Internet DRAFT - draft-ietf-dnsop-nxdomain-cut
draft-ietf-dnsop-nxdomain-cut
Domain Name System Operations (dnsop) Working Group S. Bortzmeyer
Internet-Draft AFNIC
Updates: 1034, 2308 (if approved) S. Huque
Intended status: Standards Track Verisign Labs
Expires: November 9, 2016 May 8, 2016
NXDOMAIN really means there is nothing underneath
draft-ietf-dnsop-nxdomain-cut-03
Abstract
This document states clearly that when a DNS resolver receives a
response with response code of NXDOMAIN, it means that the domain
name which is thus denied AND ALL THE NAMES UNDER IT do not exist.
REMOVE BEFORE PUBLICATION: this document should be discussed in the
IETF DNSOP (DNS Operations) group, through its mailing list. The
source of the document, as well as a list of open issues, is
currently kept at Github [1].
This documents clarifies RFC 1034 and modifies a bit RFC 2308 so it
updates both of them.
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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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 November 9, 2016.
Copyright Notice
Copyright (c) 2016 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
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Table of Contents
1. Introduction and background . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Possible issues . . . . . . . . . . . . . . . . . . . . . . . 5
5. Implementation considerations . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION 7
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix A. Why can't we just use the owner name of the returned
SOA? . . . . . . . . . . . . . . . . . . . . . . . . 9
Appendix B. Related approaches . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction and background
The DNS protocol [RFC1035] defines response code 3 as "Name Error",
or "NXDOMAIN" [RFC2308], which means that the queried domain name
does not exist in the DNS. Since domain names are represented as a
tree of labels ([RFC1034], Section 3.1), non-existence of a node
implies non-existence of the entire sub-tree rooted at this node.
The DNS iterative resolution algorithm precisely interprets the
NXDOMAIN signal in this manner. If it encounters an NXDOMAIN
response code from an authoritative server, it immediately stops
iteration and returns the NXDOMAIN response to the querier.
However, in most known existing resolvers today, a cached non-
existence for a domain is not considered "proof" that there can be no
child domains underneath. This is due to an ambiguity in [RFC1034]
that failed to distinguish Empty Non-Terminal names (ENT) ([RFC7719])
from nonexistent names. The distinction became especially important
for the development of DNSSEC, which provides proof of non-existence.
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[RFC4035], section 3.1.3.2, describes how security-aware
authoritative name servers make the distinction, but no existing RFCs
describe the behavior for recursive name servers.
This document specifies that an NXDOMAIN response for a domain name
means that no child domains underneath the queried name exist either.
And furthermore, that DNS resolvers should interpret cached non-
existence in this manner. Since the domain names are organized in a
tree, it is a simple consequence of the tree structure: non-existence
of a node implies non-existence of the entire sub-tree rooted at this
node.
1.1. Terminology
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].
"Denied name": the domain name whose existence has been denied by a
response of rcode NXDOMAIN. In most cases, it is the QNAME but,
because of [RFC6604], it is not always the case.
Other terms are defined in [RFC1034] or [RFC1035] or (like NXDOMAIN
itself) in the more recent [RFC7719].
The domain name space is conceptually defined in terms of a tree
structure. The implementation of a DNS resolver/cache MAY use a tree
or other data structures. The cache being a subset of the data in
the domain name space, it is much easier to reason about it in terms
of that tree structure and to describe things in those terms (names
under/above, descendent names, subtrees etc). In fact, the DNS
algorithm description in [RFC1034] even states an assumption that the
cache is a tree structure, so the precedent is already well
established: see its section 4.3.2 which says "The following
algorithm assumes that the RRs are organized in several tree
structures, one for each zone, and another for the cache..." So, in
this document, each time we talk of a tree or tree operations, it
refers to the model, not to the actual implementation.
2. Rules
When an iterative caching DNS resolver receives a response NXDOMAIN,
it SHOULD store it in its cache and all names and RRsets at or below
that node SHOULD then be considered to be unreachable. Subsequent
queries for such names SHOULD elicit an NXDOMAIN response.
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But if a resolver has cached data under the NXDOMAIN cut, it MAY
continue to send it as a reply. (Until the TTL of this cached data
expires.)
Another exception is that a validating resolver MAY decide to
implement this behaviour only when the NXDOMAIN response has been
validated with DNSSEC. See Section 7 for the rationale.
As an example of the consequence of these rules, consider two
successive queries to a resolver, with a non-existing domain
'foo.example': the first is for 'foo.example' (which results in an
NXDOMAIN) and the second for 'bar.foo.example' (which also results in
an NXDOMAIN). Many resolvers today will forward both queries, as
noticed in Section 8. However, following the rules in this document
("NXDOMAIN cut"), a resolver would cache the first NXDOMAIN response,
as a sign of non-existence, and then immediately return an NXDOMAIN
response for the second query, without transmitting it to an
authoritative server.
