wdiff rfc8482.alt-original rfc8482.txt

Network Working Group
Internet Engineering Task Force (IETF) J. Abley
Internet-Draft
Request for Comments: 8482 Afilias
Updates: 1034, 1035 (if approved) O. Gudmundsson
Intended status:
Category: Standards Track M. Majkowski
Expires: February 15, 2019
ISSN: 2070-1721 Cloudflare Inc.
E. Hunt
ISC
August 14, 2018
January 2019

Providing Minimal-Sized Responses to DNS Queries that have 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 performance, or other reasons.

The DNS specification does not include specific guidance for the
behaviour
behavior of DNS servers or clients in this situation. This document
aims to provide such guidance.

This document updates RFC RFCs 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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained 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.
https://www.rfc-editor.org/info/rfc8482.

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
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 ....................................................3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 ................................................3
2. Motivations for Use of ANY Queries . . . . . . . . . . . . . 3 ..............................3
3. General Approach . . . . . . . . . . . . . . . . . . . . . . 4 ................................................4
4. Behaviour Behavior of DNS Responders . . . . . . . . . . . . . . . . . 4 ......................................5
4.1. Answer with a Subset of Available RRSets . . . . . . . . 5 RRsets ...................5
4.2. Answer with a Synthesised Synthesized HINFO RRSet . . . . . . . . . . 5 RRset ......................5
4.3. Answer with Best Guess as to Intention . . . . . . . . . 6 .....................6
4.4. Behaviour with TCP Transport . . . . . . . . . . . . . . 6 Considerations ...................................6
5. Behaviour Behavior of DNS Initiators . . . . . . . . . . . . . . . . . 6 ......................................7
6. HINFO Considerations . . . . . . . . . . . . . . . . . . . . 7 ............................................7
7. Updates to RFC RFCs 1034 and RFC 1035 . . . . . . . . . . . . . . 7 ...................................7
8. Implementation Experience . . . . . . . . . . . . . . . . . . 8 .......................................8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8 .........................................8
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 ............................................9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. .....................................................9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. ......................................9
11.2. Informative References . . . . . . . . . . . . . . . . . 9
12.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9 ....................................9
Acknowledgements ..................................................10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 ................................................10

1. Introduction

The DNS specification [RFC1034] [RFC1035] does not include specific
guidance for the behaviour behavior 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]. [RFC8499].

[RFC1035] defined type 255 to be "*". However, DNS implementations
commonly use the keyword "ANY" to refer to that type code; this
document follows that common usage.

In this document, "ANY Query" query" refers to a DNS meta-query with
QTYPE=ANY. An "ANY Response" 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
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, "initiator" and the
host sending a DNS response as the responder. "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 an attempt to reduce the number of
queries needed to get information, e.g. e.g., to obtain MX, A A, and AAAA
RRSets
resource record sets (RRsets) for a mail domain in a single query. There
However, 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 If implementers assume that an ANY query will
ultimately be received by an authoritative server and will fetch all
existing RRSets, RRsets, they should include a fallback mechanism to use when
that does not happen.

ANY queries are frequently used to exploit the amplification
potential of DNS servers/resolvers servers and 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 RRsets
for a particular query name. If a DNS operator prefers operators prefer 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 response; avoiding
that processing expense might be desirable.

3. General Approach

This proposal provides a mechanism for an authority authoritative DNS server to
signal that conventional ANY queries are not supported for a
particular
QNAME, and to do QNAME. It does so in such a way that is both compatible with
and triggers desirable behaviour behavior by unmodified clients (e.g. (e.g., DNS
resolvers).

Alternative proposals for dealing with ANY queries have been
discussed. One approach proposed proposes 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 resend the same query to all available
authoritative servers, servers rather than suppress future such ANY queries for the
same QNAME.

This

The proposal described in this document avoids that outcome by
returning a non-empty RRSet RRset in the ANY response, providing which provides
resolvers with something to cache and effectively suppressing suppresses repeat
queries to the same or different
authority authoritative DNS servers.

4. Behaviour Behavior of DNS Responders

Below are the three different modes of behaviour behavior by DNS responders
when processing queries with QNAMEs that exist, QCLASS=IN QCLASS=IN, and
QTYPE=ANY. Operators/Implementers Operators and 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 RRsets at the QNAME.

2. A DNS responder can return a synthesised synthesized HINFO resource record.
See Section 6 for discussion of the use of HINFO.

