4 DNS Extensions Working Group S. Rose
6 Obsoletes: 2672 (if approved) W. Wijngaards
7 Updates: 3363,4294 NLnet Labs
8 (if approved) April 20, 2010
9 Intended status: Standards Track
10 Expires: October 22, 2010
13 Update to DNAME Redirection in the DNS
14 draft-ietf-dnsext-rfc2672bis-dname-19
18 The DNAME record provides redirection for a sub-tree of the domain
19 name tree in the DNS system. That is, all names that end with a
20 particular suffix are redirected to another part of the DNS. This is
21 a revision of the original specification in RFC 2672, also aligning
22 RFC 3363 and RFC 4294 with this revision.
26 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
27 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
28 document are to be interpreted as described in RFC 2119 [RFC2119].
32 This Internet-Draft is submitted to IETF in full conformance with the
33 provisions of BCP 78 and BCP 79.
35 Internet-Drafts are working documents of the Internet Engineering
36 Task Force (IETF), its areas, and its working groups. Note that
37 other groups may also distribute working documents as Internet-
40 Internet-Drafts are draft documents valid for a maximum of six months
41 and may be updated, replaced, or obsoleted by other documents at any
42 time. It is inappropriate to use Internet-Drafts as reference
43 material or to cite them other than as "work in progress."
45 The list of current Internet-Drafts can be accessed at
46 http://www.ietf.org/ietf/1id-abstracts.txt.
48 The list of Internet-Draft Shadow Directories can be accessed at
49 http://www.ietf.org/shadow.html.
51 This Internet-Draft will expire on October 22, 2010.
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62 Copyright (c) 2010 IETF Trust and the persons identified as the
63 document authors. All rights reserved.
65 This document is subject to BCP 78 and the IETF Trust's Legal
66 Provisions Relating to IETF Documents
67 (http://trustee.ietf.org/license-info) in effect on the date of
68 publication of this document. Please review these documents
69 carefully, as they describe your rights and restrictions with respect
70 to this document. Code Components extracted from this document must
71 include Simplified BSD License text as described in Section 4.e of
72 the Trust Legal Provisions and are provided without warranty as
73 described in the BSD License.
75 This document may contain material from IETF Documents or IETF
76 Contributions published or made publicly available before November
77 10, 2008. The person(s) controlling the copyright in some of this
78 material may not have granted the IETF Trust the right to allow
79 modifications of such material outside the IETF Standards Process.
80 Without obtaining an adequate license from the person(s) controlling
81 the copyright in such materials, this document may not be modified
82 outside the IETF Standards Process, and derivative works of it may
83 not be created outside the IETF Standards Process, except to format
84 it for publication as an RFC or to translate it into languages other
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118 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
120 2. The DNAME Resource Record . . . . . . . . . . . . . . . . . . 4
121 2.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 4
122 2.2. The DNAME Substitution . . . . . . . . . . . . . . . . . . 5
123 2.3. DNAME Owner Name Matching the QNAME . . . . . . . . . . . 7
124 2.4. Names Next to and Below a DNAME Record . . . . . . . . . . 7
125 2.5. Compression of the DNAME record. . . . . . . . . . . . . . 7
127 3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 8
128 3.1. CNAME synthesis . . . . . . . . . . . . . . . . . . . . . 8
129 3.2. Server algorithm . . . . . . . . . . . . . . . . . . . . . 9
130 3.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 11
131 3.4. Acceptance and Intermediate Storage . . . . . . . . . . . 11
133 4. DNAME Discussions in Other Documents . . . . . . . . . . . . . 11
135 5. Other Issues with DNAME . . . . . . . . . . . . . . . . . . . 13
136 5.1. Canonical hostnames cannot be below DNAME owners . . . . . 13
137 5.2. Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 13
138 5.3. DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 13
139 5.3.1. Signed DNAME, Unsigned Synthesized CNAME . . . . . . . 13
140 5.3.2. DNAME Bit in NSEC Type Map . . . . . . . . . . . . . . 14
141 5.3.3. DNAME Chains as Strong as the Weakest Link . . . . . . 14
142 5.3.4. Validators Must Understand DNAME . . . . . . . . . . . 14
143 5.3.4.1. DNAME in Bitmap Causes Invalid Name Error . . . . 14
144 5.3.4.2. Valid Name Error Response Involving DNAME in
145 Bitmap . . . . . . . . . . . . . . . . . . . . . . 15
146 5.3.4.3. Response With Synthesized CNAME . . . . . . . . . 15
148 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
150 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15
152 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 16
154 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
155 9.1. Normative References . . . . . . . . . . . . . . . . . . . 16
156 9.2. Informative References . . . . . . . . . . . . . . . . . . 17
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174 DNAME is a DNS Resource Record type originally defined in RFC 2672
175 [RFC2672]. DNAME provides redirection from a part of the DNS name
176 tree to another part of the DNS name tree.
