draft-ietf-dnsext-rfc2672bis-dname-26.txt   rfc6672.txt 
DNS Extensions Working Group S. Rose Internet Engineering Task Force (IETF) S. Rose
Internet-Draft NIST Request for Comments: 6672 NIST
Obsoletes: 2672 (if approved) W. Wijngaards Obsoletes: 2672 W. Wijngaards
Updates: 3363,4294 (if approved) NLnet Labs Updates: 3363 NLnet Labs
Intended status: Standards Track April 19, 2012 Category: Standards Track June 2012
Expires: October 21, 2012 ISSN: 2070-1721
DNAME Redirection in the DNS DNAME Redirection in the DNS
draft-ietf-dnsext-rfc2672bis-dname-26
Abstract Abstract
The DNAME record provides redirection for a sub-tree of the domain The DNAME record provides redirection for a subtree of the domain
name tree in the DNS system. That is, all names that end with a name tree in the DNS. That is, all names that end with a particular
particular suffix are redirected to another part of the DNS. This is suffix are redirected to another part of the DNS. This document
a revision to the original specification in RFC 2672 (which this obsoletes the original specification in RFC 2672 as well as updates
document obsoletes) as well as updating RFC 3363 and RFC 4294 to the document on representing IPv6 addresses in DNS (RFC 3363).
align with this revision.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED" "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This is an Internet Standards Track document.
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months This document is a product of the Internet Engineering Task Force
and may be updated, replaced, or obsoleted by other documents at any (IETF). It represents the consensus of the IETF community. It has
time. It is inappropriate to use Internet-Drafts as reference received public review and has been approved for publication by the
material or to cite them other than as "work in progress." Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
This Internet-Draft will expire on October 21, 2012. Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6672.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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modifications of such material outside the IETF Standards Process. modifications of such material outside the IETF Standards Process.
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. The DNAME Resource Record . . . . . . . . . . . . . . . . . . 5 2. The DNAME Resource Record . . . . . . . . . . . . . . . . . . 5
2.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2. The DNAME Substitution . . . . . . . . . . . . . . . . . . 6 2.2. The DNAME Substitution . . . . . . . . . . . . . . . . . . 5
2.3. DNAME Owner Name Matching the QNAME . . . . . . . . . . . 8 2.3. DNAME Owner Name Matching the QNAME . . . . . . . . . . . 6
2.4. Names Next to and Below a DNAME Record . . . . . . . . . . 8 2.4. Names next to and below a DNAME Record . . . . . . . . . . 7
2.5. Compression of the DNAME record. . . . . . . . . . . . . . 8 2.5. Compression of the DNAME Record . . . . . . . . . . . . . 7
3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. CNAME Synthesis . . . . . . . . . . . . . . . . . . . . . 8
3.1. CNAME synthesis . . . . . . . . . . . . . . . . . . . . . 9 3.2. Server Algorithm . . . . . . . . . . . . . . . . . . . . . 9
3.2. Server algorithm . . . . . . . . . . . . . . . . . . . . . 10 3.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 12 3.4. Acceptance and Intermediate Storage . . . . . . . . . . . 11
3.4. Acceptance and Intermediate Storage . . . . . . . . . . . 12 3.4.1. Resolver Algorithm . . . . . . . . . . . . . . . . . . 11
3.4.1. Resolver Algorithm . . . . . . . . . . . . . . . . . . 12 4. DNAME Discussions in Other Documents . . . . . . . . . . . . . 12
5. Other Issues with DNAME . . . . . . . . . . . . . . . . . . . 13
4. DNAME Discussions in Other Documents . . . . . . . . . . . . . 13 5.1. Canonical Hostnames Cannot Be below DNAME Owners . . . . . 13
5.2. Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 13
5. Other Issues with DNAME . . . . . . . . . . . . . . . . . . . 15 5.3. DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 14
5.1. Canonical hostnames cannot be below DNAME owners . . . . . 15 5.3.1. Signed DNAME, Unsigned Synthesized CNAME . . . . . . . 14
5.2. Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 15 5.3.2. DNAME Bit in NSEC Type Map . . . . . . . . . . . . . . 14
5.3. DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 15 5.3.3. DNAME Chains as Strong as the Weakest Link . . . . . . 14
5.3.1. Signed DNAME, Unsigned Synthesized CNAME . . . . . . . 15 5.3.4. Validators Must Understand DNAME . . . . . . . . . . . 14
5.3.2. DNAME Bit in NSEC Type Map . . . . . . . . . . . . . . 16 5.3.4.1. Invalid Name Error Response Caused by DNAME in
5.3.3. DNAME Chains as Strong as the Weakest Link . . . . . . 16 Bitmap . . . . . . . . . . . . . . . . . . . . . . 15
5.3.4. Validators Must Understand DNAME . . . . . . . . . . . 16
5.3.4.1. DNAME in Bitmap Causes Invalid Name Error . . . . 16
5.3.4.2. Valid Name Error Response Involving DNAME in 5.3.4.2. Valid Name Error Response Involving DNAME in
Bitmap . . . . . . . . . . . . . . . . . . . . . . 17 Bitmap . . . . . . . . . . . . . . . . . . . . . . 15
5.3.4.3. Response With Synthesized CNAME . . . . . . . . . 17 5.3.4.3. Response with Synthesized CNAME . . . . . . . . . 16
6. Examples of DNAME Use in a Zone . . . . . . . . . . . . . . . 16
6. Examples of DNAME Use in a Zone . . . . . . . . . . . . . . . 17 6.1. Organizational Renaming . . . . . . . . . . . . . . . . . 16
6.1. Organizational Renaming . . . . . . . . . . . . . . . . . 17 6.2. Classless Delegation of Shorter Prefixes . . . . . . . . . 17
6.2. Classless Delegation of Shorter Prefixes . . . . . . . . . 18 6.3. Network Renumbering Support . . . . . . . . . . . . . . . 17
6.3. Network Renumbering Support . . . . . . . . . . . . . . . 18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 20
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Changes from RFC 2672 . . . . . . . . . . . . . . . . 21 Appendix A. Changes from RFC 2672 . . . . . . . . . . . . . . . . 21
A.1. Changes to Server Behavior . . . . . . . . . . . . . . . . 21 A.1. Changes to Server Behavior . . . . . . . . . . . . . . . . 21
A.2. Changes to Client Behavior . . . . . . . . . . . . . . . . 22 A.2. Changes to Client Behavior . . . . . . . . . . . . . . . . 21
1. Introduction 1. Introduction
DNAME is a DNS Resource Record type originally defined in RFC 2672 DNAME is a DNS resource record type originally defined in RFC 2672
[RFC2672]. DNAME provides redirection from a part of the DNS name [RFC2672]. DNAME provides redirection from a part of the DNS name
tree to another part of the DNS name tree. tree to another part of the DNS name tree.
