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IETF RFC 2168
Resolution of Uniform Resource Identifiers using the Domain Name System
Last modified on Saturday, June 21st, 1997
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Network Working Group R. Daniel
Request for Comments: 2168 Los Alamos National Laboratory
Category: Experimental M. Mealling
Network Solutions, Inc.
June 1997
Resolution of Uniform Resource Identifiers
using the Domain Name System
 Status of this Memo
===================
This memo defines an Experimental Protocol for the Internet
community. This memo does not specify an Internet standard of any
kind. Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Abstract:
=========
Uniform Resource Locators (URLs) are the foundation of the World Wide
Web, and are a vital Internet technology. However, they have proven
to be brittle in practice. The basic problem is that URLs typically
identify a particular path to a file on a particular host. There is
no graceful way of changing the path or host once the URL has been
assigned. Neither is there a graceful way of replicating the resource
located by the URL to achieve better network utilization and/or fault
tolerance. Uniform Resource Names (URNs) have been hypothesized as a
adjunct to URLs that would overcome such problems. URNs and URLs are
both instances of a broader class of identifiers known as Uniform
Resource Identifiers (URIs).
The requirements document for URN resolution systems[15] defines the
concept of a "resolver discovery service". This document describes
the first, experimental, RDS. It is implemented by a new DNS Resource
Record, NAPTR (Naming Authority PoinTeR), that provides rules for
mapping parts of URIs to domain names. By changing the mapping
rules, we can change the host that is contacted to resolve a URI.
This will allow a more graceful handling of URLs over long time
periods, and forms the foundation for a new proposal for Uniform
Resource Names.
Daniel & Mealling Experimental PAGE 1 
RFC 2168 Resolution of URIs Using the DNS June 1997
In addition to locating resolvers, the NAPTR provides for other
naming systems to be grandfathered into the URN world, provides
independence between the name assignment system and the resolution
protocol system, and allows multiple services (Name to Location, Name
to Description, Name to Resource, ...) to be offered. In conjunction
with the SRV RR, the NAPTR record allows those services to be
replicated for the purposes of fault tolerance and load balancing.
Introduction:
=============
Uniform Resource Locators 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 to serve as persistent, location-independent
identifiers for Internet resources in order to overcome most of the
problems with URLs. RFC 1737 [1] 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 [2]
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 URNs.
Therefore our approach is to use DNS to locate "resolvers" that can
provide information on individual resources, potentially including
the resource itself. To accomplish this, we "rewrite" the URI into a
domain name following the rules provided in NAPTR records. Rewrite
rules provide considerable power, which is important when trying to
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RFC 2168 Resolution of URIs Using the DNS June 1997
meet the goals listed above. However, collections of rules can become
difficult to understand. To lessen this problem, the NAPTR rules are
*always* applied to the original URI, *never* to the output of
previous rules.
Locating a resolver through the rewrite procedure may take multiple
steps, but the beginning is always the same. The start of the URI is
scanned to extract its colon-delimited prefix. (For URNs, the prefix
is always "urn:" and we extract the following colon-delimited
namespace identifier [3]). NAPTR resolution begins by taking the
extracted string, appending the well-known suffix ".urn.net", and
querying the DNS for NAPTR records at that domain name. Based on the
results of this query, zero or more additional DNS queries may be
needed to locate resolvers for the URI. The details of the
conversation between the client and the resolver thus located are
outside the bounds of this draft. Three brief examples of this
procedure are given in the next section.
The NAPTR RR provides the level of indirection needed to keep the
naming system independent of the resolution system, its protocols,
and services. Coupled with the new SRV resource record proposal[4]
there is also the potential for replicating the resolver on multiple
hosts, overcoming some of the most significant problems of URLs. This
is an important and subtle point. Not only do the NAPTR and SRV
records allow us to replicate the resource, we can replicate the
resolvers that know about the replicated resource. Preventing a
single point of failure at the resolver level is a significant
benefit. Separating the resolution procedure from the way names are
constructed has additional benefits. Different resolution procedures
can be used over time, and resolution procedures that are determined
to be useful can be extended to deal with additional namespaces.
