Network Working Group M. Mealling
Internet-Draft Network Solutions, Inc.
Expires: December 23, 1999 R. Daniel
DATAFUSION, Inc.
June 24, 1999
The Naming Authority Pointer (NAPTR) DNS Resource Record
draft-ietf-urn-naptr-rr-03.txt
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 working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To view the entire list of Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 23, 1999.
Abstract
This document describes a DNS Resource Record which specifies a
rewrite rule that, when applied to an existing string will produce a
new domain. Reasons for rewriting a domain vary from URN Resource
Discovery Systems to moving out of date services to new domains.
This document updates the portions of RFC2168 specifically dealing
with the definition of the NAPTR record.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. NAPTR RR Format . . . . . . . . . . . . . . . . . . . . . . . 4
3. Substitution Expression Grammar . . . . . . . . . . . . . . . 8
4. The Basic NAPTR Algorithm . . . . . . . . . . . . . . . . . . 10
5. Concerning How NAPTR Uses SRV Records . . . . . . . . . . . . 11
6. Application Specifications . . . . . . . . . . . . . . . . . . 12
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.3 Example 3 . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. DNS Packet Format . . . . . . . . . . . . . . . . . . . . . . 17
9. Master File Format . . . . . . . . . . . . . . . . . . . . . . 18
10. Advice for DNS Administrators . . . . . . . . . . . . . . . . 19
11. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 22
A. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
B. Security Considerations . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
This RR was originally produced by the URN Working Group[3] as a
way to encode rule-sets in DNS so that the delegated sections of a
URI could be decomposed in such a way that they could be changed and
re-delegated over time. The result was a Resource Record that
included a regular expression that would be used by a client program
to rewrite a string into a domain name. Regular expressions were
chosen for their compactness to expressivity ratio allowing for a
great deal of information to be encoded in a rather small DNS
packet.
The function of rewriting a string according to the rules in a
record has usefulness in several different applications. This
document defines the basic assumptions to which all of those
applications must adhere to. It does not define the reasons the
rewrite is used, what the expected outcomes are, or what they are
used for. Those are specified by applications that define how they
use the NAPTR record and algorithms within their contexts.
Flags and other fields are also specified in the RR to control the
rewrite procedure in various ways or to provide information on how
to communicate with the host at the domain name that was the result
of the rewrite.
The final result is a RR that has several fields that interact in a
non-trivial but implementable way. This document specifies those
fields and their values.
This document does not define applications that utilizes this
rewrite functionality. Instead it specifies just the mechanics of
how it is done. Why its done, what the rules concerning the inputs,
and the types of rules used are reserved for other documents that
fully specify a particular application. This seperation is due to
several differrent applications all wanting to take advantage of the
rewrite rule lookup process. Each one has vastly different reasons
for why and how it uses the service, thus requiring that the
definition of the service be generic.
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.
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2. NAPTR RR Format
The format of the NAPTR RR is given below. The DNS[1]type code[2]
for NAPTR is 35.
Domain TTL Class Order Preference Flags Service Regexp Replacement
Domain
The domain name to which this resource record refers. This is the
'key' for this entry in the rule database. This value will either
be the first well known key (<something>.uri.net for example) or
a new key that is the output of a replacement or regexp rewrite.
Beyond this, it has the standard DNS requirements[1].
TTL
Standard DNS meaning.[1]
Class
Standard DNS meaning[1].
Order
A 16-bit unsigned integer specifying the order in which the NAPTR
records MUST be processed to ensure the correct ordering of
rules. Low numbers are processed before high numbers, and once a
NAPTR is found whose rule "matches" the target, the client MUST
NOT consider any NAPTRs with a higher value for order (except as
noted below for the Flags field).
Preference
A 16-bit unsigned integer that 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. A client MAY look at records with
higher preference values if it has a good reason to do so such as
not understanding the preferred protocol or service.
The important difference between Order and Preference is that
once a match is found the client MUST NOT consider records with a
different Order but they MAY process records with the same Order
but different Preferences. I.e. Preference is used to give weight
to rules that are considered the same from an authority
standpoint but not from a simple load balancing standpoint.
Flags
A <character-string> containing 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.
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At this time only four flags, "S", "A", "U", and "P", are
defined. The "S", "A" and "U" flags denote a terminal lookup.