178 The DNAME RR and the CNAME RR [RFC1034] cause a lookup to
179 (potentially) return data corresponding to a domain name different
180 from the queried domain name. The difference between the two
181 resource records is that the CNAME RR directs the lookup of data at
182 its owner to another single name, a DNAME RR directs lookups for data
183 at descendants of its owner's name to corresponding names under a
184 different (single) node of the tree.
186 Take for example, looking through a zone (see RFC 1034 [RFC1034],
187 section 4.3.2, step 3) for the domain name "foo.example.com" and a
188 DNAME resource record is found at "example.com" indicating that all
189 queries under "example.com" be directed to "example.net". The lookup
190 process will return to step 1 with the new query name of
191 "foo.example.net". Had the query name been "www.foo.example.com" the
192 new query name would be "www.foo.example.net".
194 This document is a revision of the original specification of DNAME in
195 RFC 2672 [RFC2672]. DNAME was conceived to help with the problem of
196 maintaining address-to-name mappings in a context of network
197 renumbering. With a careful set-up, a renumbering event in the
198 network causes no change to the authoritative server that has the
199 address-to-name mappings. Examples in practice are classless reverse
200 address space delegations.
202 Another usage of DNAME lies in aliasing of name spaces. For example,
203 a zone administrator may want sub-trees of the DNS to contain the
204 same information. Examples include punycode alternates for domain
207 This revision to DNAME does not change the wire format or the
208 handling of DNAME Resource Records. Discussion is added on problems
209 that may be encountered when using DNAME.
211 2. The DNAME Resource Record
215 The DNAME RR has mnemonic DNAME and type code 39 (decimal). It is
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228 Its RDATA is comprised of a single field, <target>, which contains a
229 fully qualified domain name that must be sent in uncompressed form
230 [RFC1035], [RFC3597]. The <target> field MUST be present. The
231 presentation format of <target> is that of a domain name [RFC1035].
233 <owner> <ttl> <class> DNAME <target>
235 The effect of the DNAME RR is the substitution of the record's
236 <target> for its owner name, as a suffix of a domain name. This
237 substitution is to be applied for all names below the owner name of
238 the DNAME RR. This substitution has to be applied for every DNAME RR
239 found in the resolution process, which allows fairly lengthy valid
242 Details of the substitution process, methods to avoid conflicting
243 resource records, and rules for specific corner cases are given in
244 the following subsections.
246 2.2. The DNAME Substitution
248 When following RFC 1034 [RFC1034], section 4.3.2's algorithm's third
249 step, "start matching down, label by label, in the zone" and a node
250 is found to own a DNAME resource record a DNAME substitution occurs.
251 The name being sought may be the original query name or a name that
252 is the result of a CNAME resource record being followed or a
253 previously encountered DNAME. As in the case when finding a CNAME
254 resource record or NS resource record set, the processing of a DNAME
255 will happen prior to finding the desired domain name.
257 A DNAME substitution is performed by replacing the suffix labels of
258 the name being sought matching the owner name of the DNAME resource
259 record with the string of labels in the RDATA field. The matching
260 labels end with the root label in all cases. Only whole labels are
261 replaced. See the table of examples for common cases and corner
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284 In the table below, the QNAME refers to the query name. The owner is
285 the DNAME owner domain name, and the target refers to the target of
286 the DNAME record. The result is the resulting name after performing
287 the DNAME substitution on the query name. "no match" means that the
288 query did not match the DNAME and thus no substitution is performed
289 and a possible error message is returned (if no other result is
290 possible). Thus every line contains one example substitution. In
291 the examples below, 'cyc' and 'shortloop' contain loops.
293 QNAME owner DNAME target result
294 ---------------- -------------- -------------- -----------------
295 com. example.com. example.net. <no match>
296 example.com. example.com. example.net. [0]
297 a.example.com. example.com. example.net. a.example.net.