Caveats
=======
The NAPTR proposal is the first resolution procedure to be considered
by the URN-WG. There are several concerns about the proposal which
have motivated the group to recommend it for publication as an
Experimental rather than a standards-track RFC.
First, URN resolution is new to the IETF and we wish to gain
operational experience before recommending any procedure for the
standards track. Second, the NAPTR proposal is based on DNS and
consequently inherits concerns about security and administration. The
recent advancement of the DNSSEC and secure update drafts to Proposed
Standard reduce these concerns, but we wish to experiment with those
new capabilities in the context of URN administration. A third area
of concern is the potential for a noticeable impact on the DNS. We
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RFC 2168 Resolution of URIs Using the DNS June 1997
believe that the proposal makes appropriate use of caching and
additional information, but it is best to go slow where the potential
for impact on a core system like the DNS is concerned. Fourth, the
rewrite rules in the NAPTR proposal are based on regular expressions.
Since regular expressions are difficult for humans to construct
correctly, concerns exist about the usability and maintainability of
the rules. This is especially true where international character sets
are concerned. Finally, the URN-WG is developing a requirements
document for URN Resolution Services[15], but that document is not
complete. That document needs to precede any resolution service
proposals on the standards track.
Terminology
===========
"Must" or "Shall" - Software that does not behave in the manner that
this document says it must is not conformant to this
document.
"Should" - Software that does not follow the behavior that this
document says it should may still be conformant, but is
probably broken in some fundamental way.
"May" - Implementations may or may not provide the described
behavior, while still remaining conformant to this
document.
Brief overview and examples of the NAPTR RR:
============================================
A detailed description of the NAPTR RR will be given later, but to
give a flavor for the proposal we first give a simple description of
the record and three examples of its use.
The key fields in the NAPTR RR are order, preference, service, flags,
regexp, and replacement:
* The order field specifies the order in which records MUST be
processed when multiple NAPTR records are returned in response to a
single query. A naming authority may have delegated a portion of
its namespace to another agency. Evaluating the NAPTR records in
the correct order is necessary for delegation to work properly.
* The preference field specifies the order in which records SHOULD be
processed when multiple NAPTR records have the same value of
"order". This field lets a service provider specify the order in
which resolvers are contacted, so that more capable machines are
contacted in preference to less capable ones.
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* The service field specifies the resolution protocol and resolution
service(s) that will be available if the rewrite specified by the
regexp or replacement fields is applied. Resolution protocols are
the protocols used to talk with a resolver. They will be specified
in other documents, such as [5]. Resolution services are operations
such as N2R (URN to Resource), N2L (URN to URL), N2C (URN to URC),
etc. These will be discussed in the URN Resolution Services
document[6], and their behavior in a particular resolution protocol
will be given in the specification for that protocol (see [5] for a
concrete example).
* The flags field contains modifiers that affect what happens in the
next DNS lookup, typically for optimizing the process. Flags may
also affect the interpretation of the other fields in the record,
therefore, clients MUST skip NAPTR records which contain an unknown
flag value.
* The regexp field is one of two fields used for the rewrite rules,
and is the core concept of the NAPTR record. The regexp field is a
String containing a sed-like substitution expression. (The actual
grammar for the substitution expressions is given later in this
draft). The substitution expression is applied to the original URN
to determine the next domain name to be queried. The regexp field
should be used when the domain name to be generated is conditional
on information in the URI. If the next domain name is always known,
which is anticipated to be a common occurrence, the replacement
field should be used instead.
* The replacement field is the other field that may be used for the
rewrite rule. It is an optimization of the rewrite process for the
case where the next domain name is fixed instead of being
conditional on the content of the URI. The replacement field is a
domain name (subject to compression if a DNS sender knows that a
given recipient is able to decompress names in this RR type's RDATA
field). If the rewrite is more complex than a simple substitution
of a domain name, the replacement field should be set to . and the
regexp field used.