This means that this NAPTR record is the last one and that the
flag determines what the next stage should be. The "S" flag
means that the next lookup should be for SRV records[4]. See
Section 5 for additional information on how NAPTR uses the SRV
record type. "A" means that the next lookup should be for A
records. The "U" flag means that the next step is not a DNS
lookup but that the output of the Regexp field is a URL[9].
The "P" flag says that the remainder of the application side
algorithm shall be carried out in a Protocol-specific fashion.
The new set of rules is identified by the Protocol specified in
the Services field. The record that contains the 'P' flag is the
last record that is interpreted by the rules specified in this
document. The new rules are dependent on the application for
which they are being used and the protocol specified. For
example, if the application is a URI RDS and the protocol is WIRE
then the new set of rules are governed by the algorithms
surrounding the WIRE HTTP specification and not this document.
The remaining alphabetic flags are reserved for future versions
of the NAPTR 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 NAPTRs 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 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 given target.
The "S", "A", and "U" flags are called 'terminal' flags since
they halt the looping rewrite algorithm. If those flags are not
present, clients may assume that another NAPTR RR exists at the
domain name produced by the current rewrite rule. Since the "P"
flag specifies a new algorithm, it may or may not be 'terminal'.
Thus, the client cannot assume that another NAPTR exists since
this case is determined elsewhere.
DNS servers MAY interpret these flags and 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.
Service
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Specifies the service(s) available down this rewrite path. It may
also specify the particular protocol that is used to talk with a
service. 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 2234[5]):
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.
For example, 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 beginning of a series of rules,
when it is impossible to know what services and protocols will be
offered by a particular service.
The actual format of the service request and response will be
determined by the resolution protocol, and is the subject for
other documents. 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.
The list of "valid" protocols for any given NAPTR record is any
protocol that implements some or all of the services defined for
a NAPTR application. Currently, THTTP[6] is the only protocol
that is known to make that claim at the time of publication. Any
other protocol that is to be used must have documentation
specifying:
* how it implements the services of the application
* how it is to appear in the NAPTR record (i.e., the string id
of the protocol)
The list of valid Resolution Services is defined by the documents
that specify individual NAPTR based applications. One example is
RFC-XXXX, "Resolution of Uniform Resource Identifiers using the
Domain Name System"[7].
It is worth noting that the interpretation of this field is
subject to being changed by new flags, and that the current
specification is oriented towards telling clients how to talk
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with a URN resolver.
Regexp
A STRING containing a substitution expression that is applied to
the original string held by the client in order to construct the
next domain name to lookup. The grammar of the substitution
expression is given in the next section.
The regular expressions MUST NOT be used in a cumulative fashion,
that is, they should only be applied to the original string held
by the client, never to the domain name produced by a previous
NAPTR rewrite. The latter is tempting in some applications but
experience has shown such use to be extremely fault sensitive,
very error prone, and extremely difficult to debug.
Replacement
The next NAME to query for NAPTR, SRV, or A records depending on
the value of the flags field. This MUST be a fully qualified
domain-name. Unless and until permitted by future standards
action, name compression is not to be used for this field.
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3. 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
repl = 1 * ( OCTET / backref )
backref = "\" 1POS_DIGIT
flags = "i"
POS_DIGIT = %x31-39 ; 0 is not an allowed backref
The definition of a POSIX Extended Regular Expression can be found
in [8], section 2.8.4.
The result of applying the substitution expression to the original
URI MUST result in either a string that obeys the syntax for DNS
domain-names[1] or a URI[9] if the Flags field contains a 'U'.
Since it is possible for the regexp field to be improperly
specified, such that a non-conforming domain-name can be
constructed, client software SHOULD verify that the result is a
legal DNS domain-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.
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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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4. The Basic NAPTR Algorithm
The behavior and meaning of the flags and services assume an
algorithm where the output of one rewrite is a new key that points
to another rule. This looping algorithm allows NAPTR records to
incrementally specify a complete rule. These incremental rules can
be delegated which allows other entities to specify rules so that
one entity does not need to understand _all_ rules.
The algorithm starts with a string and some known key (domain).
NAPTR records for this key are retrieved, those with unknown Flags
or inappropriate Services are discarded and the remaining records
are sorted by their Order field. Within each value of Order, the
records are further sorted by the Preferences field.
