Internet DRAFT - draft-bill-dnsop-resource-record-sets
draft-bill-dnsop-resource-record-sets
INTERNET-DRAFT Declan Ma, Ed.
Intended Status: Proposed Standard zDNS Ltd.
Expires: 2015-10-15 2015-05-22
DNS Resource Record Sets
draft-bill-dnsop-resource-record-sets-00
Abstract
RFC 2181 collected eight independent considerations and created a single
docuement to address each of them in turn. Over the following two decades
it has become clear that each of these items should be considered and evovolve
in its own right, as suggested in RFC 2181. This document extracts the exact
text from RFC 2181 and places it into its own track.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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The list of current Internet-Drafts can be accessed at
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Copyright and License 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
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to this document. Code Components extracted from this document must
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.
Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3 Sending RRs from an RRSet . . . . . . . . . . . . . . . . . . . 3
4 TTLs of RRs in an RRSet . . . . . . . . . . . . . . . . . . . . 3
5 DNSSEC Special Cases . . . . . . . . . . . . . . . . . . . . . 4
5.1 SIG records and RRSets . . . . . . . . . . . . . . . . . . 4
5.2 NXT RRs . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6 Receiving RRSets . . . . . . . . . . . . . . . . . . . . . . . 5
6.1 Ranking data . . . . . . . . . . . . . . . . . . . . . . . 6
7 Sending RRSets (reprise) . . . . . . . . . . . . . . . . . . . 7
8 Security Considerations . . . . . . . . . . . . . . . . . . . . 7
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
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1 Introduction
Each DNS Resource Record (RR) has a label, class, type, and data. It
is meaningless for two records to ever have label, class, type and
data all equal - servers should suppress such duplicates if
encountered. It is however possible for most record types to exist
with the same label, class and type, but with different data. Such a
group of records is hereby defined to be a Resource Record Set
(RRSet).
This document is intended to specify how to process DNS RRSets during
DNS operations.
2 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 RFC 2119 [RFC2119].
3 Sending RRs from an RRSet
A query for a specific (or non-specific) label, class, and type, will
always return all records in the associated RRSet - whether that be
one or more RRs. The response must be marked as "truncated" if the
entire RRSet will not fit in the response.
4 TTLs of RRs in an RRSet
Resource Records also have a time to live (TTL). It is possible for
the RRs in an RRSet to have different TTLs. No uses for this have
been found that cannot be better accomplished in other ways. This
can, however, cause partial replies (not marked "truncated") from a
caching server, where the TTLs for some but not all the RRs in the
RRSet have expired.
Consequently the use of differing TTLs in an RRSet is hereby
deprecated, the TTLs of all RRs in an RRSet must be the same.
Should a client receive a response containing RRs from an RRSet with
differing TTLs, it should treat this as an error. If the RRSet
concerned is from a non-authoritative source for this data, the
client should simply ignore the RRSet, and if the values were
required, seek to acquire them from an authoritative source. Clients
that are configured to send all queries to one, or more, particular
servers should treat those servers as authoritative for this purpose.
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Should an authoritative source send such a malformed RRSet, the
client should treat the RRs for all purposes as if all TTLs in the
RRSet had been set to the value of the lowest TTL in the RRSet. In
no case may a server send an RRSet with TTLs not all equal.
5 DNSSEC Special Cases
Two of the record types added by DNS Security (DNSSEC) [RFC2065]
require special attention when considering the formation of Resource
Record Sets. Those are the SIG and NXT records. It should be noted
that DNS Security is still very new, and there is, as yet, little
experience with it. Readers should be prepared for the information
related to DNSSEC contained in this document to become outdated as
the DNS Security specification matures.
5.1 SIG records and RRSets
A SIG record provides signature (validation) data for another RRSet
in the DNS. Where a zone has been signed, every RRSet in the zone
will have had a SIG record associated with it. The data type of the
RRSet is included in the data of the SIG RR, to indicate with which
particular RRSet this SIG record is associated. Were the rules above
applied, whenever a SIG record was included with a response to
validate that response, the SIG records for all other RRSets
associated with the appropriate node would also need to be included.
In some cases, this could be a very large number of records, not
helped by their being rather large RRs.
Thus, it is specifically permitted for the authority section to
contain only those SIG RRs with the "type covered" field equal to the
type field of an answer being returned. However, where SIG records
are being returned in the answer section, in response to a query for
SIG records, or a query for all records associated with a name
(type=ANY) the entire SIG RRSet must be included, as for any other RR
type.
Servers that receive responses containing SIG records in the
authority section, or (probably incorrectly) as additional data, must
understand that the entire RRSet has almost certainly not been
included. Thus, they must not cache that SIG record in a way that
would permit it to be returned should a query for SIG records be
received at that server. RFC2065 actually requires that SIG queries
be directed only to authoritative servers to avoid the problems that
could be caused here, and while servers exist that do not understand
the special properties of SIG records, this will remain necessary.
However, careful design of SIG record processing in new
implementations should permit this restriction to be relaxed in the
future, so resolvers do not need to treat SIG record queries
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specially.
It has been occasionally stated that a received request for a SIG
record should be forwarded to an authoritative server, rather than
being answered from data in the cache. This is not necessary - a
server that has the knowledge of SIG as a special case for processing
this way would be better to correctly cache SIG records, taking into
account their characteristics. Then the server can determine when it
is safe to reply from the cache, and when the answer is not available
and the query must be forwarded.