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Note that the client applies all the substitutions and performs all
lookups, they are not performed in the DNS servers. Note also that it
is the belief of the developers of this document that regexps should
rarely be used. The replacement field seems adequate for the vast
majority of situations. Regexps are only necessary when portions of a
namespace are to be delegated to different resolvers. Finally, note
that the regexp and replacement fields are, at present, mutually
exclusive. However, developers of client software should be aware
that a new flag might be defined which requires values in both
fields.
Example 1
---------
Consider a URN that uses the hypothetical DUNS namespace. DUNS
numbers are identifiers for approximately 30 million registered
businesses around the world, assigned and maintained by Dunn and
Bradstreet. The URN might look like:
urn:duns:002372413:annual-report-1997
The first step in the resolution process is to find out about the
DUNS namespace. The namespace identifier, "duns", is extracted from
the URN, prepended to urn.net, and the NAPTRs for duns.urn.net looked
up. It might return records of the form:
duns.urn.net
;; order pref flags service regexp replacement
IN NAPTR 100 10 "s" "dunslink+N2L+N2C" "" dunslink.udp.isi.dandb.com
IN NAPTR 100 20 "s" "rcds+N2C" "" rcds.udp.isi.dandb.com
IN NAPTR 100 30 "s" "http+N2L+N2C+N2R" "" http.tcp.isi.dandb.com
The order field contains equal values, indicating that no name
delegation order has to be followed. The preference field indicates
that the provider would like clients to use the special dunslink
protocol, followed by the RCDS protocol, and that HTTP is offered as
a last resort. All the records specify the "s" flag, which will be
explained momentarily. The service fields say that if we speak
dunslink, we will be able to issue either the N2L or N2C requests to
obtain a URL or a URC (description) of the resource. The Resource
Cataloging and Distribution Service (RCDS)[7] could be used to get a
URC for the resource, while HTTP could be used to get a URL, URC, or
the resource itself. All the records supply the next domain name to
query, none of them need to be rewritten with the aid of regular
expressions.
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RFC 2168 Resolution of URIs Using the DNS June 1997
The general case might require multiple NAPTR rewrites to locate a
resolver, but eventually we will come to the "terminal NAPTR". Once
we have the terminal NAPTR, our next probe into the DNS will be for a
SRV or A record instead of another NAPTR. Rather than probing for a
non-existent NAPTR record to terminate the loop, the flags field is
used to indicate a terminal lookup. If it has a value of "s", the
next lookup should be for SRV RRs, "a" denotes that A records should
sought. A "p" flag is also provided to indicate that the next action
is Protocol-specific, but that looking up another NAPTR will not be
part of it.
Since our example RR specified the "s" flag, it was terminal.
Assuming our client does not know the dunslink protocol, our next
action is to lookup SRV RRs for rcds.udp.isi.dandb.com, which will
tell us hosts that can provide the necessary resolution service. That
lookup might return:
;; Pref Weight Port Target
rcds.udp.isi.dandb.com IN SRV 0 0 1000 defduns.isi.dandb.com
IN SRV 0 0 1000 dbmirror.com.au
IN SRV 0 0 1000 ukmirror.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 proposal [4] for the interpretation of
the fields above).
There is opportunity for significant optimization here. We can return
the SRV records as additional information for terminal NAPTRs (and
the A records as additional information for those SRVs). While this
recursive provision of additional information is not explicitly
blessed in the DNS specifications, it is not forbidden, and BIND does
take advantage of it [8]. This is a significant optimization. In
conjunction with a long TTL for *.urn.net records, the average number
of probes to DNS for resolving DUNS URNs would approach one.
Therefore, DNS server implementors SHOULD provide additional
information with NAPTR responses. The additional information will be
either SRV or A records. If SRV records are available, their A
records should be provided as recursive additional information.
Note that the example NAPTR records above are intended to represent
the reply the client will see. They are not quite identical to what
the domain administrator would put into the zone files. For one
thing, the administrator should supply the trailing '.' character on
any FQDNs.
