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Keywords: DNS, domain name system, E.164
Network Working Group P. Faltstrom
Request for Comments: 2916 Cisco Systems Inc.
Category: Standards Track September 2000
E.164 number and DNS
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 (2000). All Rights Reserved.
Abstract
This document discusses the use of the Domain Name System (DNS) for
storage of E.164 numbers. More specifically, how DNS can be used for
identifying available services connected to one E.164 number.
Routing of the actual connection using the service selected using
these methods is not discussed.
1. Introduction
Through transformation of E.164 numbers into DNS names and the use of
existing DNS services like delegation through NS records, and use of
NAPTR [1] records in DNS [2] [3], one can look up what services are
available for a specific domain name in a decentralized way with
distributed management of the different levels in the lookup process.
1.1 Terminology
The key words "MUST", "REQUIRED", "SHOULD", "RECOMMENDED", and "MAY"
in this document are to be interpreted as described in RFC2119 [4].
2. E.164 numbers and DNS
The domain "e164.arpa" is being populated in order to provide the
infrastructure in DNS for storage of E.164 numbers. In order to
facilitate distributed operations, this domain is divided into
subdomains. Holders of E.164 numbers which want to be listed in DNS
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RFC 2916 E.164 number and DNS September 2000
should contact the appropriate zone administrator in order to be
listed, by examining the SOA resource record associated with the
zone, just like in normal DNS operations.
Of course, as with other domains, policies for such listings will be
controlled on a subdomain basis and may differ in different parts of
the world.
To find the DNS names for a specific E.164 number, the following
procedure is to be followed:
1. See that the E.164 number is written in its full form, including
the countrycode IDDD. Example: +46-8-9761234
2. Remove all non-digit characters with the exception of the leading
'+'. Example: +4689761234
3. Remove all characters with the exception of the digits. Example:
4689761234
4. Put dots (".") between each digit. Example: 4.6.8.9.7.6.1.2.3.4
5. Reverse the order of the digits. Example: 4.3.2.1.6.7.9.8.6.4
6. Append the string ".e164.arpa" to the end. Example:
4.3.2.1.6.7.9.8.6.4.e164.arpa
2.1 Special note about the '+'
The '+' is kept in stage 2 in section 2 to flag that the number which
the regular expression is operating on is a E.164 number. Future
work will be needed to determine how other numbering plans (such as
closed ones) might be identified. It is possible, but not definite,
that they would use a similar mechanism as the one described in this
document.
3. Fetching URIs given an E.164 number
For a record in DNS, the NAPTR record is used for identifying
available ways of contacting a specific node identified by that name.
Specifically, it can be used for knowing what services exists for a
specific domain name, including phone numbers by the use of the
e164.arpa domain as described above.
The identification is using the NAPTR resource record defined for use
in the URN resolution process, but it can be generalized in a way
that suits the needs specified in this document.
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RFC 2916 E.164 number and DNS September 2000
It is the string which is the result of step 2 in section 2 above
which is input to the NAPTR algorithm.
3.1 The NAPTR record
The key fields in the NAPTR RR are order, preference, service, flags,
regexp, and replacement. For a detailed description, see:
o The order field specifies the order in which records MUST be
processed when multiple NAPTR records are returned in response to
a single query.
o The preference field specifies the order in which records SHOULD
be processed when multiple NAPTR records have the same value of
"order".
o 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.
o The flags field contains modifiers that affect what happens in the
next DNS lookup, typically for optimizing the process.
o The regexp field is one of two fields used for the rewrite rules,
and is the core concept of the NAPTR record.
o The replacement field is the other field that may be used for the
rewrite rule.
Note that the client applies all the substitutions and performs all
lookups, they are not performed in the DNS servers. Note that URIs
are stored in the regexp field.
3.1.1 Specification for use of NAPTR Resource Records
The input is an E.164 encoded telephone number. The output is a
Uniform Resource Identifier in its absolute form according to the
'absoluteURI' production in the Collected ABNF found in RFC2396 [5]
An E.164 number, without any characters but leading '+' and digits,
(result of step 2 in section 2 above) is the input to the NAPTR
algorithm.
The service supported for a call is E2U.
