One document matched: draft-jabley-dnsop-refuse-any-01.xml
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<rfc category="std" ipr="trust200902" updates="1035"
docName="draft-jabley-dnsop-refuse-any-01">
<?rfc toc="yes" ?>
<?rfc symrefs="yes" ?>
<?rfc sortrefs="yes"?>
<?rfc iprnotified="no" ?>
<?rfc strict="yes" ?>
<front>
<title abbrev="Minimal Responses for ANY Queries">Providing
Minimal-Sized Responses to DNS Queries with QTYPE=ANY</title>
<author initials='J.' surname="Abley" fullname='Joe Abley'>
<organization>Dyn, Inc.</organization>
<address>
<postal>
<street>103-186 Albert Street</street>
<city>London</city>
<region>ON</region>
<code>N6A 1M1</code>
<country>Canada</country>
</postal>
<phone>+1 519 670 9327</phone>
<email>jabley@dyn.com</email>
</address>
</author>
<author initials="O." surname="Gudmundsson" fullname="Olafur Gudmundsson">
<organization>CloudFlare Inc.</organization>
<address>
<email>olafur@cloudflare.com</email>
</address>
</author>
<author initials="M." surname="Majkowski" fullname="Marek Majkowski">
<organization>CloudFlare Inc.</organization>
<address>
<email>marek@cloudflare.com</email>
</address>
</author>
<date day="12" month="October" year="2015"/>
<abstract>
<t>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.</t>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Terminology">
<t>This document uses terminology specific to the Domain Name
System (DNS), descriptions of which can be found in
<xref target="I-D.ietf-dnsop-dns-terminology"/>.</t>
<t>In this document, "ANY Query" refers to a DNS query with
QTYPE=ANY. An "ANY Response" is a response to such a query.</t>
<t>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.</t>
<t>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 <xref target="RFC2119"/>.</t>
</section>
<section title="Introduction">
<t>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.</t>
<t>The DNS specification <xref target="RFC1034"/> <xref
target="RFC1035"/> does not include specific guidance
for the behaviour of DNS servers or clients in this situation.
This document aims to provide such guidance.</t>
</section>
<section title="Motivations">
<t>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.</t>
<t>ANY queries are also frequently used to exploit the amplification
potential of DNS servers using spoofed source addresses and
UDP transport (see <xref target="RFC5358"/>). Having the ability
to return small responses to such queries makes DNS servers
less attractive amplifiers.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="General Approach">
<t>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).</t>
<t>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.</t>
<t>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.</t>
<t>This proposal specifies two different modes of behaviour
by DNS responders, and operators are free to choose
whichever mechanism best suits their environment.
<list style="numbers">
<t>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.</t>
<t>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 <xref target="hinfo"/> for discussion of the
use of HINFO.</t>
</list></t>
</section>
<section title="Behaviour of DNS Responders" anchor="responder">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>Except as described in this section, the DNS responder
MUST follow the standard algorithms when constructing a
response.</t>
</section>
<section title="Behaviour of DNS Initiators">
<t>XXX consider whether separate text here is required depending
on whether the initiator is a non-caching stub resolver or
a caching recursive resolver.</t>
<t>A DNS initator which sends a query with QTYPE=ANY and
receives a response containing an HINFO, as described in
<xref target="responder"/>, 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.</t>
<t>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 <xref
target="responder"/>.</t>
</section>
<section title="HINFO Considerations" anchor="hinfo">
<t>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.</t>
<t>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.</t>
<t>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.
</t>
</section>
<section title="Changes to RFC 1035">
<t>It is important to note that returning a subset of available
RRSets when processing an ANY query is legitimate and
consistent with <xref target="RFC1035"/>; ANY does not mean
ALL.</t>
<t>This document describes optional behaviour for both DNS
initators and responders, and implementation of the guidance
provided by this document is OPTIONAL.</t>
</section>
<section title="Security Considerations">
<t>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.</t>
<t>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.</t>
</section>
<section title="IANA Considerations">
<t>This document has no IANA actions.</t>
</section>
<section title="Acknowledgements">
<t>Evan Hunt and David Lawrence provided valuable observations.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc1034;
&rfc1035;
&rfc2119;
</references>
<references title="Informative References">
&rfc5358;
&I-D.ietf-dnsop-dns-terminology;
</references>
<section title="Editorial Notes">
<t>This section (and sub-sections) to be removed prior to
publication.</t>
<section title="Venue">
<t>An appropriate forum for discussion of this draft is
the dnsop working group.</t>
</section>
<section title="Change History">
<section title="draft-jabley-dnsop-refuse-any-01">
<t>Make signing of RRSets in answers from signed
zones mandatory.</t>
<t>Document the option of returning an existing RRSet
in place of a synthesised one.</t>
</section>
<section title="draft-jabley-dnsop-refuse-any-00">
<t>Initial draft circulated for comment.</t>
</section>
</section>
</section>
</back>
</rfc>
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