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RFC 2168 Resolution of URIs Using the DNS June 1997
Example 2
---------
Consider a URN namespace based on MIME Content-Ids. The URN might
look like this:
urn:cid:199606121851.1@mordred.gatech.edu
(Note that this example is chosen for pedagogical purposes, and does
not conform to the recently-approved CID URL scheme.)
The first step in the resolution process is to find out about the CID
namespace. The namespace identifier, cid, is extracted from the URN,
prepended to urn.net, and the NAPTR for cid.urn.net looked up. It
might return records of the form:
cid.urn.net
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "/urn:cid:.+@([^\.]+\.)(.*)$/\2/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 URN to see if it matches, which it does. The \2 part of the
substitution expression returns the string "gatech.edu". 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:
lookup(query=NAPTR, "gatech.edu").
Note that the rule does not extract the full domain name from the
CID, instead it assumes the CID comes from a host and extracts its
domain. While all hosts, such as mordred, could have their very own
NAPTR, maintaining those records for all the machines at a site as
large as Georgia Tech would be an intolerable burden. Wildcards are
not appropriate here since they only return results when there is no
exactly matching names already in the system.
The record returned from the query on "gatech.edu" might look like:
gatech.edu IN NAPTR
;; order pref flags service regexp replacement
IN NAPTR 100 50 "s" "z3950+N2L+N2C" "" z3950.tcp.gatech.edu
IN NAPTR 100 50 "s" "rcds+N2C" "" rcds.udp.gatech.edu
IN NAPTR 100 50 "s" "http+N2L+N2C+N2R" "" http.tcp.gatech.edu
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RFC 2168 Resolution of URIs Using the DNS June 1997
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:
;; 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.
Recall that the regular expression used \2 to extract a domain name
from the CID, and \. for matching the literal '.' characters
seperating the domain name components. Since '\' is the escape
character, literal occurances of a backslash must be escaped by
another backslash. For the case of the cid.urn.net record above, the
regular expression entered into the zone file should be
"/urn:cid:.+@([^\\.]+\\.)(.*)$/\\2/i". When the client code actually
receives the record, the pattern will have been converted to
"/urn:cid:.+@([^.]+\.)(.*)$/\2/i".
Example 3
---------
Even if URN systems were in place now, there would still be a
tremendous number of URLs. It should be possible to develop a URN
resolution system that can also provide location independence for
those URLs. This is related to the requirement in [1] to be able to
grandfather in names from other naming systems, such as ISO Formal
Public Identifiers, Library of Congress Call Numbers, ISBNs, ISSNs,
etc.
The NAPTR RR could also be used for URLs that have already been
assigned. 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.foo.com/software/latest-beta.exe
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RFC 2168 Resolution of URIs Using the DNS June 1997
We extract the prefix, "http", and lookup NAPTR records for
http.urn.net. This might return a record of the form
http.urn.net 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
"/http:\\/\\/([^\\/:]+)/\\1/i".).
Applying this pattern to the URL extracts "www.foo.com". Looking up
NAPTR records for that might return:
www.foo.com
;; order pref flags service regexp replacement
IN NAPTR 100 100 "s" "http+L2R" "" http.tcp.foo.com
IN NAPTR 100 100 "s" "ftp+L2R" "" ftp.tcp.foo.com
Looking up SRV records for http.tcp.foo.com would return information
on the hosts that foo.com has designated to be its mirror sites. The
client can then pick one for the user.
NAPTR RR Format
===============
The format of the NAPTR RR is given below. The DNS type code for
NAPTR is 35.
Domain TTL Class Order Preference Flags Service Regexp
Replacement
where:
Domain
The domain name this resource record refers to.
TTL
Standard DNS Time To Live field
Class
Standard DNS meaning
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RFC 2168 Resolution of URIs Using the DNS June 1997
Order
A 16-bit integer specifying the order in which the NAPTR
records MUST be processed to ensure correct delegation of
portions of the namespace over time. Low numbers are processed
before high numbers, and once a NAPTR is found that "matches"
a URN, the client MUST NOT consider any NAPTRs with a higher
value for order.
