DNS Extensions Working Group S. Rose Internet-Draft NIST Obsoletes: 2672 (if approved) W. Wijngaards Updates: 3363,4294 NLnet Labs (if approved)
April 21,May 2, 2008 Intended status: Standards Track Expires: October 23,November 3, 2008 Update to DNAME Redirection in the DNS draft-ietf-dnsext-rfc2672bis-dname-12draft-ietf-dnsext-rfc2672bis-dname-13 Status of This Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on October 23,November 3, 2008. Copyright Notice Copyright (C) The IETF Trust (2008). Abstract 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 an update toof the original specification in RFC 2672, also aligning RFC 3363 and RFC 4294 with this revision. Requirements Language 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 RFC 2119 [RFC2119]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. The DNAME Resource Record . . . . . . . . . . . . . . . . . . 3 2.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. The DNAME Substitution . . . . . . . . . . . . . . . . . . 4 2.3. DNAME Apex not Redirected itself . . . . . . . . . . . . . 5 2.4. Names Next to and Below a DNAME Record . . . . . . . . . . 6 2.5. Compression of the DNAME record. . . . . . . . . . . . . . 6 3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Server algorithmCNAME synthesis and UD bit . . . . . . . . . . . . . . . . 7 3.2. Server algorithm . . . . . 7 3.2. Wildcards. . . . . . . . . . . . . . . . 8 3.3. Wildcards . . . . . . . . 9 3.3. CNAME synthesis and UD bit. . . . . . . . . . . . . . . . 9 3.4. Acceptance and Intermediate Storage . . . . . . . . . . . 10 4. DNAME Discussions in Other Documents . . . . . . . . . . . . . 10 5. Other Issues with DNAME . . . . . . . . . . . . . . . . . . . 12 5.1. Canonical hostnames cannot be below DNAME owners . . . . . 12 5.2. Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 12 5.3. DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 12 5.3.1. DNAME bit in NSEC type map . . . . . . . . . . . . . . 12 5.3.2. Validators Must Understand DNAME . . . . . . . . . . . 12 18.104.22.168. DNAME in Bitmap Causes Invalid Name Error . . . . 13 22.214.171.124. Valid Name Error Response Involving DNAME in Bitmap . . . . . . . . . . . . . . . . . . . . . . 13 126.96.36.199. Response With Synthesized CNAME . . . . . . . . . 13 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14 7. Security Considerations . . . . . . . . . . . . . . . . . . . 14 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative References . . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . . 15 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, a DNAME RR directs lookups for data at descendents 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 [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 an update toof 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 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 and punycode alternates for domain spaces.delegations. Another usage of DNAME lies in redirection 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. DNAME is also used for the redirection of ENUM domains to another maintaining party. This update to DNAME does not change the wire format or the handling of DNAME Resource Records by existing software. A new UD (Understand Dname)DNAME) bit in the EDNS flags field can be used to signal that CNAME synthesis is not needed. Discussion is added on problems that may be encountered when using DNAME. 2. The DNAME Resource Record 2.1. Format The DNAME RR has mnemonic DNAME and type code 39 (decimal). It is not class-sensitive. 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]. <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 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 RFC 1034 [RFC1034], 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 is the case of 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). 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. <no match> 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. 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 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 of 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 Apex not Redirected itself 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 or CNAMEs. This means that DNAME RRs are not allowed at the parent side of a delegation point but are allowed at a zone apex. The reason for this decision was that one can have a DNAME at the zone apex. There still is a need to have the customary SOA and NS resource records at the zone apex. This means that DNAME does not mirror a zone completely, as it does not mirror the zone apex. These rules also allow DNAME records to be queried through RFC 1034 [RFC1034] compliant, DNAME-unaware caches. 