NIST

100 Bureau Dr. Gaithersburg 20899 MD USA +1-301-975-8439 +1-301-975-6238 scottr.nist@gmail.com

NLnet Labs

Science Park 140 1098 XG Amsterdam The Netherlands +31-20-888-4551 wouter@nlnetlabs.nl

Internet Area DNS Extensions Working Group DNS DNAME The DNAME record provides redirection for a sub-tree of the domain name tree in the DNS system. That is, all names that end with a particular suffix are redirected to another part of the DNS. This is a revision to the original specification in RFC 2672 (which this document obsoletes) as well as updating RFC 3363 and RFC 4294 to align with this revision. 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.

DNAME is a DNS Resource Record type originally defined in RFC 2672 . 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 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, 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. Take for example, looking through a zone (see RFC 1034, 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. DNAME was conceived to help with the problem of maintaining address-to-name mappings in a context of network renumbering. With a careful set-up, 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 sub-trees of the DNS to contain the same information. Examples include punycode 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.

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 , . The <target> field MUST be present. The presentation format of <target> is that of a domain name . <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.

When following RFC 1034 , section 4.3.2's algorithm's third step, "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 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>"

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 insure 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 . The DNAME record and its signature (if the zone is signed) are included in the answer as proof for the YXDOMAIN (value 6) RCODE.

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 co-exist with. 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. E.g., 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 as this would constitute data below a zone cut. 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 RFC 1034 compliant, DNAME-unaware caches.

Resource records MUST NOT exist at any sub-domain 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 sub-domain 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 , 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 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, and thus no confusion which one to follow. A server ought to refuse to load a zone that violates these rules.

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 DNSSEC validation. Although the previous DNAME specification (that is obsoleted by this specification) talked about signaling to allow compression of the target name, such signaling has never been specified and this document also does not specify this signaling behavior. RFC 2672 (obsoleted by this document) stated that the EDNS version had a meaning for understanding of DNAME and DNAME target name compression. This document revises RFC 2672, in that there is no EDNS version signaling for DNAME.

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: The DNAME is being employed as a substitution instruction. The DNAME itself matches the QTYPE and the owner name matches QNAME. When the owner name name matches the QNAME and the QTYPE matches another type owned there, the DNAME is not included in the answer. A CNAME RR with 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 , , 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 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.

Below is the server algorithm, which appeared in RFC 2672 Section 4.1. 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. 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. Start matching down, label by label, in the zone. The matching process can terminate several ways: 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. 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. 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 and exit; otherwise perform the substitution and continue. The server MUST 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. 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. 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. 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.

The use of DNAME in conjunction with wildcards is discouraged . Thus records of the form "*.example.com DNAME example.net" SHOULD NOT be used. The interaction between the expansion of the wildcard and the redirection of the DNAME is non-deterministic. Because 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.

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 which 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 of CNAME.

A resolver algorithm likewise changes to handle DNAME processing. The complete algorithm becomes: See if the answer is in local information or can be synthesized from a cached DNAME, and if so return it to the client. Find the best servers to ask. Send queries until one returns a response. Analyze the response, either: If the response answers the question or contains a name error, cache the data as well as returning it back to the client. If the response contains a better delegation to other servers, cache the delegation information, and go to step 2. 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. 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. If the response shows a server failure or other bizarre contents, delete the server from the SLIST and go back to step 3.

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

Excerpt from 10.3. MX and NS records (in RFC 2181). 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, deprecates the use of DNAME in the IPv6 reverse tree, which is then carried forward as a recommendation in . Based on the experience gained in the meantime, is revised, dropping all constraints on having DNAME RRs in these zones. This would greatly improve the manageability of the IPv6 reverse tree. These changes are made explicit below.

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

In , the reference to DNAME was left in as an editorial oversight. The paragraph "Those nodes are NOT RECOMMENDED to support the experimental A6 and DNAME Resource Records [RFC3363]."

is to be replaced by "Those nodes are NOT RECOMMENDED to support the experimental A6 Resource Record [RFC3363]."

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

The names listed as target names of MX, NS, PTR and SRV 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 , section 10.3, and RFC 1912 , section 2.4. For SRV see RFC 2782 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 can not fall under a DNAME. The same holds for NS, SRV and MX records. For example, when punycode 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. What must be done then is to have the domain names with DNAME substitution already applied to it as the MX, NS, PTR, SRV data. These are valid canonical hostnames.

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 associated with that name, then the CNAME MUST be ignored. This is similar behavior for dynamic updates to an owner name of a CNAME RR .

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

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 checking that the CNAME was properly synthesized is sufficient proof.

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

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

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

;; 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.

;; 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'.

;; 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 to query for the foo.bar.example.net A record.

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

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.

The classless scheme for in-addr.arpa delegation 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

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

The same advisory remarks concerning the choice of the "/" character apply here as in .

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 the necessity of altering the zone data 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.

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

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 . A validating resolver MUST understand DNAME, according to . The examples in illustrate this need.

The authors of this draft 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 review and comments on this document.

Major changes to server behavior from the original DNAME specification are summarized below: The rules for DNAME substitution have been clarified in Section 2. 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). The TTL for synthesized CNAME RR's is now set to the TTL of the DNAME, not zero (Section 3.1). Caching recursive servers MUST perform CNAME synthesis on behalf of clients (Section 3.4). The revised server algorithm is detailed in Section 3.2. Rules for dynamic update messages adding a DNAME or CNAME RR to a zone where a CNAME or DNAME already exists is detailed in Section 5.2

Major changes to client behavior from the original DNAME specification are summarized below: 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. DNSSEC aware clients SHOULD cache DNAME RR's and MAY cache synthesized CNAME RR's it receives 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). The revised client algorithm is detailed in Section 3.4.1.