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Network Working Group                                        M. CrawfordRequest for Comments: 2672                                      FermilabCategory: Standards Track                                    August 1999Non-Terminal DNS Name RedirectionStatus 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).Crawford                    Standards Track                     [Page 1]

RFC 2672           Non-Terminal DNS Name Redirection         August 1999   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".Crawford                    Standards Track                     [Page 2]

RFC 2672           Non-Terminal DNS Name Redirection         August 19994.1.  Processing by Servers   For a server performing non-recursive service steps 3.c and 4 ofsection 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:Crawford                    Standards Track                     [Page 3]

RFC 2672           Non-Terminal DNS Name Redirection         August 1999      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.Crawford                    Standards Track                     [Page 4]

RFC 2672           Non-Terminal DNS Name Redirection         August 1999   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 insection 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   A resolver or a server providing recursive service must be modified   to treat a DNAME as somewhat analogous to a CNAME.  The resolver   algorithm of [DNSCF]section 5.3.3 is modified to renumber step 4.d   as 4.e and insert a new 4.d.  The complete algorithm becomes:   1. See if the answer is in local information, and if so return it to      the client.   2. Find the best servers to ask.   3. Send them queries until one returns a response.   4. Analyze the response, either:      a. if the response answers the question or contains a name error,         cache the data as well as returning it back to the client.      b. if the response contains a better delegation to other servers,         cache the delegation information, and go to step 2.      c. if the response shows a CNAME and that is not the answer         itself, cache the CNAME, change the SNAME to the canonical name         in the CNAME RR and go to step 1.Crawford                    Standards Track                     [Page 5]

RFC 2672           Non-Terminal DNS Name Redirection         August 1999      d. if the response shows a DNAME and that is not the answer         itself, cache the DNAME.  If substitution of the DNAME's         <target> for its <owner> in the SNAME would overflow the legal         size for a <domain-name>, return an implementation-dependent         error to the application; otherwise perform the substitution         and go to step 1.      e. if the response shows a server failure or other bizarre         contents, delete the server from the SLIST and go back to step         3.   A resolver or recursive server which understands DNAME records but   sends non-extended queries MUST augment step 4.c by deleting from the   reply any CNAME records which have an <owner> which is a subdomain of   the <owner> of any DNAME record in the response.5.  Examples of Use5.1.  Organizational Renaming   If an organization with domain name FROBOZZ.EXAMPLE became part of an   organization with domain name ACME.EXAMPLE, it might ease transition   by placing information such as this in its old zone.       frobozz.example.  DNAME    frobozz-division.acme.example.                         MX       10       mailhub.acme.example.   The response to an extended recursive query for www.frobozz.example   would contain, in the answer section, the DNAME record shown above   and the relevant RRs for www.frobozz-division.acme.example.5.2.  Classless Delegation of Shorter Prefixes   The classless scheme for in-addr.arpa delegation [INADDR] can be   extended to prefixes shorter than 24 bits by use of the DNAME record.   For example, the prefix 192.0.8.0/22 can be delegated by the   following records.       $ORIGIN 0.192.in-addr.arpa.       8/22    NS       ns.slash-22-holder.example.       8       DNAME    8.8/22       9       DNAME    9.8/22       10      DNAME    10.8/22       11      DNAME    11.8/22Crawford                    Standards Track                     [Page 6]

RFC 2672           Non-Terminal DNS Name Redirection         August 1999   A typical entry in the resulting reverse zone for some host with   address 192.0.9.33 might be       $ORIGIN 8/22.0.192.in-addr.arpa.       33.9    PTR     somehost.slash-22-holder.example.   The same advisory remarks concerning the choice of the "/" character   apply here as in [INADDR].5.3.  Network Renumbering Support   If IPv4 network renumbering were common, maintenance of address space   delegation could be simplified by using DNAME records instead of NS   records to delegate.      $ORIGIN new-style.in-addr.arpa.      189.190           DNAME    in-addr.example.net.      $ORIGIN in-addr.example.net.      188               DNAME    in-addr.customer.example.      $ORIGIN in-addr.customer.example.      1                 PTR      www.customer.example.      2                 PTR      mailhub.customer.example.      ; etc ...   This would allow the address space 190.189.0.0/16 assigned to the ISP   "example.net" to be changed without the necessity of altering the   zone files describing the use of that space by the ISP and its   customers.   Renumbering IPv4 networks is currently so arduous a task that   updating the DNS is only a small part of the labor, so this scheme   may have a low value.  But it is hoped that in IPv6 the renumbering   task will be quite different and the DNAME mechanism may play a   useful part.6.  IANA Considerations   This document defines a new DNS Resource Record type with the   mnemonic DNAME and type code 39 (decimal).  The naming/numbering   space is defined in [DNSIS].  This name and number have already been   registered with the IANA.Crawford                    Standards Track                     [Page 7]

RFC 2672           Non-Terminal DNS Name Redirection         August 19997.  Security Considerations   The DNAME record is similar to the CNAME record with regard to the   consequences of insertion of a spoofed record into a DNS server or   resolver, differing in that the DNAME's effect covers a whole subtree   of the name space.  The facilities of [DNSSEC] are available to   authenticate this record type.8.  References   [DNSCF]  Mockapetris, P., "Domain names - concepts and facilities",            STD 13,RFC 1034, November 1987.   [DNSCLR] Elz, R. and R. Bush, "Clarifications to the DNS            Specification",RFC 2181, July 1997.   [DNSIS]  Mockapetris, P., "Domain names - implementation and            specification", STD 13,RFC 1035, November 1987.   [DNSSEC] Eastlake, 3rd, D. and C. Kaufman, "Domain Name System            Security Extensions",RFC 2065, January 1997.   [DNSUPD] Vixie, P., Ed., Thomson, S., Rekhter, Y. and J. Bound,            "Dynamic Updates in the Domain Name System",RFC 2136, April            1997.   [EDNS0]  Vixie, P., "Extensions mechanisms for DNS (EDNS0)",RFC2671, August 1999.   [INADDR] Eidnes, H., de Groot, G. and P. Vixie, "Classless IN-            ADDR.ARPA delegation",RFC 2317, March 1998.   [KWORD]  Bradner, S., "Key words for use in RFCs to Indicate            Requirement Levels,"BCP 14,RFC 2119, March 1997.   [SECDYN] D. Eastlake, 3rd, "Secure Domain Name System Dynamic            Update",RFC 2137, April 1997.9.  Author's Address   Matt Crawford   Fermilab MS 368   PO Box 500   Batavia, IL 60510   USA   Phone: +1 630 840-3461   EMail: crawdad@fnal.govCrawford                    Standards Track                     [Page 8]

RFC 2672           Non-Terminal DNS Name Redirection         August 199910.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Crawford                    Standards Track                     [Page 9]