5.2 NXT RRs
Next Resource Records (NXT) are even more peculiar. There will only
ever be one NXT record in a zone for a particular label, so
superficially, the RRSet problem is trivial. However, at a zone cut,
both the parent zone, and the child zone (superzone and subzone in
RFC2065 terminology) will have NXT records for the same name. Those
two NXT records do not form an RRSet, even where both zones are
housed at the same server. NXT RRSets always contain just a single
RR. Where both NXT records are visible, two RRSets exist. However,
servers are not required to treat this as a special case when
receiving NXT records in a response. They may elect to notice the
existence of two different NXT RRSets, and treat that as they would
two different RRSets of any other type. That is, cache one, and
ignore the other. Security aware servers will need to correctly
process the NXT record in the received response though.
6 Receiving RRSets
Servers must never merge RRs from a response with RRs in their cache
to form an RRSet. If a response contains data that would form an
RRSet with data in a server's cache the server must either ignore the
RRs in the response, or discard the entire RRSet currently in the
cache, as appropriate. Consequently the issue of TTLs varying
between the cache and a response does not cause concern, one will be
ignored. That is, one of the data sets is always incorrect if the
data from an answer differs from the data in the cache. The
challenge for the server is to determine which of the data sets is
correct, if one is, and retain that, while ignoring the other. Note
that if a server receives an answer containing an RRSet that is
identical to that in its cache, with the possible exception of the
TTL value, it may, optionally, update the TTL in its cache with the
TTL of the received answer. It should do this if the received answer
would be considered more authoritative (as discussed in the next
section) than the previously cached answer.
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6.1 Ranking data
When considering whether to accept an RRSet in a reply, or retain an
RRSet already in its cache instead, a server should consider the
relative likely trustworthiness of the various data. An
authoritative answer from a reply should replace cached data that had
been obtained from additional information in an earlier reply.
However additional information from a reply will be ignored if the
cache contains data from an authoritative answer or a zone file.
The accuracy of data available is assumed from its source.
Trustworthiness shall be, in order from most to least:
+ Data from a primary zone file, other than glue data,
+ Data from a zone transfer, other than glue,
+ The authoritative data included in the answer section of an
authoritative reply.
+ Data from the authority section of an authoritative answer,
+ Glue from a primary zone, or glue from a zone transfer,
+ Data from the answer section of a non-authoritative answer, and
non-authoritative data from the answer section of authoritative
answers,
+ Additional information from an authoritative answer,
Data from the authority section of a non-authoritative answer,
Additional information from non-authoritative answers.
Note that the answer section of an authoritative answer normally
contains only authoritative data. However when the name sought is an
alias only the record describing that alias is
necessarily authoritative. Clients should assume that other records
may have come from the server's cache. Where authoritative answers
are required, the client should query again, using the canonical name
associated with the alias.
Unauthenticated RRs received and cached from the least trustworthy of
those groupings, that is data from the additional data section, and
data from the authority section of a non-authoritative answer, should
not be cached in such a way that they would ever be returned as
answers to a received query. They may be returned as additional
information where appropriate. Ignoring this would allow the
trustworthiness of relatively untrustworthy data to be increased
without cause or excuse.
When DNS security [RFC2065] is in use, and an authenticated reply has
been received and verified, the data thus authenticated shall be
considered more trustworthy than unauthenticated data of the same
type. Note that throughout this document, "authoritative" means a
reply with the AA bit set. DNSSEC uses trusted chains of SIG and KEY
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records to determine the authenticity of data, the AA bit is almost
irrelevant. However DNSSEC aware servers must still correctly set
the AA bit in responses to enable correct operation with servers that
are not security aware (almost all currently).
Note that, glue excluded, it is impossible for data from two
correctly configured primary zone files, two correctly configured
secondary zones (data from zone transfers) or data from correctly
configured primary and secondary zones to ever conflict. Where glue
for the same name exists in multiple zones, and differs in value, the
nameserver should select data from a primary zone file in preference
to secondary, but otherwise may choose any single set of such data.
Choosing that which appears to come from a source nearer the
authoritative data source may make sense where that can be
determined. Choosing primary data over secondary allows the source
of incorrect glue data to be discovered more readily, when a problem
with such data exists. Where a server can detect from two zone files
that one or more are incorrectly configured, so as to create
conflicts, it should refuse to load the zones determined to be
erroneous, and issue suitable diagnostics.
"Glue" above includes any record in a zone file that is not properly
part of that zone, including nameserver records of delegated sub-
zones (NS records), address records that accompany those NS records
(A, AAAA, etc), and any other stray data that might appear.
7 Sending RRSets (reprise)
A Resource Record Set should only be included once in any DNS reply.
It may occur in any of the Answer, Authority, or Additional
Information sections, as required. However it should not be repeated
in the same, or any other, section, except where explicitly required
by a specification. For example, an AXFR response requires the SOA
record (always an RRSet containing a single RR) be both the first and
last record of the reply. Where duplicates are required this way,
the TTL transmitted in each case must be the same.
8 Security Considerations
It is not believed that anything in this document adds to any
security issues that may exist with the DNS, nor does it do anything
to that will necessarily lessen them. Correct implementation of the
clarifications in this document might play some small part in
limiting the spread of non-malicious bad data in the DNS, but only
DNSSEC can help with deliberate attempts to subvert DNS data.
9 References
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[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.
[RFC2065] Eastlake 3rd, D. and C. Kaufman, "Domain Name System
Security Extensions", RFC 2065, January 1997.
[RFC2199] Ramos, A., "Request for Comments Summary RFC Numbers 2100-
2199", RFC 2199, January 1998.
10 Authors' Addresses
Declan Ma, Ed.
ZDNS Ltd.
4, South 4th Street, Zhongguancun,
Haidian, Beijing 100190,
China
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