Update to DNAME Redirection in the DNS NIST 100 Bureau Dr.Gaithersburg20899MDUSA+1-301-975-8439+1-301-975-6238 scottr@nist.gov NLnet Labs Science Park 1401098 XGAmsterdamThe Netherlands+31-20-888-4551 wouter@nlnetlabs.nl
Internet Area
DNS Extensions Working Group DNSDNAME
The DNAME record provides redirection for a sub-tree of the domain
name tree in the DNS system. That is, all names that
end with a particular suffix are redirected to another part of
the DNS.
This is a revision of the original specification in RFC 2672,
also aligning RFC 3363 and RFC 4294 with this revision.
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.
DNAME is a DNS Resource Record type originally defined in
RFC 2672 . DNAME provides redirection from
a part of the DNS name tree to another part of the DNS name tree.
The DNAME RR and the CNAME RR cause a
lookup to (potentially) return data corresponding to a domain name
different from the queried domain name. The difference between
the two resource records is that the CNAME RR directs the lookup
of data at its owner to another single name, a DNAME RR directs
lookups for data at descendents of its owner's name to
corresponding names under a different (single) node of the tree.
Take for example, looking through a zone (see
RFC 1034, section 4.3.2, step 3) for
the domain name "foo.example.com" and a DNAME resource record
is found at "example.com" indicating that all queries under
"example.com" be directed to "example.net". The lookup process
will return to step 1 with the new query name of "foo.example.net".
Had the query name been "www.foo.example.com" the new query name
would be "www.foo.example.net".
This document is a revision of the original specification of DNAME
in RFC 2672.
DNAME was conceived to help with the problem of maintaining
address-to-name mappings in a context of network renumbering.
With a careful set-up, a renumbering event in the network
causes no change to the authoritative server that has the
address-to-name mappings. Examples in practice are classless
reverse address space delegations.
Another usage of DNAME lies in aliasing of name spaces. For
example, a zone administrator may want sub-trees of the DNS to
contain the same information. Examples include punycode alternates for
domain spaces.
This revision 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.
The DNAME RR has mnemonic DNAME and type code 39 (decimal).
It is not class-sensitive.
Details of the substitution process, methods to avoid conflicting
resource records, and rules for specific corner cases are given in
the following subsections.
When following RFC 1034 , section 4.3.2's
algorithm's third step, "start matching down, label by label, in
the zone" and a node is found to own a DNAME resource record
a DNAME substitution occurs. The name being sought may be the
original query name or a name that is the result of a CNAME
resource record being followed or a previously encountered DNAME.
As is the case of finding a CNAME resource record or NS resource
record set, the processing of a DNAME will happen prior to
finding the desired domain name.
A DNAME substitution is performed by replacing the suffix labels
of the name being sought matching the owner name of the DNAME
resource record with the string of labels in the RDATA field.
The matching labels end with the root label in all cases.
Only whole labels are replaced. See the table of examples for
common cases and corner cases.
It is possible for DNAMEs to form loops, just as
CNAMEs can form loops. DNAMEs and CNAMEs can chain together to
form loops. A single corner case DNAME can form a loop. Resolvers
and servers should be cautious in devoting resources to a query,
but be aware that fairly long chains of DNAMEs may be valid.
Zone content administrators should take care to insure that there
are no loops that could occur when using DNAME or DNAME/CNAME
redirection.
The domain name can get too long during substitution. For example, suppose the
target name of the DNAME RR is 250 octets in length (multiple labels), if
an incoming QNAME that has a first label over 5 octets in length, the result
would be a name over 255 octets. If this occurs the
server returns an RCODE of YXDOMAIN . The DNAME
record and its signature (if the zone is signed) are included in the answer as proof
for the YXDOMAIN (value 6) RCODE.
Unlike a CNAME RR, a DNAME RR redirects DNS names subordinate to its
owner name; the owner name of a DNAME is not redirected itself.
The domain name that owns a DNAME record is allowed to have other
resource record types at that domain name, except DNAMEs, CNAMEs
or other types that have restrictions on what they can co-exist with.
DNAME RRs are not allowed at the parent side of a delegation point
but are allowed at a zone apex.
