    Internet Engineering Task Force (IETF)                           S. Rose
    Request for Comments: 6672                                          NIST
    Obsoletes: 2672                                            W. Wijngaards
    Updates: 3363                                                 NLnet Labs
    Category: Standards Track                                      June 2012
    ISSN: 2070-1721

                          DNAME Redirection in the DNS

    Abstract

       The DNAME record provides redirection for a subtree of the domain
       name tree in the DNS.  That is, all names that end with a particular
       suffix are redirected to another part of the DNS.  This document
       obsoletes the original specification in RFC 2672 as well as updates
       the document on representing IPv6 addresses in DNS (RFC 3363).

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6672.

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    Copyright Notice

       Copyright (c) 2012 IETF Trust and the persons identified as the
       document authors.  All rights reserved.

       This document is subject to BCP 78 and the IETF Trust's Legal
       Provisions Relating to IETF Documents
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       publication of this document.  Please review these documents
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       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified
       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

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    Table of Contents

       1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
         1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
       2.  The DNAME Resource Record  . . . . . . . . . . . . . . . . . .  5
         2.1.  Format . . . . . . . . . . . . . . . . . . . . . . . . . .  5
         2.2.  The DNAME Substitution . . . . . . . . . . . . . . . . . .  5
         2.3.  DNAME Owner Name Matching the QNAME  . . . . . . . . . . .  6
         2.4.  Names next to and below a DNAME Record . . . . . . . . . .  7
         2.5.  Compression of the DNAME Record  . . . . . . . . . . . . .  7
       3.  Processing . . . . . . . . . . . . . . . . . . . . . . . . . .  8
         3.1.  CNAME Synthesis  . . . . . . . . . . . . . . . . . . . . .  8
         3.2.  Server Algorithm . . . . . . . . . . . . . . . . . . . . .  9
         3.3.  Wildcards  . . . . . . . . . . . . . . . . . . . . . . . . 10
         3.4.  Acceptance and Intermediate Storage  . . . . . . . . . . . 11
           3.4.1.  Resolver Algorithm . . . . . . . . . . . . . . . . . . 11
       4.  DNAME Discussions in Other Documents . . . . . . . . . . . . . 12
       5.  Other Issues with DNAME  . . . . . . . . . . . . . . . . . . . 13
         5.1.  Canonical Hostnames Cannot Be below DNAME Owners . . . . . 13
         5.2.  Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 13
         5.3.  DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 14
           5.3.1.  Signed DNAME, Unsigned Synthesized CNAME . . . . . . . 14
           5.3.2.  DNAME Bit in NSEC Type Map . . . . . . . . . . . . . . 14
           5.3.3.  DNAME Chains as Strong as the Weakest Link . . . . . . 14
           5.3.4.  Validators Must Understand DNAME . . . . . . . . . . . 14
             5.3.4.1.  Invalid Name Error Response Caused by DNAME in
                       Bitmap . . . . . . . . . . . . . . . . . . . . . . 15
             5.3.4.2.  Valid Name Error Response Involving DNAME in
                       Bitmap . . . . . . . . . . . . . . . . . . . . . . 15
             5.3.4.3.  Response with Synthesized CNAME  . . . . . . . . . 16
       6.  Examples of DNAME Use in a Zone  . . . . . . . . . . . . . . . 16
         6.1.  Organizational Renaming  . . . . . . . . . . . . . . . . . 16
         6.2.  Classless Delegation of Shorter Prefixes . . . . . . . . . 17
         6.3.  Network Renumbering Support  . . . . . . . . . . . . . . . 17
       7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
       8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
       9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 18
       10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
         10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
         10.2. Informative References . . . . . . . . . . . . . . . . . . 20
       Appendix A.  Changes from RFC 2672 . . . . . . . . . . . . . . . . 21
         A.1.  Changes to Server Behavior . . . . . . . . . . . . . . . . 21
         A.2.  Changes to Client Behavior . . . . . . . . . . . . . . . . 21

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    1.  Introduction

       DNAME is a DNS resource record type originally defined in RFC 2672
       [RFC2672].  DNAME provides redirection from a part of the DNS name
       tree to another part of the DNS name tree.

