Commit 2df06589 authored by Mark Andrews's avatar Mark Andrews
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new draft

parent 8461bb09
DNS Extensions Working Group S. Rose
Internet-Draft NIST
Intended status: Standards Track W. Wijngaards
Expires: February 11, 2008 NLnet Labs
August 10, 2007
Update to DNAME Redirection in the DNS
draft-ietf-dnsext-rfc2672bis-dname-04
Status of This Memo
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This Internet-Draft will expire on February 11, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
The DNAME record provides redirection for a sub-tree of the domain
name tree in the DNS system. That is, all names that end with a
particular suffix are redirected to another part of the DNS. This is
an update to the original specification in RFC 2672.
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Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The DNAME Resource Record . . . . . . . . . . . . . . . . . . 3
2.1. Format . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. The DNAME Substitution . . . . . . . . . . . . . . . . . . 4
2.3. DNAME Apex not Redirected itself . . . . . . . . . . . . . 5
2.4. Names Next to and Below a DNAME Record . . . . . . . . . . 5
2.5. Compression of the DNAME record. . . . . . . . . . . . . . 6
3. Processing . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. CNAME synthesis . . . . . . . . . . . . . . . . . . . . . 6
3.3. Acceptance and Intermediate Storage . . . . . . . . . . . 7
3.4. Server algorithm . . . . . . . . . . . . . . . . . . . . . 7
4. DNAME Discussions in Other Documents . . . . . . . . . . . . . 9
5. Other Issues with DNAME . . . . . . . . . . . . . . . . . . . 11
5.1. MX, NS and PTR Records Must Point to Target of DNAME . . . 11
5.2. Dynamic Update and DNAME . . . . . . . . . . . . . . . . . 11
5.3. DNSSEC and DNAME . . . . . . . . . . . . . . . . . . . . . 11
5.3.1. DNAME bit in NSEC/NSEC3 type map . . . . . . . . . . . 11
5.3.2. Other issues with NSEC3 and DNAME . . . . . . . . . . 12
5.3.3. Validators Must Understand DNAME . . . . . . . . . . . 12
5.3.3.1. DNAME in Bitmap Causes Invalid Name Error . . . . 12
5.3.3.2. Valid Name Error Response Involving DNAME in
Bitmap . . . . . . . . . . . . . . . . . . . . . . 12
5.3.3.3. Response With Synthesized CNAME . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
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1. Introduction
DNAME is a DNS Resource Record type. DNAME provides redirection from
a part of the DNS name tree to another part of the DNS name tree.
For example, given a query for foo.example.com and a DNAME from
example.com to example.net, the query would be redirected to
foo.example.net. With the same DNAME a query for foo.bar.example.com
would be redirected to foo.bar.example.net.
The DNAME RR is similar to the CNAME RR in that it provides
redirection. The CNAME RR only provides redirection for exactly one
name while the DNAME RR provides redirection for all names in a sub-
tree of the DNS name tree.
This document is an update to the original specification of DNAME in
RFC 2672 [RFC2672]. DNAME was conceived to help with the problem of
maintaining address-to-name mappings in a context of network
renumbering. So that with a careful set-up a renumbering event in
the network causes no change to the authoritative server that has the
address-to-name mappings. Examples in practice are classless reverse
address space delegations and punycode alternates for domain spaces.
Other usage of DNAME lies in redirection of name spaces. For
example, a zone administrator may want subtrees of the DNS to contain
the same information. DNAME is also used for redirection of ENUM
domains to another maintaining party.
This update to DNAME does not change the wire format or the handling
of DNAME Resource Records by existing software. A new UD (Understand
Dname) bit in the EDNS flags field can be used to signal that CNAME
synthesis is not needed. Discussion is added on problems that may be
encountered when using DNAME.
2. The DNAME Resource Record
2.1. Format
The DNAME RR has mnemonic DNAME and type code 39 (decimal).