There still is a need to have the customary SOA and NS
resource records at the zone apex. This means that DNAME does not
mirror a zone completely, as it does not mirror the zone apex.
These rules also allow DNAME records to be queried through RFC 1034
compliant, DNAME-unaware caches.
Resource records MUST NOT exist at any sub-domain of
the owner of a DNAME RR. To get the contents for names
subordinate to that owner name, the DNAME redirection must be invoked
and the resulting target queried. A server MAY refuse to load
a zone that has data at a sub-domain of a domain name
owning a DNAME RR. If the server does load the zone, those names
below the DNAME RR will be occluded as described in
RFC 2136 , section 7.18.
Also a server SHOULD refuse to load
a zone subordinate to the owner of a DNAME record in the ancestor zone.
See for further discussion
related to dynamic update.
DNAME is a singleton type, meaning only one DNAME is allowed per
name. The owner name of a DNAME can only have one DNAME RR, and
no CNAME RRs can exist at that name.
These rules make sure that for a single domain name only one
redirection exists, and thus no confusion which one to follow.
A server SHOULD refuse to load a zone that violates these rules.
The DNAME owner name can be compressed like any other owner name.
The DNAME RDATA target name MUST NOT be sent out in
compressed form, so that a DNAME RR can be treated as an unknown type
.
Although the previous DNAME specification
(that is obsoleted by this specification) talked about signaling
to allow compression of the target name, such signaling has never
been specified and this document also does not specify this signaling
behavior.
RFC 2672 (obsoleted by this document) stated that the EDNS version
had a meaning for understanding of DNAME and DNAME target name
compression. This document revises RFC 2672, in that there is no
EDNS version signaling for DNAME. However, the flags section of
EDNS(0) is updated with a Understand-DNAME flag by this document
(See Section 3.3).
The DNAME RR causes type NS additional section processing.
This refers to action at step 6 of the server algorithm outlined in
section 3.2.
When preparing an response, a server performing a DNAME
substitution will in all cases include the relevant DNAME RR in the
answer section.
A CNAME RR with TTL equal to the corresponding DNAME RR
is synthesized and included in the answer section for resolvers
that did not indicate understanding of DNAME in queries.
The owner name of the CNAME is the QNAME of the query. The DNSSEC
specification , ,
says that the
synthesized CNAME does not have to be signed. The DNAME has an RRSIG
and a validating resolver can check the CNAME against the DNAME
record and validate the signature over the DNAME RR.
Resolvers MUST be able to handle a synthesized CNAME TTL of zero or
equal to the TTL of the corresponding DNAME record. A TTL of zero
means that the CNAME can be discarded immediately after processing
the answer. DNAME aware resolvers can set the Understand-DNAME
(UD bit) to indicate that they can handle a response with only
a 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 of unsigned synthesized CNAMEs.
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.
Resolvers that do not implement this specification, do not set the UD bit, and
servers that do not implement this specification do not
copy the UD bit to the answer, and will not omit synthesized CNAMEs.
Servers MUST be able to answer a query for a synthesized CNAME. Like
other query types this invokes the DNAME, and synthesizes the CNAME
into the answer.
Below is the server algorithm, which appeared in RFC 2672 Section 4.1,
it is expanded to handle the UD (Understand-DNAME) bit.
Set or clear the value of recursion available in the response
depending on whether the name server is willing to provide
recursive service. If recursive service is available and
requested via the RD bit in the query, go to step 5, otherwise
step 2.
Search the available zones for the zone which is the nearest
ancestor to QNAME. If such a zone is found, go to step 3,
otherwise step 4.
Start matching down, label by label, in the zone. The matching
process can terminate several ways:
If the whole of QNAME is matched, we have found the node.
If the data at the node is a CNAME, and QTYPE does not match
CNAME, copy the CNAME RR into the answer section of the
response, change QNAME to the canonical name in the CNAME RR,
and go back to step 1.
Otherwise, copy all RRs which match QTYPE into the answer
section and go to step 6.
If a match would take us out of the authoritative data, we have
a referral. This happens when we encounter a node with NS RRs
marking cuts along the bottom of a zone.
