Network Working Group Robert Elz
Internet Draft University of Melbourne
Expiration Date: July 1997
Randy Bush
RGnet, Inc.
January 1997
Clarifications to the DNS Specification
draft-ietf-dnsind-clarify-04.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,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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ftp.isi.edu (US West Coast).
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. Five 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,
+ correct handling of zone cuts,
+ the issue of what is an authoritative, or canonical, name,
+ and the issue of what makes a valid DNS label.
The first three 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 specifications.
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This version contains corrections and clarifications as suggested on
the mailing list. Most notable is a change to the order of
preference of similar information from multiple sources.
Contents
1 Abstract ................................................... 1
2 Introduction ............................................... 2
3 Terminology ................................................ 2
4 Server Reply Source Address Selection ...................... 3
5 Resource Record Sets ....................................... 4
6 Zone Cuts .................................................. 6
7 Naming issues .............................................. 7
8 Name syntax ................................................ 9
9 Security Considerations .................................... 10
10 References ................................................. 10
11 Acknowledgements ........................................... 10
12 Authors' addresses ......................................... 11
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.
Clarifications to the DNS specification to avoid these problems are
made in this memo.
3. Terminology
This memo does not use the oft used expressions MUST, SHOULD, MAY, or
their negative forms. In some sections it may seem that a
specification is worded mildly, and hence some may infer that the
specification is optional. That is not correct. Anywhere that this
memo suggests that some action should be carried out, or must be
carried out, or that some behaviour is acceptable, or not, that is to
be considered as a fundamental aspect of this specification,
regardless of the specific words used. If some behaviour or action
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is truly optional, that will be clearly specified by the text.
4. 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 expect this usage.
Consequently they send replies from the "wrong" source address,
causing the reply to be discarded by the client.
4.1. 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 that 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.
4.2. Port Number Selection
Replies to all queries must be directed to the port from which they
were sent. When queries are 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].
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5. Resource Record Sets
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).
5.1. 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. The response must be marked as "truncated" if the
entire RRSet will not fit in the response.
5.2. 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. 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 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.
5.3. Receiving RRSets
Servers must never merge RRs from a response with RRs in their cache
to form an RRSet. If a response contains data 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 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, 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
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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.
5.3.1. 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. 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,
+ That from 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,
+ Additional information from an authoritative answer,
+ Data from the authority section of a non-authoritative answer,
+ Additional information from non-authoritative answers.
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
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 primary
zone files, two secondary zones (data from zone transfers) or data
from 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
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with such data exists.
"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.
5.4. 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.
6. Zone Cuts
A "Zone" is a set of one, or usually, more, domains collected and
treated as a unit. A "Zone Cut" is the division between one zone and
another. A zone comprises some subset of the DNS tree, rooted at a
domain known as the "origin" of the zone. The origin domain itself,
and some, or all, of its sub-domains, form the zone. The existence
of a zone cut is indicated by the presence, in the zone, of a
NameServer (NS) record for any domain other than the origin of the
zone.
6.1. Zone authority
The authoritative servers for a zone are enumerated in the NS records
for the origin of the zone, which, along with a Start of Authority
(SOA) record are the mandatory records in every zone. Such a server
is authoritative for all resource records in a zone that are not in
another zone. The NS records that indicate a zone cut are the
property of the child zone created, as are any other records for the
origin of that child zone, or any sub-domains of it. A server for a
zone should not return authoritative answers for queries related to
names in another zone, which includes the NS, and perhaps A, records
at a zone cut, unless it also happens to be a server for the other
zone.
Other than the DNSSEC cases mentioned immediately below, servers
should ignore data other than NS records, and necessary A records to
locate the servers listed in the NS records, that may happen to be
configured in a zone at a zone cut.
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6.2. DNSSEC issues
The DNS security mechanisms [RFC2065] complicate this somewhat, as
some of the new resource record types added are very unusual when
compared with other DNS RRs. In particular the NXT ("next") RR type
contains information about which names exist in a zone, and hence
which do not, and thus must necessarily relate to the zone in which
it exists. The same domain name may have different NXT records in
the parent zone and the child zone, and both are valid, and are not
an RRSet.