3. Resolver A resolver can try to give out the most likely records the
requester wants. This is not always possible 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 RRsets

A DNS responder which that receives an ANY query MAY decline to provide a
conventional ANY response, response or MAY instead send a response with a
single RRSet RRset (or a larger subset of available RRSets) RRsets) in the answer
section.

The RRSets RRsets returned in the answer section of the response MAY consist
of a single RRSet RRset owned by the name specified in the QNAME. Where
multiple RRSets RRsets exist, the responder SHOULD choose a small subset of
those avialable available 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 signaling to indicate
to a client that an incomplete subset of the available RRSets RRsets has
been returned.

4.2. Answer with a Synthesised Synthesized HINFO RRSet 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
synthesized. In this 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].
"RFC8482". 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 synthesized HINFO RR resource record 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 SHOULD be configurable by the
operator of the nameserver according to local policy, based on the
familiar balance when considerations involved in choosing a TTL value for any RR
resource record in any zone, and be specified according to local policy. zone.

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
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 response because, in general general, it is not
possible to tell with certainty whether the HINFO RRSet RRset received was
synthesised.
synthesized. In particular, systems SHOULD NOT rely upon the HINFO
RDATA described in this seection section to distinguish between synthesised synthesized
and non-synthesised non-synthesized HINFO RRSets. RRsets.

4.3. Answer with Best Guess as to Intention

In some cases 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 RRsets of RRType RRTYPE CNAME,
MX,
A A, and AAAA that are present at the owner name but while suppressing
others. This heuristic seems to work well in practice, satisfying practice; it satisfies
the needs of some applications whilst suppressing other RRSets RRsets such
as TXT and DNSKEY that can often contribute to large responses.
Whilst some applications may be satisfied by this behaviour, behavior, the
resulting responses in the general case are larger than in the
approaches described in Section Sections 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 RRset MUST be included in the response.

4.4. Behaviour with TCP Transport Considerations

A DNS responder MAY behave differently when processing ANY queries
received over different transport, e.g. transports, 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 MAY provide configuration options to allow operators to
specify different behaviour behavior over UDP and TCP. different transports.

5. Behaviour Behavior of DNS Initiators

A DNS initiator which that 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.

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, the
CPU field of the HINFO RDATA, 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 synthesized HINFO RRSet RRset in an ANY response,
once cached by the initiator, might suppress subsequent queries from
the same initiator with QTYPE=HINFO. Thus Thus, the use of HINFO in this
proposal would hence have effectively mask the HINFO RRSet RRset present in the zone.

Authority-server operators

Operators of authoritative servers 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 in passive DNS and at
recursive servers,
authority servers and in passive DNS. authoritative DNS servers.

7. Updates to RFC RFCs 1034 and RFC 1035

This document extends the specification for processing ANY queries
described in section Section 4.3.2 of [RFC1034].

It is important to note that returning a subset of available RRSets 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 Section 3.2.3 of [RFC1035]. The main difference here is that
the TC bit SHOULD NOT be set on in the response response, thus indicating that
this is not a complete answer.

This document describes optional behaviour behavior for both DNS initiators and responders, and
responders; implementation of the guidance provided by this document
is OPTIONAL.

RRSIG queries (i.e. (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.
e.g., in the case where signatures are generated on-the-fly. on the fly. RRSIG
RRSets
RRsets are not usually obtained using such explicit queries, queries but are
rather included in the responses for other RRSets RRsets that the RRSIGs
cover. This document does not specify appropriate behaviour behavior for RRSIG queries, but note that
queries; however, future such advice might well benefit from
consistency with and experience of with the approaches for ANY queries
described here.

8. Implementation Experience

In October 2015 2015, the Cloudflare Authoritative Name server authoritative nameserver
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 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
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 behavior 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 has updated the following entry in the Resource "Resource Record (RR) TYPEs
Registry [1] entry as follows:
TYPEs" registry [RR_TYPES]:

12.

11. References

12.1.

11.1. 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.

11.2. 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]

[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 7719, 8499, 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

11. 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.

[RR_TYPES]
IANA, "Domain Name System (DNS) Parameters",
<https://www.iana.org/assignments/dns-parameters>.

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
while under attack. Richard Gibson identified areas where more
detail and accuracy was were useful. A large number of other people also
provided comments and suggestions suggestions; we thank them all for the
feedback.

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

Evan Hunt
ISC
950 Charter St
Redwood City, CA 94063
USA
United States of America

Email: each@isc.org