rfc2672.txt

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Network Working Group                                        M. Crawford
Request for Comments: 2672                                      Fermilab
Category: Standards Track                                    August 1999


                   Non-Terminal DNS Name Redirection

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.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

1.  Introduction

   This document defines a new DNS Resource Record called "DNAME", which
   provides the capability to map an entire subtree of the DNS name
   space to another domain.  It differs from the CNAME record which maps
   a single node of the name space.

   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 [KWORD].

2.  Motivation

   This Resource Record and its processing rules were conceived as a
   solution to the problem of maintaining address-to-name mappings in a
   context of network renumbering.  Without the DNAME mechanism, an
   authoritative DNS server for the address-to-name mappings of some
   network must be reconfigured when that network is renumbered.  With
   DNAME, the zone can be constructed so that it needs no modification
   when renumbered.  DNAME can also be useful in other situations, such
   as when an organizational unit is renamed.

3. The DNAME Resource Record

   The DNAME RR has mnemonic DNAME and type code 39 (decimal).







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   DNAME has the following format:

      <owner> <ttl> <class> DNAME <target>

   The format is not class-sensitive.  All fields are required.  The
   RDATA field <target> is a <domain-name> [DNSIS].

   The DNAME RR causes type NS additional section processing.

   The effect of the DNAME record is the substitution of the record's
   <target> for its <owner> as a suffix of a domain name.  A "no-
   descendants" limitation governs the use of DNAMEs in a zone file:

      If a DNAME RR is present at a node N, there may be other data at N
      (except a CNAME or another DNAME), but there MUST be no data at
      any descendant of N.  This restriction applies only to records of
      the same class as the DNAME record.

   This rule assures predictable results when a DNAME record is cached
   by a server which is not authoritative for the record's zone.  It
   MUST be enforced when authoritative zone data is loaded.  Together
   with the rules for DNS zone authority [DNSCLR] it implies that DNAME
   and NS records can only coexist at the top of a zone which has only
   one node.

   The compression scheme of [DNSIS] MUST NOT be applied to the RDATA
   portion of a DNAME record unless the sending server has some way of
   knowing that the receiver understands the DNAME record format.
   Signalling such understanding is expected to be the subject of future
   DNS Extensions.

   Naming loops can be created with DNAME records or a combination of
   DNAME and CNAME records, just as they can with CNAME records alone.
   Resolvers, including resolvers embedded in DNS servers, MUST limit
   the resources they devote to any query.  Implementors should note,
   however, that fairly lengthy chains of DNAME records may be valid.

4.  Query Processing

   To exploit the DNAME mechanism the name resolution algorithms [DNSCF]
   must be modified slightly for both servers and resolvers.

   Both modified algorithms incorporate the operation of making a
   substitution on a name (either QNAME or SNAME) under control of a
   DNAME record.  This operation will be referred to as "the DNAME
   substitution".





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4.1.  Processing by Servers

   For a server performing non-recursive service steps 3.c and 4 of
   section 4.3.2 [DNSCF] are changed to check for a DNAME record before
   checking for a wildcard ("*") label, and to return certain DNAME
   records from zone data and the cache.

   DNS clients sending Extended DNS [EDNS0] queries with Version 0 or
   non-extended queries are presumed not to understand the semantics of
   the DNAME record, so a server which implements this specification,
   when answering a non-extended query, SHOULD synthesize a CNAME record
   for each DNAME record encountered during query processing to help the
   client reach the correct DNS data.  The behavior of clients and
   servers under Extended DNS versions greater than 0 will be specified
   when those versions are defined.

   The synthesized CNAME RR, if provided, MUST have

      The same CLASS as the QCLASS of the query,

      TTL equal to zero,

      An <owner> equal to the QNAME in effect at the moment the DNAME RR
      was encountered, and

      An RDATA field containing the new QNAME formed by the action of
      the DNAME substitution.

   If the server has the appropriate key on-line [DNSSEC, SECDYN], it
   MAY generate and return a SIG RR for the synthesized CNAME RR.

   The revised server algorithm is:

   1. Set or clear the value of recursion available in the response
      depending on whether the name server is willing to provide
      recursive service.  If recursive service is available and
      requested via the RD bit in the query, go to step 5, otherwise
      step 2.

   2. Search the available zones for the zone which is the nearest
      ancestor to QNAME.  If such a zone is found, go to step 3,
      otherwise step 4.

   3. Start matching down, label by label, in the zone.  The matching
      process can terminate several ways:






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      a. If the whole of QNAME is matched, we have found the node.

         If the data at the node is a CNAME, and QTYPE doesn't match
         CNAME, copy the CNAME RR into the answer section of the
         response, change QNAME to the canonical name in the CNAME RR,
         and go back to step 1.

         Otherwise, copy all RRs which match QTYPE into the answer
         section and go to step 6.

      b. If a match would take us out of the authoritative data, we have
         a referral.  This happens when we encounter a node with NS RRs
         marking cuts along the bottom of a zone.

         Copy the NS RRs for the subzone into the authority section of
         the reply.  Put whatever addresses are available into the
         additional section, using glue RRs if the addresses are not
         available from authoritative data or the cache.  Go to step 4.

      c. If at some label, a match is impossible (i.e., the
         corresponding label does not exist), look to see whether the
         last label matched has a DNAME record.

         If a DNAME record exists at that point, copy that record into
         the answer section.  If substitution of its <target> for its
         <owner> in QNAME would overflow the legal size for a <domain-
         name>, set RCODE to YXDOMAIN [DNSUPD] and exit; otherwise
         perform the substitution and continue.  If the query was not
         extended [EDNS0] with a Version indicating understanding of the
         DNAME record, the server SHOULD synthesize a CNAME record as
         described above and include it in the answer section.  Go back
         to step 1.

         If there was no DNAME record, look to see if the "*" label
         exists.

         If the "*" label does not exist, check whether the name we are
         looking for is the original QNAME in the query or a name we
         have followed due to a CNAME.  If the name is original, set an
         authoritative name error in the response and exit.  Otherwise
         just exit.

         If the "*" label does exist, match RRs at that node against
         QTYPE.  If any match, copy them into the answer section, but
         set the owner of the RR to be QNAME, and not the node with the
         "*" label.  Go to step 6.





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   4. Start matching down in the cache.  If QNAME is found in the cache,
      copy all RRs attached to it that match QTYPE into the answer
      section.  If QNAME is not found in the cache but a DNAME record is
      present at an ancestor of QNAME, copy that DNAME record into the
      answer section.  If there was no delegation from authoritative
      data, look for the best one from the cache, and put it in the
      authority section.  Go to step 6.

   5. Use the local resolver or a copy of its algorithm (see resolver
      section of this memo) to answer the query.  Store the results,
      including any intermediate CNAMEs and DNAMEs, in the answer
      section of the response.

   6. Using local data only, attempt to add other RRs which may be
      useful to the additional section of the query.  Exit.

   Note that there will be at most one ancestor with a DNAME as
   described in step 4 unless some zone's data is in violation of the
   no-descendants limitation in section 3.  An implementation might take
   advantage of this limitation by stopping the search of step 3c or
   step 4 when a DNAME record is encountered.

4.2.  Processing by Resolvers