The DNAME RR and the CNAME RR [RFC1034] cause a lookup to The DNAME RR and the CNAME RR [RFC1034] cause a lookup to
(potentially) return data corresponding to a domain name different (potentially) return data corresponding to a domain name different
from the queried domain name. The difference between the two from the queried domain name. The difference between the two
resource records is that the CNAME RR directs the lookup of data at 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 its owner to another single name, whereas a DNAME RR directs lookups
at descendants of its owner's name to corresponding names under a for data at descendants of its owner's name to corresponding names
different (single) node of the tree. under a different (single) node of the tree.
Take for example, looking through a zone (see RFC 1034 [RFC1034], For example, take looking through a zone (see RFC 1034 [RFC1034],
section 4.3.2, step 3) for the domain name "foo.example.com" and a 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 DNAME resource record is found at "example.com" indicating that all
queries under "example.com" be directed to "example.net". The lookup queries under "example.com" be directed to "example.net". The lookup
process will return to step 1 with the new query name of 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 "foo.example.net". Had the query name been "www.foo.example.com",
new query name would be "www.foo.example.net". the new query name would be "www.foo.example.net".
This document is a revision of the original specification of DNAME in This document is a revision of the original specification of DNAME in
RFC 2672 [RFC2672]. DNAME was conceived to help with the problem of RFC 2672 [RFC2672]. DNAME was conceived to help with the problem of
maintaining address-to-name mappings in a context of network maintaining address-to-name mappings in a context of network
renumbering. With a careful set-up, a renumbering event in the renumbering. With a careful setup, a renumbering event in the
network causes no change to the authoritative server that has the network causes no change to the authoritative server that has the
address-to-name mappings. Examples in practice are classless reverse address-to-name mappings. Examples in practice are classless reverse
address space delegations. address space delegations.
Another usage of DNAME lies in aliasing of name spaces. For example, 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 a zone administrator may want subtrees of the DNS to contain the same
same information. Examples include punycode [RFC3492] alternates for information. Examples include punycode [RFC3492] alternates for
domain spaces. domain spaces.
This revision of the DNAME specification does not change the wire This revision of the DNAME specification does not change the wire
format or the handling of DNAME Resource Records. Discussion is format or the handling of DNAME resource records. Discussion is
added on problems that may be encountered when using DNAME. added on problems that may be encountered when using DNAME.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED" "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
2. The DNAME Resource Record 2. The DNAME Resource Record
2.1. Format 2.1. Format
The DNAME RR has mnemonic DNAME and type code 39 (decimal). It is The DNAME RR has mnemonic DNAME and type code 39 (decimal). It is
CLASS-insensitive. CLASS-insensitive.
Its RDATA is comprised of a single field, <target>, which contains a Its RDATA is comprised of a single field, <target>, which contains a
fully qualified domain name that MUST be sent in uncompressed form fully qualified domain name that MUST be sent in uncompressed form
[RFC1035], [RFC3597]. The <target> field MUST be present. The [RFC1035] [RFC3597]. The <target> field MUST be present. The
presentation format of <target> is that of a domain name [RFC1035]. presentation format of <target> is that of a domain name [RFC1035].
The presentation format of the RR is as follows:
<owner> <ttl> <class> DNAME <target> <owner> <ttl> <class> DNAME <target>
The effect of the DNAME RR is the substitution of the record's 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 <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 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 the DNAME RR. This substitution has to be applied for every DNAME RR
found in the resolution process, which allows fairly lengthy valid found in the resolution process, which allows fairly lengthy valid
chains of DNAME RRs. chains of DNAME RRs.
Details of the substitution process, methods to avoid conflicting Details of the substitution process, methods to avoid conflicting
resource records, and rules for specific corner cases are given in resource records, and rules for specific corner cases are given in
the following subsections. the following subsections.
2.2. The DNAME Substitution 2.2. The DNAME Substitution
When following RFC 1034 [RFC1034], section 4.3.2's algorithm's third When following step 3 of the algorithm in RFC 1034 [RFC1034], Section
step, "start matching down, label by label, in the zone" and a node 4.3.2, "start matching down, label by label, in the zone" and a node
is found to own a DNAME resource record a DNAME substitution occurs. 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 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 is the result of a CNAME resource record being followed or a
previously encountered DNAME. As in the case when finding a CNAME previously encountered DNAME. As in the case when finding a CNAME
resource record or NS resource record set, the processing of a DNAME resource record or NS resource record set, the processing of a DNAME
will happen prior to finding the desired domain name. will happen prior to finding the desired domain name.
A DNAME substitution is performed by replacing the suffix labels of A DNAME substitution is performed by replacing the suffix labels of
the name being sought matching the owner name of the DNAME resource the name being sought matching the owner name of the DNAME resource
record with the string of labels in the RDATA field. The matching 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 labels end with the root label in all cases. Only whole labels are
replaced. See the table of examples for common cases and corner replaced. See the table of examples for common cases and corner
cases. cases.
In the table below, the QNAME refers to the query name. The owner is 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 owner domain name, and the target refers to the target of
the DNAME record. The result is the resulting name after performing the DNAME record. The result is the resulting name after performing
the DNAME substitution on the query name. "no match" means that the the DNAME substitution on the query name. "no match" means that the
query did not match the DNAME and thus no substitution is performed query did not match the DNAME, and thus no substitution is performed
and a possible error message is returned (if no other result is and a possible error message is returned (if no other result is
possible). Thus every line contains one example substitution. In possible). Thus, every line contains one example substitution. In
the examples below, 'cyc' and 'shortloop' contain loops. the examples below, 'cyc' and 'shortloop' contain loops.
