Network Working Group Robert Elz
Internet Draft University of Melbourne
Expiration Date: May 1997
Randy Bush
RGnet, Inc.
November 1996
Clarifications to the DNS Specification
draft-ietf-dnsind-clarify-02.txt
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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1. Abstract
This draft considers some areas that have been identified as problems
with the specification of the Domain Name System, and proposes
remedies for the defects identified. Four separate issues are
considered:
+ IP packet header address usage from multi-homed servers,
+ TTLs in sets of records with the same name, class, and type,
+ the issue of what is an authoritative, or canonical, name,
+ and the issue of what makes a valid DNS label.
The first two of these are areas where the correct behaviour has been
somewhat unclear, we seek to rectify that. The other two are already
adequately specified, however the specifications seem to be sometimes
ignored. We seek to reinforce the existing specification.
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2. Introduction
Several problem areas in the Domain Name System specification
[RFC1034, RFC1035] have been noted through the years [RFC1123]. This
draft addresses several additional problem areas. The issues here
are independent. Those issues are the question of which source
address a multi-homed DNS server should use when replying to a query,
the issue of differing TTLs for DNS records with the same label,
class and type, and the issue of canonical names, what they are, how
CNAME records relate, what names are legal in what parts of the DNS,
and what is the valid syntax of a DNS name.
Suggestions for clarifications to the DNS specification to avoid
these problems are made in this memo. The solutions proposed herein
are intended to stimulate discussion. It is possible that the sense
of either may be reversed before the next iteration of this draft,
but less likely now than it was before the previous version.
3. Server Reply Source Address Selection
Most, if not all, DNS clients, whether servers acting as clients for
the purposes of recursive query resolution, or resolvers, expect the
address from which a reply is received to be the same address as that
to which the query eliciting the reply was sent. This, along with
the identifier (ID) in the reply is used for disambiguating replies,
and filtering spurious responses. This may, or may not, have been
intended when the DNS was designed, but is now a fact of life.
Some multi-homed hosts running DNS servers fail to anticipate this
usage, and consequently send replies from the "wrong" source address,
causing the reply to be discarded by the client.
3.1. UDP Source Address Selection
To avoid these problems, servers when responding to queries using UDP
must cause the reply to be sent with the source address field in the
IP header set to the address that was in the destination address
field of the IP header of the packet containing the query causing the
response. If this would cause the response to be sent from an IP
address which is not permitted for this purpose, then the response
may be sent from any legal IP address allocated to the server. That
address should be chosen to maximise the possibility that the client
will be able to use it for further queries. Servers configured in
such a way that not all their addresses are equally reachable from
all potential clients need take particular care when responding to
queries sent to anycast, multicast, or similar, addresses.
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3.2. Port Number Selection
Replies to all queries must be directed to the port from which they
were sent. With queries received via TCP this is an inherent part of
the transport protocol, for queries received by UDP the server must
take note of the source port and use that as the destination port in
the response. Replies should always be sent from the port to which
they were directed. Except in extraordinary circumstances, this will
be the well known port assigned for DNS queries [RFC1700].
4. Resource Record Sets
Each DNS Resource Record (RR) each has a label, class, type, and
data. While 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 possible for many 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).
4.1. 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, or the response shall be marked as "truncated" if
the entire RRSet will not fit in the response.
4.2. 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, however no uses for this
have been found which 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 of 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 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.
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4.3. Receiving RRSets
Servers must never merge RRs from a response with RRs in their cache
to form an RRSet. If a response contains data which would form an
RRSet with data in a server's cache the server must either ignore the
RRs in the response, or use those to replace the existing RRSet 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, assuming that 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 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.
4.3.1. 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. That is, an
authoritative answer from a reply should replace cached data that had
been obtained from additional information in an earlier reply, but
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,
+ That from the answer section of an authoritative reply,
+ Glue from a primary zone, or glue from a zone transfer,
+ Data from the authority section of an authoritative answer,
+ Data from the answer section of a non-authoritative answer,
+ Additional information from an authoritative answer,
+ Data from the authority section of a non-authoritative answer,
+ Additional information from non-authoritative answers.
Where authenticated data has been received it shall be considered
more trustworthy than unauthenticated data of the same type.
Note that, glue excluded, it is impossible for data from two primary
zone files, two secondary zones (data from zone transfers) or data
from a primary and secondary zones to ever conflict. Wher for the
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same name exists in such 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 such data
does exist.
"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.
4.4. 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 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.
5. Naming issues
It has sometimes been inferred from some sections of the DNS
specification [RFC1034, RFC1035] that a host, or perhaps an interface
of a host, is permitted exactly one authoritative, or official, name,
called the canonical name. There is no such requirement in the DNS.