The records are examined in sorted order until a matching record is
found. A record is considered a match iff:
1. it has a Replacement field value instead of a Regexp field value.
2. or the Regexp field matches the string held by the client.
The first match MUST be the match that is used. Once a match is
found, the Services field is examined for whether or not this rule
advances toward the desired result. If so, the rule is applied to
the target string. If not, the process halts. The domain that
results from the regular expression is then used as the domain of
the next loop through the NAPTR algorithm. Note that the same target
string is used throughout the algorithm.
This looping is extremely important since it is the method by which
complex rules are broken down into manageable delegated chunks. The
flags fields simply determine at which point the looping should stop
(or other specialized behavior).
Since flags are valid at any level of the algorithm, the
degenerative case is to never loop but to look up the NAPTR and then
stop. In many specialized cases this is all that is needed.
Implementors should be aware that the degenerative case should not
become the common case.
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5. Concerning How NAPTR Uses SRV Records
When the SRV record type was originally specified it assumed that
the client did not know the specific domain-name before hand. The
client would construct a domain-name more in the form of a question
than the usual case of knowing ahead of time that the domain-name
should exist. I.e., if the client wants to know if there is a TCP
based HTTP server running at a particular domain, the client would
construct the domain-name _http._tcp.somedomain.com and ask the DNS
if that records exists.
In the case of NAPTR, the actual domain-name is specified by the
various fields in the NAPTR record. In this case the client isn't
asking a question but is instead attempting to get at information
that it has been told exists in an SRV record at that particular
domain-name. While this usage of SRV is slightly different than the
SRV authors originally intended it does not break any of the
assumptions concerning what SRV contains. Since NAPTR specifies the
entire domain a priori it could have ignored the underscore
collission avoidance convention. It was determined that such a
radical departure from how SRV was assumed to work would cause
problems in the long run. Thus the underscores were kept.
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6. Application Specifications
It should be noted that the NAPTR algorithm is the basic assumption
about how NAPTR works. The reasons for the rewrite and the expected
output and its use are specified by documents that define what
applications the NAPTR record and algorithm are used for. Any
document that defines such an application must define the following:
o The first known domain-name or how to build it
o The valid Services and Protocols
o What the expected use is for the output of the last rewrite
o The validity and/or behavior of any 'P' flag protocols.
o The general semantics surrounding why and how NAPTR and its
algorithm are being used.
Currently the only example of such a document is RFC-XXXX,
"Resolution of Uniform Resource Identifiers using the Domain Name
System"[7].
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7. Examples
NOTE: These are examples only. They are taken from ongoing work and
may not represent the end result of that work. They are here for
pedagogical reasons only.
7.1 Example 1
NAPTR was originally specified for use with the a Uniform Resource
Name Resolver Discovery System. This example details how a
particular URN would use the NAPTR record to find a resolver
service. The document that actually specifies this fully is
RFCXXXX[7].
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 CID URL scheme.)
The first step in the resolution process is to find out about the
CID namespace. The namespace identifier[3], 'cid', is extracted from
the URN, prepended to urn.net. 'cid.urn.net' then becomes the first
'known' key in the NAPTR algorithm. The process of determining the
first known key is defined by the URN RDS application document (
RFCXXXX[7]). The NAPTR records for cid.urn.net looked up and return
a single record:
cid.urn.net.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "/urn:cid:.+@([^\.]+\.)(.*)$/\2/i" .
There is only one NAPTR response, so ordering the responses is not a
problem. The replacement field is empty, so the pattern provided in
the regexp field is used . 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 where the new domain is
'gatech.edu' and the string is the same string as before.
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
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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.
;; 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.
Continuing with the example, 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
separating 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 master 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".
7.2 Example 2
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 that URNs 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
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software that the publisher wishes to mirror at multiple sites
around the world:
http://www.foo.com/software/latest-beta.exe
Using the rules specified in RFCXXXX[7] we extract the prefix,
"http", and lookup NAPTR records for http.uri.net. Note again that
RFCXXXX defines the first known key as the URI scheme appended with
the string ".uri.net". This might return a record of the form
http.uri.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.
7.3 Example 3
A non-URI example is where a NAPTR is used to specify the available
mappings from a domain-name to telephony based endpoints. In this
example the regular expression field is not used since the important
information is encoded within the services field.