QNAME owner DNAME target result QNAME owner DNAME target result
---------------- -------------- -------------- ----------------- ---------------- -------------- -------------- -----------------
com. example.com. example.net. <no match> com. example.com. example.net. <no match>
example.com. example.com. example.net. [0] example.com. example.com. example.net. [0]
a.example.com. example.com. example.net. a.example.net. a.example.com. example.com. example.net. a.example.net.
a.b.example.com. example.com. example.net. a.b.example.net. a.b.example.com. example.com. example.net. a.b.example.net.
ab.example.com. b.example.com. example.net. <no match> ab.example.com. b.example.com. example.net. <no match>
foo.example.com. example.com. example.net. foo.example.net. foo.example.com. example.com. example.net. foo.example.net.
a.x.example.com. x.example.com. example.net. a.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. 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. example.com. cyc.example.com.
cyc.example.com. example.com. c.example.com. cyc.c.example.com. cyc.example.com. example.com. c.example.com. cyc.c.example.com.
shortloop.x.x. x. . shortloop.x. shortloop.x.x. x. . shortloop.x.
shortloop.x. x. . shortloop. shortloop.x. x. . shortloop.
[0] The result depends on the QTYPE. If the QTYPE = DNAME, then [0] The result depends on the QTYPE. If the QTYPE = DNAME, then
the result is "example.com." else "<no match>" the result is "example.com.", else "<no match>".
Table 1. DNAME Substitution Examples. Table 1. DNAME Substitution Examples
It is possible for DNAMEs to form loops, just as CNAMEs can form 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 loops. DNAMEs and CNAMEs can chain together to form loops. A single
corner case DNAME can form a loop. Resolvers and servers should be corner case DNAME can form a loop. Resolvers and servers should be
cautious in devoting resources to a query, but be aware that fairly cautious in devoting resources to a query, but be aware that fairly
long chains of DNAMEs may be valid. Zone content administrators long chains of DNAMEs may be valid. Zone content administrators
should take care to insure that there are no loops that could occur should take care to ensure that there are no loops that could occur
when using DNAME or DNAME/CNAME redirection. when using DNAME or DNAME/CNAME redirection.
The domain name can get too long during substitution. For example, The domain name can get too long during substitution. For example,
suppose the target name of the DNAME RR is 250 octets in length 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 (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 octets in length, the result would be a name over 255 octets. If
this occurs the server returns an RCODE of YXDOMAIN [RFC2136]. The this occurs, the server returns an RCODE of YXDOMAIN [RFC2136]. The
DNAME record and its signature (if the zone is signed) are included DNAME record and its signature (if the zone is signed) are included
in the answer as proof for the YXDOMAIN (value 6) RCODE. in the answer as proof for the YXDOMAIN (value 6) RCODE.
2.3. DNAME Owner Name Matching the QNAME 2.3. DNAME Owner Name Matching the QNAME
Unlike a CNAME RR, a DNAME RR redirects DNS names subordinate to its 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 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 domain name that owns a DNAME record is allowed to have other
resource record types at that domain name, except DNAMEs, CNAMEs or resource record types at that domain name, except DNAMEs, CNAMEs, or
other types that have restrictions on what they can co-exist with. other types that have restrictions on what they can coexist with.
When there is a match of the QTYPE to a type (or types) also owned by 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 the owner name, the response is sourced from the owner name. For
QTYPE of ANY would return the (available) types at the owner name, example, a QTYPE of ANY would return the (available) types at the
not the target name. owner name, not the target name.
DNAME RRs MUST NOT appear at the same owner name as an NS RR unless 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 the owner name is the zone apex; if it is not the zone apex, then the
zone cut. NS RR signifies a delegation point, and the DNAME RR must in that
case appear below the zone cut at the zone apex of the child zone.
If a DNAME record is present at the zone apex, there is still a need 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. 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 Such a DNAME cannot be used to mirror a zone completely, as it does
not mirror the zone apex. not mirror the zone apex.
These rules also allow DNAME records to be queried through RFC 1034 These rules also allow DNAME records to be queried through caches
[RFC1034] compliant, DNAME-unaware caches. that are RFC 1034 [RFC1034] compliant and are DNAME unaware.
2.4. Names Next to and Below a DNAME Record 2.4. Names next to and below a DNAME Record
Resource records MUST NOT exist at any sub-domain of the owner of a Resource records MUST NOT exist at any subdomain of the owner of a
DNAME RR. To get the contents for names subordinate to that owner DNAME RR. To get the contents for names subordinate to that owner
name, the DNAME redirection must be invoked and the resulting target 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- queried. A server MAY refuse to load a zone that has data at a
domain of a domain name owning a DNAME RR. If the server does load subdomain of a domain name owning a DNAME RR. If the server does
the zone, those names below the DNAME RR will be occluded as load the zone, those names below the DNAME RR will be occluded as
described in RFC 2136 [RFC2136], section 7.18. Also a server ought described in RFC 2136 [RFC2136], Section 7.18. Also, a server ought
to refuse to load a zone subordinate to the owner of a DNAME record to refuse to load a zone subordinate to the owner of a DNAME record
in the ancestor zone. See Section 5.2 for further discussion related in the ancestor zone. See Section 5.2 for further discussion related
to dynamic update. to dynamic update.
DNAME is a singleton type, meaning only one DNAME is allowed per 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 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 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 single domain name, only one redirection exists; thus, there's no
which one to follow. A server ought to refuse to load a zone that confusion about which one to follow. A server ought to refuse to
violates these rules. load a zone that violates these rules.
2.5. Compression of the DNAME record. 2.5. Compression of the DNAME Record
The DNAME owner name can be compressed like any other owner name. 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 The DNAME RDATA target name MUST NOT be sent out in compressed form
and MUST be downcased for DNSSEC validation. and MUST be downcased for DNS Security Extensions (DNSSEC)
validation.
Although the previous DNAME specification [RFC2672] (that is Although the previous DNAME specification [RFC2672] (that is
obsoleted by this specification) talked about signaling to allow obsoleted by this specification) talked about signaling to allow
compression of the target name, such signaling has never been compression of the target name, such signaling has never been
specified and this document also does not specify this signaling specified, nor is it specified in this document.
behavior.
RFC 2672 (obsoleted by this document) stated that the EDNS version RFC 2672 (obsoleted by this document) states that the Extended DNS
had a meaning for understanding of DNAME and DNAME target name (EDNS) version has a means for understanding DNAME and DNAME target
compression. This document revises RFC 2672, in that there is no name compression. This document revises RFC 2672, in that there is
EDNS version signaling for DNAME. no EDNS version signaling for DNAME.
3. Processing 3. Processing
3.1. CNAME synthesis 3.1. CNAME Synthesis
When preparing a response, a server performing a DNAME substitution When preparing a response, a server performing a DNAME substitution
will in all cases include the relevant DNAME RR in the answer will, in all cases, include the relevant DNAME RR in the answer
section. Relevant cases includes the following: section. Relevant cases includes the following:
1. The DNAME is being employed as a substitution instruction. 1. The DNAME is being employed as a substitution instruction.
