Internet DRAFT - draft-ietf-dnsop-resolver-priming
draft-ietf-dnsop-resolver-priming
Network Working Group P. Koch
Internet-Draft DENIC eG
Intended status: Best Current Practice M. Larson
Expires: June 27, 2017 P. Hoffman
ICANN
December 24, 2016
Initializing a DNS Resolver with Priming Queries
draft-ietf-dnsop-resolver-priming-11
Abstract
This document describes the queries that a DNS resolver should emit
to initialize its cache. The result is that the resolver gets both a
current NS RRSet for the root zone and the necessary address
information for reaching the root servers.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 27, 2017.
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
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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.
1. Introduction
Recursive DNS resolvers need a starting point to resolve queries.
[RFC1034] describes a common scenario for recursive resolvers: they
begin with an empty cache and some configuration for finding the
names and addresses of the DNS root servers. [RFC1034] describes
that configuration as a list of servers that will give authoritative
answers to queries about the root. This has become a common
implementation choice for recursive resolvers, and is the topic of
this document.
This document describes the steps needed for this common
implementation choice. Note that this is not the only way to start a
recursive name server with an empty cache, but it is the only one
described in [RFC1034]. Some implementers have chosen other
directions, some of which work well and others of which fail
(sometimes disastrously) under different conditions. For example, an
implementation that only gets the addresses of the root name servers
from configuration, not from the DNS as described in this document,
will have stale data that could cause slower resolution.
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].
This document only deals with recursive name servers (recursive
resolvers, resolvers) for the IN class.
2. Description of Priming
Priming is the act of finding the list of root servers from a
configuration that lists some or all of the purported IP addresses of
some or all of those root servers. A recursive resolver starts with
no information about the root servers, and ends up with a list of
their names and their addresses.
Priming is described in Sections 5.3.2 and 5.3.3 of [RFC1034]. The
scenario used in that description, that of a recursive server that is
also authoritative, is no longer as common.
The configured list of IP addresses for the root servers usually
comes from the vendor or distributor of the recursive server
software. This list is usually correct and complete when shipped,
but may become out of date over time.
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The list of root server operators and the domain name associated with
each one has been stable since 1997. However, there are address
changes for the root server domain names, both for IPv4 and IPv6
addresses. However, research shows that after those addresses
change, some resolvers never get the new 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.
Root server change is the main reason that resolvers need to do
priming instead of just going from a configured list to get a full
and accurate list of root servers.
3. Priming Queries
A priming query is a DNS query used to get the root server
information in a resolver. It has a QNAME of "." and a QTYPE of NS,
and is sent to one of the addresses in the configuration for the
recursive resolver. The priming query can be sent over either UDP or
TCP. If the query is sent over UDP, the source port SHOULD be
randomly selected (see [RFC5452]). The RD bit MAY be set to 0 or 1,
although the meaning of it being set to 1 is undefined for priming
queries.
The recursive resolver SHOULD use EDNS0 [RFC6891] for priming queries
and SHOULD announce and handle a reassembly size of at least 1024
octets [RFC3226]. Doing so allows responses that cover the size of a
full priming response (see Section 4.2) for the current set of root
servers. See Section 3.3 for discussion of setting the DNSSEC OK
(DO) bit (defined in [RFC4033]).
3.1. Repeating Priming Queries
The recursive resolver SHOULD send a priming query only when it is
needed, such as when the resolver starts with an empty cache and when
the NS RRset for the root zone has expired. Because the NS records
for the root are not special, the recursive resolver expires those NS
records according to their TTL values. (Note that a recursive
resolver MAY pre-fetch the NS RRset before it expires.)
If a priming query does not get a response, the recursive resolver
needs to retry the query with a different target address from the
configuration.
3.2. Target Selection
In order to spread the load across all the root server domain names,
the recursive resolver SHOULD select the target for a priming query
randomly from the list of addresses. The recursive resolver might
choose either IPv4 and IPv6 addresses based on its knowledge of
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whether the system on which it is running has adequate connectivity
on either type of address.