       The DNAME RR and the CNAME RR [RFC1034] cause a lookup to
       (potentially) return data corresponding to a domain name different
       from the queried domain name.  The difference between the two
       resource records is that the CNAME RR directs the lookup of data at
       its owner to another single name, whereas a DNAME RR directs lookups
       for data at descendants of its owner's name to corresponding names
       under a different (single) node of the tree.

       For example, take looking through a zone (see RFC 1034 [RFC1034],
       Section 4.3.2, step 3) for the domain name "foo.example.com", and a
       DNAME resource record is found at "example.com" indicating that all
       queries under "example.com" be directed to "example.net".  The lookup
       process will return to step 1 with the new query name of
       "foo.example.net".  Had the query name been "www.foo.example.com",
       the new query name would be "www.foo.example.net".

       This document is a revision of the original specification of DNAME in
       RFC 2672 [RFC2672].  DNAME was conceived to help with the problem of
       maintaining address-to-name mappings in a context of network
       renumbering.  With a careful setup, a renumbering event in the
       network causes no change to the authoritative server that has the
       address-to-name mappings.  Examples in practice are classless reverse
       address space delegations.

       Another usage of DNAME lies in aliasing of name spaces.  For example,
       a zone administrator may want subtrees of the DNS to contain the same
       information.  Examples include punycode [RFC3492] alternates for
       domain spaces.

       This revision of the DNAME specification does not change the wire
       format or the handling of DNAME resource records.  Discussion is
       added on problems that may be encountered when using DNAME.

    1.1.  Requirements Language

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED" "NOT RECOMMENDED", "MAY", and
       "OPTIONAL" in this document are to be interpreted as described in RFC
       2119 [RFC2119].

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    2.  The DNAME Resource Record

    2.1.  Format

       The DNAME RR has mnemonic DNAME and type code 39 (decimal).  It is
       CLASS-insensitive.

       Its RDATA is comprised of a single field, <target>, which contains a
       fully qualified domain name that MUST be sent in uncompressed form
       [RFC1035] [RFC3597].  The <target> field MUST be present.  The
       presentation format of <target> is that of a domain name [RFC1035].
       The presentation format of the RR is as follows:

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

       The effect of the DNAME RR is the substitution of the record's
       <target> for its owner name, as a suffix of a domain name.  This
       substitution is to be applied for all names below the owner name of
       the DNAME RR.  This substitution has to be applied for every DNAME RR
       found in the resolution process, which allows fairly lengthy valid
       chains of DNAME RRs.

       Details of the substitution process, methods to avoid conflicting
       resource records, and rules for specific corner cases are given in
       the following subsections.

    2.2.  The DNAME Substitution

       When following step 3 of the algorithm in RFC 1034 [RFC1034], Section
       4.3.2, "start matching down, label by label, in the zone" and a node
       is found to own a DNAME resource record, a DNAME substitution occurs.
       The name being sought may be the original query name or a name that
       is the result of a CNAME resource record being followed or a
       previously encountered DNAME.  As in the case when finding a CNAME
       resource record or NS resource record set, the processing of a DNAME
       will happen prior to finding the desired domain name.

       A DNAME substitution is performed by replacing the suffix labels of
       the name being sought matching the owner name of the DNAME resource
       record with the string of labels in the RDATA field.  The matching
       labels end with the root label in all cases.  Only whole labels are
       replaced.  See the table of examples for common cases and corner
       cases.

       In the table below, the QNAME refers to the query name.  The owner is
       the DNAME owner domain name, and the target refers to the target of
       the DNAME record.  The result is the resulting name after performing
       the DNAME substitution on the query name. "no match" means that the

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       query did not match the DNAME, and thus no substitution is performed
       and a possible error message is returned (if no other result is
       possible).  Thus, every line contains one example substitution.  In
       the examples below, 'cyc' and 'shortloop' contain loops.