2. The DNAME itself matches the QTYPE and the owner name matches 2. The DNAME itself matches the QTYPE, and the owner name matches
QNAME. QNAME.
When the owner name name matches the QNAME and the QTYPE matches When the owner name matches the QNAME and the QTYPE matches another
another type owned there, the DNAME is not included in the answer. type owned there, the DNAME is not included in the answer.
A CNAME RR with TTL equal to the corresponding DNAME RR is A CNAME RR with Time to Live (TTL) equal to the corresponding DNAME
synthesized and included in the answer section when the DNAME is RR is synthesized and included in the answer section when the DNAME
employed as a substitution instruction. The owner name of the CNAME is employed as a substitution instruction. The owner name of the
is the QNAME of the query. The DNSSEC specification [RFC4033], CNAME is the QNAME of the query. The DNSSEC specification ([RFC4033]
[RFC4034], [RFC4035] says that the synthesized CNAME does not have to [RFC4034] [RFC4035]) says that the synthesized CNAME does not have to
be signed. The signed DNAME has an RRSIG and a validating resolver be signed. The signed DNAME has an RRSIG, and a validating resolver
can check the CNAME against the DNAME record and validate the can check the CNAME against the DNAME record and validate the
signature over the DNAME RR. signature over the DNAME RR.
Servers MUST be able to answer a query for a synthesized CNAME. Like Servers MUST be able to answer a query for a synthesized CNAME. Like
other query types this invokes the DNAME, and then the server other query types, this invokes the DNAME, and then the server
synthesizes the CNAME and places it into the answer section. If the 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 server in question is a cache, the synthesized CNAME's TTL SHOULD be
equal to the decremented TTL of the cached DNAME. equal to the decremented TTL of the cached DNAME.
Resolvers MUST be able to handle a synthesized CNAME TTL of zero or Resolvers MUST be able to handle a synthesized CNAME TTL of zero or a
equal to the TTL of the corresponding DNAME record (as some older value equal to the TTL of the corresponding DNAME record (as some
authoritative server implementations set the TTL of synthesized older, authoritative server implementations set the TTL of
CNAMEs to zero). A TTL of zero means that the CNAME can be discarded synthesized CNAMEs to zero). A TTL of zero means that the CNAME can
immediately after processing the answer. be discarded immediately after processing the answer.
3.2. Server algorithm 3.2. Server Algorithm
Below is the server algorithm, which appeared in RFC 2672 Section Below is the revised version of the server algorithm, which appears
4.1. in RFC 2672, Section 4.1.
1. Set or clear the value of recursion available in the response 1. Set or clear the value of recursion available in the response
depending on whether the name server is willing to provide depending on whether the name server is willing to provide
recursive service. If recursive service is available and recursive service. If recursive service is available and
requested via the RD bit in the query, go to step 5, otherwise requested via the RD bit in the query, go to step 5; otherwise,
step 2. step 2.
2. Search the available zones for the zone which is the nearest 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, ancestor to QNAME. If such a zone is found, go to step 3;
otherwise step 4. otherwise, step 4.
3. Start matching down, label by label, in the zone. The matching 3. Start matching down, label by label, in the zone. The matching
process can terminate several ways: process can terminate several ways:
A. If the whole of QNAME is matched, we have found the node. 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 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 CNAME, copy the CNAME RR into the answer section of the
response, change QNAME to the canonical name in the CNAME RR, response, change QNAME to the canonical name in the CNAME RR,
and go back to step 1. and go back to step 1.
skipping to change at page 11, line 8 skipping to change at page 9, line 50
available from authoritative data or the cache. Go to step available from authoritative data or the cache. Go to step
4. 4.
C. If at some label, a match is impossible (i.e., the C. If at some label, a match is impossible (i.e., the
corresponding label does not exist), look to see whether the corresponding label does not exist), look to see whether the
last label matched has a DNAME record. last label matched has a DNAME record.
If a DNAME record exists at that point, copy that record into If a DNAME record exists at that point, copy that record into
the answer section. If substitution of its <target> for its the answer section. If substitution of its <target> for its
<owner> in QNAME would overflow the legal size for a <domain- <owner> in QNAME would overflow the legal size for a <domain-
name>, set RCODE to YXDOMAIN [RFC2136] and exit; otherwise name>, set RCODE to YXDOMAIN [RFC2136] and exit; otherwise,
perform the substitution and continue. The server MUST perform the substitution and continue. The server MUST
synthesize a CNAME record as described above and include it synthesize a CNAME record as described above and include it
in the answer section. Go back to step 1. in the answer section. Go back to step 1.
If there was no DNAME record, look to see if the "*" label If there was no DNAME record, look to see if the "*" label
exists. exists.
If the "*" label does not exist, check whether the name we If the "*" label does not exist, check whether the name we
are looking for is the original QNAME in the query or a name 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 we have followed due to a CNAME or DNAME. If the name is
original, set an authoritative name error in the response and original, set an authoritative name error in the response and
exit. Otherwise just exit. exit. Otherwise, just exit.
If the "*" label does exist, match RRs at that node against If the "*" label does exist, match RRs at that node against
QTYPE. If any match, copy them into the answer section, but 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 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 the "*" label. If the data at the node with the "*" label is
a CNAME, and QTYPE doesn't match CNAME, copy the CNAME RR a CNAME, and QTYPE doesn't match CNAME, copy the CNAME RR
into the answer section of the response changing the owner into the answer section of the response changing the owner
name to the QNAME, change QNAME to the canonical name in the 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. CNAME RR, and go back to step 1. Otherwise, go to step 6.
4. Start matching down in the cache. If QNAME is found in the 4. Start matching down in the cache. If QNAME is found in the
cache, copy all RRs attached to it that match QTYPE into the 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 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 record is present at an ancestor of QNAME, copy that DNAME record
into the answer section. If there was no delegation from into the answer section. If there was no delegation from
authoritative data, look for the best one from the cache, and put authoritative data, look for the best one from the cache, and put
it in the authority section. Go to step 6. it in the authority section. Go to step 6.
5. Use the local resolver or a copy of its algorithm to answer the 5. Use the local resolver or a copy of its algorithm to answer the
query. Store the results, including any intermediate CNAMEs and query. Store the results, including any intermediate CNAMEs and
DNAMEs, in the answer section of the response. DNAMEs, in the answer section of the response.