5.1. CNAME records
The DNS CNAME ("canonical name") record exists to provide the
canonical name associated with an alias name. There may be only one
such canonical name for any one alias. That name must be a name that
exists elsewhere in the DNS. The alias name must have no other data
in the DNS. That is, for any label in the DNS (any domain name)
exactly one of the following is true:
+ one CNAME record exists
+ other records exist, possibly many records, none of them being
CNAME records
+ the name does not exist at all.
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5.1.1. CNAME terminology
It has been traditional to refer to the label of a CNAME record as "a
CNAME". This is unfortunate, as "CNAME" is an abbreviation of
"canonical name", and the label of a CNAME record is most certainly
not a canonical name. It is, however, an entrenched usage, care must
therefore be taken to be very clear whether the label, or the value
(the canonical name) of a CNAME resource record is intended. In this
document, the label of a CNAME resource record will always be
referred to as an alias.
5.2. PTR records
Confusion about canonical names has lead to a belief that a PTR
record should have exactly one RR in its RRSet. This is incorrect,
the relevant section of RFC1034 (section 3.6.2) indicates that the
value of a PTR record should be a canonical name. That is, it should
not be an alias. There is no implication in that section that only
one PTR record is permitted for a name, and no such restriction
should be inferred.
5.3. MX and NS records
The domain name used as the value of a NS resource record, or part of
the value of a MX resource record should not be an alias. Not only
is the specification quite clear on this point, but using an alias in
either of these positions neither works as well as might be hoped,
nor well fulfills the ambition that may have led to this approach.
Searching for either NS or MX records causes "additional section
processing" in which address records associated with the value of the
record sought are appended to the answer. This helps avoid needless
extra queries which are easily anticipated when the first was made.
Additional section processing does not include CNAME records, let
alone the address records that may be associated with the canonical
name derived from the alias. Thus, if an alias is used as the value
of an NS or MX record, no address will be returned together with the
NS or MX value. This can cause extra queries, and extra network
burden, on every query, that could have been trivially avoided by
resolving the alias and placing the canonical name directly in the
affected record just once when it was updated or installed. In some
particular hard cases the lack of the additional section address
records in the results of a NS lookup can actually cause the request
to fail.
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6. Name syntax
Occasionally it is assumed that the Domain Name System serves only
the purpose of mapping Internet host names to data, and mapping
Internet addresses to host names. This is not correct, the DNS is a
general (if somewhat limited) hierarchical database, and can store
almost any kind of data, for almost any purpose.
The DNS itself places only one restriction upon the particular labels
that can be used to identify resource records. That one restriction
relates to the length of the label and the full name. Any one label
is limited to 63 octets, and a full name is limited to 255 octets
(including the separators). That restriction aside, any binary
string whatever can be used as the label of any resource record, and
as the value of one of the records that includes a domain name as
some or all of its value (SOA, NS, MX, PTR, CNAME, SRV, and any
others that may be added). Implementations of the DNS protocols must
not place any restrictions on the labels that can be used.
Note however, that the various applications that make use of DNS data
can have restrictions imposed upon what particular data is acceptable
in their environment. For example, that any binary label can have an
MX record does not imply that any binary name can be used as the host
part of an e-mail address. Clients of the DNS can impose whatever
restrictions are appropriate to their circumstances to the values
they use as keys for DNS lookup requests, and to the values returned
by the DNS.
See also [RFC1123] section 6.1.3.5.
7. Security Considerations
This document does not consider security.
In particular, nothing in section 3 is any way related to, or useful
for, any security related purposes.
Section 4.3.1 is also not related to security. Security of DNS data
will be obtained by the Secure DNS [DNSSEC], which is orthogonal to
this memo.
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 lessen them.
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8. References
[RFC1034] Domain Names - Concepts and Facilities, (STD 13)
P. Mockapetris, ISI, November 1987.
[RFC1035] Domain Names - Implementation and Specification (STD 13)
P. Mockapetris, ISI, November 1987
[RFC1123] Requirements for Internet hosts - application and support,
(STD 3) R. Braden, January 1989
[RFC1700] Assigned Numbers (STD 2)
J. Reynolds, J. Postel, October 1994.
[DNSSEC] Domain Name System Security Extensions,
D. E. Eastlake, 3rd, C. W. Kaufman,
Work in Progress (soon to be an RFC), August 1996.
9. Acknowledgements
This memo arose from discussions in the DNSIND working group of the
IETF in 1995 and 1996, the members of that working group are largely
responsible for the ideas captured herein.
10. Authors' addresses
Robert Elz
Computer Science
University of Melbourne
Parkville, Victoria, 3052
Australia.
EMail: kre@munnari.OZ.AU
Randy Bush
RGnet, Inc.
10361 NE Sasquatch Lane
Bainbridge Island, Washington, 98110
United States.
EMail: randy@psg.com
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