Network Working Group M. Mealling

Network Working Group M. Mealling
Request for Comments: 3403 VeriSign
Obsoletes: 2915, 2168 October 2002
Category: Standards Track

Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

This document describes a Dynamic Delegation Discovery System (DDDS)
Database using the Domain Name System (DNS) as a distributed database
of Rules. The Keys are domain-names and the Rules are encoded using
the Naming Authority Pointer (NAPTR) Resource Record (RR).

Since this document obsoletes RFC 2915, it is the official
specification for the NAPTR DNS Resource Record. It is also part of
a series that is completely specified in "Dynamic Delegation
Discovery System (DDDS) Part One: The Comprehensive DDDS" (RFC 3401).
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
3. DDDS Database Specification . . . . . . . . . . . . . . . . 3
4. NAPTR RR Format . . . . . . . . . . . . . . . . . . . . . . 5
4.1 Packet Format . . . . . . . . . . . . . . . . . . . . . . . 5
4.2 Additional Information Processing . . . . . . . . . . . . . 7
4.2.1 Additional Section processing by DNS servers . . . . . . . . 7
4.2.2 Additional Section processing by resolver/applications . . . 7
4.3 Master File Format . . . . . . . . . . . . . . . . . . . . . 7
5. Application Specifications . . . . . . . . . . . . . . . . . 8
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1 URN Example . . . . . . . . . . . . . . . . . . . . . . . . 8
6.2 E164 Example . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Advice for DNS Administrators . . . . . . . . . . . . . . . 10
8. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 11
10. Security Considerations . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . 13
Full Copyright Statement . . . . . . . . . . . . . . . . . . 14

1. Introduction

The Dynamic Delegation Discovery System (DDDS) 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 the way in which the Domain Name System (DNS)
is used as a data store for the Rules that allow a DDDS Application
to function. It does not specify any particular application or usage
scenario. The entire series of documents is specified in "Dynamic
Delegation Discovery System (DDDS) Part One: The Comprehensive DDDS"
(RFC 3401) [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.

The Naming Authority Pointer (NAPTR) DNS Resource Record (RR)
specified here was originally produced by the URN Working Group as a
way to encode rule-sets in DNS so that the delegated sections of a
Uniform Resource Identifiers (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.

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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.

Over time this process was generalized for other Applications and
Rule Databases. This document defines a Rules Database absent any
particular Application as there may be several Applications all
taking advantage of this particular Rules Database.

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 [6].

All other terminology, especially capitalized terms, is taken from
[3].

3. DDDS Database Specification

General Description:
This database uses the Domain Name System (DNS) as specified in
[8] and [7].

The character set used to specify the various values of the NAPTR
records is UTF-8 [17]. Care must be taken to ensure that, in the
case where either the input or the output to the substitution
expression contains code points outside of the ASCII/Unicode
equivalence in UTF-8, any UTF-8 is interpreted as a series of
code-points instead of as a series of bytes. This is to ensure
that the internationalized features of the POSIX Extended Regular
Expressions are able to match their intended code-points.
Substitution expressions MUST NOT be written where they depend on
a specific POSIX locale since this would cause substitution
expressions to loose their ability to be universally applicable.

All DNS resource records have a Time To Live (TTL) associated with
them. When the number of seconds has passed since the record was
retrieved the record is no longer valid and a new query must be
used to retrieve the new records. Thus, as mentioned in the DDDS
Algorithm, there can be the case where a given Rule expires. In
the case where an application attempts to fall back to previously
retrieved sets of Rules (either in the case of a bad delegation
path or some network or server failure) the application MUST
ensure that none of the records it is relying on have expired. In
the case where even a single record has expired, the application
is required to start over at the beginning of the algorithm.

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Key Format:
A Key is a validly constructed DNS domain-name.

Lookup Request:
In order to request a set of rules for a given Key, the client
issues a request, following standard DNS rules, for NAPTR Resource
Records for the given domain-name.

Lookup Response:
The response to a request for a given Key (domain-name) will be a
series of NAPTR records. The format of a NAPTR Resource Record
can be found in Section 4.

Rule Insertion Procedure:
Rules are inserted by adding new records to the appropriate DNS
zone. If a Rule produces a Key that exists in a particular zone
then only the entity that has administrative control of that zone
can specify the Rule associated with that Key.

Collision Avoidance:
In the case where two Applications may use this Database (which is
actually the case with the ENUM and URI Resolution Applications,
Section 6.2), there is a chance of collision between rules where
two NAPTR records appear in the same domain but they apply to more
than one Application. There are three ways to avoid collisions:

* create a new zone within the domain in common that contains
only NAPTR records that are appropriate for the application.
E.g., all URI Resolution records would exist under
urires.example.com and all ENUM records would be under
enum.example.com. In the case where this is not possible due
to lack of control over the upstream delegation the second
method is used.

