Network Working Group M. Mealling
Internet-Draft Verisign
Expires: April 28, 2002 October 28, 2001
Dynamic Delegation Discovery System (DDDS) Part Four: The URI
Resolution Application
draft-ietf-urn-uri-res-ddds-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
A specification for taking a URI and locating an authoritative server
for information about that URI. The method used to locate that
authoritative server is the Dynamic Delegation Discovery System.
This document is part of a series that is specified in "Dynamic
Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS
Standard" (RFC WWWW). It is very important to note that it is
impossible to read and understand any document in this series without
reading the others.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The Distinction between URNs and URLs . . . . . . . . . . . 6
4. The URI and URN Resolution Application Specifications . . . 7
4.1 Application Unique String . . . . . . . . . . . . . . . . . 7
4.2 First Well Known Rule . . . . . . . . . . . . . . . . . . . 7
4.3 Flags . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.4 Services Parameters . . . . . . . . . . . . . . . . . . . . 8
4.4.1 Services . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.4.2 protocols . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.5 Valid Databases . . . . . . . . . . . . . . . . . . . . . . 9
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.1 An example using a URN . . . . . . . . . . . . . . . . . . . 11
5.2 CID URI Scheme Example . . . . . . . . . . . . . . . . . . . 12
5.3 Resolving an HTTP URI Scheme . . . . . . . . . . . . . . . . 14
6. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . 21
A. Pseudo Code . . . . . . . . . . . . . . . . . . . . . . . . 22
Full Copyright Statement . . . . . . . . . . . . . . . . . . 25
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1. Introduction
The Dynamic Delegation Discovery System is used to implement lazy
binding of strings to data, in order to support dynamically
configured delegation systems. The DDDS functions by mapping some
unique string to data stored within a DDDS Database by iteratively
applying string transformation rules until a terminal condition is
reached.
This document describes a DDDS Application for resolving URIs. It
does not define the DDDS Algorithm or a Database. The entire series
of documents that do so are specified in "Dynamic Delegation
Discovery System (DDDS) Part One: The Comprehensive DDDS Standard"
(RFC WWWW) [1]. It is very important to note that it is impossible
to read and understand any document in that series without reading
the related documents.
Uniform Resource Identifiers have been a significant advance in
retrieving Internet-accessible resources. However, their brittle
nature over time has been recognized for several years. The Uniform
Resource Identifier working group proposed the development of Uniform
Resource Names [8] to serve as persistent, location-independent
identifiers for Internet resources in order to overcome most of the
problems with URLs. RFC 1737 [6] sets forth requirements on URNs.
During the lifetime of the URI-WG, a number of URN proposals were
generated. The developers of several of those proposals met in a
series of meetings, resulting in a compromise known as the Knoxville
framework. The major principle behind the Knoxville framework is
that the resolution system must be separate from the way names are
assigned. This is in marked contrast to most URLs, which identify
the host to contact and the protocol to use. Readers are referred to
[7]for background on the Knoxville framework and for additional
information on the context and purpose of this proposal.
Separating the way names are resolved from the way they are
constructed provides several benefits. It allows multiple naming
approaches and resolution approaches to compete, as it allows
different protocols and resolvers to be used. There is just one
problem with such a separation - how do we resolve a name when it
can't give us directions to its resolver?
For the short term, DNS is the obvious candidate for the resolution
framework, since it is widely deployed and understood. However, it
is not appropriate to use DNS to maintain information on a per-
resource basis. First of all, DNS was never intended to handle that
many records. Second, the limited record size is inappropriate for
catalog information. Third, domain names are not appropriate as
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URNs.
Therefore our approach is to use the DDDS to locate "resolvers" that
can provide information on individual resources, potentially
including the resource itself. To accomplish this, we "rewrite" the
URI into a Key following the rules found in the Dynamic Delegation
Discovery System (DDDS). This document describes URI Resolution as
an application of the DDDS and specifies the use of at least one
Database based on DNS.
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2. Terminology
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.
All capitalized terms are taken from the vocabulary found in the DDDS
algorithm specification found in RFC ZZZZ [3].
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3. The Distinction between URNs and URLs
From the point of view of this system, there is no theoretical
difference between resolving URIs in the general case and URNs in the
specific case. Operationally however, there is a difference that
stems from URI resolution possibly not becoming of widespread use.
