One document matched: draft-ietf-dnsop-qname-minimisation-01.txt
Differences from draft-ietf-dnsop-qname-minimisation-00.txt
Domain Name System Operations (dnsop) Working Group S. Bortzmeyer
Internet-Draft AFNIC
Intended status: Experimental February 15, 2015
Expires: August 19, 2015
DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-01
Abstract
This document describes one of the techniques that could be used to
improve DNS privacy (see [I-D.ietf-dprive-problem-statement]), a
technique called "qname minimisation".
Discussions of the document should take place on the DNSOP working
group mailing list [dnsop].
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
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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 August 19, 2015.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. Qname minimisation . . . . . . . . . . . . . . . . . . . . . 2
3. Operational considerations . . . . . . . . . . . . . . . . . 3
4. Performance implications . . . . . . . . . . . . . . . . . . 5
5. Security considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
7.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. An algorithm to find the zone cut . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction and background
The problem statement is exposed in
[I-D.ietf-dprive-problem-statement] TODO: add a reference to the
specific section when ietf-dprive-problem-statement will be published
as RFC. The terminology ("qname", "resolver", etc) is also defined
in this companion document. This specific solution is not intended
to fully solve the DNS privacy problem; instead, it should be viewed
as one tool amongst many.
It follows the principle explained in section 6.1 of [RFC6973]: the
less data you send out, the less privacy problems you'll get.
2. Qname minimisation
The idea is to minimise the amount of data sent from the DNS
resolver. When a resolver receives the query "What is the AAAA
record for www.example.com?", it sends to the root (assuming a cold
resolver, whose cache is empty) the very same question. Sending
"What are the NS records for .com?" would be sufficient (since it
will be the answer from the root anyway). To do so would be
compatible with the current DNS system and therefore could be easily
deployable, since it is an unilateral change to the resolvers, it
does not change the protocol. Because of that, resolver implementers
may do qname minmisation in slightly different ways.
If "minimisation" is too long, you can write it "m10n".
To do such minimisation, the resolver needs to know the zone cut
[RFC2181]. Zone cuts do not necessarily exist at every label
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boundary. If we take the name 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 the resolver already knows the
name servers of .example, when it receives the query "What is the
AAAA record of www.foo.bar.example", it does not always know if the
request should be sent to the name servers of bar.example or to those
of example. [RFC2181] suggests a method to find the zone cut
(section 6), so resolvers may try it.
Note that DNSSEC-validating resolvers already have access to this
information, since they have to find the zone cut (the DNSKEY record
set is just below, the DS record set just above).
Minimising the amount of data sent also, in part, addresses the case
of a wire sniffer as well the case of privacy invasion by the
servers.
One should note that the behaviour suggested here (minimising the
amount of data sent in qnames) is NOT forbidden by the [RFC1034]
(section 5.3.3) or [RFC1035] (section 7.2). Sending the full qname
to the authoritative name server is a tradition, not a protocol
requirment. This tradition comes[mockapetris-history] from a desire
to optimize the number of requests, when the same name server is
authoritative for many zones in a given name (something which was
more common in the old days, where the same name servers served .com
and the root) or when the same name server is both recursive and
authoritative (something which is strongly discouraged now).
Whatever the merits of this choice at this time, the DNS is quite
different now.
As mentioned before, there are several ways to implement qname
minimisation. Two main strategies are the aggressive one and the
lazy one. In the aggressive one, the resolver only sends NS queries
as long as it does not know the zone cuts. This is the safest, from
a privacy point of view. The lazy way "piggybacks" on the
traditional resolution code. It sends traditional full qnames and
learn the zone cuts from the referrals received, then switching to NS
queries. This leaks more data but probably requires less changes in
the existing resolver codebase.
3. Operational considerations
The administrators of the forwarders, and of the authoritative name
servers, will get less data, which will reduce the utility of the
statistics they can produce (such as the percentage of the various
qtypes). On the other hand, it may decrease their legal
responsibility.
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Some broken name servers do not react properly to qtype=NS requests.
For instance, some authoritative name servers embedded in load
balancers reply properly to A queries but send REFUSED to NS queries.
REMOVE THIS SENTENCE BEFORE PUBLICATION: As an example of today, look
at www.ratp.fr (not ratp.fr). This behaviour is a gross protocol
violation, and there is no need to stop improving the DNS because of
such brokenness. However, qname minimisation may still work with
such domains since they are only leaf domains (no need to send them
NS requests). Such setup breaks more than just qname minimisation.
It breaks negative answers, since the servers don't return the
correct SOA, and it also breaks anything dependent upon NS and SOA
records existing at the top of the zone.
A problem can also appear when a name server does not react properly
to ENT (Empty Non-Terminals). If ent.example.com has no resource
records but foobar.ent.example.com does, then ent.example.com is an
ENT. A query, whatever the qtype, for ent.example.com must return
NODATA (NOERROR / ANSWER: 0). However, some broken name servers
return NXDOMAIN for ENTs. REMOVE THIS SENTENCE BEFORE PUBLICATION:
As an example of today, look at com.akadns.net or www.upenn.edu with
its delegations to Akamai. If a resolver queries only
foobar.ent.example.com, everything will be OK but, if it implements
qname minimisation, it may query ent.example.com and get a NXDOMAIN.
See also section 3 of [I-D.vixie-dnsext-resimprove] for the other bad
consequences of this brokenness.
