Internet DRAFT - draft-ietf-dnsop-qname-minimisation
draft-ietf-dnsop-qname-minimisation
Domain Name System Operations (dnsop) Working Group S. Bortzmeyer
Internet-Draft AFNIC
Intended status: Experimental January 8, 2016
Expires: July 11, 2016
DNS query name minimisation to improve privacy
draft-ietf-dnsop-qname-minimisation-09
Abstract
This document describes a technique to improve DNS privacy, a
technique called "QNAME minimisation", where the DNS resolver no
longer sends the full original QNAME to the upstream name server.
Status of This Memo
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Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
2. QNAME minimisation . . . . . . . . . . . . . . . . . . . . . 3
3. Possible issues . . . . . . . . . . . . . . . . . . . . . . . 4
4. Protocol and compatibility discussion . . . . . . . . . . . . 5
5. Operational considerations . . . . . . . . . . . . . . . . . 5
6. Performance considerations . . . . . . . . . . . . . . . . . 6
7. On the experimentation . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
11.1. Normative References . . . . . . . . . . . . . . . . . . 7
11.2. Informative References . . . . . . . . . . . . . . . . . 8
11.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. An algorithm to perform QNAME minimisation . . . . . 9
Appendix B. Alternatives . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and background
The problem statement is described in [RFC7626]. 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.
QNAME minimisation follows the principle explained in section 6.1 of
[RFC6973]: the less data you send out, the fewer privacy problems you
have.
Currently, 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 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.
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2. QNAME minimisation
The idea is to minimise the amount of data sent from the DNS resolver
to the authoritative name server. In the example in the previous
section, sending "What are the NS records for .com?" would have been
sufficient (since it will be the answer from the root anyway). The
rest of this section describes the recommended way to do QNAME
minimisation, the one which maximimes privacy benefits (other
alternatives are discussed in appendixes).
A resolver which implements QNAME minimisation, and which does not
have already the answer in its cache, instead of sending the full
QNAME and the original QTYPE upstream, sends a request to the name
server authoritative for the closest known ancestor of the original
QNAME. The request is done with:
the QTYPE NS,
the QNAME which is the original QNAME, stripped to just one label
more than the zone for which the server is authoritative.
For example, a resolver receives a request to resolve
foo.bar.baz.example. Let's assume it already knows that
ns1.nic.example is authoritative for .example and the resolver does
not know a more specific authoritative name server. It will send the
query QTYPE=NS,QNAME=baz.example to ns1.nic.example.
The minimising resolver works perfectly when it knows the zone cut
(zone cuts are described in section 6 of [RFC2181]). But zone cuts
do not necessarily exist at every label 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 where the zone cut will
be. To find it out, it will query the .example name servers for the
NS records for bar.example. It will get a NODATA response,
indicating there is no zone cut at that point, so it has to to query
the .example name servers again with one more label, and so on.
(Appendix A describes this algorithm in deeper details.)
Since the information about the zone cuts will be stored in the
resolver's cache, the performance cost is probably reasonable.
Section 6 discusses this performance discrepancy further.
Note that DNSSEC-validating resolvers already have access to this
information, since they have to know the zone cut (the DNSKEY record
set is just below, the DS record set just above).
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3. Possible issues
QNAME minimisation is legal, since the original DNS RFC do not
mandate sending the full QNAME. So, in theory, it should work
without any problems. However, in practice, some problems may occur
(see an analysis in [huque-qnamemin] and an interesting discussion in
[huque-qnamestorify]).
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.
This behaviour is a protocol violation, and there is no need to stop
improving the DNS because of such behaviour. 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.
Another way to deal with such incorrect name servers would be to try
with QTYPE=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.
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 name servers incorrectly
return NXDOMAIN for ENTs. 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 bad behaviour.
A possible solution, currently implemented in Knot, is to retry with
the full query when you receive a NXDOMAIN. It works but it is not
ideal for privacy.
Other practices that do not conform to the DNS protocol standards may
pose a problem: there is a common DNS trick used by some 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:
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*.example. 60 IN A 192.0.2.6
(They could just wildcard all of "*.", which would be sufficient. We
don't know why they don't do it.)
This lets them have 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 not give the right result, which may
endanger QNAME minimisation (it will be a problem for DNSSEC, too).
4. Protocol and compatibility discussion
QNAME minimisation 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 it is an
unilateral change, resolver implementers may do QNAME minimisation in
slightly different ways, see the appendices for examples.)
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). As said in
Section 1, the current method, sending the full QNAME, is not
mandated by the DNS protocol.