Preference
A 16-bit integer which specifies the order in which NAPTR
records with equal "order" values SHOULD be processed, low
numbers being processed before high numbers. This is similar
to the preference field in an MX record, and is used so domain
administrators can direct clients towards more capable hosts
or lighter weight protocols.
Flags
A String giving flags to control aspects of the rewriting and
interpretation of the fields in the record. Flags are single
characters from the set [A-Z0-9]. The case of the alphabetic
characters is not significant.
At this time only three flags, "S", "A", and "P", are defined.
"S" means that the next lookup should be for SRV records
instead of NAPTR records. "A" means that the next lookup
should be for A records. The "P" flag says that the remainder
of the resolution shall be carried out in a Protocol-specific
fashion, and we should not do any more DNS queries.
The remaining alphabetic flags are reserved. The numeric flags
may be used for local experimentation. The S, A, 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 NAPTRs would be more
likely to signal such an error than a client such as a
browser). We anticipate 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 record. This test takes precedence even
over the "order" field. 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 URN.
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RFC 2168 Resolution of URIs Using the DNS June 1997
Service
Specifies the resolution service(s) available down this
rewrite path. It may also specify the particular protocol that
is used to talk with a resolver. A protocol MUST be specified
if the flags field states that the NAPTR is terminal. If a
protocol is specified, but the flags field does not state that
the NAPTR is terminal, the next lookup MUST be for a NAPTR.
The client MAY choose not to perform the next lookup if the
protocol is unknown, but that behavior MUST NOT be relied
upon.
The service field may take any of the values below (using the
Augmented BNF of RFC 822[9]):
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.
// The current set of "known" strings are:
// protocol = "rcds" / "thttp" / "hdl" / "rwhois" / "z3950"
// rs = "N2L" / "N2Ls" / "N2R" / "N2Rs" / "N2C"
// / "N2Ns" / "L2R" / "L2Ns" / "L2Ls" / "L2C"
i.e. an optional protocol specification followed by 0 or more
resolution services. Each resolution service is indicated by
an initial '+' character.
Note that the empty string is also a valid service field. This
will typically be seen at the top levels of a namespace, when
it is impossible to know what services and protocols will be
offered by a particular publisher within that name space.
At this time the known protocols are rcds[7], hdl[10] (binary,
UDP-based protocols), thttp[5] (a textual, TCP-based
protocol), rwhois[11] (textual, UDP or TCP based), and
Z39.50[12] (binary, TCP-based). More will be allowed later.
The names of the protocols must be formed from the characters
[a-Z0-9]. Case of the characters is not significant.
The service requests currently allowed will be described in
more detail in [6], but in brief they are:
N2L - Given a URN, return a URL
N2Ls - Given a URN, return a set of URLs
N2R - Given a URN, return an instance of the resource.
N2Rs - Given a URN, return multiple instances of the
resource, typically encoded using
multipart/alternative.
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RFC 2168 Resolution of URIs Using the DNS June 1997
N2C - Given a URN, return a collection of meta-
information on the named resource. The format of
this response is the subject of another document.
N2Ns - Given a URN, return all URNs that are also
identifers for the resource.
L2R - Given a URL, return the resource.
L2Ns - Given a URL, return all the URNs that are
identifiers for the resource.
L2Ls - Given a URL, return all the URLs for instances of
of the same resource.
L2C - Given a URL, return a description of the
resource.
The actual format of the service request and response will be
determined by the resolution protocol, and is the subject for
other documents (e.g. [5]). Protocols need not offer all
services. The labels for service requests shall be formed from
the set of characters [A-Z0-9]. The case of the alphabetic
characters is not significant.
Regexp
A STRING containing a substitution expression that is applied
to the original URI in order to construct the next domain name
to lookup. The grammar of the substitution expression is given
in the next section.
