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Versions: 00 01 02 03 rfc2672                                           
DNSIND Working Group                                       Matt Crawford
Internet Draft                                                  Fermilab
                                                       September 1, 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 and may be updated, replaced, or obsoleted by other documents
    at any time.  It is inappropriate to use Internet- Drafts as
    reference material or to cite them other than as "work in progress."

    To view the entire list of current Internet-Drafts, please check the
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    Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).

    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",
    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

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    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:

       <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.

    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

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    DNAME record.  This operation will be referred to as "the DNAME

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 1 or
    greater are presumed to understand the semantics of the DNAME
    record.  The sender of a Version 0 or a non-extended query may not
    understand DNAME, 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 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

        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 with Version 1 or greater, 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

            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

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        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.

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

        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

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            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 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 might be

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        $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 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

    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.  References

    [DNSCF]  P.V. Mockapetris, "Domain names - concepts and facilities",
             RFC 1034.

    [DNSCLR] R. Elz, R. Bush, "Clarifications to the DNS Specification",
             RFC 2181.

    [DNSIS]  P.V. Mockapetris, "Domain names - implementation and
             specification", RFC 1035.

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    [DNSSEC] D. Eastlake, 3rd, C. Kaufman, "Domain Name System Security
             Extensions", RFC 2065.

    [DNSUPD] P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, "Dynamic
             Updates in the Domain Name System", RFC 2136.

    [EDNS0]  P. Vixie, "Extensions mechanisms for DNS (EDNS0)", Work in

    [INADDR] H. Eidnes, G. de Groot, P. Vixie, "Classless IN-ADDR.ARPA
             delegation", RFC 2317.

    [KWORD]  S. Bradner, "Key words for use in RFCs to Indicate
             Requirement Levels," RFC 2119.

    [SECDYN] D. Eastlake, 3rd, "Secure Domain Name System Dynamic
             Update", RFC 2137.

7.  Author's Address

    Matt Crawford
    Fermilab MS 368
    PO Box 500
    Batavia, IL 60510

    Phone: +1 630 840-3461

    EMail: crawdad@fnal.gov

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