6. Using local data only, attempt to add other RRs which may be 6. Using local data only, attempt to add other RRs that may be
useful to the additional section of the query. Exit. useful to the additional section of the query. Exit.
Note that there will be at most one ancestor with a DNAME as 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 described in step 4 unless some zone's data is in violation of the
no-descendants limitation in section 3. An implementation might take no-descendants limitation in Section 3. An implementation might take
advantage of this limitation by stopping the search of step 3c or advantage of this limitation by stopping the search of step 3c or
step 4 when a DNAME record is encountered. step 4 when a DNAME record is encountered.
3.3. Wildcards 3.3. Wildcards
The use of DNAME in conjunction with wildcards is discouraged The use of DNAME in conjunction with wildcards is discouraged
[RFC4592]. Thus records of the form "*.example.com DNAME [RFC4592]. Thus, records of the form "*.example.com DNAME
example.net" SHOULD NOT be used. example.net" SHOULD NOT be used.
The interaction between the expansion of the wildcard and the The interaction between the expansion of the wildcard and the
redirection of the DNAME is non-deterministic. Because the redirection of the DNAME is non-deterministic. Due to the fact that
processing is non-deterministic, DNSSEC validating resolvers may not the processing is non-deterministic, DNSSEC validating resolvers may
be able to validate a wildcarded DNAME. not be able to validate a wildcarded DNAME.
A server MAY give a warning that the behavior is unspecified if such 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 a wildcarded DNAME is loaded. The server MAY refuse it, refuse to
load the zone or refuse dynamic updates. load the zone, or refuse dynamic updates.
3.4. Acceptance and Intermediate Storage 3.4. Acceptance and Intermediate Storage
Recursive caching name servers can encounter data at names below the 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 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. where data from before and after the change resides in the cache.
This conflict situation is a transitional phase that ends when the 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 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 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, drop the old data, or drop the longer domain name. In any approach,
consistency returns after the older data TTL times out. consistency returns after the older data TTL times out.
Recursive caching name servers MUST perform CNAME synthesis on behalf Recursive caching name servers MUST perform CNAME synthesis on behalf
of clients. of clients.
If a recursive caching name server encounters a DNSSEC validated If a recursive caching name server encounters a DNSSEC validated
DNAME RR which contradicts information already in the cache DNAME RR that contradicts information already in the cache (excluding
(excluding CNAME records), it SHOULD cache the DNAME RR, but it MAY CNAME records), it SHOULD cache the DNAME RR, but it MAY cache the
cache the CNAME record received along with it, subject to the rules CNAME record received along with it, subject to the rules for CNAME.
for CNAME. If the DNAME RR cannot be validated via DNSSEC (i.e. not If the DNAME RR cannot be validated via DNSSEC (i.e., not BOGUS, but
BOGUS, but not able to validate), the recursive caching server SHOULD not able to validate), the recursive caching server SHOULD NOT cache
NOT cache the DNAME RR but MAY cache the CNAME record received along the DNAME RR but MAY cache the CNAME record received along with it,
with it, subject to the rules of CNAME. subject to the rules for CNAME.
3.4.1. Resolver Algorithm 3.4.1. Resolver Algorithm
A resolver algorithm likewise changes to handle DNAME processing. Below is the revised version of the resolver algorithm, which appears
The complete algorithm becomes: in RFC 2672, Section 4.2.
1. See if the answer is in local information or can be synthesized 1. See if the answer is in local information or can be synthesized
from a cached DNAME, and if so return it to the client. from a cached DNAME; if so, return it to the client.
2. Find the best servers to ask. 2. Find the best servers to ask.
3. Send queries until one returns a response. 3. Send queries until one returns a response.
4. Analyze the response, either: 4. Analyze the response, either:
A. If the response answers the question or contains a name A. If the response answers the question or contains a name
error, cache the data as well as returning it back to the error, cache the data as well as return it back to the
client. client.
B. If the response contains a better delegation to other B. If the response contains a better delegation to other
servers, cache the delegation information, and go to step 2. servers, cache the delegation information, and go to step 2.
C. If the response shows a CNAME and that is not the answer C. If the response shows a CNAME and that is not the answer
itself, cache the CNAME, change the SNAME to the canonical itself, cache the CNAME, change the SNAME to the canonical
name in the CNAME RR and go to step 1. name in the CNAME RR, and go to step 1.
D. If the response shows a DNAME and that is not the answer D. If the response shows a DNAME and that is not the answer
itself, cache the DNAME (upon successful DNSSEC validation if itself, cache the DNAME (upon successful DNSSEC validation if
the client is a validating resolver). If substitution of the the client is a validating resolver). If substitution of the
DNAME's target name for its owner name in the SNAME would DNAME's target name for its owner name in the SNAME would
overflow the legal size for a domain name, return an overflow the legal size for a domain name, return an
implementation-dependent error to the application; otherwise implementation-dependent error to the application; otherwise,
perform the substitution and go to step 1. perform the substitution and go to step 1.
E. If the response shows a server failure or other bizarre E. If the response shows a server failure or other bizarre
contents, delete the server from the SLIST and go back to contents, delete the server from the SLIST and go back to
step 3. step 3.
4. DNAME Discussions in Other Documents 4. DNAME Discussions in Other Documents
In [RFC2181], in Section 10.3., the discussion on MX and NS records In Section 10.3 of [RFC2181], the discussion on MX and NS records
touches on redirection by CNAMEs, but this also holds for DNAMEs. touches on redirection by CNAMEs, but this also holds for DNAMEs.
Excerpt from 10.3. MX and NS records (in RFC 2181). Section 10.3 ("MX and NS records") of [RFC2181] states:
The domain name used as the value of a NS resource record, 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 or part of the value of a MX resource record must not be
an alias. Not only is the specification clear on this an alias. Not only is the specification clear on this
point, but using an alias in either of these positions point, but using an alias in either of these positions
neither works as well as might be hoped, nor well fulfills neither works as well as might be hoped, nor well fulfills
the ambition that may have led to this approach. This the ambition that may have led to this approach. This
domain name must have as its value one or more address domain name must have as its value one or more address
records. Currently those will be A records, however in records. Currently those will be A records, however in
the future other record types giving addressing the future other record types giving addressing
information may be acceptable. It can also have other information may be acceptable. It can also have other
RRs, but never a CNAME RR. RRs, but never a CNAME RR.