Copy the NS RRs for the sub-zone into the authority section of
the reply. Put whatever addresses are available into the
additional section, using glue RRs if the addresses are not
available from authoritative data or the cache. Go to step 4.
If at some label, a match is impossible (i.e., the
corresponding label does not exist), look to see whether the
last label matched has a DNAME record.
If a DNAME record exists at that point, copy that record into
the answer section. If substitution of its <target> for its
<owner> in QNAME would overflow the legal size for a <domain-
name>, set RCODE to YXDOMAIN and exit;
otherwise
perform the substitution and continue.
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 we
have followed due to a CNAME or DNAME. If the name is original, set an
authoritative name error in the response and exit. Otherwise
just exit.
If the "*" label does exist, match RRs at that node against
QTYPE. If any match, copy them into the answer section, but
set the owner of the RR to be QNAME, and not the node with
the "*" label. If the data at the node with the "*" label is a CNAME,
and QTYPE doesn't match CNAME, copy the CNAME RR into the answer
section of the response changing the owner name to the QNAME,
change QNAME to the canonical name in the CNAME RR, and go back to
step 1. Otherwise, Go to step 6.
Start matching down in the cache. If QNAME is found in the cache,
copy all RRs attached to it that match QTYPE into the answer
section. If QNAME is not found in the cache but a DNAME record is
present at an ancestor of QNAME, copy that DNAME record into the
answer section. If there was no delegation from authoritative
data, look for the best one from the cache, and put it in the
authority section. Go to step 6.
Use the local resolver or a copy of its algorithm
to answer the query. Store the results,
including any intermediate CNAMEs and DNAMEs, in the answer
section of the response.
Using local data only, attempt to add other RRs which may be
useful to the additional section of the query. Exit.
Note that there will be at most one ancestor with a DNAME as
described in step 4 unless some zone's data is in violation of the
no-descendants limitation in section 3. An implementation might take
advantage of this limitation by stopping the search of step 3c or
step 4 when a DNAME record is encountered.
The use of DNAME in conjunction with wildcards is discouraged
. Thus records of the form
"*.example.com DNAME example.net" SHOULD NOT be used.
The interaction between the expansion of the wildcard and the
redirection of the DNAME is non-deterministic.
Because the processing is non-deterministic, DNSSEC validating
resolvers may not be able to validate a wildcarded DNAME.
A server MAY give a warning that the behavior is unspecified
if such a wildcarded DNAME is loaded. The server MAY refuse it,
refuse to load the zone or refuse dynamic updates.
Recursive caching name servers can encounter data at names below the owner name of a
DNAME RR, due to a change at the authoritative server where data from
before and after the change resides in the cache. This conflict
situation is a transitional phase, that ends when the old data
times out. The caching name server can opt to store both old and new data and
treat each as if the other did not exist, or drop the old data, or
drop the longer domain name. In any approach, consistency returns
after the older data TTL times out.
Recursive caching name servers MUST perform CNAME synthesis on behalf of DNAME-ignorant
clients. A recursive caching name server that understands DNAMEs can send out queries
on behalf of clients with the UD bit set (See ).
After receiving the answers the recursive caching name server sends replies to DNAME
ignorant clients that include DNAMEs and synthesized CNAMEs.
If a recursive caching name server encounters a DNAME RR which
contradicts information already in the cache (excluding CNAME
records), it SHOULD NOT cache the DNAME RR, but it MAY cache the
CNAME record received along with it or synthesized from the DNAME
record, subject to the rules for CNAME caching.
In , in Section 10.3., the discussion
on MX and NS records touches on redirection by CNAMEs, but this
also holds for DNAMEs.
The DNAME RR is discussed in RFC 3363, section 4, on A6 and DNAME.
The opening premise of this section is demonstrably wrong, and so
the conclusion based on that premise is wrong. In particular,
deprecates the use of DNAME in the IPv6
reverse tree, which is then carried forward as a recommendation in
. Based on the experience gained in the
meantime, 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.
There are several issues to be aware of about the use of DNAME.
The names listed as target names of MX, NS, PTR and SRV
records must
be canonical hostnames. This means no CNAME or DNAME redirection
may be present during DNS lookup of the address records for the host.
This is discussed in RFC 2181 ,
section 10.3, and RFC 1912 , section 2.4.