If URN resolution is collapsed into generic URI resolution, URNs may
suffer by the lack of adoption of URI resolution.
The solution is to allow for shortcutting for URN resolution. In the
following specification generic URI resolution starts by inserting
rules for known URI schemes into the 'uri.arpa' registry. For the
'URN:' URI scheme, one of the rules found in 'uri.arpa' would be for
the 'urn' URI scheme. This rule would simply delegate to the
'urn.arpa' zone for additional NAPTRs based on the URN namespace.
Essentially, the URI Resolution Rewrite Rule for 'URN:' is the URN
Resolution Application's First Well Known Rule.
Therefore, this document specifies two DDDS Applications. One is for
URI Resolution and the other is for URN Resolution. Both are
technically identical but by separating the two URN Resolution can
still proceed without the dependency.
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4. The URI and URN Resolution Application Specifications
This template defines the URI and URN Resolution DDDS Application
according to the rules and requirements found in [3]. The DDDS
database used by this Application is found in [4] which is the
document that defines the NAPTR DNS Resource Record type.
4.1 Application Unique String
The Application Unique String is the Uniform Resource Identifier or
Uniform Resource Name for which an authoritative server is being
located. This URI or URN MUST be canonicalized and hex encoded
according to the "absolute-uri" production found in the Collected
ABNF from RFC 2396 [15].
4.2 First Well Known Rule
In the URI case, the first known key is created by taking the URI
scheme. In the URN case, the first known key is the Namespace
Identifier. For example, the URI 'http://www.example.com/' would
have a 'http' as its Key. The URN 'urn:foo:foospace' would have
'foo' as its first Key.
4.3 Flags
At this time only four flags, "S", "A", "U", and "P", are defined.
The "S", "A" and "U" flags are for a terminal lookup. This means
that the Rule is the last one and that the flag determines what the
next stage should be. The "S" flag means that the output of this
Rule is a domain-name for which one or more SRV [9] records exist.
See Section 5 for additional information on how URI and URN
Resolution use the SRV record type. "A" means that the output of the
Rule is a domain-name and should be used to lookup either A, AAAA, or
A6 records for that domain. The "U" flag means that the output of
the Rule is a URI [15].
The "P" flag says that the remainder of the DDDS Algorithm is ignored
and that the rest of the process is application specific and outside
the scope of this document. An application can use the Protocol part
found in the Services field to identify which Application specific
set of rules that should be followed next. The record that contains
the 'P' flag is the last record that is interpreted by the rules in
this document. One might think that this would also make the "P"
flag an indicator of a terminal lookup but this would be incorrect
since a "terminal" Rule is a DDDS concept and this flag indicates
that anything after this rule does not adhere to DDDS concepts at
all.
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The remaining alphabetic flags are reserved for future versions of
this specification. The numeric flags may be used for local
experimentation. The S, A, U and P flags are all mutually exclusive,
and resolution libraries MAY signal an error if more than one is
given. (Experimental code and code for assisting in the creation of
Rewrite Rules would be more likely to signal such an error than a
client such as a browser). It is anticipated that multiple flags
will be allowed in the future, so implementers MUST NOT assume that
the flags field can only contain 0 or 1 characters. Finally, if a
client encounters a record with an unknown flag, it MUST ignore it
and move to the next Rule. This test takes precedence over any
ordering since flags can control the interpretation placed on fields.
A novel flag might change the interpretation of the regexp and/or
replacement fields such that it is impossible to determine if a
record matched a given target.
The "S", "A", and "U" flags are called 'terminal' flags since they
halt the looping DDDS algorithm. If those flags are not present,
clients may assume that another Rule exists at the Key produced by
the current Rewrite Rule.
4.4 Services Parameters
Service Parameters for this Application take the form of a string of
characters that follow this ABNF:
service_field = [ [protocol] *("+" rs)]
protocol = ALPHA *31ALPHANUM
rs = ALPHA *31ALPHANUM
; The protocol and rs fields are limited to 32
; characters and must start with an alphabetic.
In other words, an optional protocol specification followed by 0 or
more resolution services. Each resolution service is indicated by an
initial '+' character.
The empty string is also valid. This will typically be seen at the
beginning of a series of Rules, when it is impossible to know what
services and protocols will be offered at the end of a particular
delegation path.
