DNSIND Working Group Matt Crawford
Internet Draft Fermilab
March 21, 1999
Non-Terminal DNS Name Redirection
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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reference material or to cite them other than as "work in progress."
To view the list Internet-Draft Shadow Directories, see
1. Introduction
This document defines a new DNS Resource Record called ``DNAME'',
which provides the capability to map an entire subtree of the DNS
name space to another domain. It differs from the CNAME record
which maps a single node of the name space.
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 [KWORD].
2. Motivation
This Resource Record and its processing rules were conceived as a
solution to the problem of maintaining address-to-name mappings in a
context of network renumbering. Without the DNAME mechanism, an
authoritative DNS server for the address-to-name mappings of some
network must be reconfigured when that network is renumbered. With
DNAME, the zone can be constructed so that it needs no modification
when renumbered. DNAME can also be useful in other situations, such
as when an organizational unit is renamed.
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3. The DNAME Resource Record
The DNAME RR has mnemonic DNAME and type code 39 (decimal).
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> [DNSIS].
The DNAME RR causes type NS additional section processing.
The effect of the DNAME record is the substitution of the record's
<target> for its <owner> as a suffix of a domain name. A "no-
descendants" limitation governs the use of DNAMEs in a zone file:
If a DNAME RR is present at a node N, there may be other data at
N (except a CNAME or another DNAME), but there MUST be no data
at any descendant of N. This restriction applies only to
records of the same class as the DNAME record.
This rule assures predictable results when a DNAME record is cached
by a server which is not authoritative for the record's zone. It
MUST be enforced when authoritative zone data is loaded. Together
with the rules for DNS zone authority [DNSCLR] it implies that DNAME
and NS records can only coexist at the top of a zone which has only
one node.
The compression scheme of [DNSIS] MUST NOT be applied to the RDATA
portion of a DNAME record unless the sending server has some way of
knowing that the receiver understands the DNAME record format.
Signalling such understanding is expected to be the subject of
future DNS Extensions.
Naming loops can be created with DNAME records or a combination of
DNAME and CNAME records, just as they can with CNAME records alone.
Resolvers, including resolvers embedded in DNS servers, MUST limit
the resources they devote to any query. Implementors should note,
however, that fairly lengthy chains of DNAME records may be valid.
4. Query Processing
To exploit the DNAME mechanism the name resolution algorithms
[DNSCF] must be modified slightly for both servers and resolvers.
Both modified algorithms incorporate the operation of making a
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substitution on a name (either QNAME or SNAME) under control of a
DNAME record. This operation will be referred to as "the DNAME
substitution".
4.1. Processing by Servers
For a server performing non-recursive service steps 3.c and 4 of
section 4.3.2 [DNSCF] are changed to check for a DNAME record before
checking for a wildcard ("*") label, and to return certain DNAME
records from zone data and the cache.
DNS clients sending Extended DNS [EDNS0] queries with Version 0 or
non-extended queries are presumed not to understand the semantics of
the DNAME record, so a server which implements this specification,
when answering a non-extended query, SHOULD synthesize a CNAME
record for each DNAME record encountered during query processing to
help the client reach the correct DNS data. The behavior of clients
and servers under Extended DNS versions greater than 0 will be
specified when those versions are defined.
The synthesized CNAME RR, if provided, MUST have
The same CLASS as the QCLASS of the query,
TTL equal to zero,
An <owner> equal to the QNAME in effect at the moment the DNAME
RR was encountered, and
An RDATA field containing the new QNAME formed by the action of
the DNAME substitution.
If the server has the appropriate key on-line [DNSSEC, SECDYN], it
MAY generate and return a SIG RR for the synthesized CNAME RR.
The revised server algorithm is:
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.
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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 doesn't 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 [DNSUPD] and exit;
otherwise perform the substitution and continue. If the
query was not extended [EDNS0] with a Version indicating
understanding of the DNAME record, the server SHOULD
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. 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
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set the owner of the RR to be QNAME, and not the node with
the "*" label. 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 (see resolver
section of this memo) 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.
4.2. Processing by Resolvers
A resolver or a server providing recursive service must be modified
to treat a DNAME as somewhat analogous to a CNAME. The resolver
algorithm of [DNSCF] section 5.3.3 is modified to renumber step 4.d
1. See if the answer is in local information, and if so return it
to the client.