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RFC 2916 E.164 number and DNS September 2000
3.1.2 Specification of Service E2U (E.164 to URI)
* Name: E.164 to URI
* Mnemonic: E2U
* Number of Operands: 1
* Type of Each Operand: First operand is an E.164 number.
* Format of Each Operand: First operand is the E.164 number in the
form as specified in step 2 in section 2 in this document.
* Algorithm: Opaque
* Output: One or more URIs
* Error Conditions:
o E.164 number not in the numbering plan
o E.164 number in the numbering plan, but no URIs exist for
that number
o Service unavailable
* Security Considerations:
o Malicious Redirection
One of the fundamental dangers related to any service such
as this is that a malicious entry in a resolver's database
will cause clients to resolve the E.164 into the wrong URI.
The possible intent may be to cause the client to retrieve
a resource containing fraudulent or damaging material.
o Denial of Service
By removing the URI to which the E.164 maps, a malicious
intruder may remove the client's ability to access the
resource.
This operation is used to map a one E.164 number to a list of URIs.
The first well-known step in the resolution process is to remove all
non-digits apart from the leading '+' from the E.164 number as
described in step 1 and 2 in section 2 of this document.
3.2 Examples
3.2.1 Example 1
$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
IN NAPTR 100 10 "u" "sip+E2U" "!^.*$!sip:info@tele2.se!" .
IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:info@tele2.se!" .
This describes that the domain 4.3.2.1.6.7.9.8.6.4.e164.arpa is
preferably contacted by SIP, and secondly by SMTP.
In both cases, the next step in the resolution process is to use the
resolution mechanism for each of the protocols, (SIP and SMTP) to
know what node to contact for each.
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RFC 2916 E.164 number and DNS September 2000
3.2.2 Example 2
$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:paf@swip.net!" .
IN NAPTR 102 10 "u" "mailto+E2U" "!^.*$!mailto:paf@swip.net!" .
IN NAPTR 102 10 "u" "tel+E2U" "!^.*$!tel:+4689761234!" .
Note that the preferred method is to use the SIP protocol, but the
result of the rewrite of the NAPTR record is a URI (the "u" flag in
the NAPTR record). In the case of the protocol SIP, the URI might be
a SIP URI, which is resolved as described in RFC 2543 [6]. In the
case of the "tel" URI scheme [7], the procedure is restarted with
this new E.164 number. The client is responsible for loop detection.
The rest of the resolution of the routing is done as described above.
3.2.3 Example 3
$ORIGIN 6.4.e164.arpa.
* IN NAPTR 100 10 "u" "ldap+E2U" "!^+46(.*)$!ldap://ldap.se/cn=01!" .
We see in this example that information about all E.164 numbers in
the 46 countrycode (for Sweden) exists in an LDAP server, and the
search to do is specified by the LDAP URI [8].
4. IANA Considerations
This memo requests that the IANA delegate the E164.ARPA domain
following instructions to be provided by the IAB. Names within this
zone are to be delegated to parties according to the ITU
recommendation E.164. The names allocated should be hierarchic in
accordance with ITU Recommendation E.164, and the codes should
assigned in accordance with that Recommendation.
Delegations in the zone e164.arpa (not delegations in delegated
domains of e164.arpa) should be done after Expert Review, and the
IESG will appoint a designated expert.
5. Security Considerations
As this system is built on top of DNS, one can not be sure that the
information one get back from DNS is more secure than any DNS query.
To solve that, the use of DNSSEC [9] for securing and verifying zones
is to be recommended.
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RFC 2916 E.164 number and DNS September 2000
The caching in DNS can make the propagation time for a change take
the same amount of time as the time to live for the NAPTR records in
the zone that is changed. The use of this in an environment where
IP-addresses are for hire (for example, when using DHCP [11]) must
therefore be done very carefully.
There are a number of countries (and other numbering environments) in
which there are multiple providers of call routing and number/name-
translation services. In these areas, any system that permits users,
or putative agents for users, to change routing or supplier
information may provide incentives for changes that are actually
unauthorized (and, in some cases, for denial of legitimate change
requests). Such environments should be designed with adequate
mechanisms for identification and authentication of those requesting
changes and for authorization of those changes.