2.4. Names Next to and Below a DNAME Record Resource records MUST NOT exist at any domain name subordinate to the owner of a DNAME RR. To get the contents for names subordinate to that owner, the DNAME redirection must be invoked and the resulting target queried. A server MAY refuse to load a zone that has data at a domain name subordinate to a domain name owning a DNAME RR. If the server does load the zone, those names below the DNAME RR will be occluded, RFC 2136 [RFC2136], section 7.18. Also a server SHOULD 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, and thus no confusion which one to follow. A server SHOULD 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, so that a DNAME RR can be treated as an unknown type [RFC3597]. Although the previous DNAME specification [RFC2672] (that is obsoleted by this specification) talked about signaling to allow compression of the target name, such signaling is not specified. RFC 2672 stated that the EDNS version had a meaning for understanding of DNAME and DNAME target name compression. This document updates RFC 2672, in that there is no EDNS version signaling for DNAME as of yet.DNAME. However, the flags section of EDNS(0) is updated with a Understand-DNAMEUnderstand- DNAME flag by this document (See Section 3.3). 3. Processing The DNAME RR causes type NS additional section processing. 3.1. Server algorithm Below theCNAME synthesis and UD bit When preparing an response, a server algorithm, which appearedupon performing a DNAME substitution will in RFC 2672 Section 4.1, is expanded to handle the UD (Understand Dname) bit. 1. Set or clearall cases include the value of recursion availableDNAME RR used in the response depending on whether the name server is willinganswer section. A CNAME RR record with TTL equal to provide recursive service. If recursive servicethe corresponding DNAME RR is availablesynthesized and requested via the RD bitincluded in the query, go to step 5, otherwise step 2. 2. Search the available zonesanswer section 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.old resolvers. The matching process can terminate several ways: A. If the wholeowner name of QNAMEthe CNAME is matched, we have foundthe node. IfQNAME of the data atquery. DNSSEC [RFC4033], [RFC4034], [RFC4035] says that the node is a CNAME, and QTYPEsynthesized CNAME does not match CNAME, copyhave to be signed. The DNAME has an RRSIG and a validating resolver can check the CNAME RR intoagainst the answer section ofDNAME record and validate 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 goDNAME record. Resolvers MUST be able 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 ofhandle a zone. Copy the NS RRs for the sub-zone into the authority sectionsynthesized CNAME TTL of the reply. Put whatever addresses are available into the additional section, using glue RRs if the addresses are not available from authoritative datazero or the cache. Goequal to step 4. C. If at some label, a match is impossible (i.e.,the corresponding label does not exist), look to see whetherTTL of the last label matched has acorresponding DNAME record. If a DNAME record exists at that point, copyA TTL of zero means that record intothe answer section. If substitution of its <target> for its <owner> in QNAME would overflowCNAME can be discarded immediately after processing the legal size for a <domain- name>,answer. DNAME aware resolvers can set RCODE to YXDOMAIN [RFC2136] and exit; otherwise perform the substitution and continue. Ifthe EDNS OPT record is present inUnderstand-DNAME (UD bit) to receive a response with only the queryDNAME RR and theno synthesized CNAMEs. The UD bit is set, the server MAY copy the UD bit topart of the answerEDNS OPT record,[RFC2671] extended RCODE and Flags field. It is used to omit CNAME synthesis. Else theserver MUST synthesize a CNAME record as described aboveprocessing, transmission and include itresolver processing of unsigned synthesized CNAMEs. Resolvers can set this in the answer section. Go back to step 1. If there was no DNAME record, looka query to see if the "*" label exists. If the "*" label does not exist, check whether the name we are looking for isrequest omission of the original QNAME insynthesized CNAMEs. Servers copy the query or a name we have followed dueUD bit to a CNAME or DNAME. If the name is original, set an authoritative name error inthe responseresponse, and exit. Otherwise just exit. If the "*" label does exist, match RRs at that node against QTYPE. If any match, copy them intocan omit synthesized CNAMEs from the answer section, butanswer. Older resolvers do not set the owner ofUD bit, and older servers do not copy the RRUD bit to be QNAME,the answer, and will not omit synthesized CNAMEs. Updated EDNS extended RCODE and Flags field. +0 (MSB) +1 (LSB) +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 0: | EXTENDED-RCODE | VERSION | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 2: |DO|UD| Z | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ Servers MUST be able to answer a query for a synthesized CNAME. Like other query types this invokes the node with the "*" label. IfDNAME, and synthesizes the data atCNAME into the node withanswer. 3.2. Server algorithm Below the "*" labelserver algorithm, which appeared in RFC 2672 Section 4.1, is a CNAME, and QTYPE doesn't match CNAME, copyexpanded to handle the CNAME RR intoUD (Understand DNAME) bit. 1. Set or clear the answer sectionvalue of recursion available in the response changingdepending on whether the ownername server is willing to provide recursive service. If recursive service is available and requested via the QNAME, change QNAME to the canonical nameRD bit in the CNAME RR, andquery, go backto step 1. Otherwise, Go5, 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 6.3, otherwise step 4. 