Network Working Group P. Koch
Internet-Draft DENIC eG
Intended status: Best Current Practice M. Larson
Expires: August 18, 2014 Dyn, Inc.
February 14, 2014
Initializing a DNS Resolver with Priming Queries
draft-ietf-dnsop-resolver-priming-04
Abstract
This document describes the initial queries a DNS resolver is
supposed to emit to initialize its cache with a current NS RRSet for
the root zone as well as the necessary address information.
Status of This Memo
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1. Introduction
Domain Name System (DNS) resolvers need a starting point to resolve
queries. [RFC1034], section 5.3.2, defines the SBELT structure in a
full resolver as:
``a "safety belt" structure of the same form as SLIST, which is
initialized from a configuration file, and lists servers which
should be used when the resolver doesn't have any local
information to guide name server selection. The match count will
be -1 to indicate that no labels are known to match.''
Section 5.3.3 of [RFC1034] adds
``the usual choice is two of the root servers and two of the
servers for the host's domain''
Today's practice generally seperates serving and resolving
functionality, so the servers ``for the host's domain'' might no
longer be an appropriate choice, even if they were only intended to
resolve ``local'' names, especially since the SBELT structure does
not distinguish between local and global information. In addition,
DNS server implementations have for a long time been seeded with not
only two but an exhaustive list of the root servers' addresses. This
list is either supplied as a configuration file (root "hints", an
excerpt of the DNS root zone) or even compiled into the software.
The list of root name servers has been rather stable over the last
fifteen years. After the last four servers had been added and moved
to their final (network) destinations in 1997, there have been only
five address changes affecting the L (twice), J, B, and D servers.
Research is available for B [Mann2006] and J [BLKT2004], which shows
that several months or even years after the change had become
effective, traffic is still received on the old addresses.
Therefore, it is important that resolvers be able to cope with
change, even without relying upon configuration updates to be applied
by their operator.
Work by the ICANN SSAC and RSSAC committees, [SSAC016] and [SSAC017],
aiming at adding AAAA RRs for the root name servers' names, deals
with priming queries and so does a draft on DNSSEC Trust Anchor
maintenance [I-D.ietf-dnsop-dnssec-trust-anchor]. However, it turned
out that despite having been practiced for a long time, priming
queries have not yet been documented as an important resolver
feature.
The following sections cover parameters of both the priming query and
the response to be sent by a root name server.
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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].
2. Priming Queries
This document only deals with recursive name servers (recursive
resolvers, resolvers) for the IN CLASS.
2.1. Parameters of a Priming Query
A priming query SHOULD use a QNAME of "." and a QTYPE of NS. The
priming query MUST be sent over UDP (section 6.1.3.2 of [RFC1123]).
The UDP source port SHOULD be randomly selected [RFC5452]. The RD
bit MUST NOT be set.
The resolver SHOULD also use EDNS0 [RFC6891] and SHOULD announce and
handle a reassembly size of at least 1024 octets [RFC3226]. This is
to cover the size of a full priming response (see Section 3.3).
2.2. Repeating Priming Queries
A resolver SHOULD NOT originate a priming query more often than once
per day (or whenever the resolver starts). It SHOULD adhere to the
TTL values given in the priming response. To avoid amnesia, the
resolver MAY proactively re-prime before the old root NS RRSet
expires from the cache, but only after 75 percent of the NS RRSet's
TTL (or of the A/AAAA RRSets' TTL, whichever is lower) have passed.
Should the priming query time out, the resolver SHOULD retry with a
different target address.
2.3. Target Selection
A resolver MUST select the target for a priming query randomly from
the list of addresses (IPv4 and IPv6) available in its SBELT
structure and it MUST ensure that all targets are selected with equal
probability even upon startup. For resending the priming query to a
different server the random selection SHOULD also be used.
2.4. DNSSEC with Priming Queries
The resolver SHOULD NOT set the DNSSEC OK [RFC4033] bit.