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.
5. 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) [kaliski-minimum].
DNS administrators are reminded that the data on DNS requests that
they store may have legal consequences, depending on your
jurisdiction (check with your local lawyer).
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6. Performance considerations
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.example
followed by B.example followed by C.example, the result will be three
NXDOMAINs, since .example does not exist in the root zone. Under
query name minimisation, the root name servers would hear only one
question (for .example 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.example or C.example
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.
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). Let's assume a resolver which knows only the name servers
of .example. Without QNAME minimisation, it would send these
.example nameservers a query for
www.host.group.department.example.com and immediately get a specific
referral or an answer, without the need for more queries to probe for
the zone cut. For such a name, a cold resolver with QNAME
minimisation will, depending how QNAME minimisation is implemented,
send more queries, one per label. Once the cache is warm, there will
be no difference with a traditional resolver. Actual testing is
described in [huque-qnamemin]. Such deep domains are specially
common under ip6.arpa.
7. On the experimentation
This document has status "Experimental". Since the beginning of time
(or DNS), the fully qualified host name was always sent to the
authoritative name servers. There was a concern that changing this
behavior may engage the Law of Unintended Consequences. Hence this
status.
The idea about the experiment is to observe QNAME minimisation in
action with multiple resolvers, various authoritative name servers,
etc.
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8. IANA Considerations
This document has no actions for IANA.
9. 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 as 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. Also, the effect of QNAME minimisation on
wire sniffers depends on whether the sniffer is, on the DNS path.)
QNAME minimisation offers zero protection against the recursive
resolver, which still sees the full request coming from the stub
resolver.
All the alternatives mentioned in Appendix B decrease privacy in the
hope of improving performance. They must not be used if you want the
maximum privacy.
10. Acknowledgments
Thanks to Olaf Kolkman for the original idea during a KLM flight from
Amsterdam to Vancouver, although the concept is probably much older
[1]. Thanks for Shumon Huque and Marek Vavrusa for implementation
and testing. 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 and for discussion of the algorithm.
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.
11. References
11.1. 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>.
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[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>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973, DOI
10.17487/RFC6973, July 2013,
<http://www.rfc-editor.org/info/rfc6973>.
[RFC7626] Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
DOI 10.17487/RFC7626, August 2015,
<http://www.rfc-editor.org/info/rfc7626>.
11.2. Informative References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<http://www.rfc-editor.org/info/rfc2181>.
[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.
[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,
<http://blogs.verisigninc.com/blog/entry/
minimum_disclosure_what_information_does>.
[huque-qnamemin]
Huque, S., "Query name minimization and authoritative
server behavior", May 2015, <https://indico.dns-
oarc.net/event/21/contribution/9>.
[huque-qnamestorify]
Huque, S., "Qname Minimization @ DNS-OARC", May 2015,
<https://storify.com/shuque/qname-minimization-dns-oarc>.
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11.3. URIs
[1] https://lists.dns-oarc.net/pipermail/dns-
operations/2010-February/005003.html
Appendix A. An algorithm to perform QNAME minimisation
This algorithm performs name resolution with QNAME minimisation in
presence of not-yet-known zone cuts.
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 the cuts. This is just a possible help for
implementors, it is not intended to be normative:
(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 ANCESTOR.
(2) Initialize CHILD to the same as ANCESTOR.
(3) If CHILD is the same as the QNAME, resolve the original query
using ANCESTOR'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 ANCESTOR'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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Appendix B. Alternatives
Remember that QNAME minimisation is unilateral so a resolver is not
forced to implement it exactly as described here.
There are several ways to perform QNAME minimisation. The one in
Section 2 is the suggested one. It can be called the aggressive
algorithm, since 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. Another possible algorithm, not fully studied at this
time, could be to "piggyback" on the traditional resolution code. At
startup, 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 could require
fewer changes in the existing resolver codebase.
In the above specification, the original QTYPE is replaced by NS (or
may be A, if too many servers react incorrectly to NS requests),
which is the best approach to preserve privacy. But this erases
information about the relative use of the various QTYPEs, which may
be interesting for researchers (for instance if they try to follow
IPv6 deployment by counting the percentage of AAAA vs. A queries). A
variant of QNAME minimisation would be to keep the original QTYPE.
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.)
To address the "number of queries" issue, described in Section 6, a
possible solution is to always use the traditional algorithm when the
cache is cold and then to move to QNAME minimisation (precisely
defining what is "hot" or "cold" is left to the implementer). This
will decrease the privacy but will guarantee no degradation of
performance.
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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