298 a.b.example.com. example.com. example.net. a.b.example.net.
299 ab.example.com. b.example.com. example.net. <no match>
300 foo.example.com. example.com. example.net. foo.example.net.
301 a.x.example.com. x.example.com. example.net. a.example.net.
302 a.example.com. example.com. y.example.net. a.y.example.net.
303 cyc.example.com. example.com. example.com. cyc.example.com.
304 cyc.example.com. example.com. c.example.com. cyc.c.example.com.
305 shortloop.x.x. x. . shortloop.x.
306 shortloop.x. x. . shortloop.
308 [0] The result depends on the QTYPE. If the QTYPE = DNAME, then
309 the result is "example.com." else "<no match>"
311 Table 1. DNAME Substitution Examples.
313 It is possible for DNAMEs to form loops, just as CNAMEs can form
314 loops. DNAMEs and CNAMEs can chain together to form loops. A single
315 corner case DNAME can form a loop. Resolvers and servers should be
316 cautious in devoting resources to a query, but be aware that fairly
317 long chains of DNAMEs may be valid. Zone content administrators
318 should take care to insure that there are no loops that could occur
319 when using DNAME or DNAME/CNAME redirection.
321 The domain name can get too long during substitution. For example,
322 suppose the target name of the DNAME RR is 250 octets in length
323 (multiple labels), if an incoming QNAME that has a first label over 5
324 octets in length, the result would be a name over 255 octets. If
325 this occurs the server returns an RCODE of YXDOMAIN [RFC2136]. The
326 DNAME record and its signature (if the zone is signed) are included
327 in the answer as proof for the YXDOMAIN (value 6) RCODE.
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340 2.3. DNAME Owner Name Matching the QNAME
342 Unlike a CNAME RR, a DNAME RR redirects DNS names subordinate to its
343 owner name; the owner name of a DNAME is not redirected itself. The
344 domain name that owns a DNAME record is allowed to have other
345 resource record types at that domain name, except DNAMEs, CNAMEs or
346 other types that have restrictions on what they can co-exist with.
347 When there is a match of the QTYPE to a type (or types) also owned by
348 the owner name the response is sourced from the owner name. E.g., a
349 QTYPE of ANY would return the (available) types at the owner name,
352 DNAME RRs MUST NOT appear at the same owner name as an NS RR unless
353 the owner name is the zone apex as this would constitute data below a
356 If a DNAME record is present at the zone apex, there is still a need
357 to have the customary SOA and NS resource records there as well.
358 Such a DNAME cannot be used to mirror a zone completely, as it does
359 not mirror the zone apex.
361 These rules also allow DNAME records to be queried through RFC 1034
362 [RFC1034] compliant, DNAME-unaware caches.
364 2.4. Names Next to and Below a DNAME Record
366 Resource records MUST NOT exist at any sub-domain of the owner of a
367 DNAME RR. To get the contents for names subordinate to that owner
368 name, the DNAME redirection must be invoked and the resulting target
369 queried. A server MAY refuse to load a zone that has data at a sub-
370 domain of a domain name owning a DNAME RR. If the server does load
371 the zone, those names below the DNAME RR will be occluded as
372 described in RFC 2136 [RFC2136], section 7.18. Also a server SHOULD
373 refuse to load a zone subordinate to the owner of a DNAME record in
374 the ancestor zone. See Section 5.2 for further discussion related to
377 DNAME is a singleton type, meaning only one DNAME is allowed per
378 name. The owner name of a DNAME can only have one DNAME RR, and no
379 CNAME RRs can exist at that name. These rules make sure that for a
380 single domain name only one redirection exists, and thus no confusion
381 which one to follow. A server SHOULD refuse to load a zone that
382 violates these rules.
384 2.5. Compression of the DNAME record.
386 The DNAME owner name can be compressed like any other owner name.
387 The DNAME RDATA target name MUST NOT be sent out in compressed form,
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396 so that a DNAME RR can be treated as an unknown type [RFC3597].