* write the regular expression such that it contains enough of
the Application Unique string to disambiguate it from any
other. For example, the URI Resolution Application would be
able to use the scheme name on the left hand side to anchor the
regular expression match to that scheme. An ENUM specific
record in that same zone would be able to anchor the left hand
side of the match with the "+" character which is defined by
ENUM to be at the beginning of every Application Unique String.
This way a given Application Unique String can only match one
or the other record, not both.

* if two Application use different Flags or Services values then
a record from another Application will be ignored since it
doesn't apply to the Services/Flags in question.

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4. NAPTR RR Format

4.1 Packet Format

The packet format of the NAPTR RR is given below. The DNS type code
for NAPTR is 35.

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 /
/ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

<character-string> and <domain-name> as used here are defined in RFC
1035 [7].

ORDER
A 16-bit unsigned integer specifying the order in which the NAPTR
records MUST be processed in order to accurately represent the
ordered list of Rules. The ordering is from lowest to highest.
If two records have the same order value then they are considered
to be the same rule and should be selected based on the
combination of the Preference values and Services offered.

PREFERENCE
Although it is called "preference" in deference to DNS
terminology, this field is equivalent to the Priority value in the
DDDS Algorithm. It is 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 supporting some protocol or service very well.

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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. The only exception to this is noted in
the second important Note in the DDDS algorithm specification
concerning allowing clients to use more complex Service
determination between steps 3 and 4 in the algorithm. Preference
is used to give communicate a higher quality of service to rules
that are considered the same from an authority standpoint but not
from a simple load balancing standpoint.

It is important to note that DNS contains several load balancing
mechanisms and if load balancing among otherwise equal services
should be needed then methods such as SRV records or multiple A
records should be utilized to accomplish load balancing.

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-Z and 0-9. The case of the
alphabetic characters is not significant. The field can be empty.

It is up to the Application specifying how it is using this
Database to define the Flags in this field. It must define which
ones are terminal and which ones are not.

SERVICES
A <character-string> that specifies the Service Parameters
applicable to this this delegation path. It is up to the
Application Specification to specify the values found in this
field.

REGEXP
A <character-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. See the DDDS Algorithm
specification for the syntax of this field.

As stated in the DDDS algorithm, 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.

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REPLACEMENT
A <domain-name> which is the next domain-name to query for
depending on the potential values found in the flags field. This
field is used when the regular expression is a simple replacement
operation. Any value in this field MUST be a fully qualified
domain-name. Name compression is not to be used for this field.

This field and the REGEXP field together make up the Substitution
Expression in the DDDS Algorithm. It is simply a historical
optimization specifically for DNS compression that this field
exists. The fields are also mutually exclusive. If a record is
returned that has values for both fields then it is considered to
be in error and SHOULD be either ignored or an error returned.

4.2 Additional Information Processing

Additional section processing requires upgraded DNS servers, thus it
will take many years before applications can expect to see relevant
records in the additional information section.

4.2.1 Additional Section Processing by DNS Servers

DNS servers MAY add RRsets to the additional information section that
are relevant to the answer and have the same authenticity as the data
in the answer section. Generally this will be made up of A and SRV
records but the exact records depends on the application.

4.2.2 Additional Section Processing by Resolver/Applications

Applications MAY inspect the Additional Information section for
relevant records but Applications MUST NOT require that records of
any type be in the Additional Information section of any DNS response
in order for clients to function. All Applications must be capable
of handling responses from nameservers that never fill in the
Additional Information part of a response.

4.3 Master File Format

The master file format follows the standard rules in RFC-1035. 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 7 for how to correctly enter and escape the regular
expression.

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5. Application Specifications

This DDDS Database is usable by any application that makes use of the
DDDS algorithm. In addition to the items required to specify a DDDS
Application, an application wishing to use this Database must also
define the following values:

o What domain the Key that is produced by the First Well Known Rule
belongs to. Any application must ensure that its rules do not
collide with rules used by another application making use of this
Database. For example, the 'foo' application might have all of
its First Well Known Keys be found in the 'foo.net' zone.

o What the allowed values for the Services and Protocols fields are.

o What the expected output is of the terminal rewrite rule in
addition to how the Flags are actually encoded and utilized.

6. Examples

6.1 URN Example

The NAPTR record was originally created for use with the Uniform
Resource Name (URN) Resolver Discovery Service (RDS) [15]. This
example details how a particular URN would use the NAPTR record to
find a resolver service that can answer questions about the URN. See
[2] for the definitive specification for this Application.

Consider a URN namespace based on MIME Content-Ids (this is very
hypothetical so do not rely on this). The URN might look like this:

urn:cid:[email protected]

This Application's First Well Known Rule is to extract the characters
between the first and second colon. For this URN that would be
'cid'. The Application also specifies that, in order to build a
Database-valid Key, the string 'urn.arpa' should be appended to the
result of the First Well Known Rule. The result is 'cid.urn.arpa'.
Next, the client queries the DNS for NAPTR records for the domain-
name 'cid.urn.arpa'. The result is a single record:

cid.urn.arpa.
;; order pref flags service regexp replacement
IN NAPTR 100 10 "" "" "!^urn:cid:.+@([^\.]+\.)(.*)$!\2!i" .

