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rfc:rfc2181

Network Working Group R. Elz Request for Comments: 2181 University of Melbourne Updates: 1034, 1035, 1123 R. Bush Category: Standards Track RGnet, Inc.

                                                             July 1997
              Clarifications to the DNS Specification

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

1. Abstract

 This document considers some areas that have been identified as
 problems with the specification of the Domain Name System, and
 proposes remedies for the defects identified.  Eight 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,
   + three minor issues concerning SOA records and their use,
   + the precise definition of the Time to Live (TTL)
   + Use of the TC (truncated) header bit
   + the issue of what is an authoritative, or canonical, name,
   + and the issue of what makes a valid DNS label.
 The first six 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.

Elz & Bush Standards Track [Page 1] RFC 2181 Clarifications to the DNS Specification July 1997

Contents

  1  Abstract  ...................................................   1
  2  Introduction  ...............................................   2
  3  Terminology  ................................................   3
  4  Server Reply Source Address Selection  ......................   3
  5  Resource Record Sets  .......................................   4
  6  Zone Cuts  ..................................................   8
  7  SOA RRs  ....................................................  10
  8  Time to Live (TTL)  .........................................  10
  9  The TC (truncated) header bit  ..............................  11
 10  Naming issues  ..............................................  11
 11  Name syntax  ................................................  13
 12  Security Considerations  ....................................  14
 13  References  .................................................  14
 14  Acknowledgements  ...........................................  15
 15  Authors' Addresses  .........................................  15

2. Introduction

 Several problem areas in the Domain Name System specification
 [RFC1034, RFC1035] have been noted through the years [RFC1123].  This
 document 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.  A minor ambiguity in RFC1034 concerned with SOA
 records is also corrected, as is one in the definition of the TTL
 (Time To Live) and some possible confusion in use of the TC bit.

Elz & Bush Standards Track [Page 2] RFC 2181 Clarifications to the DNS Specification July 1997

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
 is truly optional, that will be clearly specified by the text.

4. Server Reply Source Address Selection

 Most, if not all, DNS clients, 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 is true for servers acting as
 clients for the purposes of recursive query resolution, as well as
 simple resolver clients.  The address, 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 generate a reply using a
 source address that is not the same as the destination address from
 the client's request packet.  Such replies will be discarded by the
 client because the source address of the reply does not match that of
 a host to which the client sent the original request.  That is, it
 appears to be an unsolicited response.

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.

Elz & Bush Standards Track [Page 3] RFC 2181 Clarifications to the DNS Specification July 1997

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].

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 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.
 Should an authoritative source send such a malformed RRSet, the

Elz & Bush Standards Track [Page 4] RFC 2181 Clarifications to the DNS Specification July 1997

 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.

5.3. 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.

5.3.1. 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
 specially.

Elz & Bush Standards Track [Page 5] RFC 2181 Clarifications to the DNS Specification July 1997

 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.

5.3.2. 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.

5.4. 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.

Elz & Bush Standards Track [Page 6] RFC 2181 Clarifications to the DNS Specification July 1997

5.4.1. 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 (see section 10.1.1) 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

Elz & Bush Standards Track [Page 7] RFC 2181 Clarifications to the DNS Specification July 1997

 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.

5.5. 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.

6. Zone Cuts

 The DNS tree is divided into "zones", which are collections of
 domains that are treated as a unit for certain management purposes.
 Zones are delimited by "zone cuts".  Each zone cut separates a
 "child" zone (below the cut) from a "parent" zone (above the cut).
 The domain name that appears at the top of a zone (just below the cut
 that separates the zone from its parent) is called the zone's
 "origin".  The name of the zone is the same as the name of the domain
 at the zone's origin.  Each zone comprises that subset of the DNS
 tree that is at or below the zone's origin, and that is above the

Elz & Bush Standards Track [Page 8] RFC 2181 Clarifications to the DNS Specification July 1997

 cuts that separate the zone from its children (if any).  The
 existence of a zone cut is indicated in the parent zone by the
 existence of NS records specifying the origin of the child zone.  A
 child zone does not contain any explicit reference to its parent.

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.

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.  See also section 5.3.2.
 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 5.4.
 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 records will be
 present, and authoritative, in that zone, but should also always be
 present in the parent zone (if secure).

Elz & Bush Standards Track [Page 9] RFC 2181 Clarifications to the DNS Specification July 1997

 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. SOA RRs

 Three minor issues concerning the Start of Zone of Authority (SOA)
 Resource Record need some clarification.

7.1. Placement of SOA RRs in authoritative answers

 RFC1034, in section 3.7, indicates that the authority section of an
 authoritative answer may contain the SOA record for the zone from
 which the answer was obtained.  When discussing negative caching,
 RFC1034 section 4.3.4 refers to this technique but mentions the
 additional section of the response.  The former is correct, as is
 implied by the example shown in section 6.2.5 of RFC1034.  SOA
 records, if added, are to be placed in the authority section.