398 Although the previous DNAME specification [RFC2672] (that is
399 obsoleted by this specification) talked about signaling to allow
400 compression of the target name, such signaling has never been
401 specified and this document also does not specify this signaling
404 RFC 2672 (obsoleted by this document) stated that the EDNS version
405 had a meaning for understanding of DNAME and DNAME target name
406 compression. This document revises RFC 2672, in that there is no
407 EDNS version signaling for DNAME.
411 The DNAME RR causes type NS additional section processing. This
412 refers to action at step 6 of the server algorithm outlined in
417 When preparing a response, a server performing a DNAME substitution
418 will in all cases include the relevant DNAME RR in the answer
419 section. Relevant includes the following cases:
421 1. The DNAME is being employed as a substitution instruction.
423 2. The DNAME itself matches the QTYPE and the owner name matches
426 When the owner name name matches the QNAME and the QTYPE matches
427 another type owned there, the DNAME is not included in the answer.
429 A CNAME RR with TTL equal to the corresponding DNAME RR is
430 synthesized and included in the answer section when the DNAME is
431 employed as a substitution instruction. The owner name of the CNAME
432 is the QNAME of the query. The DNSSEC specification [RFC4033],
433 [RFC4034], [RFC4035] says that the synthesized CNAME does not have to
434 be signed. The DNAME has an RRSIG and a validating resolver can
435 check the CNAME against the DNAME record and validate the signature
438 Servers MUST be able to answer a query for a synthesized CNAME. Like
439 other query types this invokes the DNAME, and synthesizes the CNAME
440 into the answer. If the server in question is a cache, the
441 synthesized CNAME's TTL SHOULD be equal to the decremented TTL of the
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452 Resolvers MUST be able to handle a synthesized CNAME TTL of zero or
453 equal to the TTL of the corresponding DNAME record (as some older
454 authoritative server implementations set the TTL of synthesized
455 CNAMEs to zero). A TTL of zero means that the CNAME can be discarded
456 immediately after processing the answer.
458 3.2. Server algorithm
460 Below is the server algorithm, which appeared in RFC 2672 Section
463 1. Set or clear the value of recursion available in the response
464 depending on whether the name server is willing to provide
465 recursive service. If recursive service is available and
466 requested via the RD bit in the query, go to step 5, otherwise
470 2. Search the available zones for the zone which is the nearest
471 ancestor to QNAME. If such a zone is found, go to step 3,
475 3. Start matching down, label by label, in the zone. The matching
476 process can terminate several ways:
479 A. If the whole of QNAME is matched, we have found the node.
481 If the data at the node is a CNAME, and QTYPE does not match
482 CNAME, copy the CNAME RR into the answer section of the
483 response, change QNAME to the canonical name in the CNAME RR,
484 and go back to step 1.
486 Otherwise, copy all RRs which match QTYPE into the answer
487 section and go to step 6.
490 B. If a match would take us out of the authoritative data, we
491 have a referral. This happens when we encounter a node with
492 NS RRs marking cuts along the bottom of a zone.
494 Copy the NS RRs for the sub-zone into the authority section
495 of the reply. Put whatever addresses are available into the
496 additional section, using glue RRs if the addresses are not
497 available from authoritative data or the cache. Go to step
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508 C. If at some label, a match is impossible (i.e., the
509 corresponding label does not exist), look to see whether the
510 last label matched has a DNAME record.
512 If a DNAME record exists at that point, copy that record into
513 the answer section. If substitution of its <target> for its
514 <owner> in QNAME would overflow the legal size for a <domain-
515 name>, set RCODE to YXDOMAIN [RFC2136] and exit; otherwise
516 perform the substitution and continue. The server MUST
517 synthesize a CNAME record as described above and include it
518 in the answer section. Go back to step 1.
520 If there was no DNAME record, look to see if the "*" label
523 If the "*" label does not exist, check whether the name we
524 are looking for is the original QNAME in the query or a name
525 we have followed due to a CNAME or DNAME. If the name is
526 original, set an authoritative name error in the response and
527 exit. Otherwise just exit.
529 If the "*" label does exist, match RRs at that node against
530 QTYPE. If any match, copy them into the answer section, but
531 set the owner of the RR to be QNAME, and not the node with
532 the "*" label. If the data at the node with the "*" label is
533 a CNAME, and QTYPE doesn't match CNAME, copy the CNAME RR
534 into the answer section of the response changing the owner
535 name to the QNAME, change QNAME to the canonical name in the
536 CNAME RR, and go back to step 1. Otherwise, Go to step 6.
539 4. Start matching down in the cache. If QNAME is found in the
540 cache, copy all RRs attached to it that match QTYPE into the
541 answer section. If QNAME is not found in the cache but a DNAME
542 record is present at an ancestor of QNAME, copy that DNAME record
543 into the answer section. If there was no delegation from
544 authoritative data, look for the best one from the cache, and put
545 it in the authority section. Go to step 6.