Replacement
The next NAME to query for NAPTR, SRV, or A records depending
on the value of the flags field. As mentioned above, this may
be compressed.
Substitution Expression Grammar:
================================
The content of the regexp field is a substitution expression. True
sed(1) substitution expressions are not appropriate for use in this
application for a variety of reasons, therefore the contents of the
regexp field MUST follow the grammar below:
subst_expr = delim-char ere delim-char repl delim-char *flags
delim-char = "/" / "!" / ... (Any non-digit or non-flag character other
than backslash '\'. All occurances of a delim_char in a
subst_expr must be the same character.)
ere = POSIX Extended Regular Expression (see [13], section
2.8.4)
repl = dns_str / backref / repl dns_str / repl backref
dns_str = 1*DNS_CHAR
backref = "\" 1POS_DIGIT
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flags = "i"
DNS_CHAR = "-" / "0" / ... / "9" / "a" / ... / "z" / "A" / ... / "Z"
POS_DIGIT = "1" / "2" / ... / "9" ; 0 is not an allowed backref
value domain name (see RFC 1123 [14]).
The result of applying the substitution expression to the original
URI MUST result in a string that obeys the syntax for DNS host names
[14]. Since it is possible for the regexp field to be improperly
specified, such that a non-conforming host name can be constructed,
client software SHOULD verify that the result is a legal host name
before making queries on it.
Backref expressions in the repl portion of the substitution
expression are replaced by the (possibly empty) string of characters
enclosed by '(' and ')' in the ERE portion of the substitution
expression. N is a single digit from 1 through 9, inclusive. It
specifies the N'th backref expression, the one that begins with the
N'th '(' and continues to the matching ')'. For example, the ERE
(A(B(C)DE)(F)G)
has backref expressions:
\1 = ABCDEFG
\2 = BCDE
\3 = C
\4 = F
\5..\9 = error - no matching subexpression
The "i" flag indicates that the ERE matching SHALL be performed in a
case-insensitive fashion. Furthermore, any backref replacements MAY
be normalized to lower case when the "i" flag is given.
The first character in the substitution expression shall be used as
the character that delimits the components of the substitution
expression. There must be exactly three non-escaped occurrences of
the delimiter character in a substitution expression. Since escaped
occurrences of the delimiter character will be interpreted as
occurrences of that character, digits MUST NOT be used as delimiters.
Backrefs would be confused with literal digits were this allowed.
Similarly, if flags are specified in the substitution expression, the
delimiter character must not also be a flag character.
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RFC 2168 Resolution of URIs Using the DNS June 1997
Advice to domain administrators:
================================
Beware of regular expressions. Not only are they a pain to get
correct on their own, but there is the previously mentioned
interaction with DNS. Any backslashes in a regexp must be entered
twice in a zone file in order to appear once in a query response.
More seriously, the need for double backslashes has probably not been
tested by all implementors of DNS servers. We anticipate that urn.net
will be the heaviest user of regexps. Only when delegating portions
of namespaces should the typical domain administrator need to use
regexps.
On a related note, beware of interactions with the shell when
manipulating regexps from the command line. Since '\' is a common
escape character in shells, there is a good chance that when you
think you are saying "\\" you are actually saying "\". Similar
caveats apply to characters such as
The "a" flag allows the next lookup to be for A records rather than
SRV records. Since there is no place for a port specification in the
NAPTR record, when the "A" flag is used the specified protocol must
be running on its default port.
The URN Sytnax draft defines a canonical form for each URN, which
requires %encoding characters outside a limited repertoire. The
regular expressions MUST be written to operate on that canonical
form. Since international character sets will end up with extensive
use of %encoded characters, regular expressions operating on them
will be essentially impossible to read or write by hand.
Usage
=====
For the edification of implementers, pseudocode for a client routine
using NAPTRs is given below. This code is provided merely as a
convience, 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.net
sprintf(key, "%s.urn.net", extractNS(URN));
do {
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RFC 2168 Resolution of URIs Using the DNS June 1997
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;
}
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;
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RFC 2168 Resolution of URIs Using the DNS June 1997
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.