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Since there is only one record, 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 "example.com". Since the flags field
is empty, the lookup is not terminal and our next probe to DNS is for
more NAPTR records where the new domain is 'example.com'.

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 'bar', could have their very own
NAPTR, maintaining those records for all the machines at a site could
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 "example.com" might look like:

example.com.
;; order pref flags service regexp replacement
IN NAPTR 100 50 "a" "z3950+N2L+N2C" "" cidserver.example.com.
IN NAPTR 100 50 "a" "rcds+N2C" "" cidserver.example.com.
IN NAPTR 100 50 "s" "http+N2L+N2C+N2R" "" www.example.com.

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 Application defines the flag 'a' to mean a
terminal lookup and that the output of the rewrite will be a domain-
name for which an A record should be queried. Once the client has
done that, it has the following information: the host, its IP
address, the protocol, and the services available via that protocol.
Given these bits of information the client has enough to be able to
contact that server and ask it questions about the URN.

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 occurrences of a backslash must be escaped by
another backslash. For the case of the cid.urn.arpa 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".

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6.2 E164 Example

The ENUM Working Group in the IETF has specified a service that
allows a telephone number to be mapped to a URI [18]. The
Application Unique String for the ENUM Application is the E.164
telephone number with the dashes removed. The First Well Known Rule
is to remove all characters from the the telephone number and then
use the entire number as the first Key. For example, the phone
number "770-555-1212" represented as an E.164 number would be "+1-
770-555-1212". Converted to the Key it would be "17705551212".

The ENUM Application at present only uses this Database. It
specifies that, in order to convert the first Key into a form valid
for this Database, periods are inserted between each digit, the
entire Key is inverted and then "e164.arpa" is appended to the end.
The above telephone number would then read
"2.1.2.1.5.5.5.0.7.7.1.e164.arpa.". This domain-name is then used to
retrieve Rewrite Rules as NAPTR records.

For this example telephone number we might get back the following
NAPTR records:

$ORIGIN 2.1.2.1.5.5.5.0.7.7.1.e164.arpa.
IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!sip:[email protected]!i" .
IN NAPTR 102 10 "u" "smtp+E2U" "!^.*$!mailto:[email protected]!i" .

Both the ENUM [18] and URI Resolution [4] Applications use the 'u'
flag. This flag states that the Rule is terminal and that the output
is a URI which contains the information needed to contact that
telephone service. ENUM also uses the same format for its Service
Parameters. These state that the available protocols used to access
that telephone's service are either the Session Initiation Protocol
or SMTP mail.

7. 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.

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In order to mitigate zone file problems, administrators should
encourage those writing rewrite rules to utilize the 'default
delimiter' feature of the regular expression. In the DDDS
specification the regular expression starts with the character that
is to be the delimiter. Hence if the first character of the regular
expression is an exclamation mark ('!') for example then the regular
expression can usually be written with fewer backslashes.

8. 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 Service Parameter combination.

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.

If the lookup after a rewrite fails, clients are strongly encouraged
to report a failure, rather than backing up to pursue other rewrite
paths.

9. IANA Considerations

The values for the Services and Flags fields will be determined by
the Application that makes use of this DDDS Database. Those values
may require a registration mechanism and thus may need some IANA
resources. This specification by itself does not.

10. Security Considerations

The NAPTR record, like any other DNS record, can be signed and
validated according to the procedures specified in DNSSEC.

This Database makes 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 since arbitrary code can be included and
subsequently processed.

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References

[1] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.

[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.

[3] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
2002.

[4] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI) Resolution
Application", RFC 3404, October 2002.

[5] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Five: URI.ARPA Assignment Procedures", RFC 3405, October 2002.

[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[7] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.

[8] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.

[9] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.

[10] Crocker, D., "Augmented BNF for Syntax Specifications: ABNF",
RFC 2234, November 1997.

[11] Daniel, R., "A Trivial Convention for using HTTP in URN
Resolution", RFC 2169, June 1997.

[12] 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.

[13] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

[14] Moats, R., "URN Syntax", RFC 2141, May 1997.

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RFC 3403 DDDS DNS Database October 2002

[15] Sollins, K., "Architectural Principles of Uniform Resource Name
Resolution", RFC 2276, January 1998.

[16] Daniel, R. and M. Mealling, "Resolution of Uniform Resource
Identifiers using the Domain Name System", RFC 2168, June 1997.

[17] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.

[18] Faltstrom, P., "E.164 number and DNS", RFC 2916, September 2000.

Author's Address

Michael Mealling
VeriSign
21345 Ridgetop Circle
Sterling, VA 20166
US

EMail: [email protected]
URI: http://www.verisignlabs.com

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Full Copyright Statement

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Acknowledgement

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