The DNAME RR is discussed in RFC 3363, section 4, on A6 and DNAME. 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 The opening premise of this section is demonstrably wrong, and so the
conclusion based on that premise is wrong. In particular, [RFC3363] conclusion based on that premise is wrong. In particular, [RFC3363]
deprecates the use of DNAME in the IPv6 reverse tree, which is then deprecates the use of DNAME in the IPv6 reverse tree. Based on the
carried forward as a recommendation in [RFC4294]. Based on the
experience gained in the meantime, [RFC3363] is revised, dropping all experience gained in the meantime, [RFC3363] is revised, dropping all
constraints on having DNAME RRs in these zones. This would greatly constraints on having DNAME RRs in these zones [RFC6434]. This would
improve the manageability of the IPv6 reverse tree. These changes greatly improve the manageability of the IPv6 reverse tree. These
are made explicit below. changes are made explicit below.
In [RFC3363], the paragraph In [RFC3363], the following paragraph is updated by this document,
and the use of DNAME RRs in the reverse tree is no longer deprecated.
"The issues for DNAME in the reverse mapping tree appears to be 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 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 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, 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 the intent of this document is that use of DNAME RRs in the reverse
tree be deprecated." tree be deprecated.
is updated by this document and the use of DNAME RRs in the reverse
tree is no longer deprecated.
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 5. Other Issues with DNAME
There are several issues to be aware of about the use of DNAME. There are several issues to be aware of about the use of DNAME.
5.1. Canonical hostnames cannot be below DNAME owners 5.1. Canonical Hostnames Cannot Be below DNAME Owners
The names listed as target names of MX, NS, PTR and SRV [RFC2782] The names listed as target names of MX, NS, PTR, and SRV [RFC2782]
records must be canonical hostnames. This means no CNAME or DNAME records must be canonical hostnames. This means no CNAME or DNAME
redirection may be present during DNS lookup of the address records redirection may be present during DNS lookup of the address records
for the host. This is discussed in RFC 2181 [RFC2181], section 10.3, 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] and RFC 1912 [RFC1912], Section 2.4. For SRV, see RFC 2782
page 4. [RFC2782], page 4.
The upshot of this is that although the lookup of a PTR record can 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 involve DNAMEs, the name listed in the PTR record cannot fall under a
a DNAME. The same holds for NS, SRV and MX records. For example, DNAME. The same holds for NS, SRV, and MX records. For example,
when punycode [RFC3492] alternates for a zone use DNAME then the NS, when punycode [RFC3492] alternates for a zone use DNAME, then the NS,
MX, SRV and PTR records that point to that zone must use names that 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 are not aliases in their RDATA. Then, what must be done is to have
the domain names with DNAME substitution already applied to it as the the domain names with DNAME substitution already applied to it as the
MX, NS, PTR, SRV data. These are valid canonical hostnames. MX, NS, PTR, and SRV data. These are valid canonical hostnames.
5.2. Dynamic Update and DNAME 5.2. Dynamic Update and DNAME
DNAME records can be added, changed and removed in a zone using DNAME records can be added, changed, and removed in a zone using
dynamic update transactions. Adding a DNAME RR to a zone occludes dynamic update transactions. Adding a DNAME RR to a zone occludes
any domain names that may exist under the added DNAME. any domain names that may exist under the added DNAME.
If a dynamic update message attempts to add a DNAME with a given 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 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 MUST ignore the DNAME. If a DNAME is already associated with that
name, then it is replaced with the new DNAME. Otherwise, add the 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 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 associated with that name, then the CNAME MUST be ignored. Similar
similar behavior for dynamic updates to an owner name of a CNAME RR behavior occurs for dynamic updates to an owner name of a CNAME RR
[RFC2136]. [RFC2136].
5.3. DNSSEC and DNAME 5.3. DNSSEC and DNAME
The following subsections specify the behavior of implementations The following subsections specify the behavior of implementations
that understand both DNSSEC and DNAME (synthesis). that understand both DNSSEC and DNAME (synthesis).
5.3.1. Signed DNAME, Unsigned Synthesized CNAME 5.3.1. Signed DNAME, Unsigned Synthesized CNAME
In any response, a signed DNAME RR indicates a non-terminal In any response, a signed DNAME RR indicates a non-terminal
redirection of the query. There might or might not be a server redirection of the query. There might or might not be a server-
synthesized CNAME in the answer section; if there is, the CNAME will synthesized CNAME in the answer section; if there is, the CNAME will
never be signed. For a DNSSEC validator, verification of the DNAME never be signed. For a DNSSEC validator, verification of the DNAME
RR and then checking that the CNAME was properly synthesized is RR and then that the CNAME was properly synthesized is sufficient
sufficient proof. proof.
5.3.2. DNAME Bit in NSEC Type Map 5.3.2. DNAME Bit in NSEC Type Map
In any negative response, the NSEC or NSEC3 [RFC5155] record type bit In any negative response, the NSEC or NSEC3 [RFC5155] record type
map SHOULD be checked to see that there was no DNAME that could have bitmap SHOULD be checked to see that there was no DNAME that could
been applied. If the DNAME bit in the type bit map is set and the have been applied. If the DNAME bit in the type bitmap is set and
query name is a sub-domain of the closest encloser that is asserted, the query name is a subdomain of the closest encloser that is
then DNAME substitution should have been done, but the substitution asserted, then DNAME substitution should have been done, but the
has not been done as specified. substitution has not been done as specified.
5.3.3. DNAME Chains as Strong as the Weakest Link 5.3.3. DNAME Chains as Strong as the Weakest Link
A response can contain a chain of DNAME and CNAME redirections. That 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 chain can end in a positive answer or a negative reply (no name error
data error) reply. Each step in that chain results in resource or no data error). Each step in that chain results in resource
records added to the answer or authority section of the response. records being added to the answer or authority section of the
Only if all steps are secure can the AD bit be set for the response. response. Only if all steps are secure can the AD (Authentic Data)
If one of the steps is bogus, the result is bogus. bit be set for the response. If one of the steps is bogus, the
result is bogus.
5.3.4. Validators Must Understand DNAME 5.3.4. Validators Must Understand DNAME
Below are examples of why DNSSEC validators MUST understand DNAME. Below are examples of why DNSSEC validators MUST understand DNAME.
In the examples below, SOA records, wildcard denial NSECs and other In the examples, SOA records, wildcard denial NSECs, and other
material not under discussion has been omitted or shortened. material not under discussion have been omitted or shortened.