548 5. Use the local resolver or a copy of its algorithm to answer the
549 query. Store the results, including any intermediate CNAMEs and
550 DNAMEs, in the answer section of the response.
553 6. Using local data only, attempt to add other RRs which may be
554 useful to the additional section of the query. Exit.
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564 Note that there will be at most one ancestor with a DNAME as
565 described in step 4 unless some zone's data is in violation of the
566 no-descendants limitation in section 3. An implementation might take
567 advantage of this limitation by stopping the search of step 3c or
568 step 4 when a DNAME record is encountered.
572 The use of DNAME in conjunction with wildcards is discouraged
573 [RFC4592]. Thus records of the form "*.example.com DNAME
574 example.net" SHOULD NOT be used.
576 The interaction between the expansion of the wildcard and the
577 redirection of the DNAME is non-deterministic. Because the
578 processing is non-deterministic, DNSSEC validating resolvers may not
579 be able to validate a wildcarded DNAME.
581 A server MAY give a warning that the behavior is unspecified if such
582 a wildcarded DNAME is loaded. The server MAY refuse it, refuse to
583 load the zone or refuse dynamic updates.
585 3.4. Acceptance and Intermediate Storage
587 Recursive caching name servers can encounter data at names below the
588 owner name of a DNAME RR, due to a change at the authoritative server
589 where data from before and after the change resides in the cache.
590 This conflict situation is a transitional phase that ends when the
591 old data times out. The caching name server can opt to store both
592 old and new data and treat each as if the other did not exist, or
593 drop the old data, or drop the longer domain name. In any approach,
594 consistency returns after the older data TTL times out.
596 Recursive caching name servers MUST perform CNAME synthesis on behalf
599 If a recursive caching name server encounters a DNAME RR which
600 contradicts information already in the cache (excluding CNAME
601 records), it SHOULD NOT cache the DNAME RR, but it MAY cache the
602 CNAME record received along with it, subject to the rules for CNAME.
604 4. DNAME Discussions in Other Documents
606 In [RFC2181], in Section 10.3., the discussion on MX and NS records
607 touches on redirection by CNAMEs, but this also holds for DNAMEs.
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620 Excerpt from 10.3. MX and NS records (in RFC 2181).
622 The domain name used as the value of a NS resource record,
623 or part of the value of a MX resource record must not be
624 an alias. Not only is the specification clear on this
625 point, but using an alias in either of these positions
626 neither works as well as might be hoped, nor well fulfills
627 the ambition that may have led to this approach. This
628 domain name must have as its value one or more address
629 records. Currently those will be A records, however in
630 the future other record types giving addressing
631 information may be acceptable. It can also have other
632 RRs, but never a CNAME RR.
634 The DNAME RR is discussed in RFC 3363, section 4, on A6 and DNAME.
635 The opening premise of this section is demonstrably wrong, and so the
636 conclusion based on that premise is wrong. In particular, [RFC3363]
637 deprecates the use of DNAME in the IPv6 reverse tree, which is then
638 carried forward as a recommendation in [RFC4294]. Based on the
639 experience gained in the meantime, [RFC3363] should be revised,
640 dropping all constraints on having DNAME RRs in these zones. This
641 would greatly improve the manageability of the IPv6 reverse tree.
642 These changes are made explicit below.
644 In [RFC3363], the paragraph
646 "The issues for DNAME in the reverse mapping tree appears to be
647 closely tied to the need to use fragmented A6 in the main tree: if
648 one is necessary, so is the other, and if one isn't necessary, the
649 other isn't either. Therefore, in moving RFC 2874 to experimental,
650 the intent of this document is that use of DNAME RRs in the reverse
653 is to be replaced with the word "DELETED".