If the first request is for 'bar.foo.example' and the second for
'baz.foo.example', the first NXDOMAIN response won't tell anything
about 'baz.foo.example' and therefore the second query will be
transmitted as it was before the use of "NXDOMAIN cut" optimisation
(see Appendix A).
These rules replace the second paragraph of section 5 of [RFC2308].
Otherwise, this document does not update any other parts of
[RFC2308]. The fact that a subtree does not exist is not forever:
[RFC2308], section 3, already describes the amount of time that an
NXDOMAIN response may be cached (the "negative TTL").
If the NXDOMAIN response due to a cached non-existence is from a
DNSSEC signed zone, then it will have accompanying NSEC or NSEC3
records that authenticate the non-existence of the name. For a
descendant name of the original NXDOMAIN name, the same set of NSEC
or NSEC3 records proves the non-existence of the descendant name.
The iterative, caching resolver MUST return these NSEC or NSEC3
records in the response to the triggering query if the query had the
DNSSEC OK (DO) bit set.
Warning: if there is a chain of CNAME (or DNAME), the name which does
not exist is the last of the chain ([RFC6604]) and not the QNAME.
The NXDOMAIN stored in the cache is for the denied name, not always
for the QNAME.
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3. Benefits
The main benefit is a better efficiency of the caches. In the
example above, the resolver sends only one query instead of two, the
second one being answered from the cache. This will benefit the
entire DNS ecosystem, since the authoritative name servers will have
less unnecessary traffic to process.
The correct behavior (in [RFC1034] and made clearer in this document)
is specially useful when combined with QNAME minimisation [RFC7816]
since it will allow a resolver to stop searching as soon as an
NXDOMAIN is encountered.
"NXDOMAIN cut" may also help mitigate certain types of random QNAME
attacks [joost-dnsterror] [balakrichenan-dafa888], where there is a
fixed suffix which does not exist. In these attacks against the
authoritative name server, queries are sent to resolvers for a QNAME
composed of a fixed suffix ("dafa888.wf" in one of the articles
above), which is typically nonexistent, and a random prefix,
different for each request. A resolver receiving these requests have
to forward them to the authoritative servers. With "NXDOMAIN cut", a
system administrator would just have to send to the resolver a query
for the fixed suffix, the resolver would get a NXDOMAIN and then
would stop forwarding the queries. (It would be better if the SOA
record in the NXDOMAIN response were sufficient to find the non-
existing domain but it is not the case, see Appendix A.)
4. Possible issues
Let's assume the TLD example exists but foobar.example is not
delegated (so the example's name servers will reply NXDOMAIN for a
query about anything.foobar.example). A system administrator decides
to name the internal machines of his organization under
office.foobar.example and uses a trick of his resolver to forward
requests about this zone to his local authoritative name servers.
"NXDOMAIN cut" would create problems here, since, depending on the
order of requests to the resolver, it may have cached the non-
existence from example and therefore "deleted" everything under.
This document assumes that such setup is rare and does not need to be
supported.
Another issue that may happen: today, we see broken authoritative
name servers which reply to ENT ([RFC7719], section 6) with NXDOMAIN
instead of the normal NODATA ([RFC7719], section 3).
RFC-EDITOR: REMOVE THE PARAGRAPH BEFORE PUBLICATION. An example
today is mta2._domainkey.cbs.nl (which exists) where querying
_domainkey.cbs.nl yields NXDOMAIN. Another example is www.upenn.edu,
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redirected to www.upenn.edu-dscg.edgesuite.net while a query for edu-
dscg.edgesuite.net returns NXDOMAIN.
Such name servers are definitely wrong and have always been. Their
behaviour is incompatible with DNSSEC. Given the advantages of
"NXDOMAIN cut", there is little reason to support this behavior.
5. Implementation considerations
This section is non-normative, and is composed only of various things
which may be useful for implementors. A recursive resolver may
implement its cache in many ways. The most obvious one is a tree
data structure, because it fits the data model of domain names. But,
in practice, other implementations are possible, as well as various
optimisations (such as a tree, augmented by an index of some common
domain names).
If a resolver implements its cache as a tree (without any
optimisation), one way to follow the rules of Section 2 is, when
receiving the NXDOMAIN, to prune the subtree of positive cache
entries at that node, or to delete all individual cache entries for
names below that node. Then, when searching downward in its cache,
this iterative caching DNS resolver stop searching if it encounters a
cached non-existence.
Some resolver may have a cache which is NOT organized as a tree (but,
for instance, as a dictionary) and therefore have a reason to ignore
the rules of Section 2. So these rules are a SHOULD and not a MUST.
6. IANA Considerations
This document has no actions for IANA.
7. Security Considerations
The technique described here may help against a denial-of-service
attack named "random qnames" and described in Section 3.