The format of the DNAME record has not changed from the original
specification in RFC 2672. DNAME has the following format:
<owner> <ttl> <class> DNAME <target>
The format is not class-sensitive. All fields are required. The
RDATA field target is a domain name. The RDATA field target name
MUST be sent uncompressed [RFC3597].
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The DNAME RR causes type NS additional section processing.
2.2. The DNAME Substitution
DNAMEs cause a name substitution to happen to query names. This is
called the DNAME substitution. The suffix owner name of the DNAME is
replaced by the target of the DNAME. The owner name of the DNAME is
not itself redirected, only domain names below the owner name are
redirected. Only whole labels are replaced. A name is considered
below the owner name if it has more labels than the owner name, and
the labels of the owner name appear as the suffix of the name. See
the table of examples for common cases and corner cases.
In the table below, the QNAME refers to the query name. The owner is
the DNAME owner domain name, and the target refers to the target of
the DNAME record. The result is the resulting name after performing
the DNAME substitution on the query name. "no match" means that the
query did not match the DNAME and thus no substitution is performed
and a possible error message is returned (if no other result is
possible). In the examples below, 'cyc' and 'shortloop' contain
loops.
QNAME owner DNAME target result
---------------- -------------- -------------- -----------------
com. example.com. example.net. <no match>
example.com. example.com. example.net. <no match>
a.example.com. example.com. example.net. a.example.net.
a.b.example.com. example.com. example.net. a.b.example.net.
ab.example.com. b.example.com. example.net. <no match>
foo.example.com. example.com. example.net. foo.example.net.
a.x.example.com. x.example.com. example.net. a.example.net.
a.example.com. example.com. y.example.net. a.y.example.net.
cyc.example.com. example.com. example.com. cyc.example.com.
cyc.example.com. example.com. c.example.com. cyc.c.example.com.
shortloop.x.x. x. . shortloop.x.
shortloop.x. x. . shortloop.
Table 1. DNAME Substitution Examples.
It is possible for DNAMEs to form loops. Just like CNAMEs can form
loops. DNAMEs and CNAMEs can chain together to form loops. A single
corner case DNAME can form a loop. Resolvers and servers should be
cautious in devoting resources to a query, but be aware that fairly
long chains of DNAMEs may be valid. Zone content administrators
should take care to insure that there are no loops that could occur
when using DNAME or DNAME/CNAME redirection.
The domain name can get too long during substitution. For example,
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suppose the target name of the DNAME RR is 250 octets in length
(multiple labels), if an incoming QNAME that has a first label over 5
octets in length, the result of the result would be a name over 255
octets. If this occurs the server returns an RCODE of YXDOMAIN
[RFC2136]. The DNAME record and its signature (if the zone is
signed) are included in the answer as proof for the YXDOMAIN (value
6) RCODE.
2.3. DNAME Apex not Redirected itself
The owner name of a DNAME is not redirected itself. The reason for
the original decision was that one can have a DNAME at the zone apex
without problem. Then use this DNAME at the zone apex to point
queries to the target zone. There still is a need to have the
customary SOA and NS resource records at the zone apex. This means
that DNAME does not mirror a zone completely, as it does not mirror
the zone apex.
Another reason for excluding the DNAME owner from the DNAME
substitution is that one can then query for the DNAME through RFC
1034 [RFC1034] caches.
This means that a DNAME RR is not allowed at the same domain name as
NS records unless there is also a SOA record present. DNAME RRs are
not allowed at the parent side of a delegation point but are allowed
at a zone apex.
2.4. Names Next to and Below a DNAME Record
Other resource records MUST NOT exist below the owner of a DNAME RR.
To get the contents for names subordinate to that owner, the DNAME
redirection must be invoked and the resulting target queried. A
server SHOULD refuse to load a zone that has data below a domain name
owning a DNAME RR. Also a server SHOULD refuse to load a zone
subordinate to the owner of a DNAME record in the ancestor zone.