3. Start matching downdown, label by label, in the cache.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 cache,CNAME RR, and go back to step 1. Otherwise, copy all RRs attached to it thatwhich match QTYPE into the answer section.section and go to step 6. B. If QNAMEa 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 found inexist), look to see whether the cache butlast label matched has a DNAME record. If a DNAME record is presentexists at an ancestor of QNAME,that point, copy that DNAMErecord into the answer section. If there was no delegation from authoritative data, looksubstitution of its <target> for its <owner> in QNAME would overflow the best one fromlegal size for a <domain- name>, set RCODE to YXDOMAIN [RFC2136] and exit; otherwise perform the cache,substitution and put itcontinue. If the EDNS OPT record is present in the authority section. Go to step 6. 5. Usequery and the local resolver or aUD bit is set, the server MAY copy of its algorithmthe UD bit to answerthe query. Storeanswer EDNS OPT record, and omit CNAME synthesis. Else the results, including any intermediate CNAMEsserver MUST synthesize a CNAME record as described above and DNAMEs,include it in the answer section of the response. 6. Using local data only, attemptsection. Go back to add other RRs which may be usefulstep 1. If there was no DNAME record, look to see if the additional section of"*" label exists. If 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"*" label does not exist, check whether the name we are looking for is in violation ofthe no-descendants limitationoriginal QNAME in section 3. An implementation might take advantage of this limitation by stoppingthe search of step 3cquery or step 4 whena DNAME recordname we have followed due to a CNAME or DNAME. If the name is encountered. 3.2. Wildcards The use of DNAMEoriginal, set an authoritative name error in conjunction with wildcards is discouraged [RFC4592]. Thus records of the form "*.example.com DNAME example.net" SHOULD NOT be used. The interaction between the expansion ofthe wildcardresponse and exit. Otherwise just exit. If the redirection of"*" label does exist, match RRs at that node against QTYPE. If any match, copy them into the DNAME is non-deterministic. Becauseanswer section, but set the processing is non-deterministic, DNSSEC validating resolvers may not be able to validate a wildcarded DNAME. A server MAY give a warning thatowner of the behavior is unspecified if such a wildcarded DNAME is loaded. The server MAY refuse it, refuseRR to load or refuse dynamic update. 3.3. CNAME synthesisbe QNAME, and UD bit When preparing an response, a server upon performing a DNAME substitution will in all cases includenot the DNAME RR used innode with the "*" label. If the data at the answer section. A CNAME RR recordnode with TTL equal tothe corresponding DNAME RR"*" label is synthesizeda CNAME, and included inQTYPE doesn't match CNAME, copy the CNAME RR into the answer section for old resolvers. Theof the response changing the owner name ofto the QNAME, change QNAME to the canonical name in the CNAME isRR, and go back to step 1. Otherwise, Go to step 6. 4. Start matching down in the cache. If QNAME ofis found in the query. DNSSEC [RFC4033], [RFC4034], [RFC4035] sayscache, copy all RRs attached to it that match QTYPE into the synthesized CNAME doesanswer section. If QNAME is not have to be signed. Thefound in the cache but a DNAME hasrecord is present at an RRSIG and a validating resolver can check the CNAME against theancestor 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 validateput it in the DNAME record. Resolvers MUST be ableauthority section. Go to handlestep 6. 5. Use the local resolver or a synthesized CNAME TTLcopy of zero or equalits algorithm to answer the TTL ofquery. Store the corresponding DNAME record. A TTLresults, including any intermediate CNAMEs and DNAMEs, in the answer section of zero means thatthe CNAME canresponse. 6. Using local data only, attempt to add other RRs which may be discarded immediately after processinguseful to the answer. DNAME aware resolvers can setadditional section of the Understand-DNAME (UD bit) to receive a responsequery. Exit. Note that there will be at most one ancestor with only thea DNAME RR and no synthesized CNAMEs. The UD bitas described in step 4 unless some zone's data is partin violation of the EDNS [RFC2671] extended RCODE and Flags field. It is used to omit server processing, transmission and resolver processingno-descendants limitation in section 3. An implementation might take advantage of unsigned synthesized CNAMEs. Resolvers can setthis inlimitation by stopping the search of step 3c or step 4 when a query to request omissionDNAME record is encountered. 