Discussion: Delegations in referral responses are not signed, so
consequently the priming response is not validated, either. For that
to work, the priming response would also have to be self-contained in
that it would allow the resolver to not only validate the NS RRSet
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(with the root DNSKEY RRSet and the root NS RRSet's signature), but
also the A and AAAA RRSets. All this information cannot be
guaranteed to be either present at the root name servers or fit into
the priming reponse even with the largest feasible EDNS0 buffer size.
In fact, in today's Internet, with the root name servers' names under
"ROOT-SERVERS.NET.", this isn't even true for the top level domain
involved. So, even though a poisoned priming response could
drastically influence the resolver's operations, there is little a
DNSSEC enhanced priming response could achieve without the whole
validation chain. This would probably call for a different naming
scheme (see section 6.1 of [I-D.koch-dns-glue-clarifications]).
3. Priming Responses
A root name server cannot distinguish a priming query from any other
query for the root NS RRSet, except that QTYPE NS would not usually
be part of the DNS resolution process.
3.1. Expected Properties of the Priming Response
The priming response can be expected to have an RCODE of NOERROR and
the AA bit set. Also, there should be an NS RRSet in the answer
section (since the NS RRSet originates from the root zone), an empty
authority section (since the NS RRSet already appears in the answer
section) and an additional section with A and AAAA RRSets for the
root name servers pointed at by the NS RRSet. Resolver software
SHOULD NOT expect a fixed number of 13 NS RRs, since "internal" root
server setups in split DNS configurations might use a different
number of servers. Resolver software SHOULD warn the operator about
any change in the number or names of name servers or their addresses
compared to the SBELT information.
3.2. Use of the Priming Response
A resolver MAY use the priming response as it would use any other
data fed to its cache. However, it SHOULD NOT use the SBELT
information directly in any responses it hands out.
3.3. Completeness of the Response
Assuming an upper bound of thirteen root name servers and one address
each for IPv4 and IPv6, the combined size of all the A and AAAA
RRSets is 13 * (16 + 28) == 572, independent of the naming scheme.
Not even counting the NS RRSet, this value exceeds the original 512
octet payload limit.
For an EDNS response, a resolver SHOULD consider the address
information found in the additional section complete for any
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particular server that appears at all. In other words: if the
additional section only has an A RRSet for a server, the resolver
SHOULD assume that no AAAA RRSet exists. This is to avoid repeated
unnecessary queries for names of name servers that do not or not yet
offer IPv6 service, or, in perspective, will have ceased IPv4
service.
If the resolver did not announce a reassembly size larger than 512
octets, this assumption is invalid. Simple re-issuing of the priming
query does not help with those root name servers that respond with a
fixed order of addresses in the additional section. Instead the
resolver ought to issue direct queries for A and AAAA RRSets for the
remaining names. In today's environment these RRSets would be
authoritatively available from the root name servers.
4. Requirements for Root Name Servers and the Root Zone
The operational requirements for root name servers are described in
[RFC2870]. This section specifies additional guidance for the
configuration of and software deployed at the root name servers.
All DNS root name servers need to be able to provide for all
addresses of all root name servers. This can easily achieved by
keeping all root name server names in a single zone and by making all
root name servers authoritative for that zone.
If the response packet does not provide for more than 512 octets due
to lack of EDNS0 support, A RRSets SHOULD be given preference over
AAAA RRSets when filling the additional section.
[[Issue 1: EDNS0 is used as an indication of AAAA understanding on
the side of the client. What to do with payload sizes indicated by
EDNS0 that are smaller than 1024, is open to discussion. At the time
of writing, some root name servers will fill the additional section
with all available A RRSets, only adding some AAAA RRSets, when
queried over IPv4 without EDNS0. Other servers will deliver more
AAAA RRSets, therefore withholding some A RRSets completely
[RFC4472].]]
To ensure equal availability the A and AAAA RRSets for the root name
servers' names SHOULD have identical TTL values at the authoritative
source.