DNAME is a singleton type, meaning only one DNAME is allowed per
name. The owner name of a DNAME can only have one DNAME RR, and no
CNAME RRs can exist at that name. These rules make sure that for a
single domain name only one redirection exists, and thus no confusion
which one to follow. A server SHOULD refuse to load a zone that
violates these rules.
The domain name that owns a DNAME record is allowed to have other
resource record types at that domain name, except DNAMEs or CNAMEs.
These rules allow DNAME records to be queried through DNAME unaware
caches.
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2.5. Compression of the DNAME record.
The DNAME owner name can be compressed like any other owner name.
The DNAME RDATA target name MUST NOT be sent out in compressed form,
so that a DNAME RR can be treated as an unknown type.
Although the previous specification [RFC2672] talked about signaling
to allow compression of the target name, no such signaling is
explicitly specified.
RFC2672 stated that the EDNS version had a meaning for understanding
of DNAME and DNAME target name compression. This document updates
RFC2672, in that there is no EDNS version signaling for DNAME as of
yet. However, the flags section of EDNS(0) is updated with a
Understand-DNAME flag by this document (See Section 3.2).
3. Processing
3.1. Wildcards
The use of DNAME in conjunction with wildcards is discouraged
[RFC4592]. Thus records of the form "*.example.com DNAME
example.net" SHOULD NOT be used.
The interaction between the expansion of the wildcard and the
redirection of the DNAME is non-deterministic. Because the
processing is non-deterministic, DNSSEC validating resolvers may not
be able to validate a wildcarded DNAME.
A server MAY give a warning that the behavior is unspecified if such
a wildcarded DNAME is loaded.
3.2. CNAME synthesis
On the server side, the DNAME RR record is always included in the
answer section of a query. A CNAME RR record with TTL 0 is
synthesized for old resolvers, specifically for the QNAME in the
query. DNSSEC [RFC4033], [RFC4034], [RFC4035] says that the
synthesized CNAME does not have to be signed. The DNAME has an RRSIG
and a validating resolver can check the CNAME against the DNAME
record and validate the DNAME record.
It does not make sense for the authoritative server to follow the
chain of DNAMEs, CNAMEs and wildcards outside of the zone of the
query, as modern resolvers will remove out-of-zone information from
the answer.
Resolvers MUST be able to handle a synthesized CNAME TTL of zero or
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equal to the TTL of the corresponding DNAME record. The TTL of zero
means that the CNAME can be discarded immediately after processing
the answer. DNSSEC aware resolvers can set the Understand-DNAME (UD
bit) to receive a response with only the DNAME RR and no synthesized
CNAMEs.
The UD bit is part of the EDNS extended RCODE and Flags field. It is
used to omit server processing, transmission and resolver processing
the unsigned synthesized CNAMEs when DNSSEC validation is performed.
Resolvers can set this in a query to request omission of the
synthesized CNAMEs. Servers copy the UD bit to the response, and can
omit synthesized CNAMEs from the answer. Older resolvers do not set
the UD bit, and older servers do not copy the UD bit to the answer,
and will not omit synthesized CNAMEs.
Updated EDNS extended RCODE and Flags field.
+0 (MSB) +1 (LSB)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
0: | EXTENDED-RCODE | VERSION |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
2: |DO|UD| Z |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Servers MUST be able to answer a query for a synthesized CNAME. An
answer containing the synthesized CNAME cannot contain an error
(since a CNAME has been followed), as per RFC 1034 CNAME rules.
3.3. Acceptance and Intermediate Storage
DNS Caches MUST NOT allow data to be cached below the owner of a
DNAME RR, except CNAME records or perhaps NSEC3 records and their
signatures. CNAME records below the owner of a DNAME MUST be re-
synthesized from the DNAME, or checked against the DNAME record
before sending them out. This improves consistency of the DNAME and
CNAME records below the owner of the DNAME.