4.4.1 Services
The service identifiers that make up the 'rs' production are generic
for both URI and URN resolution since the input value types itself
based on the URI scheme. The list of valid services are defined in
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[11].
Examples of some of these services are:
I2L: given a URI return one URL that identifies a location where the
original URI can be found
I2Ls: given a URI return one or more URLs that identify multiple
locations where the original URI can be found
I2R: given a URI return one instance of the resource identified by
that URI.
I2Rs: given a URI return one or more instances of the resources
identified by that URI.
I2C: given a URI return one instance of a description of that
resource.
I2N: given a URI return one URN that names the resource (Caution:
equality with respect to URNs is non-trivial. See [6]for examples
of why.)
4.4.2 protocols
The protocol identifiers that are valid for the 'protocol' production
MUST be defined by documents that are specific to URI resolution. At
present the THTTP [10] protocol is the only such specification.
It is extremely important to realize that simply specifying any
protocol in the services field is insufficient since there are
additional semantics surrounding URI resolution that are not defined
within the protocols. For example, if Z39.50 were to be specified as
a valid protocol it would have to additionally define how it would
encode requests for specific services, how the URI is encoded, and
what information is returned.
4.5 Valid Databases
At present only one DDDS Database is specified for this Application.
"Dynamic Delegation Discovery System (DDDS) Part Three: The DNS
Database" (RFC ZZZZ) [4] specifies a DDDS Database that uses the
NAPTR DNS resource record to contain the rewrite rules. The Keys for
this database are encoded as domain-names.
The output of the First Well Known Rule for the URI Resolution
Application is the URI's scheme. In order to convert this to a
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unique key in this Database the string 'uri.arpa.' is appended to the
end. This domain-name is used to request NAPTR records which
produces new keys in the form of domain-names.
The output of the First Well Known Rule of the URN Resolution
Application is the URN's namespace id. In order to convert this to a
unique key in this Database the string 'urn.arpa.' is appended to the
end. This domain-name is used to request NAPTR records which
produces new keys in the form of domain-names.
DNS servers MAY interpret Flag values and use that information to
include appropriate SRV and A records in the Additional Information
portion of the DNS packet. Clients are encouraged to check for
additional information but are not required to do so. See the
Additional Information Processing section of RFC YYYY for more
information on NAPTR records and the Additional Information section
of a DNS response packet.
The character set used to encode the substitution expression is UTF-
8. The allowed input characters are all those characters that are
allowed anywhere in a URI. The characters allowed to be in a Key are
those that are currently defined for DNS domain-names. The "i" flag
to the substitution expression is used to denote that, where
appropriate for the code points in question, any matches should be
done in a case-insensitive way.
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5. Examples
5.1 An example using a URN
Consider a URN that uses the hypothetical FOO namespace. FOO numbers
are identifiers for approximately 30 million registered businesses
around the world, assigned and maintained by Fred, Otto and Orvil,
Inc. The URN might look like:
urn:foo:002372413:annual-report-1997
The first step in the resolution process is to find out about the FOO
namespace. The namespace identifier [8], "foo", is extracted from
the URN and prepended to 'urn.arpa', producing 'foo.urn.arpa'. The
DNS is queried for NAPTR records for this domain which produces the
following results:
foo.urn.arpa.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "foolink+I2L+I2C" "" foolink.udp.example.com.
IN NAPTR 100 20 "s" "rcds+I2C" "" rcds.udp.example.com.
IN NAPTR 100 30 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.example.com.
The order field contains equal values, indicating that no order has
to be followed. The preference field indicates that the provider
would like clients to use the special 'foolink' protocol, followed by
the RCDS protocol, and that THTTP is offered as a last resort. All
the records specify the "s" flag which means that the record is
terminal and that the next step is to retrieve an SRV record from DNS
for the given domain-name.
The service fields say that if we speak of foolink, we will be able
to issue either the I2L, I2C or I2R requests to obtain a URL or ask
some complicated questions about the resource. The Resource
Cataloging and Distribution Service (RCDS) [12] could be used to get
some metadata for the resource, while THTTP could be used to get a
URL for the current location of the resource.
Assuming our client does not know the foolink protocol but does know
the RCDS protocol, our next action is to lookup SRV RRs for
rcds.udp.example.com, which will tell us hosts that can provide the
necessary resolution service. That lookup might return:
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;; Pref Weight Port Target
rcds.udp.example.com IN SRV 0 0 1000 deffoo.example.com.