5.3.4.1. DNAME in Bitmap Causes Invalid Name Error 5.3.4.1. Invalid Name Error Response Caused by DNAME in Bitmap
;; Header: QR AA RCODE=3(NXDOMAIN) ;; Header: QR AA RCODE=3(NXDOMAIN)
;; OPT PSEUDOSECTION: ;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096 ; EDNS: version: 0, flags: do; udp: 4096
;; Question ;; Question
foo.bar.example.com. IN A foo.bar.example.com. IN A
;; Authority ;; Authority
bar.example.com. NSEC dub.example.com. A DNAME bar.example.com. NSEC dub.example.com. A DNAME
bar.example.com. RRSIG NSEC [valid signature] bar.example.com. RRSIG NSEC [valid signature]
If this is the received response, then only by understanding that the 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 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 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 BOGUS reply from an attacker that collated existing records from the
DNS to create a confusing reply. DNS to create a confusing reply.
If the DNAME bit had not been set in the NSEC record above then the 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. answer would have validated as a correct name error response.
5.3.4.2. Valid Name Error Response Involving DNAME in Bitmap 5.3.4.2. Valid Name Error Response Involving DNAME in Bitmap
;; Header: QR AA RCODE=3(NXDOMAIN) ;; Header: QR AA RCODE=3(NXDOMAIN)
;; OPT PSEUDOSECTION: ;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096 ; EDNS: version: 0, flags: do; udp: 4096
;; Question ;; Question
cee.example.com. IN A cee.example.com. IN A
;; Authority ;; Authority
bar.example.com. NSEC dub.example.com. A DNAME bar.example.com. NSEC dub.example.com. A DNAME
bar.example.com. RRSIG NSEC [valid signature] bar.example.com. RRSIG NSEC [valid signature]
This response has the same NSEC records as the example above, but This response has the same NSEC records as the example above, but
with this query name (cee.example.com), the answer is validated, with this query name (cee.example.com), the answer is validated,
because 'cee' does not get redirected by the DNAME at 'bar'. because 'cee' does not get redirected by the DNAME at 'bar'.
5.3.4.3. Response With Synthesized CNAME 5.3.4.3. Response with Synthesized CNAME
;; Header: QR AA RCODE=0(NOERROR) ;; Header: QR AA RCODE=0(NOERROR)
;; OPT PSEUDOSECTION: ;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 4096 ; EDNS: version: 0, flags: do; udp: 4096
;; Question ;; Question
foo.bar.example.com. IN A foo.bar.example.com. IN A
;; Answer ;; Answer
bar.example.com. DNAME bar.example.net. bar.example.com. DNAME bar.example.net.
bar.example.com. RRSIG DNAME [valid signature] bar.example.com. RRSIG DNAME [valid signature]
foo.bar.example.com. CNAME foo.bar.example.net. foo.bar.example.com. CNAME foo.bar.example.net.
The response shown above has the synthesized CNAME included. The response shown above has the synthesized CNAME included.
However, the CNAME has no signature, since the server does not sign However, the CNAME has no signature, since the server does not sign
online. So this response cannot be trusted. It could be altered by 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 an attacker to be foo.bar.example.com CNAME bla.bla.example. The
DNAME record does have its signature included, since it does not DNAME record does have its signature included, since it does not
change. The validator must verify the DNAME signature and then change. The validator must verify the DNAME signature and then
recursively resolve further to query for the foo.bar.example.net A recursively resolve further in order to query for the
record. foo.bar.example.net A record.
6. Examples of DNAME Use in a Zone 6. Examples of DNAME Use in a Zone
Below are some examples of the use of DNAME in a zone. These Below are some examples of the use of DNAME in a zone. These
examples are by no means exhaustive. examples are by no means exhaustive.
6.1. Organizational Renaming 6.1. Organizational Renaming
If an organization with domain name FROBOZZ.EXAMPLE.NET became part If an organization with domain name FROBOZZ.EXAMPLE.NET became part
of an organization with domain name ACME.EXAMPLE.COM, it might ease of an organization with domain name ACME.EXAMPLE.COM, it might ease
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following records. following records.
$ORIGIN 0.192.in-addr.arpa. $ORIGIN 0.192.in-addr.arpa.
8/22 NS ns.slash-22-holder.example.com. 8/22 NS ns.slash-22-holder.example.com.
8 DNAME 8.8/22 8 DNAME 8.8/22
9 DNAME 9.8/22 9 DNAME 9.8/22
10 DNAME 10.8/22 10 DNAME 10.8/22
11 DNAME 11.8/22 11 DNAME 11.8/22
A typical entry in the resulting reverse zone for some host with A typical entry in the resulting reverse zone for some host with
address 192.0.9.33 might be address 192.0.9.33 might be as follows:
$ORIGIN 8/22.0.192.in-addr.arpa. $ORIGIN 8/22.0.192.in-addr.arpa.
33.9 PTR somehost.slash-22-holder.example.com. 33.9 PTR somehost.slash-22-holder.example.com.
The same advisory remarks concerning the choice of the "/" character The advisory remarks in [RFC2317] concerning the choice of the "/"
apply here as in [RFC2317] . character apply here as well.
6.3. Network Renumbering Support 6.3. Network Renumbering Support
If IPv4 network renumbering were common, maintenance of address space If IPv4 network renumbering were common, maintenance of address space
delegation could be simplified by using DNAME records instead of NS delegation could be simplified by using DNAME records instead of NS
records to delegate. records to delegate.
$ORIGIN new-style.in-addr.arpa. $ORIGIN new-style.in-addr.arpa.
189.190 DNAME in-addr.example.net. 189.190 DNAME in-addr.example.net.
$ORIGIN in-addr.example.net. $ORIGIN in-addr.example.net.
188 DNAME in-addr.customer.example.com. 188 DNAME in-addr.customer.example.com.
$ORIGIN in-addr.customer.example. $ORIGIN in-addr.customer.example.
1 PTR www.customer.example.com 1 PTR www.customer.example.com
2 PTR mailhub.customer.example.com. 2 PTR mailhub.customer.example.com.
; etc ... ; etc ...
This would allow the address space 190.189.0.0/16 assigned to the ISP 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 "example.net" to be changed without having to alter the zone data
zone data describing the use of that space by the ISP and its describing the use of that space by the ISP and its customers.
customers.
Renumbering IPv4 networks is currently so arduous a task that Renumbering IPv4 networks is currently so arduous a task that
updating the DNS is only a small part of the labor, so this scheme 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 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 task will be quite different, and the DNAME mechanism may play a
useful part. useful part.