Note that this recommended method is not the only way to choose from
the list in a recursive resolver's configuration. Two other common
methods include picking the first from the list, and remembering
which address in the list gave the fastest response earlier and using
that one. There are probably other methods in use today. However,
the random method listed above SHOULD be used for priming.
3.3. DNSSEC with Priming Queries
The resolver MAY set the DNSSEC OK (DO) bit. At the time this
document is being published, there is little use to performing DNSSEC
validation on the priming query. Currently all root name server
names end in "root-servers.net" and the AAAA and A RRsets for the
root server names reside in the "root-servers.net" zone. All root
servers are also authoritative for this zone, allowing priming
responses to include the appropriate root name server A and AAAA
RRsets. But because the "root-servers.net" zone is not currently
signed, these RRsets cannot be validated.
A man-in-the-middle attack on the priming query could direct a
resolver to a rogue root name server. Note, however, that a
validating resolver will not accept responses from rogue root name
servers if they are different from the real responses because the
resolver has a trust anchor for the root and the answers from the
root are signed. Thus, if there is a man-in-the-middle attack on the
priming query, the only result for a validating resolver will be a
denial of service, not the resolver's accepting the bad responses.
If the "root-servers.net" zone is later signed, or if the root
servers are named in a different zone and that zone is signed, having
DNSSEC validation for the priming queries might be valuable.
4. Priming Responses
A priming query is a normal DNS query. Thus, a root name server
cannot distinguish a priming query from any other query for the root
NS RRSet. Thus, the root server's response will also be a normal DNS
response.
4.1. Expected Properties of the Priming Response
The priming response is expected to have an RCODE of NOERROR, and to
have the AA bit set. Also, it is expected to have an NS RRSet in the
Answer section (because the NS RRSet originates from the root zone),
and an empty Authority section (because the NS RRSet already appears
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in the Answer section). There will also be an Additional section
with A and/or AAAA RRSets for the root name servers pointed at by the
NS RRSet.
Resolver software SHOULD treat the response to the priming query as a
normal DNS response, just as it would use any other data fed to its
cache. Resolver software SHOULD NOT expect exactly 13 NS RRs because
historically some root servers have returned fewer.
4.2. Completeness of the Response
There are currently 13 root servers. All have one IPv4 address and
one IPv6 address. Not even counting the NS RRSet, the combined size
of all the A and AAAA RRSets exceeds the original 512 octet payload
limit from [RFC1035].
In the event of a response where the Additional section omits certain
root server address information, 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 recursive
resolver needs to issue direct queries for A and AAAA RRSets for the
remaining names. Currently, these RRSets would be authoritatively
available from the root name servers.
5. Security Considerations
Spoofing a response to a priming query can be used to redirect all of
the queries originating from a victim recursive resolver to one or
more servers for the attacker. Until the responses to priming
queries are protected with DNSSEC, there is no definitive way to
prevent such redirection.
An on-path attacker who sees a priming query coming from a resolver
can inject false answers before a root server can give correct
answers. If the attacker's answers are accepted, this can set up the
ability to give further false answers for future queries to the
resolver. False answers for root servers are more dangerous than,
say, false answers for TLDs, because the root is the highest node of
the DNS. See Section 3.3 for more discussion.
In both of the scenarios above, a validating resolver will be able to
detect the attack if its chain of queries comes to a zone that is
signed, but not for those that are unsigned.
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6. IANA Considerations
None.
7. 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>.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226,
DOI 10.17487/RFC3226, December 2001,
<http://www.rfc-editor.org/info/rfc3226>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>.
[RFC5452] Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452,
DOI 10.17487/RFC5452, January 2009,
<http://www.rfc-editor.org/info/rfc5452>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891,
DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
Appendix A. Acknowledgements
This document is the product of the DNSOP WG and benefitted from the
reviews done there.
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Authors' Addresses
Peter Koch
DENIC eG
Kaiserstrasse 75-77
Frankfurt 60329
DE
Phone: +49 69 27235 0
Email: pk@DENIC.DE
Matt Larson
ICANN
Email: matt.larson@icann.org
Paul Hoffman
ICANN
Email: paul.hoffman@icann.org
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