        QNAME            owner  DNAME   target         result
        ---------------- -------------- -------------- -----------------
        com.             example.com.   example.net.   <no match>
        example.com.     example.com.   example.net.   [0]
        a.example.com.   example.com.   example.net.   a.example.net.
        a.b.example.com. example.com.   example.net.   a.b.example.net.
        ab.example.com.  b.example.com. example.net.   <no match>
        foo.example.com. example.com.   example.net.   foo.example.net.
        a.x.example.com. x.example.com. example.net.   a.example.net.
        a.example.com.   example.com.   y.example.net. a.y.example.net.
        cyc.example.com. example.com.   example.com.   cyc.example.com.
        cyc.example.com. example.com.   c.example.com. cyc.c.example.com.
        shortloop.x.x.   x.             .              shortloop.x.
        shortloop.x.     x.             .              shortloop.

       [0] The result depends on the QTYPE.  If the QTYPE = DNAME, then
           the result is "example.com.", else "<no match>".

                       Table 1. DNAME Substitution Examples

       It is possible for DNAMEs to form loops, just as CNAMEs can form
       loops.  DNAMEs and CNAMEs can chain together to form loops.  A single
       corner case DNAME can form a loop.  Resolvers and servers should be
       cautious in devoting resources to a query, but be aware that fairly
       long chains of DNAMEs may be valid.  Zone content administrators
       should take care to ensure that there are no loops that could occur
       when using DNAME or DNAME/CNAME redirection.

       The domain name can get too long during substitution.  For example,
       suppose the target name of the DNAME RR is 250 octets in length
       (multiple labels), if an incoming QNAME that has a first label over 5
       octets in length, the result would be a name over 255 octets.  If
       this occurs, the server returns an RCODE of YXDOMAIN [RFC2136].  The
       DNAME record and its signature (if the zone is signed) are included
       in the answer as proof for the YXDOMAIN (value 6) RCODE.

    2.3.  DNAME Owner Name Matching the QNAME

       Unlike a CNAME RR, a DNAME RR redirects DNS names subordinate to its
       owner name; the owner name of a DNAME is not redirected itself.  The
       domain name that owns a DNAME record is allowed to have other
       resource record types at that domain name, except DNAMEs, CNAMEs, or
       other types that have restrictions on what they can coexist with.

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       When there is a match of the QTYPE to a type (or types) also owned by
       the owner name, the response is sourced from the owner name.  For
       example, a QTYPE of ANY would return the (available) types at the
       owner name, not the target name.

       DNAME RRs MUST NOT appear at the same owner name as an NS RR unless
       the owner name is the zone apex; if it is not the zone apex, then the
       NS RR signifies a delegation point, and the DNAME RR must in that
       case appear below the zone cut at the zone apex of the child zone.

       If a DNAME record is present at the zone apex, there is still a need
       to have the customary SOA and NS resource records there as well.
       Such a DNAME cannot be used to mirror a zone completely, as it does
       not mirror the zone apex.

       These rules also allow DNAME records to be queried through caches
       that are RFC 1034 [RFC1034] compliant and are DNAME unaware.

    2.4.  Names next to and below a DNAME Record

       Resource records MUST NOT exist at any subdomain of the owner of a
       DNAME RR.  To get the contents for names subordinate to that owner
       name, the DNAME redirection must be invoked and the resulting target
       queried.  A server MAY refuse to load a zone that has data at a
       subdomain of a domain name owning a DNAME RR.  If the server does
       load the zone, those names below the DNAME RR will be occluded as
       described in RFC 2136 [RFC2136], Section 7.18.  Also, a server ought
       to refuse to load a zone subordinate to the owner of a DNAME record
       in the ancestor zone.  See Section 5.2 for further discussion related
       to dynamic update.