IN SRV 0 0 1000 dbexample.com.au.
IN SRV 0 0 1000 ukexample.com.uk.
telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their RCDS server. (The
reader is referred to the SRV specification [9] for the
interpretation of the fields above).
There is opportunity for significant optimization here. RFC YYYY
defines that Additional Information section may be available. In
this case the the SRV records may be returned as additional
information for terminal NAPTRs lookups (as well as the A records for
those SRVs). This is a significant optimization. In conjunction
with a long TTL for *.urn.arpa records, the average number of probes
to DNS for resolving most URIs would approach one.
Note that the example NAPTR records above are intended to represent
the result of a NAPTR lookup using some client software like
nslookup; zone administrators should consult the documentation
accompanying their domain name servers to verify the precise syntax
they should use for zone files.
Also note that there could have been an additional first step where
the URN was resolved as a generic URI by looking up urn.uri.arpa.
The resulting rule would have specified that the NID be extracted
from the URN and 'urn.arpa' appended to it resulting in the new key
'foo.urn.arpa' which is the first step from above.
5.2 CID URI Scheme Example
Consider a URI scheme based on MIME Content-Ids. The URI might look
like this:
cid:199606121851.1@bar.example.com
(Note that this example is chosen for pedagogical purposes, and does
not conform to the CID URL scheme.)
The first step in the resolution process is to find out about the CID
scheme. The schem is extracted from the URI, prepended to
'uri.arpa', and the NAPTR for 'cid.uri.arpa' looked up in the DNS.
It might return records of the form:
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cid.uri.arpa.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "!cid:.+@(.*)$!\1!i" .
We have only one NAPTR response, so ordering the responses is not a
problem. The replacement field is empty, so we check the regexp
field and use the pattern provided there. We apply that regexp to
the entire URI to see if it matches, which it does. The \1 part of
the substitution expression returns the string "bar.example.com".
Since the flags field does not contain "s" or "a", the lookup is not
terminal and our next probe to DNS is for more NAPTR records at the
domain-name 'bar.example.com'.
At first glance this would make it sound as though a NAPTR would need
to exist for every single past and future domain-name in a particular
zone. This is partially correct but is handled very nicely by the
DNS wildcard convention. For each delegated zone in the example.com
domain, simply add the following records with the label set to "*".
In the case where the substitution expression from above generates a
domain-name that doesn't explicitly have a NAPTR record for it, the
ones listed below will be returned. If a particular domain-name was
an exception to this rule, simply add it to the zone explicitly and
those records will be returned instead of the default records.
The record returned from the query on "bar.example.com" might look
like:
*
;; order pref flags service regexp replacement
IN NAPTR 100 50 "s" "z3950+I2L+I2C" "" z3950.tcp.gatech.edu.
IN NAPTR 100 50 "s" "rcds+I2C" "" rcds.udp.gatech.edu.
IN NAPTR 100 50 "s" "thttp+I2L+I2C+I2R" "" thttp.tcp.gatech.edu.
Continuing with our example, we note that the values of the order and
preference fields are equal in all records, so the client is free to
pick any record. The flags field tells us that these are the last
NAPTR patterns we should see, and after the rewrite (a simple
replacement in this case) we should look up SRV records to get
information on the hosts that can provide the necessary service.
Assuming we prefer the Z39.50 protocol, our lookup might return:
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;; Pref Weight Port Target
z3950.tcp.gatech.edu IN SRV 0 0 1000 z3950.gatech.edu.
IN SRV 0 0 1000 z3950.cc.gatech.edu.
IN SRV 0 0 1000 z3950.uga.edu.
telling us three hosts that could actually do the resolution, and
giving us the port we should use to talk to their Z39.50 server.
5.3 Resolving an HTTP URI Scheme
Even if URN systems were in place now, there would still be a
tremendous number of URLs. It should be possible to develop a URI
resolution system that can also provide location independence for
those URLs.
Assume we have the URL for a very popular piece of software that the
publisher wishes to mirror at multiple sites around the world:
http://www.example.com/software/latest-beta.exe
We extract the prefix, "http", and lookup NAPTR records for
'http.uri.arpa'. This might return a record of the form:
http.uri.arpa. IN NAPTR
;; order pref flags service regexp replacement
100 90 "" "" "!http://([^/:]+)!\1!i" .