DNS Caches MUST perform CNAME synthesis on behalf of DNAME-ignorant
clients. A DNS Cache that understands DNAMEs can send out queries on
behalf of clients with the UD bit set. After receiving the answers
the DNS Cache sends replies to DNAME ignorant clients that include
DNAMEs and synthesized CNAMEs.
3.4. Server algorithm
Below the server algorithm, which appeared in RFC 2672 Section 4.1,
is expanded to handle the UD bit.
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1. Set or clear the value of recursion available in the response
depending on whether the name server is willing to provide
recursive service. If recursive service is available and
requested via the RD bit in the query, go to step 5, otherwise
step 2.
2. Search the available zones for the zone which is the nearest
ancestor to QNAME. If such a zone is found, go to step 3,
otherwise step 4.
3. Start matching down, label by label, in the zone. The matching
process can terminate several ways:
A. If the whole of QNAME is matched, we have found the node.
If the data at the node is a CNAME, and QTYPE does not match
CNAME, copy the CNAME RR into the answer section of the
response, change QNAME to the canonical name in the CNAME RR,
and go back to step 1.
Otherwise, copy all RRs which match QTYPE into the answer
section and go to step 6.
B. If a match would take us out of the authoritative data, we
have a referral. This happens when we encounter a node with
NS RRs marking cuts along the bottom of a zone.
Copy the NS RRs for the subzone into the authority section of
the reply. Put whatever addresses are available into the
additional section, using glue RRs if the addresses are not
available from authoritative data or the cache. Go to step
4.
C. If at some label, a match is impossible (i.e., the
corresponding label does not exist), look to see whether the
last label matched has a DNAME record.
If a DNAME record exists at that point, copy that record into
the answer section. If substitution of its <target> for its
<owner> in QNAME would overflow the legal size for a <domain-
name>, set RCODE to YXDOMAIN [RFC2136] and exit; otherwise
perform the substitution and continue. If the EDNS OPT
record is present in the query and the UD bit is set, the
server MAY copy the UD bit to the answer EDNS OPT record, and
omit CNAME synthesis. Else the server MUST synthesize a
CNAME record as described above and include it in the answer
section. Go back to step 1.
If there was no DNAME record, look to see if the "*" label
exists.
If the "*" label does not exist, check whether the name we
are looking for is the original QNAME in the query or a name
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we have followed due to a CNAME or DNAME. If the name is
original, set an authoritative name error in the response and
exit. Otherwise just exit.
If the "*" label does exist, match RRs at that node against
QTYPE. If any match, copy them into the answer section, but
set the owner of the RR to be QNAME, and not the node with
the "*" label. If the data at the node with the "*" label is
a CNAME, and QTYPE doesn't match CNAME, copy the CNAME RR
into the answer section of the response changing the owner
name to the QNAME, change QNAME to the canonical name in the
CNAME RR, and go back to step 1. Otherwise, Go to step 6.
4. Start matching down in the cache. If QNAME is found in the
cache, copy all RRs attached to it that match QTYPE into the
answer section. If QNAME is not found in the cache but a DNAME
record is present at an ancestor of QNAME, copy that DNAME record
into the answer section. If there was no delegation from
authoritative data, look for the best one from the cache, and put
it in the authority section. Go to step 6.
5. Use the local resolver or a copy of its algorithm to answer the
query. Store the results, including any intermediate CNAMEs and
DNAMEs, in the answer section of the response.
6. Using local data only, attempt to add other RRs which may be
useful to the additional section of the query. Exit.
Note that there will be at most one ancestor with a DNAME as
described in step 4 unless some zone's data is in violation of the
no-descendants limitation in section 3. An implementation might take
advantage of this limitation by stopping the search of step 3c or
step 4 when a DNAME record is encountered.
4. DNAME Discussions in Other Documents
In [RFC2181], in Section 10.3., the discussion on MX and NS records
touches on redirection by CNAMEs, but this also holds for DNAMEs.