2. Find the best servers to ask.
3. Send them queries until one returns a response.
4. Analyze the response, either:
a. if the response answers the question or contains a name
error, cache the data as well as returning it back to the
client.
b. if the response contains a better delegation to other
servers, cache the delegation information, and go to step 2.
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c. if the response shows a CNAME and that is not the answer
itself, cache the CNAME, change the SNAME to the canonical
name in the CNAME RR and go to step 1.
d. if the response shows a DNAME and that is not the answer
itself, cache the DNAME. If substitution of the DNAME's
<target> for its <owner> in the SNAME would overflow the
legal size for a <domain-name>, return an implementation-
dependent error to the application; otherwise perform the
substitution and go to step 1.
e. if the response shows a server failure or other bizarre
contents, delete the server from the SLIST and go back to
step 3.
A resolver or recursive server which understands DNAME records but
sends non-extended queries MUST augment step 4.c by deleting from
the reply any CNAME records which have an <owner> which is a
subdomain of the <owner> of any DNAME record in the response.
5. Examples of Use
5.1. Organizational Renaming
If an organization with domain name FROBOZZ.EXAMPLE became part of
an organization with domain name ACME.EXAMPLE, it might ease
transition by placing information such as this in its old zone.
MX 10 mailhub.acme.example.
The response to an extended recursive query for www.frobozz.example
would contain, in the answer section, the DNAME record shown above
and the relevant RRs for www.frobozz-division.acme.example.
5.2. Classless Delegation of Shorter Prefixes
The classless scheme for in-addr.arpa delegation [INADDR] can be
extended to prefixes shorter than 24 bits by use of the DNAME
record. For example, the prefix 192.0.8.0/22 can be delegated by
the following records.
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$ORIGIN 0.192.in-addr.arpa.
8/22 NS ns.slash-22-holder.example.
8 DNAME 8.8/22
9 DNAME 9.8/22
10 DNAME 10.8/22
11 DNAME 11.8/22
A typical entry in the resulting reverse zone for some host with
address 192.0.9.33 might be
$ORIGIN 8/22.0.192.in-addr.arpa.
33.9 PTR somehost.slash-22-holder.example.
The same advisory remarks concerning the choice of the "/" character
apply here as in [INADDR].
5.3. Network Renumbering Support
If IPv4 network renumbering were common, maintenance of address
space delegation could be simplified by using DNAME records instead
of NS records to delegate.
$ORIGIN new-style.in-addr.arpa.
189.190 DNAME in-addr.example.net.
$ORIGIN in-addr.example.net.
188 DNAME in-addr.customer.example.
$ORIGIN in-addr.customer.example.
1 PTR www.customer.example.
; etc ...
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 zone files describing the use of that space by the ISP and its
customers.
Renumbering IPv4 networks is currently so arduous a task that
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
task will be quite different and the DNAME mechanism may play a
useful part.
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6. IANA Considerations
This document defines a new DNS Resource Record type with the
mnemonic DNAME and type code 39 (decimal). The naming/numbering
space is defined in [DNSIS]. This name and number have already been
registered with the IANA.
7. Security Considerations
The DNAME record is similar to the CNAME record with regard to the
consequences of insertion of a spoofed record into a DNS server or
resolver, differing in that the DNAME's effect covers a whole
subtree of the name space. The facilities of [DNSSEC] are available
to authenticate this record type.
8. References
[DNSCF] P.V. Mockapetris, "Domain names - concepts and facilities",
RFC 1034.
[DNSCLR] R. Elz, R. Bush, "Clarifications to the DNS Specification",
RFC 2181.
[DNSIS] P.V. Mockapetris, "Domain names - implementation and
specification", RFC 1035.
[DNSSEC] D. Eastlake, 3rd, C. Kaufman, "Domain Name System Security
Extensions", RFC 2065.
[DNSUPD] P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136.
[EDNS0] P. Vixie, "Extensions mechanisms for DNS (EDNS0)", Currently
[INADDR] H. Eidnes, G. de Groot, P. Vixie, "Classless IN-ADDR.ARPA
delegation", RFC 2317.
[KWORD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
[SECDYN] D. Eastlake, 3rd, "Secure Domain Name System Dynamic
Update", RFC 2137.
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9. Author's Address
Matt Crawford
Fermilab MS 368
PO Box 500
Batavia, IL 60510
USA
Phone: +1 630 840-3461
EMail: crawdad@fnal.gov
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