This expression returns everything after the first double slash and
before the next slash or colon. (We use the '!' character to delimit
the parts of the substitution expression. Otherwise we would have to
use backslashes to escape the forward slashes, and would have a
regexp in the zone file that looked like this:
"/http:\\/\\/([^\\/:]+)/\\1/i").
Applying this pattern to the URL extracts "www.example.com". Looking
up NAPTR records for that might return:
www.example.com.
;; order pref flags service regexp replacement
IN NAPTR 100 100 "s" "thttp+L2R" "" thttp.example.com.
IN NAPTR 100 100 "s" "ftp+L2R" "" ftp.example.com.
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Looking up SRV records for thttp.example.com would return information
on the hosts that example.com has designated to be its mirror sites.
The client can then pick one for the user.
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6. Notes
o Registration procedures for the 'urn.arpa' and 'uri.arpa' DNS
zones are specified in "Dynamic Delegation Discovery System
(DDDS) Part Five: URI.ARPA Assignment Procedures" (RFC VVVV) [5].
o If a record at a particular order matches the URI, but the client
doesn't know the specified protocol and service, the client SHOULD
continue to examine records that have the same order. The client
MUST NOT consider records with a higher value of order. This is
necessary to make delegation of portions of the namespace work.
The order field is what lets site administrators say "all requests
for URIs matching pattern x go to server 1, all others go to
server 2".
o Note that SRV RRs impose additional requirements on clients.
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7. IANA Considerations
The use of the "urn.arpa" and "uri.arpa" zones requires registration
policies and procedures to be followed and for the operation of those
DNS zones to be maintained. These policies and procedures are
spelled out in a "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures (RFC VVVV)" [5]. The operation
of those zones imposes operational and administrative
responsibilities on the IANA.
The registration method used for values in the Services and Flags
fields is for a specification to be approved by the IESG and
published as either an Informational or standards track RFC.
The registration policies for URLs is found in RFC2717 [17]. URN NID
registration policies are found in RFC2611 [16].
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8. Security Considerations
The use of "urn.arpa" and "uri.arpa" as the registry for namespaces
is subject to denial of service attacks, as well as other DNS
spoofing attacks. The interactions with DNSSEC are currently being
studied. It is expected that NAPTR records will be signed with SIG
records once the DNSSEC work is deployed.
The rewrite rules make identifiers from other namespaces subject to
the same attacks as normal domain names. Since they have not been
easily resolvable before, this may or may not be considered a
problem.
Regular expressions should be checked for sanity, not blindly passed
to something like PERL.
This document has discussed a way of locating a resolver, but has not
discussed any detail of how the communication with the resolver takes
place. There are significant security considerations attached to the
communication with a resolver. Those considerations are outside the
scope of this document, and must be addressed by the specifications
for particular resolver communication protocols.
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9. Acknowledgments
The editors would like to thank Keith Moore for all his consultations
during the development of this draft. We would also like to thank
Paul Vixie for his assistance in debugging our implementation, and
his answers on our questions. Finally, we would like to acknowledge
our enormous intellectual debt to the participants in the Knoxville
series of meetings, as well as to the participants in the URI and URN
working groups.
Specific recognition is given to Ron Daniel who was co-author on the
original versions of these documents. His early implementations and
clarity of thinking was invaluable in clearing up many of the
potential boundary cases.
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References
[1] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS Standard", RFC WWWW, draft-ietf-
urn-ddds-toc-00.txt (work in progress), October 2001.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC XXXX, draft-ietf-urn-ddds-05.txt (work
in progress), May 2000.
[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The DNS Database", RFC ZZZZ, draft-ietf-urn-dns-ddds-
database-07.txt (work in progress), May 2000.
[4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The URI Resolution Application", RFC YYYY, draft-ietf-
urn-uri-res-ddds-05.txt (work in progress), October 2000.
[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures", RFC VVVV, draft-ietf-
urn-net-procedures-09.txt (work in progress), October 2001.
[6] Sollins, K. and L. Masinter, "Functional Requirements for
Uniform Resource Names", RFC 1737, December 1994.