For SRV see RFC 2782 page 4.
The upshot of this is that although the lookup of a PTR record can
involve DNAMEs, the name listed in the PTR record can not fall under
a DNAME. The same holds for NS, SRV and MX records. For example,
when punycode alternates for a zone use DNAME then the
NS, MX, SRV and PTR records that point to that zone must use names
without punycode in their RDATA.
What must be done then is to have the domain names with DNAME
substitution already applied to it as the MX, NS, PTR, SRV data.
These are valid canonical hostnames.
DNAME records can be added, changed and removed in a zone using
dynamic update transactions. Adding a DNAME RR to a zone occludes
any domain names that may exist under the added DNAME.
A server MUST reject a dynamic update message that attempts to
add a DNAME RR at a name that already has a CNAME RR or another DNAME
RR associated with that name.
The following is for implementations that understand both DNSSEC and DNAME (synthesis).
In any response, a signed DNAME RR indicates a non-terminal
redirection of the query. There might or might not be a server
synthesized CNAME in the answer section, if there is, the CNAME
will never be signed. For a DNSSEC validator, verification
of the DNAME RR and then checking that the CNAME was properly
synthesized is sufficient proof.
In any negative response, the NSEC or NSEC3
record type bit map SHOULD be checked to see that there was no
DNAME that could have been applied. If the DNAME bit in the type
bit map is set and the query name is a subdomain of the closest
encloser that is asserted, then DNAME substitution should have
been done, but the substitution has not been done as specified.
A response can contain a chain of DNAME and CNAME redirections.
That chain can end in a positive answer or a negative (no name error or
no data error) reply. Each step in that chain results in resource
records added to the answer or authority section of the response.
Only if all steps are secure can the AD bit be set for the response.
If one of the steps is bogus, the result is bogus.
Below are examples of why DNSSEC validators MUST understand DNAME.
In the examples below, SOA records, wildcard denial NSECs and
other material not under discussion has been omitted.
If this is the received response, then only by understanding that the
DNAME bit in the NSEC bitmap means that foo.bar.example.com needed to have been
redirected by the DNAME, the validator can see that it is a BOGUS reply
from an attacker that collated existing records from the DNS
to create a confusing reply.
If the DNAME bit had not been set in the NSEC record above then
the answer would have validated as a correct name error response.
This response has the same NSEC records as the example above,
but with this query name (cee.example.com),
the answer is validated, because 'cee' does
not get redirected by the DNAME at 'bar'.
The response shown above has the synthesized CNAME included.
However, the CNAME has no signature, since the server does not
sign online. So this response cannot be trusted. It could be altered by
an attacker to be foo.bar.example.com CNAME bla.bla.example.
The DNAME record does have its signature included, since it
does not change. The validator must verify
the DNAME signature and then recursively resolve further to
query for the foo.bar.example.net A record.
The DNAME Resource Record type code 39 (decimal) originally has been
registered by [RFC2672]. IANA should update the DNS resource record
registry to point to this document for RR type 39.
This draft requests the second highest bit in the EDNS flags
field for the Understand-DNAME (UD) flag as described in Section 3.1.
DNAME redirects queries elsewhere, which may impact security based
on policy and the security status of the zone with the DNAME and
the redirection zone's security status. For validating resolvers,
the lowest security status of the links in the chain of CNAME and
DNAME redirections is applied to the result.
If a validating resolver accepts wildcarded DNAMEs, this creates
security issues. Since the processing of a wildcarded DNAME is
non-deterministic and the CNAME that was substituted by the
server has no signature, the resolver may choose a different
result than what the server meant, and consequently end up at
the wrong destination. Use of wildcarded DNAMEs is discouraged in
any case .
A validating resolver MUST understand DNAME, according to
. In RFC 4034
examples are given that illustrate this need.
The authors of this draft would like to acknowledge Matt Larson
for beginning this effort to address the issues related to the
DNAME RR type. The authors would also like to acknowledge Paul Vixie,
Ed Lewis, Mark Andrews, Mike StJohns, Niall O'Reilly, Sam Weiler,
Alfred Hoenes and Kevin Darcy
for their review and comments on this document.