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).
 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".
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:
 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.
 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
 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.
 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 Use
5.1. 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 11.8/22
 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.
7. 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)", RFC
 2671, 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.gov
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