DNSIND Working Group Matt Crawford
Internet Draft Fermilab
March 6, 1998
Non-Terminal DNS Name Redirection
<draft-ietf-dnsind-dname-00.txt>
Status of this Memo
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Distribution of this memo is unlimited.
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
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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.
3. The DNAME Resource Record
The DNAME RR has mnemonic DNAME and type code TBA (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. Two limiting
rules govern the use of DNAMEs.
Rule One
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, except for the wildcard node *.N which MAY
have data intended to help old DNS clients which do not understand
the DNAME record. These restrictions apply only to records of the
same class as the DNAME record.
Rule Two
When resolving a query, it is valid to encounter more than one
DNAME record along the way ONLY IF every DNAME record encountered
has fewer labels in its <target> than in its <owner>. (Note that
in the bit-string label [BITLBL], each bit is a separate label.)
If a single DNAME is encountered there is no special restriction
on the number of labels in its <target>.
Rule One assures predictable results when a DNAME record is cached
by a server which is not authoritative for the record's zone. Rule
Two prevents DNAME loops.
Rule One MUST be enforced when authoritative zone data is loaded.
Rule Two MUST be enforced during query processing.
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4. Query Processing
To exploit the DNAME mechanism the name resolution algorithms
[DNSCF] must be modified slightly for both servers and resolvers.
In both cases a conceptual per-query variable DFLAG is introduced to
enforce Rule Two. Implementations MAY use other means to enforce
the rule. DFLAG DFLAG's value is
0 when processing of a query begins and whenever no DNAME has been
encountered;
1 when one or more DNAME records have been encountered, and each
had fewer labels in its <target> than in its <owner>;
2 when a DNAME record has been encountered which had at least as
many labels in its <target> as in its <owner>.
Both modified algorithms incorporate the operation of making a
substitution on a name (either QNAME or SNAME) under control of a
DNAME record. For conciseness, this operation is elaborated here
and will be referred to as "procedure S".
S1. If DFLAG = 2, Rule Two is violated. Go to step S4.
S2. If DFLAG = 1 and the DNAME record's <target>'s does not have
fewer labels than its <owner>, Rule Two is violated. Go to step
S4.
S3. If substituting the DNAME's <target> for its <owner> in the name
being operated on would overflow the legal size for a <domain-
name>, go to step S4. Otherwise make the substitution.
If the <target> has fewer labels than the <owner>, set DFLAG to
1, otherwise set DFLAG to 2. Return.
S4. In a resolver, return an implementation-dependent error to the
application. In a server, copy the DNAME record to the answer
section, set RCODE to YXDOMAIN [DNSUPD], and exit.
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 the cache. The revised algorithm is:
1. Set or clear the value of recursion available in the response
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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 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, substitute its <target> for its |
<owner> in QNAME according to procedure S, and 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.
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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. 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. Using 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 Rule
One.
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
as 4.e and insert a new 4.d. The complete algorithm becomes:
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.
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b. if the response contains a better delegation to other
servers, cache the delegation information, and go to step 2.
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 the DNAME, substitute the |
DNAME's <target> for its <owner> in the SNAME according to |
procedure S 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.
5. Examples of Use
If an organization with domain name FROBOZZ.TLD became part of an
organization with domain name ACME.TLD, it might ease transition by
placing information such as this in its old zone.
frobozz.tld. DNAME frobozz-division.acme.tld.
MX mailhub.acme.tld.
*.frobozz.tld. CNAME www.frobozz-division.acme.tld.
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.xy.
$ORIGIN in-addr.customer.xy.
1 PTR www.customer.xy.
2 PTR mailhub.customer.xy.
; etc ...
This would allow the address space 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.
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6. Discussion (in draft only)
Why does [DNSCF] section 4.3.2 step 3.a say "go back to step 1"
rather than "go back to step 2"?
Would a DNAME with a special <target> be of any use in securely
denying anything in a subtree below its <owner>? If so, a special
rule could be added about a null label in the <target>, or
<target>=<owner>.
The meaning of NS and DNAME records together at the same node is
unclear. Perhaps this ought to be forbidden.
YXDOMAIN seems like a reasonable error code to overload for name-
length overflow due to DNAME substitution. But a case for NXDOMAIN
could be made on the grounds that the resulting too-long name is
sure not to exist.
Rule Two could be modified to allow up to one (or any fixed finite
number) of DNAMEs which don't decrease the label count AND any
number of DNAMEs which do decrease it. Would that help anything?
7. Acknowledgments
Donald Eastlake 3rd suggested the wildcard subdomain to help old
clients get something useful.
8. References
[BITLBL]M. Crawford, "Binary Labels in the Domain Name System",
currently draft-ietf-dnsind-binary-labels-00.txt.
[DNSCF] P.V. Mockapetris, "Domain names - concepts and facilities",
RFC 1034.
[DNSIS] P.V. Mockapetris, "Domain names - implementation and
specification", RFC 1035.
[DNSUPD]P. Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System", RFC 2136.
[KWORD] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels," RFC 2119.
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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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