655 In [RFC4294], the reference to DNAME was left in as an editorial
656 oversight. The paragraph
658 "Those nodes are NOT RECOMMENDED to support the experimental A6 and
659 DNAME Resource Records [RFC3363]."
663 "Those nodes are NOT RECOMMENDED to support the experimental
664 A6 Resource Record [RFC3363]."
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676 5. Other Issues with DNAME
678 There are several issues to be aware of about the use of DNAME.
680 5.1. Canonical hostnames cannot be below DNAME owners
682 The names listed as target names of MX, NS, PTR and SRV [RFC2782]
683 records must be canonical hostnames. This means no CNAME or DNAME
684 redirection may be present during DNS lookup of the address records
685 for the host. This is discussed in RFC 2181 [RFC2181], section 10.3,
686 and RFC 1912 [RFC1912], section 2.4. For SRV see RFC 2782 [RFC2782]
689 The upshot of this is that although the lookup of a PTR record can
690 involve DNAMEs, the name listed in the PTR record can not fall under
691 a DNAME. The same holds for NS, SRV and MX records. For example,
692 when punycode alternates for a zone use DNAME then the NS, MX, SRV
693 and PTR records that point to that zone must use names without
694 punycode in their RDATA. What must be done then is to have the
695 domain names with DNAME substitution already applied to it as the MX,
696 NS, PTR, SRV data. These are valid canonical hostnames.
698 5.2. Dynamic Update and DNAME
700 DNAME records can be added, changed and removed in a zone using
701 dynamic update transactions. Adding a DNAME RR to a zone occludes
702 any domain names that may exist under the added DNAME.
704 A server MUST ignore a dynamic update message that attempts to add a
705 non-DNAME/CNAME RR at a name that already has a DNAME RR associated
706 with that name. Otherwise, replace the DNAME RR with the DNAME (or
707 CNAME) update RR. This is similar behavior to dynamic updates to an
708 owner name of a CNAME RR [RFC2136].
710 5.3. DNSSEC and DNAME
712 The following subsections specify the behavior of implementations
713 that understand both DNSSEC and DNAME (synthesis).
715 5.3.1. Signed DNAME, Unsigned Synthesized CNAME
717 In any response, a signed DNAME RR indicates a non-terminal
718 redirection of the query. There might or might not be a server
719 synthesized CNAME in the answer section; if there is, the CNAME will
720 never be signed. For a DNSSEC validator, verification of the DNAME
721 RR and then checking that the CNAME was properly synthesized is
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732 5.3.2. DNAME Bit in NSEC Type Map
734 In any negative response, the NSEC or NSEC3 [RFC5155] record type bit
735 map SHOULD be checked to see that there was no DNAME that could have
736 been applied. If the DNAME bit in the type bit map is set and the
737 query name is a sub-domain of the closest encloser that is asserted,
738 then DNAME substitution should have been done, but the substitution
739 has not been done as specified.
741 5.3.3. DNAME Chains as Strong as the Weakest Link
743 A response can contain a chain of DNAME and CNAME redirections. That
744 chain can end in a positive answer or a negative (no name error or no
745 data error) reply. Each step in that chain results in resource
746 records added to the answer or authority section of the response.
747 Only if all steps are secure can the AD bit be set for the response.
748 If one of the steps is bogus, the result is bogus.
750 5.3.4. Validators Must Understand DNAME
752 Below are examples of why DNSSEC validators MUST understand DNAME.
753 In the examples below, SOA records, wildcard denial NSECs and other
754 material not under discussion has been omitted or shortened.
756 5.3.4.1. DNAME in Bitmap Causes Invalid Name Error
758 ;; Header: QR AA RCODE=3(NXDOMAIN)
759 ;; OPT PSEUDOSECTION:
760 ; EDNS: version: 0, flags: do; udp: 4096
763 foo.bar.example.com. IN A
765 bar.example.com. NSEC dub.example.com. A DNAME
766 bar.example.com. RRSIG NSEC [valid signature]
768 If this is the received response, then only by understanding that the
769 DNAME bit in the NSEC bitmap means that foo.bar.example.com needed to
770 have been redirected by the DNAME, the validator can see that it is a
771 BOGUS reply from an attacker that collated existing records from the
772 DNS to create a confusing reply.
774 If the DNAME bit had not been set in the NSEC record above then the
775 answer would have validated as a correct name error response.
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788 5.3.4.2. Valid Name Error Response Involving DNAME in Bitmap
790 ;; Header: QR AA RCODE=3(NXDOMAIN)
791 ;; OPT PSEUDOSECTION:
792 ; EDNS: version: 0, flags: do; udp: 4096
795 cee.example.com. IN A
797 bar.example.com. NSEC dub.example.com. A DNAME
798 bar.example.com. RRSIG NSEC [valid signature]
800 This response has the same NSEC records as the example above, but
801 with this query name (cee.example.com), the answer is validated,
802 because 'cee' does not get redirected by the DNAME at 'bar'.