       DNAME is a singleton type, meaning only one DNAME is allowed per
       name.  The owner name of a DNAME can only have one DNAME RR, and no
       CNAME RRs can exist at that name.  These rules make sure that for a
       single domain name, only one redirection exists; thus, there's no
       confusion about which one to follow.  A server ought to refuse to
       load a zone that violates these rules.

    2.5.  Compression of the DNAME Record

       The DNAME owner name can be compressed like any other owner name.
       The DNAME RDATA target name MUST NOT be sent out in compressed form
       and MUST be downcased for DNS Security Extensions (DNSSEC)
       validation.

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       Although the previous DNAME specification [RFC2672] (that is
       obsoleted by this specification) talked about signaling to allow
       compression of the target name, such signaling has never been
       specified, nor is it specified in this document.

       RFC 2672 (obsoleted by this document) states that the Extended DNS
       (EDNS) version has a means for understanding DNAME and DNAME target
       name compression.  This document revises RFC 2672, in that there is
       no EDNS version signaling for DNAME.

    3.  Processing

    3.1.  CNAME Synthesis

       When preparing a response, a server performing a DNAME substitution
       will, in all cases, include the relevant DNAME RR in the answer
       section.  Relevant cases includes the following:

       1.  The DNAME is being employed as a substitution instruction.

       2.  The DNAME itself matches the QTYPE, and the owner name matches
           QNAME.

       When the owner name matches the QNAME and the QTYPE matches another
       type owned there, the DNAME is not included in the answer.

       A CNAME RR with Time to Live (TTL) equal to the corresponding DNAME
       RR is synthesized and included in the answer section when the DNAME
       is employed as a substitution instruction.  The owner name of the
       CNAME is the QNAME of the query.  The DNSSEC specification ([RFC4033]
       [RFC4034] [RFC4035]) says that the synthesized CNAME does not have to
       be signed.  The signed DNAME has an RRSIG, and a validating resolver
       can check the CNAME against the DNAME record and validate the
       signature over the DNAME RR.

       Servers MUST be able to answer a query for a synthesized CNAME.  Like
       other query types, this invokes the DNAME, and then the server
       synthesizes the CNAME and places it into the answer section.  If the
       server in question is a cache, the synthesized CNAME's TTL SHOULD be
       equal to the decremented TTL of the cached DNAME.

       Resolvers MUST be able to handle a synthesized CNAME TTL of zero or a
       value equal to the TTL of the corresponding DNAME record (as some
       older, authoritative server implementations set the TTL of
       synthesized CNAMEs to zero).  A TTL of zero means that the CNAME can
       be discarded immediately after processing the answer.

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    3.2.  Server Algorithm

       Below is the revised version of the server algorithm, which appears
       in RFC 2672, Section 4.1.

       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 does not 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 sub-zone 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 [RFC2136] and exit; otherwise,
               perform the substitution and continue.  The server MUST

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               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 or DNAME.  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.  If the data at the node with the "*" label is
               a CNAME, and QTYPE doesn't match CNAME, copy the CNAME RR
               into the answer section of the response changing the owner
               name to the QNAME, change QNAME to the canonical name in the
               CNAME RR, and go back to step 1.  Otherwise, 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 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 that 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.

    3.3.  Wildcards

       The use of DNAME in conjunction with wildcards is discouraged
       [RFC4592].  Thus, records of the form "*.example.com DNAME
       example.net" SHOULD NOT be used.

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       The interaction between the expansion of the wildcard and the
       redirection of the DNAME is non-deterministic.  Due to the fact that
       the processing is non-deterministic, DNSSEC validating resolvers may
       not be able to validate a wildcarded DNAME.

       A server MAY give a warning that the behavior is unspecified if such
       a wildcarded DNAME is loaded.  The server MAY refuse it, refuse to
       load the zone, or refuse dynamic updates.

    3.4.  Acceptance and Intermediate Storage

       Recursive caching name servers can encounter data at names below the
       owner name of a DNAME RR, due to a change at the authoritative server
       where data from before and after the change resides in the cache.
       This conflict situation is a transitional phase that ends when the
       old data times out.  The caching name server can opt to store both
       old and new data and treat each as if the other did not exist, or
       drop the old data, or drop the longer domain name.  In any approach,
       consistency returns after the older data TTL times out.