}
die with an "unable to find a host" error;
}
Notes:
======
- A client MUST process multiple NAPTR records in the order
specified by the "order" field, it MUST NOT simply use the first
record that provides a known protocol and service combination.
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RFC 2168 Resolution of URIs Using the DNS June 1997
- 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".
(A match is defined as:
1) The NAPTR provides a replacement domain name
or
2) The regular expression matches the URN
)
- When multiple RRs have the same "order", the client should use
the value of the preference field to select the next NAPTR to
consider. However, because of preferred protocols or services,
estimates of network distance and bandwidth, etc. clients may
use different criteria to sort the records.
- If the lookup after a rewrite fails, clients are strongly
encouraged to report a failure, rather than backing up to pursue
other rewrite paths.
- When a namespace is to be delegated among a set of resolvers,
regexps must be used. Each regexp appears in a separate NAPTR
RR. Administrators should do as little delegation as possible,
because of limitations on the size of DNS responses.
- Note that SRV RRs impose additional requirements on clients.
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.
References:
===========
[1] Sollins, Karen and Larry Masinter, "Functional Requirements
for Uniform Resource Names", RFC 1737, Dec. 1994.
[2] The URN Implementors, Uniform Resource Names: A Progress Report,
http://www.dlib.org/dlib/february96/02arms.html, D-Lib Magazine,
February 1996.
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RFC 2168 Resolution of URIs Using the DNS June 1997
[3] Moats, Ryan, "URN Syntax", RFC 2141, May 1997.
[4] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying
the location of services (DNS SRV)", RFC 2052, October 1996.
[5] Daniel, Jr., Ron, "A Trivial Convention for using HTTP in URN
Resolution", RFC 2169, June 1997.
[6] URN-WG, "URN Resolution Services", Work in Progress.
[7] Moore, Keith, Shirley Browne, Jason Cox, and Jonathan Gettler,
Resource Cataloging and Distribution System, Technical Report
CS-97-346, University of Tennessee, Knoxville, December 1996
[8] Paul Vixie, personal communication.
[9] Crocker, Dave H. "Standard for the Format of ARPA Internet Text
Messages", RFC 822, August 1982.
[10] Orth, Charles and Bill Arms; Handle Resolution Protocol
Specification, http://www.handle.net/docs/client_spec.html
[11] Williamson, S., M. Kosters, D. Blacka, J. Singh, K. Zeilstra,
"Referral Whois Protocol (RWhois)", RFC 2167, June 1997.
[12] Information Retrieval (Z39.50): Application Service Definition
and Protocol Specification, ANSI/NISO Z39.50-1995, July 1995.
[13] IEEE Standard for Information Technology - Portable Operating
System Interface (POSIX) - Part 2: Shell and Utilities (Vol. 1);
IEEE Std 1003.2-1992; The Institute of Electrical and
Electronics Engineers; New York; 1993. ISBN:1-55937-255-9
[14] Braden, R., "Requirements for Internet Hosts - Application and
and Support", RFC 1123, Oct. 1989.
[15] Sollins, Karen, "Requirements and a Framework for URN Resolution
Systems", November 1996, Work in Progress.
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RFC 2168 Resolution of URIs Using the DNS June 1997
Security Considerations
=======================
The use of "urn.net" as the registry for URN 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.
Author Contact Information:
===========================
Ron Daniel
Los Alamos National Laboratory
MS B287
Los Alamos, NM, USA, 87545
voice: +1 505 665 0597
fax: +1 505 665 4939
email: rdaniel@lanl.gov
Michael Mealling
Network Solutions
505 Huntmar Park Drive
Herndon, VA 22070
voice: (703) 742-0400
fax: (703) 742-9552
email: michaelm@internic.net
URL: http://www.netsol.com/
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Resolution of Uniform Resource Identifiers using the Domain Name System
RFC TOTAL SIZE: 46528 bytes
PUBLICATION DATE: Saturday, June 21st, 1997
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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