An implementation of the subset-mode response to ANY queries was
implemented in NSD 4.1 in 2016.
An implementation of a single RRSet response to an ANY query was made
for BIND9 by Tony Finch, and that functionality was subsequently made
available in production releases starting in BIND 9.11.
9. Security Considerations
Queries with QTYPE=ANY are frequently observed as part of reflection
attacks, since a relatively small query can be used to elicit a large
response; this is a desirable characteristic if the goal is to
maximize the amplification potential of a DNS server as part of a
volumetric attack. The ability of a DNS operator to suppress such
responses on a particular server makes that server a less useful
amplifier.
The optional behaviour described in this document to reduce the size
of responses to queries with QTYPE=ANY is compatible with the use of
DNSSEC by both initiator and responder.
10. IANA Considerations
The IANA is requested to update the Resource Record (RR) TYPEs
Registry [1] entry as follows:
+------+-------+-------------------------------+--------------------+
| Type | Value | Meaning | Reference |
+------+-------+-------------------------------+--------------------+
| * | 255 | A request for some or all | [RFC1035][RFC6895] |
| | | records the server has | [This Document] |
| | | available | |
+------+-------+-------------------------------+--------------------+
11. Acknowledgements
David Lawrence provided valuable observations and concrete
suggestions. Jeremy Laidman helped make the document better. Tony
Finch realized that this document was valuable and implemented it
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while under attack. Richard Gibson identified areas where more
detail and accuracy was useful. A large number of other people also
provided comments and suggestions we thank them all for the feedback.
12. References
12.1. 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>.
[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>.
12.2. Informative References
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
DOI 10.17487/RFC5358, October 2008,
<https://www.rfc-editor.org/info/rfc5358>.
[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>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.
12.3. URIs
[1] http://www.iana.org/assignments/dns-parameters/dns-
parameters.xhtml#dns-parameters-4
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Appendix A. Editorial Notes
This section (and sub-sections) to be removed prior to publication.
A.1. Change History
A.1.1. draft-ietf-dnsop-refuse-any-07
Address AD's concerns: more colour to describe updates to 1034/1035
in the abstract; don't rely upon HINFO RDATA formatting; language
cleanup around guess intent. Add Evan as author (originator of the
"choose one record" response idea).
A.1.2. draft-ietf-dnsop-refuse-any-06
Update RFC 1034 as well as RFC 1035; define the term "conventional
ANY response"; soften and qualify ANY does not mean ALL; note that
the subset mode response lacks signalling.
A.1.3. draft-ietf-dnsop-refuse-any-05
Minor editorial changes. Soften advice on RRSIG queries. Version
bump.
A.1.4. draft-ietf-dnsop-refuse-any-04
These are the changes requested during WGLC. The title has been
updated for readability The behavior section now contains description
of three different approaches in order of preference. Text added on
behavior over TCP. The document is clear in how it updates from
RFC1035. Minor adjustments for readability and remove redundancy.
A.1.5. draft-ietf-dnsop-refuse-any-03
Change section name to "Updates to RFC1034", few minor grammar
changes suggested by Matthew Pounsett and Tony Finch.
Text clarifications, reflecting experience, added implementation
experience.
A.1.6. draft-ietf-dnsop-refuse-any-02
Added suggestion to call out RRSIG is optional when DO=0.
Number of text suggestions from Jeremy Laidman.
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A.1.7. draft-ietf-dnsop-refuse-any-01
Add IANA Considerations
A.1.8. draft-ietf-dnsop-refuse-any-00
Re-submitted with a different name following adoption at the dnsop WG
meeting convened at IETF 94.
A.1.9. draft-jabley-dnsop-refuse-any-01
Make signing of RRSets in answers from signed zones mandatory.
Document the option of returning an existing RRSet in place of a
synthesised one.
A.1.10. draft-jabley-dnsop-refuse-any-00
Initial draft circulated for comment.
Authors' Addresses
Joe Abley
Afilias
300-184 York Street
London, ON N6A 1B5
Canada
Phone: +1 519 670 9327
Email: jabley@afilias.info
Olafur Gudmundsson
Cloudflare Inc.
Email: olafur+ietf@cloudflare.com
Marek Majkowski
Cloudflare Inc.
Email: marek@cloudflare.com
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Evan Hunt
ISC
950 Charter St
Redwood City, CA 94063
USA
Email: each@isc.org
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