       Recursive caching name servers MUST perform CNAME synthesis on behalf
       of clients.

       If a recursive caching name server encounters a DNSSEC validated
       DNAME RR that contradicts information already in the cache (excluding
       CNAME records), it SHOULD cache the DNAME RR, but it MAY cache the
       CNAME record received along with it, subject to the rules for CNAME.
       If the DNAME RR cannot be validated via DNSSEC (i.e., not BOGUS, but
       not able to validate), the recursive caching server SHOULD NOT cache
       the DNAME RR but MAY cache the CNAME record received along with it,
       subject to the rules for CNAME.

    3.4.1.  Resolver Algorithm

       Below is the revised version of the resolver algorithm, which appears
       in RFC 2672, Section 4.2.

       1.  See if the answer is in local information or can be synthesized
           from a cached DNAME; if so, return it to the client.

       2.  Find the best servers to ask.

       3.  Send queries until one returns a response.

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       4.  Analyze the response, either:

           A.  If the response answers the question or contains a name
               error, cache the data as well as return 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 (upon successful DNSSEC validation if
               the client is a validating resolver).  If substitution of the
               DNAME's target name for its owner name 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.

    4.  DNAME Discussions in Other Documents

       In Section 10.3 of [RFC2181], the discussion on MX and NS records
       touches on redirection by CNAMEs, but this also holds for DNAMEs.

       Section 10.3 ("MX and NS records") of [RFC2181] states:

               The domain name used as the value of a NS resource record,
               or part of the value of a MX resource record must not be
               an alias.  Not only is the specification clear on this
               point, but using an alias in either of these positions
               neither works as well as might be hoped, nor well fulfills
               the ambition that may have led to this approach.  This
               domain name must have as its value one or more address
               records.  Currently those will be A records, however in
               the future other record types giving addressing
               information may be acceptable.  It can also have other
               RRs, but never a CNAME RR.

       The DNAME RR is discussed in RFC 3363, Section 4, on A6 and DNAME.
       The opening premise of this section is demonstrably wrong, and so the
       conclusion based on that premise is wrong.  In particular, [RFC3363]
       deprecates the use of DNAME in the IPv6 reverse tree.  Based on the

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       experience gained in the meantime, [RFC3363] is revised, dropping all
       constraints on having DNAME RRs in these zones [RFC6434].  This would
       greatly improve the manageability of the IPv6 reverse tree.  These
       changes are made explicit below.

       In [RFC3363], the following paragraph is updated by this document,
       and the use of DNAME RRs in the reverse tree is no longer deprecated.

         The issues for DNAME in the reverse mapping tree appears to be
         closely tied to the need to use fragmented A6 in the main tree: if
         one is necessary, so is the other, and if one isn't necessary, the
         other isn't either.  Therefore, in moving RFC 2874 to experimental,
         the intent of this document is that use of DNAME RRs in the reverse
         tree be deprecated.

    5.  Other Issues with DNAME

       There are several issues to be aware of about the use of DNAME.

    5.1.  Canonical Hostnames Cannot Be below DNAME Owners

       The names listed as target names of MX, NS, PTR, and SRV [RFC2782]
       records must be canonical hostnames.  This means no CNAME or DNAME
       redirection may be present during DNS lookup of the address records
       for the host.  This is discussed in RFC 2181 [RFC2181], Section 10.3,
       and RFC 1912 [RFC1912], Section 2.4.  For SRV, see RFC 2782
       [RFC2782], page 4.

       The upshot of this is that although the lookup of a PTR record can
       involve DNAMEs, the name listed in the PTR record cannot fall under a
       DNAME.  The same holds for NS, SRV, and MX records.  For example,
       when punycode [RFC3492] alternates for a zone use DNAME, then the NS,
       MX, SRV, and PTR records that point to that zone must use names that
       are not aliases in their RDATA.  Then, what must be done is to have
       the domain names with DNAME substitution already applied to it as the
       MX, NS, PTR, and SRV data.  These are valid canonical hostnames.