804 5.3.4.3. Response With Synthesized CNAME
806 ;; Header: QR AA RCODE=0(NOERROR)
807 ;; OPT PSEUDOSECTION:
808 ; EDNS: version: 0, flags: do; udp: 4096
811 foo.bar.example.com. IN A
813 bar.example.com. DNAME bar.example.net.
814 bar.example.com. RRSIG DNAME [valid signature]
815 foo.bar.example.com. CNAME foo.bar.example.net.
817 The response shown above has the synthesized CNAME included.
818 However, the CNAME has no signature, since the server does not sign
819 online. So this response cannot be trusted. It could be altered by
820 an attacker to be foo.bar.example.com CNAME bla.bla.example. The
821 DNAME record does have its signature included, since it does not
822 change. The validator must verify the DNAME signature and then
823 recursively resolve further to query for the foo.bar.example.net A
826 6. IANA Considerations
828 The DNAME Resource Record type code 39 (decimal) originally has been
829 registered by [RFC2672]. IANA should update the DNS resource record
830 registry to point to this document for RR type 39.
832 7. Security Considerations
834 DNAME redirects queries elsewhere, which may impact security based on
835 policy and the security status of the zone with the DNAME and the
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841 Internet-Draft DNAME Redirection April 2010
844 redirection zone's security status. For validating resolvers, the
845 lowest security status of the links in the chain of CNAME and DNAME
846 redirections is applied to the result.
848 If a validating resolver accepts wildcarded DNAMEs, this creates
849 security issues. Since the processing of a wildcarded DNAME is non-
850 deterministic and the CNAME that was substituted by the server has no
851 signature, the resolver may choose a different result than what the
852 server meant, and consequently end up at the wrong destination. Use
853 of wildcarded DNAMEs is discouraged in any case [RFC4592].
855 A validating resolver MUST understand DNAME, according to [RFC4034].
856 The examples in Section 5.3.4 illustrate this need.
860 The authors of this draft would like to acknowledge Matt Larson for
861 beginning this effort to address the issues related to the DNAME RR
862 type. The authors would also like to acknowledge Paul Vixie, Ed
863 Lewis, Mark Andrews, Mike StJohns, Niall O'Reilly, Sam Weiler, Alfred
864 Hoenes and Kevin Darcy for their review and comments on this
869 9.1. Normative References
871 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
872 STD 13, RFC 1034, November 1987.
874 [RFC1035] Mockapetris, P., "Domain names - implementation and
875 specification", STD 13, RFC 1035, November 1987.
877 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
878 Requirement Levels", BCP 14, RFC 2119, March 1997.
880 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
881 "Dynamic Updates in the Domain Name System (DNS UPDATE)",
882 RFC 2136, April 1997.
884 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
885 Specification", RFC 2181, July 1997.
887 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
888 specifying the location of services (DNS SRV)", RFC 2782,
891 [RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
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897 Internet-Draft DNAME Redirection April 2010
900 (RR) Types", RFC 3597, September 2003.
902 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
903 Rose, "DNS Security Introduction and Requirements",
904 RFC 4033, March 2005.
906 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
907 Rose, "Resource Records for the DNS Security Extensions",
908 RFC 4034, March 2005.
910 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
911 Rose, "Protocol Modifications for the DNS Security
912 Extensions", RFC 4035, March 2005.
914 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
915 System", RFC 4592, July 2006.
917 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
918 Security (DNSSEC) Hashed Authenticated Denial of
919 Existence", RFC 5155, March 2008.
921 9.2. Informative References
923 [RFC1912] Barr, D., "Common DNS Operational and Configuration
924 Errors", RFC 1912, February 1996.
926 [RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
927 RFC 2672, August 1999.
929 [RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
930 Hain, "Representing Internet Protocol version 6 (IPv6)
931 Addresses in the Domain Name System (DNS)", RFC 3363,
934 [RFC4294] Loughney, J., "IPv6 Node Requirements", RFC 4294,
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953 Internet-Draft DNAME Redirection April 2010
961 Gaithersburg, MD 20899
964 Phone: +1-301-975-8439
966 EMail: scottr.nist@gmail.com
975 Phone: +31-20-888-4551
976 EMail: wouter@nlnetlabs.nl
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