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Excerpt from 10.3. MX and NS records (in RFC 2181).
The domain name used as the value of a NS resource record,
or part of the value of a MX resource record must not be
an alias. Not only is the specification clear on this
point, but using an alias in either of these positions
neither works as well as might be hoped, nor well fulfills
the ambition that may have led to this approach. This
domain name must have as its value one or more address
records. Currently those will be A records, however in
the future other record types giving addressing
information may be acceptable. It can also have other
RRs, but never a CNAME RR.
The DNAME RR is discussed in RFC 3363, section 4, on A6 and DNAME.
[RFC3363] does NOT RECOMMENDED the use of DNAME in the IPv6 reverse
tree. (Hence, all references to DNAME should have been removed from
[RFC4294].) Based on the experience gained in the meantime, RFC 3363
should be revised, dropping all constraints on having DNAME RRs in
these zones. This would greatly improve the manageability of the
IPv6 reverse tree. These changes are made explicit below.
In [RFC3363], section 4, DNAME is not recommended for the IPv6
reverse tree. The opening premise of this section is demonstrably
wrong. Everything that follows from that premise is also invalid.
In [RFC3363], the paragraph
"The issues for DNAME in the reverse mapping tree appears to be
closely tied to the need to use fragmented A6 in the main tree: if
one is necessary, so is the other, and if one isn't necessary, the
other isn't either. Therefore, in moving RFC 2874 to experimental,
the intent of this document is that use of DNAME RRs in the reverse
tree be deprecated."
is to be replaced with the word "DELETED".
In [RFC4294], the reference to DNAME was left in as a editorial
oversight. The paragraph
"Those nodes are NOT RECOMMENDED to support the experimental A6 and
DNAME Resource Records [RFC3363]."
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is to be replaced by
"Those nodes are NOT RECOMMENDED to support the experimental
A6 Resource Record [RFC3363]."
5. Other Issues with DNAME
There are several issues to be aware of about the use of DNAME.
5.1. MX, NS and PTR Records Must Point to Target of DNAME
The names listed as target names of MX, NS and PTR records must be
canonical hostnames. This means no CNAME or DNAME redirection may be
present during DNS lookup of the address records for the host. This
is discussed in RFC 2181 [RFC2181], section 10.3, and RFC 1912
[RFC1912], section 2.4.
The upshot of this is that although the lookup of a PTR record can
involve DNAMEs, the name listed in the PTR record can not fall under
a DNAME. The same holds for NS and MX records. For example, when
punycode alternates for a zone use DNAME then the NS, MX and PTR
records that point to that zone must use names without punycode in
their RDATA. What must be done then is to have the domain names with
DNAME substitution already applied to it as the MX, NS, PTR data.
These are valid canonical hostnames.
5.2. Dynamic Update and DNAME
Zones containing a DNAME RR MUST NOT accept a dynamic update message
that would add a record or delegation with a name existing under a
DNAME.
A server MUST return an error message with RCODE=REFUSED [RFC2136] in
response to a dynamic update message that would add a resource record
under a DNAME in the zone.
5.3. DNSSEC and DNAME
5.3.1. DNAME bit in NSEC/NSEC3 type map
When a validator checks the NSEC/NSEC3 RRs returned on a name error
response, it SHOULD check that the DNAME bit is not set. If the
DNAME bit is set then the DNAME substitution should have been done,
but has not.
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5.3.2. Other issues with NSEC3 and DNAME
NSEC3 records and their signatures are allowed to exist below the
owner name of a DNAME RR. This is because of the nature of NSEC3 RRs
in DNSSEC, which creates hashed owner names that exist below the apex
name of the zone. This is an exception to the rule that there MUST
NOT be any other RRs under the owner name of a DNAME RR, if the DNAME
RR is owned by the zone apex domain name.
Queries for NSEC3 owner names are redirected as if there were no such