    5.2.  Dynamic Update and DNAME

       DNAME records can be added, changed, and removed in a zone using
       dynamic update transactions.  Adding a DNAME RR to a zone occludes
       any domain names that may exist under the added DNAME.

       If a dynamic update message attempts to add a DNAME with a given
       owner name, but a CNAME is associated with that name, then the server
       MUST ignore the DNAME.  If a DNAME is already associated with that
       name, then it is replaced with the new DNAME.  Otherwise, add the
       DNAME.  If a CNAME is added with a given owner name, but a DNAME is

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       associated with that name, then the CNAME MUST be ignored.  Similar
       behavior occurs for dynamic updates to an owner name of a CNAME RR
       [RFC2136].

    5.3.  DNSSEC and DNAME

       The following subsections specify the behavior of implementations
       that understand both DNSSEC and DNAME (synthesis).

    5.3.1.  Signed DNAME, Unsigned Synthesized CNAME

       In any response, a signed DNAME RR indicates a non-terminal
       redirection of the query.  There might or might not be a server-
       synthesized CNAME in the answer section; if there is, the CNAME will
       never be signed.  For a DNSSEC validator, verification of the DNAME
       RR and then that the CNAME was properly synthesized is sufficient
       proof.

    5.3.2.  DNAME Bit in NSEC Type Map

       In any negative response, the NSEC or NSEC3 [RFC5155] record type
       bitmap SHOULD be checked to see that there was no DNAME that could
       have been applied.  If the DNAME bit in the type bitmap is set and
       the query name is a subdomain of the closest encloser that is
       asserted, then DNAME substitution should have been done, but the
       substitution has not been done as specified.

    5.3.3.  DNAME Chains as Strong as the Weakest Link

       A response can contain a chain of DNAME and CNAME redirections.  That
       chain can end in a positive answer or a negative reply (no name error
       or no data error).  Each step in that chain results in resource
       records being added to the answer or authority section of the
       response.  Only if all steps are secure can the AD (Authentic Data)
       bit be set for the response.  If one of the steps is bogus, the
       result is bogus.

    5.3.4.  Validators Must Understand DNAME

       Below are examples of why DNSSEC validators MUST understand DNAME.
       In the examples, SOA records, wildcard denial NSECs, and other
       material not under discussion have been omitted or shortened.

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    5.3.4.1.  Invalid Name Error Response Caused by DNAME in Bitmap

       ;; Header: QR AA RCODE=3(NXDOMAIN)
       ;; OPT PSEUDOSECTION:
       ; EDNS: version: 0, flags: do; udp: 4096

       ;; Question
       foo.bar.example.com. IN A
       ;; Authority
       bar.example.com. NSEC dub.example.com. A DNAME
       bar.example.com. RRSIG NSEC [valid signature]

       If this is the received response, then only by understanding that the
       DNAME bit in the NSEC bitmap means that foo.bar.example.com needed to
       have been redirected by the DNAME, the validator can see that it is a
       BOGUS reply from an attacker that collated existing records from the
       DNS to create a confusing reply.

       If the DNAME bit had not been set in the NSEC record above, then the
       answer would have validated as a correct name error response.

    5.3.4.2.  Valid Name Error Response Involving DNAME in Bitmap

       ;; Header: QR AA RCODE=3(NXDOMAIN)
       ;; OPT PSEUDOSECTION:
       ; EDNS: version: 0, flags: do; udp: 4096

       ;; Question
       cee.example.com. IN A
       ;; Authority
       bar.example.com. NSEC dub.example.com. A DNAME
       bar.example.com. RRSIG NSEC [valid signature]

       This response has the same NSEC records as the example above, but
       with this query name (cee.example.com), the answer is validated,
       because 'cee' does not get redirected by the DNAME at 'bar'.