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[[Issue 2: Do the TTLs for the root NS RRSet and address RRSets in
the root and the ROOT-SERVERS.NET. zones need to be aligned? In real
life responses, the address RRSet's TTL values vary by name server
implementation. Is this diversity we can live with? Should the
authoritative source prevail? Is it therefore a protocol issue
rather than an operational choice of parameters?]]
5. Security Considerations
This document deals with priming a DNS resolver's cache. The usual
DNS caveats apply. Use of DNSSEC with priming queries is discussed
in section 2.4.
Spoofing a response to a priming query can be used to redirect all
queries originating from a victim resolver, therefore any difference
between the inital SBELT list and the priming response SHOULD be
brought to the operators' attention. There is also a chance that the
random target selection chooses the address of a retired root name
server. Operational measures to prevent reuse of these addresses are
out of the scope of this document.
6. IANA Considerations
This document does not propose any new IANA registry nor does it ask
for any allocation from an existing IANA registry.
However, this document deals with requirements for the root zone and
root server operations.
[[Issue3: related to issue 2 - any recommendation on the "." NS
RRSet TTL or the TTLs of the respective A and/or AAAA RRSets might go
here.]]
7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
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[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, January 2009.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
7.2. Informative References
[BLKT2004]
Barber, P., Larson, M., Kosters, M., and P. Toscano, "Life
and Times of J-Root", NANOG 32, October 2004.
[I-D.ietf-dnsop-dnssec-trust-anchor]
Larson, M. and O. Gudmundsson, "DNSSEC Trust Anchor
Configuration and Maintenance", draft-ietf-dnsop-dnssec-
trust-anchor-04 (work in progress), October 2010.
[I-D.koch-dns-glue-clarifications]
Koch, P., "DNS Glue RR Survey and Terminology
Clarification", draft-koch-dns-glue-clarifications-04
(work in progress), July 2010.
[Mann2006]
Manning, B., "persistent queries and phantom nameservers",
WIDE/CAIDA Workshop , October 2006.
[RFC2870] Bush, R., Karrenberg, D., Kosters, M., and R. Plzak, "Root
Name Server Operational Requirements", BCP 40, RFC 2870,
June 2000.
[RFC4472] Durand, A., Ihren, J., and P. Savola, "Operational
Considerations and Issues with IPv6 DNS", RFC 4472, April
2006.
[RFC4697] Larson, M. and P. Barber, "Observed DNS Resolution
Misbehavior", BCP 123, RFC 4697, October 2006.
[SSAC016] ICANN Security and Stability Advisory Committee, "Testing
Firewalls for IPv6 and EDNS0 Support", SSAC 016, January
2007.
[SSAC017] ICANN Security and Stability Advisory Committee, "Testing
Recursive Name Servers for IPv6 and EDNS0 Support", SSAC
017, February 2007.
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Appendix A. Document Revision History
This section is to be removed should the draft be published.
$Id: draft-ietf-dnsop-resolver-priming.xml,v 1.7 2014/02/14 23:27:23
pk Exp $
A.1. -04 WG Document
Revived. Updated EDNS0 reference. Minor edits for clarity.
A.2. -03 WG Document
Revived. Resolved most open issues [[]] as per previous WG
discussion. Minor edits on history and wording. All root servers
authoritative for ROOT-SERVERS.NET.
A.3. -02 WG Document
Revived. Changed use of DNSSEC OK in the priming query as per the WG
discussion.
A.4. -01 WG Document
Revived with minor edits. Open issues marked [[]].
A.5. -00 WG Document
Reposted as WG document with minor edits.
Added re-primimg proposal and A/AAAA TTL considerations.
A.6. Initial Document
First draft
Authors' Addresses
Peter Koch
DENIC eG
Kaiserstrasse 75-77
Frankfurt 60329
DE
Phone: +49 69 27235 0
Email: pk@DENIC.DE
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Matt Larson
Dyn, Inc.
150 Dow St
Manchester, NH 03101
USA
Email: mlarson@dyn.com
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