[7] Arms, B., "The URN Implementors, Uniform Resource Names: A
Progress Report", D-Lib Magazine, February 1996.
[8] Moats, R., "URN Syntax", RFC 2141, May 1997.
[9] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[10] Daniel, R., "A Trivial Convention for using HTTP in URN
Resolution", RFC 2169, June 1997.
[11] Mealling, M., "URI Resolution Services Necessary for URN
Resolution", RFC 2483, January 1999.
[12] Moore, K., Browne, S., Cox, J. and J. Gettler, "Resource
Cataloging and Distribution System", Technical Report CS-97-
346, December 1996.
[13] Sollins, K., "Architectural Principles of Uniform Resource Name
Resolution", RFC 2276, January 1998.
[14] Daniel, R. and M. Mealling, "Resolution of Uniform Resource
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Identifiers using the Domain Name System", RFC 2168, June 1997.
[15] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[16] Daigle, L., van Gulik, D., Iannella, R. and P. Falstrom, "URN
Namespace Definition Mechanisms", RFC 2611, BCP 33, June 1999.
[17] Petke, R. and I. King, "Registration Procedures for URL Scheme
Names", RFC 2717, BCP 35, November 1999.
[18] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, August 2000.
Author's Address
Michael Mealling
Verisign
505 Huntmar Park Drive
Herndon, VA 22070
US
Phone: +1 770 921-2251
EMail: michaelm@netsol.com
URI: http://www.verisign.com
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Appendix A. Pseudo Code
For the edification of implementers, pseudocode for a client routine
using NAPTRs is given below. This code is provided merely as a
convenience, it does not have any weight as a standard way to process
NAPTR records. Also, as is the case with pseudocode, it has never
been executed and may contain logical errors. You have been warned.
//
// findResolver(URN)
// Given a URN, find a host that can resolve it.
//
findResolver(string URN) {
// prepend prefix to urn.arpa
sprintf(key, "%s.urn.arpa", extractNS(URN));
do {
rewrite_flag = false;
terminal = false;
if (key has been seen) {
quit with a loop detected error
}
add key to list of "seens"
records = lookup(type=NAPTR, key); // get all NAPTR RRs for 'key'
discard any records with an unknown value in the "flags" field.
sort NAPTR records by "order" field and "preference" field
(with "order" being more significant than "preference").
n_naptrs = number of NAPTR records in response.
curr_order = records[0].order;
max_order = records[n_naptrs-1].order;
// Process current batch of NAPTRs according to "order" field.
for (j=0; j < n_naptrs && records[j].order <= max_order; j++) {
if (unknown_flag) // skip this record and go to next one
continue;
newkey = rewrite(URN, naptr[j].replacement, naptr[j].regexp);
if (!newkey) // Skip to next record if the rewrite didn't
match continue;
// We did do a rewrite, shrink max_order to current value
// so that delegation works properly
max_order = naptr[j].order;
// Will we know what to do with the protocol and services
// specified in the NAPTR? If not, try next record.
if(!isKnownProto(naptr[j].services)) {
continue;
}
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if(!isKnownService(naptr[j].services)) {
continue;
}
// At this point we have a successful rewrite and we will
// know how to speak the protocol and request a known
// resolution service. Before we do the next lookup, check
// some optimization possibilities.
if (strcasecmp(flags, "S")
|| strcasecmp(flags, "P"))
|| strcasecmp(flags, "A")) {
terminal = true;
services = naptr[j].services;
addnl = any SRV and/or A records returned as additional
info for naptr[j].
}
key = newkey;
rewriteflag = true;
break;
}
} while (rewriteflag && !terminal);
// Did we not find our way to a resolver?
if (!rewrite_flag) {
report an error
return NULL;
}
// Leave rest to another protocol?
if (strcasecmp(flags, "P")) {
return key as host to talk to;
}
// If not, keep plugging
if (!addnl) { // No SRVs came in as additional info, look them up
srvs = lookup(type=SRV, key);
}
sort SRV records by preference, weight, ...
foreach (SRV record) { // in order of preference
try contacting srv[j].target using the protocol and one of the
resolution service requests from the "services" field of the
last NAPTR record.
if (successful)
return (target, protocol, service);
// Actually we would probably return a result, but this
// code was supposed to just tell us a good host to talk to.
}
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die with an "unable to find a host" error;
}
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Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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