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    5.3.4.3.  Response with Synthesized CNAME

       ;; Header: QR AA RCODE=0(NOERROR)
       ;; OPT PSEUDOSECTION:
       ; EDNS: version: 0, flags: do; udp: 4096

       ;; Question
       foo.bar.example.com. IN A
       ;; Answer
       bar.example.com. DNAME bar.example.net.
       bar.example.com. RRSIG DNAME [valid signature]
       foo.bar.example.com. CNAME foo.bar.example.net.

       The response shown above has the synthesized CNAME included.
       However, the CNAME has no signature, since the server does not sign
       online.  So this response cannot be trusted.  It could be altered by
       an attacker to be foo.bar.example.com CNAME bla.bla.example.  The
       DNAME record does have its signature included, since it does not
       change.  The validator must verify the DNAME signature and then
       recursively resolve further in order to query for the
       foo.bar.example.net A record.

    6.  Examples of DNAME Use in a Zone

       Below are some examples of the use of DNAME in a zone.  These
       examples are by no means exhaustive.

    6.1.  Organizational Renaming

       If an organization with domain name FROBOZZ.EXAMPLE.NET became part
       of an organization with domain name ACME.EXAMPLE.COM, it might ease
       transition by placing information such as this in its old zone.

           frobozz.example.net.  DNAME    frobozz-division.acme.example.com.
                                 MX       10       mailhub.acme.example.com.

       The response to an extended recursive query for
       www.frobozz.example.net would contain, in the answer section, the
       DNAME record shown above and the relevant RRs for www.frobozz-
       division.acme.example.com.

       If an organization wants to have aliases for names, for a different
       spelling or language, the same example applies.  Note that the MX RR
       at the zone apex is not redirected and has to be repeated in the
       target zone.  Also note that the services at mailhub or www.frobozz-
       division.acme.example.com. have to recognize and handle the aliases.

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    6.2.  Classless Delegation of Shorter Prefixes

       The classless scheme for in-addr.arpa delegation [RFC2317] 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.com.
           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 as follows:

            $ORIGIN 8/22.0.192.in-addr.arpa.
            33.9    PTR     somehost.slash-22-holder.example.com.

       The advisory remarks in [RFC2317] concerning the choice of the "/"
       character apply here as well.

    6.3.  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.com.

           $ORIGIN in-addr.customer.example.
           1                 PTR      www.customer.example.com
           2                 PTR      mailhub.customer.example.com.
           ; etc ...

       This would allow the address space 190.189.0.0/16 assigned to the ISP
       "example.net" to be changed without having to alter the zone data
       describing the use of that space by the ISP and its customers.

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    RFC 6672                    DNAME Redirection                  June 2012

       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.

    7.  IANA Considerations

       The DNAME resource record type code 39 (decimal) originally was
       registered by [RFC2672] in the DNS Resource Record (RR) Types
       registry table at http://www.iana.org/assignments/dns-parameters.
       IANA has updated the DNS resource record registry to point to this
       document for RR type 39.

    8.  Security Considerations

       DNAME redirects queries elsewhere, which may impact security based on
       policy and the security status of the zone with the DNAME and the
       redirection zone's security status.  For validating resolvers, the
       lowest security status of the links in the chain of CNAME and DNAME
       redirections is applied to the result.

       If a validating resolver accepts wildcarded DNAMEs, this creates
       security issues.  Since the processing of a wildcarded DNAME is non-
       deterministic and the CNAME that was substituted by the server has no
       signature, the resolver may choose a different result than what the
       server meant, and consequently end up at the wrong destination.  Use
       of wildcarded DNAMEs is discouraged in any case [RFC4592].

       A validating resolver MUST understand DNAME, according to [RFC4034].
       The examples in Section 5.3.4 illustrate this need.

    9.  Acknowledgments

       The authors of this document would like to acknowledge Matt Larson
       for beginning this effort to address the issues related to the DNAME
       RR type.  The authors would also like to acknowledge Paul Vixie, Ed
       Lewis, Mark Andrews, Mike StJohns, Niall O'Reilly, Sam Weiler, Alfred
       Hoenes, and Kevin Darcy for their reviews and comments on this
       document.