If a resolver does not validate the answers with DNSSEC, or if the
zone is not signed, the resolver can of course be poisoned with a
false NXDOMAIN, thus "deleting" a part of the domain name tree. This
denial-of-service attack is already possible without the rules of
this document (but "NXDOMAIN cut" may increase its effects). The
only solution is to use DNSSEC.
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8. Implementation status - RFC EDITOR: 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, practically all existing DNS resolvers are conservative
by default: they consider a NXDOMAIN as only significant for the
denied name itself, not for the names under. Almost all the current
recursive servers will send upstream a query for out-of-cache
sub.example.com even if their cache contains an NXDOMAIN for
example.com.
There are a few exceptions. The Unbound resolver has a configuration
parameter called "harden-below-nxdomain" [2], which if set to "yes"
turns on "NXDOMAIN cut" behavior ("only DNSSEC-secure nxdomains are
used", see Section 7). The PowerDNS recursor has optional partial
support for "NXDOMAIN cut", for the root domain only, with its "root-
nx-trust" setting, described as [3] "If set, an NXDOMAIN from the
root-servers will serve as a blanket NXDOMAIN for the entire TLD the
query belonged to. The effect of this is far fewer queries to the
root-servers.".
9. Acknowledgments
The main idea is in this document is taken from
[I-D.vixie-dnsext-resimprove], section 3, "Stopping Downward Cache
Search on NXDOMAIN". Thanks to its authors, Paul Vixie, Rodney
Joffe, and Frederico Neves. Additionally Tony Finch, John Levine,
Jinmei Tatuya, Bob Harold and Duane Wessels provided valuable
feedback and suggestions.
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10. References
10.1. 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>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<http://www.rfc-editor.org/info/rfc2308>.
[RFC6604] Eastlake 3rd, D., "xNAME RCODE and Status Bits
Clarification", RFC 6604, DOI 10.17487/RFC6604, April
2012, <http://www.rfc-editor.org/info/rfc6604>.
10.2. Informative References
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running
Code: The Implementation Status Section", RFC 6982,
DOI 10.17487/RFC6982, July 2013,
<http://www.rfc-editor.org/info/rfc6982>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
[RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
<http://www.rfc-editor.org/info/rfc7816>.
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[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.
[I-D.fujiwara-dnsop-nsec-aggressiveuse]
Fujiwara, K. and A. Kato, "Aggressive use of NSEC/NSEC3",
draft-fujiwara-dnsop-nsec-aggressiveuse-03 (work in
progress), March 2016.
[joost-dnsterror]
Joost, M., "About DNS Attacks and ICMP Destination
Unreachable Reports", December 2014,
<http://www.michael-joost.de/dnsterror.html>.
[balakrichenan-dafa888]
Balakrichenan, S., "Disturbance in the DNS - "Random
qnames", the dafa888 DoS attack"", October 2014,
<https://indico.dns-oarc.net/event/20/session/3/
contribution/37>.
10.3. URIs
[1] https://github.com/bortzmeyer/ietf-dnsop-nxdomain
[2] https://www.unbound.net/documentation/unbound.conf.html
[3] https://doc.powerdns.com/md/recursor/settings/#root-nx-trust
Appendix A. Why can't we just use the owner name of the returned SOA?
In this document, we deduce the non-existence of a domain only for
NXDOMAIN answers where the denied name was this exact domain. If a
resolver sends a query to the name servers of the TLD example, and
asks the MX record for www.foobar.example, and receives a NXDOMAIN,
it can only register the fact that www.foobar.example (and everything
underneath) does not exist. Even if the accompanying SOA record is
for example only, one cannot infer that foobar.example is
nonexistent. The accompanying SOA indicates the apex of the zone,
not the closest existing domain name.
RFC-EDITOR: REMOVE BEFORE PUBLICATION: to use a real example today,
ask the authoritative name servers of the TLD fr about
anything.which.does.not.exist.gouv.fr. The SOA will indicate fr (the
apex) even while gouv.fr does exist (there is no zone cut between
gouv.fr and fr).
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Deducing the non-existence of a node from the SOA in the NXDOMAIN
reply may certainly help with random qnames attacks but this is out-
of-scope for this document. It would require to address the problems
mentioned in the first paragraph of this section. A possible
solution would be, when receiving a NXDOMAIN with a SOA which is more
than one label up in the tree, to send requests for the domains which
are between the QNAME and the owner name of the SOA. (A resolver
which does DNSSEC validation or QNAME minimisation will need to do
it, anyway.)
Appendix B. Related approaches
The document [I-D.fujiwara-dnsop-nsec-aggressiveuse] describes
another way to address some of the same concerns (decreasing the
traffic for non-existing domain names). Unlike "NXDOMAIN cut", it
requires DNSSEC but it is more powerful since it can synthesize
NXDOMAINs for domains that were not queried.
Authors' Addresses
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/
Shumon Huque
Verisign Labs
12061 Bluemont Way
Reston 20190
USA
Email: shuque@verisign.com
URI: http://www.verisignlabs.com/
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