7.2. TTLs on SOA RRs

 It may be observed that in section 3.2.1 of RFC1035, which defines
 the format of a Resource Record, that the definition of the TTL field
 contains a throw away line which states that the TTL of an SOA record
 should always be sent as zero to prevent caching.  This is mentioned
 nowhere else, and has not generally been implemented.
 Implementations should not assume that SOA records will have a TTL of
 zero, nor are they required to send SOA records with a TTL of zero.

7.3. The SOA.MNAME field

 It is quite clear in the specifications, yet seems to have been
 widely ignored, that the MNAME field of the SOA record should contain
 the name of the primary (master) server for the zone identified by
 the SOA.  It should not contain the name of the zone itself.  That
 information would be useless, as to discover it, one needs to start
 with the domain name of the SOA record - that is the name of the
 zone.

8. Time to Live (TTL)

 The definition of values appropriate to the TTL field in STD 13 is
 not as clear as it could be, with respect to how many significant
 bits exist, and whether the value is signed or unsigned.  It is
 hereby specified that a TTL value is an unsigned number, with a
 minimum value of 0, and a maximum value of 2147483647.  That is, a
 maximum of 2^31 - 1.  When transmitted, this value shall be encoded
 in the less significant 31 bits of the 32 bit TTL field, with the

Elz & Bush Standards Track [Page 10] RFC 2181 Clarifications to the DNS Specification July 1997

 most significant, or sign, bit set to zero.
 Implementations should treat TTL values received with the most
 significant bit set as if the entire value received was zero.
 Implementations are always free to place an upper bound on any TTL
 received, and treat any larger values as if they were that upper
 bound.  The TTL specifies a maximum time to live, not a mandatory
 time to live.

9. The TC (truncated) header bit

 The TC bit should be set in responses only when an RRSet is required
 as a part of the response, but could not be included in its entirety.
 The TC bit should not be set merely because some extra information
 could have been included, but there was insufficient room.  This
 includes the results of additional section processing.  In such cases
 the entire RRSet that will not fit in the response should be omitted,
 and the reply sent as is, with the TC bit clear.  If the recipient of
 the reply needs the omitted data, it can construct a query for that
 data and send that separately.
 Where TC is set, the partial RRSet that would not completely fit may
 be left in the response.  When a DNS client receives a reply with TC
 set, it should ignore that response, and query again, using a
 mechanism, such as a TCP connection, that will permit larger replies.

10. 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.

10.1. CNAME resource 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 there are some rare
 applications for aliases with the accompanying canonical name
 undefined in the DNS.  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:

Elz & Bush Standards Track [Page 11] RFC 2181 Clarifications to the DNS Specification July 1997

   + one CNAME record exists, optionally accompanied by SIG, NXT, and
     KEY RRs,
   + one or more records exist, none being CNAME records,
   + the name exists, but has no associated RRs of any type,
   + the name does not exist at all.

10.1.1. 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.

10.2. 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, which it should not be,
 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.

10.3. 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.

Elz & Bush Standards Track [Page 12] RFC 2181 Clarifications to the DNS Specification July 1997

 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 for the DNS administrator 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.

11. 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

Elz & Bush Standards Track [Page 13] RFC 2181 Clarifications to the DNS Specification July 1997

 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.  If the client has such restrictions, it is
 solely responsible for validating the data from the DNS to ensure
 that it conforms before it makes any use of that data.
 See also [RFC1123] section 6.1.3.5.

12. 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 5.4.1 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 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.

13. References

 [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.
 [RFC1123]   Braden, R., "Requirements for Internet Hosts - application
             and support", STD 3, RFC 1123, January 1989.
 [RFC1700]   Reynolds, J., Postel, J., "Assigned Numbers",
             STD 2, RFC 1700, October 1994.
 [RFC2065]   Eastlake, D., Kaufman, C., "Domain Name System Security
             Extensions", RFC 2065, January 1997.

Elz & Bush Standards Track [Page 14] RFC 2181 Clarifications to the DNS Specification July 1997

14. 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, and Olafur Gudmundsson, for help with the
 DNSSEC issues in this document, and to John Gilmore for pointing out
 where the clarifications were not necessarily clarifying.  Bob Halley
 suggested clarifying the placement of SOA records in authoritative
 answers, and provided the references.  Michael Patton, as usual, and
 Mark Andrews, Alan Barrett and Stan Barber provided much assistance
 with many details.  Josh Littlefield helped make sure that the
 clarifications didn't cause problems in some irritating corner cases.

15. Authors' Addresses

 Robert Elz
 Computer Science
 University of Melbourne
 Parkville, Victoria, 3052
 Australia.
 EMail: kre@munnari.OZ.AU
 Randy Bush
 RGnet, Inc.
 5147 Crystal Springs Drive NE
 Bainbridge Island, Washington,  98110
 United States.
 EMail: randy@psg.com

Elz & Bush Standards Track [Page 15]

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