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    10.  References

    10.1.  Normative References

       [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
                  STD 13, RFC 1034, November 1987.

       [RFC1035]  Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, November 1987.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
                  "Dynamic Updates in the Domain Name System (DNS UPDATE)",
                  RFC 2136, April 1997.

       [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
                  Specification", RFC 2181, July 1997.

       [RFC2317]  Eidnes, H., de Groot, G., and P. Vixie, "Classless IN-
                  ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.

       [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
                  specifying the location of services (DNS SRV)", RFC 2782,
                  February 2000.

       [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
                  (RR) Types", RFC 3597, September 2003.

       [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "DNS Security Introduction and Requirements",
                  RFC 4033, March 2005.

       [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "Resource Records for the DNS Security Extensions",
                  RFC 4034, March 2005.

       [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
                  Rose, "Protocol Modifications for the DNS Security
                  Extensions", RFC 4035, March 2005.

       [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
                  System", RFC 4592, July 2006.

       [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
                  Security (DNSSEC) Hashed Authenticated Denial of
                  Existence", RFC 5155, March 2008.

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    RFC 6672                    DNAME Redirection                  June 2012

    10.2.  Informative References

       [RFC1912]  Barr, D., "Common DNS Operational and Configuration
                  Errors", RFC 1912, February 1996.

       [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
                  RFC 2672, August 1999.

       [RFC3363]  Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
                  Hain, "Representing Internet Protocol version 6 (IPv6)
                  Addresses in the Domain Name System (DNS)", RFC 3363,
                  August 2002.

       [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
                  for Internationalized Domain Names in Applications
                  (IDNA)", RFC 3492, March 2003.

       [RFC6434]  Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
                  Requirements", RFC 6434, December 2011.

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    RFC 6672                    DNAME Redirection                  June 2012

    Appendix A.  Changes from RFC 2672

    A.1.  Changes to Server Behavior

       Major changes to server behavior from the original DNAME
       specification are summarized below:

       o  The rules for DNAME substitution have been clarified in
          Section 2.2.

       o  The EDNS option to signal DNAME understanding and compression has
          never been specified, and this document clarifies that there is no
          signaling method (Section 2.5).

       o  The TTL for synthesized CNAME RRs is now set to the TTL of the
          DNAME, not zero (Section 3.1).

       o  Recursive caching servers MUST perform CNAME synthesis on behalf
          of clients (Section 3.4).

       o  The revised server algorithm is detailed in Section 3.2.

       o  Rules for dynamic update messages adding a DNAME or CNAME RR to a
          zone where a CNAME or DNAME already exists are detailed in
          Section 5.2.

    A.2.  Changes to Client Behavior

       Major changes to client behavior from the original DNAME
       specification are summarized below:

       o  Clients MUST be able to accept synthesized CNAME RR's with a TTL
          of either zero or the TTL of the DNAME RR that accompanies the
          CNAME RR.

       o  DNSSEC-aware clients SHOULD cache DNAME RRs and MAY cache
          synthesized CNAME RRs they receive in the same response.  DNSSEC-
          aware clients SHOULD also check the NSEC/NSEC3 type bitmap to
          verify that DNAME redirection is to be done.  DNSSEC validators
          MUST understand DNAME (Section 5.3).

       o  The revised client algorithm is detailed in Section 3.4.1.

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    Authors' Addresses

       Scott Rose
       NIST
       100 Bureau Dr.
       Gaithersburg, MD  20899
       USA

       Phone: +1-301-975-8439
       Fax:   +1-301-975-6238
       EMail: scott.rose@nist.gov

       Wouter Wijngaards
       NLnet Labs
       Science Park 140
       Amsterdam  1098 XH
       The Netherlands

       Phone: +31-20-888-4551
       EMail: wouter@nlnetlabs.nl

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