0.0.0.4.6.2.6.5.8.6.4.e164.int.
IN NAPTR 100 10 "s" "h323call+N2R" "" _h323._udp.tele2.se.
IN NAPTR 102 10 "s" "potscall+N2R" "" _potscall._tcp.tele2.se.
IN NAPTR 102 10 "s" "smtp+N2R" "" _smtp._tcp.tele2.se.
In these examples the domain is an encoded E164 telephone number.
The services field specifies that, for this particular telephone
number, the services that are available are h323call, potscall and
smtp; and that "tele2.se" is the target that provides those
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services. Since the flag is "s", the next step should be a query for
an SRV record which will contain specific information about the
"tele2.se" domain.
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8. DNS Packet Format
The packet format for the NAPTR record is as follows
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ORDER |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PREFERENCE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ FLAGS /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ SERVICES /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ REGEXP /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
/ REPLACEMENT /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
where:
FLAGS A <character-string> which contains various flags.
SERVICES A <character-string> which contains protocol and service
identifiers.
REGEXP A <character-string> which contains a regular expression.
REPLACEMENT A <domain-name> which specifies the new value in the
case where the regular expression is a simple replacement
operation.
<character-string> and <domain-name> as used here are defined in
RFC1035[1].
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9. Master File Format
The master file format follows the standard rules in RFC-1035[1].
Order and preference, being 16-bit unsigned integers, shall be an
integer between 0 and 65535. The Flags and Services and Regexp
fields are all quoted <character-string>s. Since the Regexp field
can contain numerous backslashes and thus should be treated with
care. See Section 10 for how to correctly enter and escape the
regular expression.
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10. Advice for DNS Administrators
Beware of regular expressions. Not only are they difficult 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.
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 Syntax 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.
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11. Notes
o 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.
o When multiple RRs have the same "order" and all other criteria
being equal, the client should use the value of the preference
field to select the next NAPTR to consider. However, because it
will often be the case where preferred protocols or services
exist, clients may use this additional criteria to sort
the records.
o If the lookup after a rewrite fails, clients are strongly
encouraged to report a failure, rather than backing up to pursue
other rewrite paths.
o Note that SRV RRs impose additional requirements on clients.
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12. 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.
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References
[1] Mockapetris, P., "Domain names - implementation and
specification", RFC 1035, STD 13, November 1987.
[2] Mockapetris, P., "Domain names - concepts and facilities", RFC
1034, STD 13, November 1987.
[3] Moats, R., "URN Syntax", RFC 2141, May 1997.
[4] Eastlake, D., Gulbrandsen, A., "A DNS RR for specifying the
location of services (DNS SRV)", January 1999.
[5] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[6] Daniel, R., "A Trivial Convention for using HTTP in URN
Resolution", RFC 2169, June 1997.
[7] Mealling, M., Daniel, R., "Resolution of Uniform Resource
Identifiers using the Domain Name System", July 1999.
[8] IEEE, "IEEE Standard for Information Technology - Portable
Operating System Interface (POSIX) - Part 2: Shell and
Utilities (Vol. 1)", IEEE Std 1003.2-1992, January 1993.
[9] Berners-Lee, T., Fielding, R., Masinter, L., "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
Authors' Addresses
Michael Mealling
Network Solutions, Inc.
505 Huntmar Park Drive
Herndon, VA 22070
US
Phone: +1 770 935 5492
EMail: michaelm@netsol.com
URI: http://www.netsol.com
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Ron Daniel
DATAFUSION, Inc.
139 Townsend Street, Ste. 100
San Francisco, CA 94107
US
Phone: +1 415 222 0100
EMail: rdaniel@datafusion.net
URI: http://www.datafusion.net
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Appendix A. IANA Considerations
The only registration function that impacts the IANA is for the
values that are standardized for the Services and Flags fields. To
extend the valid values of the Flags field beyond what is specified
in this document requires a published specification that is approved
by the IESG.
The values for the Services field will be determined by the
application that makes use of the NAPTR record. Those values must be
specified in a published specification and approved by the IESG.
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Appendix B. Security Considerations
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 service, but has not
discussed any detail of how the communication with that service
takes place. There are significant security considerations attached
to the communication with a service. Those considerations are
outside the scope of this document, and must be addressed by the
specifications for particular communication protocols.
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Full Copyright Statement
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