7. IANA Considerations 7. IANA Considerations
The DNAME Resource Record type code 39 (decimal) originally has been The DNAME resource record type code 39 (decimal) originally was
registered by [RFC2672] in the DNS Resource Record (RR) Types registered by [RFC2672] in the DNS Resource Record (RR) Types
registry table at http://www.iana.org/assignments/dns-parameters. registry table at http://www.iana.org/assignments/dns-parameters.
IANA should update the DNS resource record registry to point to this IANA has updated the DNS resource record registry to point to this
document for RR type 39. document for RR type 39.
8. Security Considerations 8. Security Considerations
DNAME redirects queries elsewhere, which may impact security based on DNAME redirects queries elsewhere, which may impact security based on
policy and the security status of the zone with the DNAME and the policy and the security status of the zone with the DNAME and the
redirection zone's security status. For validating resolvers, the redirection zone's security status. For validating resolvers, the
lowest security status of the links in the chain of CNAME and DNAME lowest security status of the links in the chain of CNAME and DNAME
redirections is applied to the result. redirections is applied to the result.
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deterministic and the CNAME that was substituted by the server has no deterministic and the CNAME that was substituted by the server has no
signature, the resolver may choose a different result than what the signature, the resolver may choose a different result than what the
server meant, and consequently end up at the wrong destination. Use server meant, and consequently end up at the wrong destination. Use
of wildcarded DNAMEs is discouraged in any case [RFC4592]. of wildcarded DNAMEs is discouraged in any case [RFC4592].
A validating resolver MUST understand DNAME, according to [RFC4034]. A validating resolver MUST understand DNAME, according to [RFC4034].
The examples in Section 5.3.4 illustrate this need. The examples in Section 5.3.4 illustrate this need.
9. Acknowledgments 9. Acknowledgments
The authors of this draft would like to acknowledge Matt Larson for The authors of this document would like to acknowledge Matt Larson
beginning this effort to address the issues related to the DNAME RR for beginning this effort to address the issues related to the DNAME
type. The authors would also like to acknowledge Paul Vixie, Ed RR type. The authors would also like to acknowledge Paul Vixie, Ed
Lewis, Mark Andrews, Mike StJohns, Niall O'Reilly, Sam Weiler, Alfred Lewis, Mark Andrews, Mike StJohns, Niall O'Reilly, Sam Weiler, Alfred
Hoenes and Kevin Darcy for their review and comments on this Hoenes, and Kevin Darcy for their reviews and comments on this
document. document.
10. References 10. References
10.1. Normative References 10.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987. STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
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[RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T. [RFC3363] Bush, R., Durand, A., Fink, B., Gudmundsson, O., and T.
Hain, "Representing Internet Protocol version 6 (IPv6) Hain, "Representing Internet Protocol version 6 (IPv6)
Addresses in the Domain Name System (DNS)", RFC 3363, Addresses in the Domain Name System (DNS)", RFC 3363,
August 2002. August 2002.
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, March 2003. (IDNA)", RFC 3492, March 2003.
[RFC4294] Loughney, J., "IPv6 Node Requirements", RFC 4294, [RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
April 2006. Requirements", RFC 6434, December 2011.
Appendix A. Changes from RFC 2672 Appendix A. Changes from RFC 2672
A.1. Changes to Server Behavior A.1. Changes to Server Behavior
Major changes to server behavior from the original DNAME Major changes to server behavior from the original DNAME
specification are summarized below: specification are summarized below:
o The rules for DNAME substitution have been clarified in Section 2. o The rules for DNAME substitution have been clarified in
Section 2.2.
o The EDNS option to signal DNAME understanding and compression has o The EDNS option to signal DNAME understanding and compression has
never been specified, and this document clarifies that there is no never been specified, and this document clarifies that there is no
signaling method (Section 2.5). signaling method (Section 2.5).
o The TTL for synthesized CNAME RR's is now set to the TTL of the o The TTL for synthesized CNAME RRs is now set to the TTL of the
DNAME, not zero (Section 3.1). DNAME, not zero (Section 3.1).
o Caching recursive servers MUST perform CNAME synthesis on behalf o Recursive caching servers MUST perform CNAME synthesis on behalf
of clients (Section 3.4). of clients (Section 3.4).
o The revised server algorithm is detailed in Section 3.2. o The revised server algorithm is detailed in Section 3.2.
o Rules for dynamic update messages adding a DNAME or CNAME RR to a o 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 zone where a CNAME or DNAME already exists are detailed in
5.2 Section 5.2.
A.2. Changes to Client Behavior A.2. Changes to Client Behavior
Major changes to client behavior from the original DNAME Major changes to client behavior from the original DNAME
specification are summarized below: specification are summarized below:
o Clients MUST be able to accept synthesized CNAME RR's with a TTL o Clients MUST be able to accept synthesized CNAME RR's with a TTL
of either zero or the TTL of the DNAME RR that accompanies the of either zero or the TTL of the DNAME RR that accompanies the
CNAME RR. CNAME RR.
o DNSSEC aware clients SHOULD cache DNAME RR's and MAY cache o DNSSEC-aware clients SHOULD cache DNAME RRs and MAY cache
synthesized CNAME RR's it receives in the same response. DNSSEC synthesized CNAME RRs they receive in the same response. DNSSEC-
aware clients SHOULD also check the NSEC/NSEC3 type bitmap to aware clients SHOULD also check the NSEC/NSEC3 type bitmap to
verify that DNAME redirection is to be done. DNSSEC validators verify that DNAME redirection is to be done. DNSSEC validators
MUST understand DNAME (Section 5.3). MUST understand DNAME (Section 5.3).
o The revised client algorithm is detailed in Section 3.4.1. o The revised client algorithm is detailed in Section 3.4.1.
Authors' Addresses Authors' Addresses
Scott Rose Scott Rose
NIST NIST
100 Bureau Dr. 100 Bureau Dr.
Gaithersburg, MD 20899 Gaithersburg, MD 20899
USA USA
Phone: +1-301-975-8439 Phone: +1-301-975-8439
Fax: +1-301-975-6238 Fax: +1-301-975-6238
EMail: scottr.nist@gmail.com EMail: scott.rose@nist.gov
Wouter Wijngaards Wouter Wijngaards
NLnet Labs NLnet Labs
Science Park 140 Science Park 140
Amsterdam 1098 XG Amsterdam 1098 XH
The Netherlands The Netherlands
Phone: +31-20-888-4551 Phone: +31-20-888-4551
EMail: wouter@nlnetlabs.nl EMail: wouter@nlnetlabs.nl
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