Domain Name System Operations (dnsop) Working Group S. Bortzmeyer
Internet-Draft AFNIC
Intended status: Experimental March 4, 2015
Expires: September 5, 2015
DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-02
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".
REMOVE BEFORE PUBLICATION 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 September 5, 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
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
Bortzmeyer Expires September 5, 2015 [Page 1]
Internet-Draft Qname minimisation March 2015
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 . . . . . . . . . . . . . . . . . . . 6
6. Implementation status - REMOVE BEFORE PUBLICATION . . . . . . 6
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Appendix A. An algorithm to find the zone cut . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 9
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 fewer privacy problems you'll get.
2. Qname minimisation
The idea is to minimise the amount of data sent from the DNS
resolver. Under current practice, 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). This
is compatible with the current DNS system and therefore can easily be
deployed; since it is a unilateral change to the resolver, it does
not change the protocol. Because of that, resolver implementers may
do qname minmisation in slightly different ways.
To do such minimisation, the resolver needs to know the zone cut
[RFC2181]. Zone cuts do not necessarily exist at every label
boundary. If we take the name www.foo.bar.example, it is possible
Bortzmeyer Expires September 5, 2015 [Page 2]
Internet-Draft Qname minimisation March 2015
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 whether
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).
One should note that the behaviour suggested here (minimising the
amount of data sent in qnames from the resolver) 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 requirement. 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.
It may be noticed that many documents explaining the DNS and intended
for a wide audience, incorrectly describe the resolution process as
using qname minimisation, for instance by showing a request going to
the root, with just the TLD in the query. As a result, these
documents may confuse the privacy analysis of the users who see them.
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
learns the zone cuts from the referrals received, then switches to NS
queries asking only for the minimum domain name. This leaks more
data but probably requires fewer 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
Bortzmeyer Expires September 5, 2015 [Page 3]
Internet-Draft Qname minimisation March 2015
responsibility in some jurisdictions. (TODO: do we keep any mention
of legal issues? We're not lawyers.)
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: 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 for each TLD 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.)
Bortzmeyer Expires September 5, 2015 [Page 4]
Internet-Draft Qname minimisation March 2015
This lets them turn up many 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 the
hoster's nameservers, and the Web hoster doesn't have to provision
anything in order to make the customer's domain resolve. NS queries
to the hoster will therefore do not give the right result, which may
endanger qname minimisation (it will be a problem for DNSSEC, too).
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 the cache is
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 inferior to 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.
Bortzmeyer Expires September 5, 2015 [Page 5]
Internet-Draft Qname minimisation March 2015
5. Security considerations
Qname minimisation's benefits are clear in the case where you want to
decrease exposure to the authoritative name server. But 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.
(Encryption is of course a better defense against wire sniffers but,
unlike qname minimisation, it changes the protocol and cannot be
deployed unilaterally.)
Qname minimisation offers zero protection against the recursive
resolver, which still sees the full request coming from the stub
resolver.
At this stage, this document does not recommend one of the two qname
minimisation approaches (aggressive or lazy) against the other.
No security consequence (besides privacy improvment) is known at this
time.
6. Implementation status - REMOVE BEFORE PUBLICATION
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in [RFC6982].
The description of implementations in this section is intended to
assist the IETF in its decision processes in progressing drafts to
RFCs. Please note that the listing of any individual implementation
here does not imply endorsement by the IETF. Furthermore, no effort
has been spent to verify the information presented here that was
supplied by IETF contributors. This is not intended as, and must not
be construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
According to [RFC6982], "this will allow reviewers and working groups
to assign due consideration to documents that have the benefit of
running code, which may serve as evidence of valuable experimentation
and feedback that have made the implemented protocols more mature.
It is up to the individual working groups to use this information as
they see fit".
As of today, no production resolver implements qname minimisation.
For Unbound, see ticket 648 [1].
The algorithm to find the zone cuts described in Appendix A is
implemented with qname minimisation in the sample code zonecut.go
Bortzmeyer Expires September 5, 2015 [Page 6]
Internet-Draft Qname minimisation March 2015
[2]. It is also implemented, for a much longer time, in an option of
dig, "dig +trace", but without qname minimisation.
7. Acknowledgments
Thanks to Olaf Kolkman for the original idea although the concept is
probably much older [3]. Thanks to Mark Andrews and Francis Dupont
for the interesting discussions. Thanks to Brian Dickson, Warren
Kumari, Evan Hunt 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 minimisation. 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.
8. References
8.1. 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.
8.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982, July
2013.
Bortzmeyer Expires September 5, 2015 [Page 7]
Internet-Draft Qname minimisation March 2015
[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.
[dnsop] IETF, , "The DNSOP working group of IETF", March 2014,
.
[mockapetris-history]
Mockapetris, P., "Private discussion", January 2015.
[kaliski-minimum]
Kaliski, B., "Minimum Disclosure: What Information Does a
Name Server Need to Do Its Job?", March 2015,
.
8.3. URIs
[1] https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=648
[2] https://github.com/bortzmeyer/my-IETF-work/blob/master/draft-
ietf-dnsop-qname-minimisation/zonecut.go
[3] 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.
Bortzmeyer Expires September 5, 2015 [Page 8]
Internet-Draft Qname minimisation March 2015
(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.
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/
Bortzmeyer Expires September 5, 2015 [Page 9]