Another way to deal with such broken name servers would be to try
with A requests (A being chosen because it is the most common and
hence a qtype which will be always accepted, while a qtype NS may
ruffle the feathers of some middleboxes). Instead of querying name
servers with a query "NS example.com", we could use "A _.example.com"
and see if we get a referral.
Other strange and illegal practices may pose a problem: for instance,
there is a common DNS anti-pattern used by low-end web hosters that
also do DNS hosting that exploits the fact that the DNS protocol
(pre-DNSSEC) allows certain serious misconfigurations, such as parent
and child zones disagreeing on the location of a zone cut.
Basically, they have a single zone with wildcards like:
*.example. 60 IN A 192.0.2.6
(It is not known why they don't just wildcard all of "*." and be done
with it.)
This lets them turn up tons of web hosting customers without having
to configure thousands of individual zones on their nameservers.
They just tell the prospective customer to point their NS records at
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their nameservers, and the Web hoster doesn't have to provision
anything in order to make the customer's domain resolve.
Qname minimisation can decrease performance in some cases, for
instance for a deep domain name (like
www.host.group.department.example.com where
host.group.department.example.com is hosted on example.com's name
servers). For such a name, a cold resolver will, depending how qname
minimisation is implemented, send more queries. Once warm, there
will be no difference with a traditional resolver. A possible
solution is to always use the traditional algorithm when the cache is
cold and then to move to qname minimisation. This will decrease the
privacy a bit but will guarantee no degradation of performance.
Another useful optimisation may be, in the spirit of the HAMMER idea
[I-D.wkumari-dnsop-hammer] to probe in advance for the introduction
of zone cuts where none previously existed (i.e. confirm their
continued absence, or discover them.)
4. Performance implications
The main goal of qname minimisation is to improve privacy by sending
less data. However, it may have other advantages. For instance, if
a root name server receives a query from some resolver for A.CORP
followed by B.CORP followed by C.CORP, the result will be three
NXDOMAINs, since .CORP does not exist in the root zone. Under query
name minimisation, the root name servers would hear only one question
(for .CORP itself) to which they could answer NXDOMAIN, thus opening
up a negative caching opportunity in which the full resolver could
know a priori that neither B.CORP or C.CORP could exist. Thus in
this common case the total number of upstream queries under qname
minimisation would be counter-intuitively less than the number of
queries under the traditional iteration (as described in the DNS
standard).
Qname minimisation may also improve look-up performance for TLD
operators. For a typical TLD, delegation-only, and with delegations
just under the TLD, a 2-label QNAME query is optimal for finding the
delegation owner name.
5. Security considerations
No security consequence (besides privacy improvment) is known at this
time.
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6. Acknowledgments
Thanks to Olaf Kolkman for the original idea although the concept is
probably much older [1]. Thanks to Mark Andrews and Francis Dupont
for the interesting discussions. Thanks to Brian Dickson, Warren
Kumari and David Conrad for remarks and suggestions. Thanks to
Mohsen Souissi for proofreading. Thanks to Tony Finch for the zone
cut algorithm in Appendix A. Thanks to Paul Vixie for pointing out
that there are practical advantages (besides privacy) to qname m10n.
Thanks to Phillip Hallam-Baker for the fallback on A queries, to deal
with broken servers. Thanks to Robert Edmonds for an interesting
anti-pattern.
7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, July
2013.
[I-D.ietf-dprive-problem-statement]
Bortzmeyer, S., "DNS privacy considerations", draft-ietf-
dprive-problem-statement-01 (work in progress), January
2015.
7.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[I-D.wkumari-dnsop-hammer]
Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly
Automated Method for Maintaining Expiring Records", draft-
wkumari-dnsop-hammer-01 (work in progress), July 2014.
[I-D.vixie-dnsext-resimprove]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June
2010.
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[dnsop] IETF, , "The DNSOP working group of IETF", March 2014,
<https://datatracker.ietf.org/wg/dnsop/charter/>.
[mockapetris-history]
Mockapetris, P., "Private discussion", January 2015.
7.3. URIs
[1] https://lists.dns-oarc.net/pipermail/dns-
operations/2010-February/005003.html
Appendix A. An algorithm to find the zone cut
Although a validating resolver already has the logic to find the zone
cut, other resolvers may be interested by this algorithm to follow in
order to locate this cut:
(0) If the query can be answered from the cache, do so, otherwise
iterate as follows:
(1) Find closest enclosing NS RRset in your cache. The owner of
this NS RRset will be a suffix of the QNAME - the longest suffix
of any NS RRset in the cache. Call this PARENT.
(2) Initialize CHILD to the same as PARENT.
(3) If CHILD is the same as the QNAME, resolve the original query
using PARENT's name servers, and finish.
(4) Otherwise, add a label from the QNAME to the start of CHILD.
(5) If you have a negative cache entry for the NS RRset at CHILD,
go back to step 3.
(6) Query for CHILD IN NS using PARENT's name servers. The
response can be:
(6a) A referral. Cache the NS RRset from the authority section
and go back to step 1.
(6b) An authoritative answer. Cache the NS RRset from the
answer section and go back to step 1.
(6c) An NXDOMAIN answer. Return an NXDOMAIN answer in response
to the original query and stop.
(6d) A NOERROR/NODATA answer. Cache this negative answer and
go back to step 3.
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Author's Address
Stephane Bortzmeyer
AFNIC
1, rue Stephenson
Montigny-le-Bretonneux 78180
France
Phone: +33 1 39 30 83 46
Email: bortzmeyer+ietf@nic.fr
URI: http://www.afnic.fr/
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