6. Acknowledgements
Support and ideas have come from people at Ericsson, Bjorn Larsson
and the group which implemented this scheme in their lab to see that
it worked. Input has also come from ITU-T SG2, Working Party 1/2
(Numbering, Routing, Global Mobility and Service Definition), the
ENUM working group in the IETF, John Klensin and Leif Sunnegardh.
References
[1] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
[2] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Berners-Lee, T., Fielding, R.T. and L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", RFC 2396, August
1998.
[6] Handley, M., Schulzrinne, H., Schooler, E. and J. Rosenberg,
"SIP: Session Initiation Protocol", RFC 2543, March 1999.
[7] Vaha-Sipila, A., "URLs for Telephone Calls", RFC 2806, April
2000.
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RFC 2916 E.164 number and DNS September 2000
[8] Howes, T. and M. Smith, "An LDAP URL Format", RFC 1959, June
1996.
[9] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[10] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[11] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
Author's Address
Patrik Faltstrom
Cisco Systems Inc
170 W Tasman Drive SJ-13/2
San Jose CA 95134
USA
EMail: paf@cisco.com
URI: http://www.cisco.com
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RFC 2916 E.164 number and DNS September 2000
Appendix A. Scenario
Say that the content of the e164.arpa zone is the following:
$ORIGIN e164.arpa.
6.4 IN NS ns.regulator-e164.example.se.
The regulator has in turn given a series of 10000 numbers to the
telco with the name Telco-A. The regulator because of that has in
his DNS.
$ORIGIN 6.4.e164.arpa.
6.7.9.8 IN NS ns.telco-a.example.se.
A user named Sven Svensson has from Telco A got the phone number
+46-8-9761234. The user gets the service of running DNS from the
company Redirection Service. Sven Svensson has asked Telco A to
point out Redirection Service as the authoritative source for
information about the number +46-8-9761234. Telco A because of this
puts in his DNS the following.
$ORIGIN 6.7.9.8.6.4.e164.arpa.
4.3.2.1 IN NS ns.redirection-service.example.se.
Sven Svensson has already plain telephony from Telco A, but also a
SIP service from the company Sip Service which provides Sven with
the SIP URI "sip:sven@sips.se". The ISP with the name
ISP A runs email and webpages for Sven, under the email address
sven@ispa.se, and URI http://svensson.ispa.se.
The DNS for the redirection service because of this contains the
following.
$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:sven@sips.se!" .
IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:sven@ispa.se!" .
IN NAPTR 10 10 "u" "http+E2U" "!^.*$!http://svensson.ispa.se!" .
IN NAPTR 10 10 "u" "tel+E2U" "!^.*$!tel:+46-8-9761234!" .
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RFC 2916 E.164 number and DNS September 2000
A user, John Smith, want to contact Sven Svensson, he to start with
only has the E.164 number of Sven, i.e. +46-8-9761234. He takes the
number, and enters the number in his communication client, which
happen to know how to handle the SIP protocol. The client removes
the dashes, and ends up with the E.164 number +4689761234. That is
what is used in the algorithm for NAPTR records, which is as
follows.
The client converts the E.164 number into the domain name
4.3.2.1.6.7.9.8.6.4.e164.arpa., and queries for NAPTR records for
this domainname. Using DNS mechanisms which includes following the
NS record referrals, the following records are returned:
$ORIGIN 4.3.2.1.6.7.9.8.6.4.e164.arpa.
IN NAPTR 10 10 "u" "sip+E2U" "!^.*$!sip:sven@sips.se" .
IN NAPTR 10 10 "u" "mailto+E2U" "!^.*$!mailto:sven@ispa.se" .
IN NAPTR 10 10 "u" "http+E2U" "!^.*$!http://svensson.ispa.se" .
IN NAPTR 10 10 "u" "tel+E2U" "!^.*$!tel:+46-8-9761234" .
Because the client knows sip, the first record above is selected,
and the regular expression "!^.*$!sip:sven@sips.se" is applied to
the original string, "+4689761234". The output is "sip:sven@sips.se"
which is used according to SIP resolution.
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