Since NXT records are intended to be automatically generated, rather
than configured by DNS operators, servers may, but are not required
to, retain all differing NXT records they receive regardless of the
rules in section 5.3.
For a secure parent zone to securely indicate that a subzone is
insecure, DNSSEC requires that a KEY RR indicating that the subzone
is insecure, and the parent zone's authenticating SIG RR(s) be
present in the parent zone, as they by definition cannot be in the
subzone. Where a subzone is secure, the KEY and SIG can be
duplicated in both zone files, but should always be present in the
subzone.
Note that in none of these cases should a server for the parent zone,
not also being a server for the subzone, set the AA bit in any
response for a label at a zone cut.
7. 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.
7.1. 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 should generally be
a name that exists elsewhere in the DNS, though some applications for
aliases with no accompanying canonical name exist. An alias name
(label of a CNAME record) may, if DNSSEC is in use, have SIG, NXT,
and KEY RRs, but may have no other data. That is, for any label in
the DNS (any domain name) exactly one of the following is true:
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+ one CNAME record exists, optionally accompanied by SIG, NXT, and
KEY RRs,
+ one or more records exist, none being CNAME records,
+ the name does not exist at all.
7.1.1. 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.
7.2. 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. No such restriction should
be inferred.
Note that while the value of a PTR record must not be an alias, there
is no requirement that the process of resolving a PTR record not
encounter any aliases. The label that is being looked up for a PTR
value might have a CNAME record. That is, it might be an alias. The
value of that CNAME RR, if not another alias, will give the location
where the PTR record is found. That record gives the result of the
PTR type lookup. This final result, the value of the PTR RR, is the
label which must not be an alias.
7.3. 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 must not be an alias. Not only is
the specification 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. This
domain name must have as its value one or more address records.
Currently those will be A records, however in the future other record
types giving addressing information may be acceptable. It can also
have other RRs, but never a CNAME RR.
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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 that 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 with the NS or MX
value. This can cause extra queries, and extra network burden, on
every query. It is trivial to avoid this by resolving the alias and
placing the canonical name directly in the affected record just once
when it is updated or installed. In some particular hard cases the
lack of the additional section address records in the results of a NS
lookup can cause the request to fail.
8. 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 on 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. The length of
any one label is limited to between 1 and 63 octets. A full domain
name is limited to 255 octets (including the separators). The zero
length full name is defined as representing the root of the DNS tree,
and is typically written and displayed as ".". Those restrictions
aside, any binary string whatever can be used as the label of any
resource record. Similarly, any binary string can serve as the value
of any record that includes a domain name as some or all of its value
(SOA, NS, MX, PTR, CNAME, and any others that may be added).
Implementations of the DNS protocols must not place any restrictions
on the labels that can be used. In particular, DNS servers must not
refuse to serve a zone because it contains labels that might not be
acceptable to some DNS client programs. A DNS server may be
configurable to issue warnings when loading, or even to refuse to
load, a primary zone containing labels that might be considered
questionable, however this should not happen by default.
Note however, that the various applications that make use of DNS data
can have restrictions imposed on what particular values are
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
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whatever restrictions are appropriate to their circumstances on the
values they use as keys for DNS lookup requests, and on the values
returned by the DNS.
See also [RFC1123] section 6.1.3.5.
9. Security Considerations
This document does not consider security.
In particular, nothing in section 4 is any way related to, or useful
for, any security related purposes.
Section 5.3.1 is also not related to security. Security of DNS data
will be obtained by the Secure DNS [RFC2065], which is mostly
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.
10. 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.
[RFC2065] Domain Name System Security Extensions,
D. E. Eastlake, 3rd, C. W. Kaufman, January 1997.
11. 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. Particular thanks to
Donald E. Eastlake, 3rd, for help with the DNSSEC issues in this
document, and to John Gilmore for pointing out where the
clarifications were not necessarily clarifying.
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12. 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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