Internet DRAFT - draft-jabley-dnsop-refuse-any
draft-jabley-dnsop-refuse-any
Network Working Group J. Abley
Internet-Draft Dyn, Inc.
Updates: 1035 (if approved) O. Gudmundsson
Intended status: Standards Track M. Majkowski
Expires: April 14, 2016 CloudFlare Inc.
October 12, 2015
Providing Minimal-Sized Responses to DNS Queries with QTYPE=ANY
draft-jabley-dnsop-refuse-any-01
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.
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
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This Internet-Draft will expire on April 14, 2016.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Motivations . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. General Approach . . . . . . . . . . . . . . . . . . . . . . . 6
5. Behaviour of DNS Responders . . . . . . . . . . . . . . . . . 7
6. Behaviour of DNS Initiators . . . . . . . . . . . . . . . . . 8
7. HINFO Considerations . . . . . . . . . . . . . . . . . . . . . 9
8. Changes to RFC 1035 . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
12.1. Normative References . . . . . . . . . . . . . . . . . . 14
12.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Editorial Notes . . . . . . . . . . . . . . . . . . . 15
A.1. Venue . . . . . . . . . . . . . . . . . . . . . . . . . . 15
A.2. Change History . . . . . . . . . . . . . . . . . . . . . 15
A.2.1. draft-jabley-dnsop-refuse-any-01 . . . . . . . . . . . 15
A.2.2. draft-jabley-dnsop-refuse-any-00 . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Terminology
This document uses terminology specific to the Domain Name System
(DNS), descriptions of which can be found in
[I-D.ietf-dnsop-dns-terminology].
In this document, "ANY Query" refers to a DNS query with QTYPE=ANY.
An "ANY Response" is a response to such a query.
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", "MAY" and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. 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.
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3. Motivations
ANY queries are legitimately used for debugging and checking the
state of a DNS server for a particular owner 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, although there is no documented guidance
available for this use case and some implementations have been
observed that appear not to function as perhaps their developers
expected.
ANY queries are also frequently used to exploit the amplification
potential of DNS servers 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 return all RRSets for a particular
owner name. A DNS zone maintainer might prefer not to send full ANY
responses to reduce the potential for such information leaks.
Some authoritative-only DNS server implementations require additional
processing in order to send a conventional ANY response, and avoiding
that processing expense may be desirable.
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4. 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
authortitaive 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 caused them to 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.
This proposal specifies two different modes of behaviour by DNS
responders, and operators are free to choose whichever mechanism best
suits their environment.
1. A DNS responder may choose to search for an owner name that
matches the QNAME and, if that name owns multiple RRs, return
just one of them.
2. A DNS responder for whom a search for an owner name with an
existing resource record is expensive may instead synthesise an
HINFO resource record and return that instead. See Section 7 for
discussion of the use of HINFO.
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5. Behaviour of DNS Responders
A DNS responder which receives an ANY query MAY decline to provide a
conventional response, and MAY instead send a response with a single
RRSet in the answer section.
The RRSet returned in the answer section of the response MAY be a
single RRSet owned by the name specified in the QNAME. Where
mulitple RRSets exist, the responder MAY choose a small one to reduce
its amplification potential.
If there is no CNAME present at the owner name matching the QNAME,
the resource record returned in the response MAY instead 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 minimise
the size of the response.
The TTL encoded for a synthesised 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 TTLs
for any RR in any zone, and should be specified according to local
policy.
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 section MUST be returned.
Except as described in this section, the DNS responder MUST follow
the standard algorithms when constructing a response.
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6. Behaviour of DNS Initiators
XXX consider whether separate text here is required depending on
whether the initiator is a non-caching stub resolver or a caching
recursive resolver.
A DNS initator which sends a query with QTYPE=ANY and receives a
response containing an HINFO, as described in Section 5, MAY cache
the HINFO response in the normal way. Such cached HINFO 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, as described in Section 5.
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7. HINFO Considerations
In the case where a zone that contains HINFO RRSets is served from an
authority server that does not provide conventional ANY responses, it
is possible that the HINFO RRSet in an ANY response, once cached by
the initiator, might suppress subsequent queries from the same
initiator with QTYPE=HINFO. The use of HINFO in this proposal would
hence have effectively masked the HINFO RRSet present in the zone.
Authority-server operators who serve zones that rely upon
conventional use of the HINFO RRType might sensibly choose not to
deploy the mechanism described in this document.
The HINFO RRType is believed to be rarely used in the DNS at the time
of writing, based on observations made both at recursive servers and
authority servers.
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8. Changes to RFC 1035
It is important to note that returning a subset of available RRSets
when processing an ANY query is legitimate and consistent with
[RFC1035]; ANY does not mean ALL.
This document describes optional behaviour for both DNS initators and
responders, and implementation of the guidance provided by this
document is OPTIONAL.
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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
maximise 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.
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10. IANA Considerations
This document has no IANA actions.
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11. Acknowledgements
Evan Hunt and David Lawrence provided valuable observations.
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12. References
12.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://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,
<http://www.rfc-editor.org/info/rfc2119>.
12.2. Informative References
[I-D.ietf-dnsop-dns-terminology]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-dns-terminology-05 (work in
progress), September 2015.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
DOI 10.17487/RFC5358, October 2008,
<http://www.rfc-editor.org/info/rfc5358>.
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Appendix A. Editorial Notes
This section (and sub-sections) to be removed prior to publication.
A.1. Venue
An appropriate forum for discussion of this draft is the dnsop
working group.
A.2. Change History
A.2.1. 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.2.2. draft-jabley-dnsop-refuse-any-00
Initial draft circulated for comment.
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Authors' Addresses
Joe Abley
Dyn, Inc.
103-186 Albert Street
London, ON N6A 1M1
Canada
Phone: +1 519 670 9327
Email: jabley@dyn.com
Olafur Gudmundsson
CloudFlare Inc.
Email: olafur@cloudflare.com
Marek Majkowski
CloudFlare Inc.
Email: marek@cloudflare.com
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