3.3. Wildcards The use of DNAME in conjunction with wildcards is discouraged [RFC4592]. Thus records of the synthesized CNAMEs. Servers copyform "*.example.com DNAME example.net" SHOULD NOT be used. The interaction between the UD bit toexpansion of the response,wildcard and can omit synthesized CNAMEs from the answer. Older resolvers do not setthe UD bit, and older servers do not copyredirection of the UD bit toDNAME is non-deterministic. Because the answer, and willprocessing is non-deterministic, DNSSEC validating resolvers may not omit synthesized CNAMEs. Updated EDNS extended RCODE and Flags field. +0 (MSB) +1 (LSB) +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 0: | EXTENDED-RCODE | VERSION | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 2: |DO|UD| Z | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ Servers MUSTbe able to answervalidate a query forwildcarded DNAME. A server MAY give a synthesized CNAME. Like other query types this invokes the DNAME, and synthesizes the CNAME intowarning that the answer.behavior is unspecified if such a wildcarded DNAME is loaded. The server MAY refuse it, refuse to load or refuse dynamic update. 3.4. Acceptance and Intermediate Storage DNS caches 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 cache 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. DNS caches MUST perform CNAME synthesis on behalf of DNAME-ignorant clients. A DNS cache that understands DNAMEs can send out queries on behalf of clients with the UD bit set.set (See Section 3.1). After receiving the answers the DNS cache sends replies to DNAME ignorant clients that include DNAMEs and synthesized CNAMEs. 4. DNAME Discussions in Other Documents In [RFC2181], 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, [RFC3363] deprecates the use of DNAME in the IPv6 reverse tree, which is then carried forward as a recommendation in [RFC4294]. Based on the experience gained in the meantime, [RFC3363] should be 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 [RFC3363], 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 to be replaced with the word "DELETED". In [RFC4294], 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]." 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 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 without punycode 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. 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. A server MUST ignore a dynamic update message that attempts to add a DNAME RR at a name that already has a CNAME RR or another DNAME RR associated with that name. 5.3. DNSSEC and DNAME 5.3.1. DNAME bit in NSEC type map When a validator checks the NSEC RRs returned on a name error response, it SHOULD check that the DNAME bit is not set. If the DNAME bit is set then the DNAME substitution should have been done, but has not. 5.3.2. Validators Must Understand DNAME Examples of why DNSSEC validators MUST understand DNAME. 188.8.131.52. DNAME in Bitmap Causes Invalid Name Error ;; Header: QR AA DO RCODE=3(NXDOMAIN) ;; Question foo.bar.example.com. IN A ;; Answer bar.example.com. NSEC dub.example.com. A DNAME bar.example.com. RRSIG NSEC [valid signature] If this is the response, then only by understanding that the DNAME bit 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. 184.108.40.206. Valid Name Error Response Involving DNAME in Bitmap ;; Header: QR AA DO RCODE=3(NXDOMAIN) ;; Question cee.example.com. IN A ;; Answer bar.example.com. NSEC dub.example.com. A DNAME bar.example.com. RRSIG NSEC [valid signature] This reply 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'. 220.127.116.11. Response With Synthesized CNAME ;; Header: QR AA DO RCODE=0(NOERROR) ;; 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 answer shown above has the synthesized CNAME included. However, the CNAME has no signature, since the server does not sign online. So it 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 for every query name. The validator must verify the DNAME signature and then recursively resolve further to query for the foo.bar.example.net A record. 6. IANA Considerations The DNAME Resource Record type code 39 (decimal) originally has been registered by [RFC2672]. IANA should update the DNS resource record registry to point to this document for RR type 39. This draft requests the second highest bit in the EDNS flags field for the Understand-DNAME (UD) flag. 7. 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. 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]. In Section 5.3.2 examples are given that illustrate this need. 8. Acknowledgments 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 HoenesHines and Kevin Darcy for their review and comments on this document. 9. References 9.1. 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. [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671, August 1999. [RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC 2672, August 1999.[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. 9.2. 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. [RFC4294] Loughney, J., "IPv6 Node Requirements", RFC 4294, April 2006. Authors' Addresses Scott Rose NIST 100 Bureau Dr. Gaithersburg, MD 20899 USA Phone: +1-301-975-8439 Fax: +1-301-975-6238 EMail: email@example.com Wouter Wijngaards NLnet Labs Kruislaan 419 Amsterdam 1098 VA The Netherlands Phone: +31-20-888-4551 EMail: firstname.lastname@example.org Full Copyright Statement Copyright (C) The IETF Trust (2008). 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