DNSIND Working Group Matt Crawford Internet Draft Fermilab March 6, 1998 Non-Terminal DNS Name Redirection 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet Drafts Shadow Directories on ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific Rim). Distribution of this memo is unlimited. 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 Expires September 11, 1998 Crawford [Page 1] Internet Draft Non-Terminal Nicknames March 6, 1998 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 TBA (decimal). DNAME has the following format: DNAME The format is not class-sensitive. All fields are required. The RDATA field is a [DNSIS]. The DNAME RR causes type NS additional section processing. The effect of the DNAME record is the substitution of the record's for its as a suffix of a domain name. Two limiting rules govern the use of DNAMEs. Rule One 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, except for the wildcard node *.N which MAY have data intended to help old DNS clients which do not understand the DNAME record. These restrictions apply only to records of the same class as the DNAME record. Rule Two When resolving a query, it is valid to encounter more than one DNAME record along the way ONLY IF every DNAME record encountered has fewer labels in its than in its . (Note that in the bit-string label [BITLBL], each bit is a separate label.) If a single DNAME is encountered there is no special restriction on the number of labels in its . Rule One assures predictable results when a DNAME record is cached by a server which is not authoritative for the record's zone. Rule Two prevents DNAME loops. Rule One MUST be enforced when authoritative zone data is loaded. Rule Two MUST be enforced during query processing. Expires September 11, 1998 Crawford [Page 2] Internet Draft Non-Terminal Nicknames March 6, 1998 4. Query Processing To exploit the DNAME mechanism the name resolution algorithms [DNSCF] must be modified slightly for both servers and resolvers. In both cases a conceptual per-query variable DFLAG is introduced to enforce Rule Two. Implementations MAY use other means to enforce the rule. DFLAG DFLAG's value is 0 when processing of a query begins and whenever no DNAME has been encountered; 1 when one or more DNAME records have been encountered, and each had fewer labels in its than in its ; 2 when a DNAME record has been encountered which had at least as many labels in its as in its . Both modified algorithms incorporate the operation of making a substitution on a name (either QNAME or SNAME) under control of a DNAME record. For conciseness, this operation is elaborated here and will be referred to as "procedure S". S1. If DFLAG = 2, Rule Two is violated. Go to step S4. S2. If DFLAG = 1 and the DNAME record's 's does not have fewer labels than its , Rule Two is violated. Go to step S4. S3. If substituting the DNAME's for its in the name being operated on would overflow the legal size for a , go to step S4. Otherwise make the substitution. If the has fewer labels than the , set DFLAG to 1, otherwise set DFLAG to 2. Return. S4. In a resolver, return an implementation-dependent error to the application. In a server, copy the DNAME record to the answer section, set RCODE to YXDOMAIN [DNSUPD], and exit. 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 the cache. The revised algorithm is: 1. Set or clear the value of recursion available in the response Expires September 11, 1998 Crawford [Page 3] Internet Draft Non-Terminal Nicknames March 6, 1998 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, substitute its for its | in QNAME according to procedure S, and 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. Expires September 11, 1998 Crawford [Page 4] Internet Draft Non-Terminal Nicknames March 6, 1998 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. Using 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 Rule One. 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. Expires September 11, 1998 Crawford [Page 5] Internet Draft Non-Terminal Nicknames March 6, 1998 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 the DNAME, substitute the | DNAME's for its in the SNAME according to | procedure S 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. 5. Examples of Use If an organization with domain name FROBOZZ.TLD became part of an organization with domain name ACME.TLD, it might ease transition by placing information such as this in its old zone. frobozz.tld. DNAME frobozz-division.acme.tld. MX mailhub.acme.tld. *.frobozz.tld. CNAME www.frobozz-division.acme.tld. 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.xy. $ORIGIN in-addr.customer.xy. 1 PTR www.customer.xy. 2 PTR mailhub.customer.xy. ; etc ... This would allow the address space 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. Expires September 11, 1998 Crawford [Page 6] Internet Draft Non-Terminal Nicknames March 6, 1998 6. Discussion (in draft only) Why does [DNSCF] section 4.3.2 step 3.a say "go back to step 1" rather than "go back to step 2"? Would a DNAME with a special be of any use in securely denying anything in a subtree below its ? If so, a special rule could be added about a null label in the , or =. The meaning of NS and DNAME records together at the same node is unclear. Perhaps this ought to be forbidden. YXDOMAIN seems like a reasonable error code to overload for name- length overflow due to DNAME substitution. But a case for NXDOMAIN could be made on the grounds that the resulting too-long name is sure not to exist. Rule Two could be modified to allow up to one (or any fixed finite number) of DNAMEs which don't decrease the label count AND any number of DNAMEs which do decrease it. Would that help anything? 7. Acknowledgments Donald Eastlake 3rd suggested the wildcard subdomain to help old clients get something useful. 8. References [BITLBL]M. Crawford, "Binary Labels in the Domain Name System", currently draft-ietf-dnsind-binary-labels-00.txt. [DNSCF] P.V. Mockapetris, "Domain names - concepts and facilities", RFC 1034. [DNSIS] P.V. Mockapetris, "Domain names - implementation and specification", RFC 1035. [DNSUPD]P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, "Dynamic Updates in the Domain Name System", RFC 2136. [KWORD] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119. Expires September 11, 1998 Crawford [Page 7] Internet Draft Non-Terminal Nicknames March 6, 1998 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 Expires September 11, 1998 Crawford [Page 8]