Network Working Group P. Faltstrom
Request for Comments: 3761 Cisco Systems, Inc.
Obsoletes: 2916 M. Mealling
Category: Standards Track VeriSign
April 2004 The E.164 to Uniform Resource Identifiers (URI)
Dynamic Delegation Discovery System (DDDS) Application (ENUM)
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 (C) The Internet Society (2004). All Rights Reserved.
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. It
specifically obsoletes RFC 2916 to bring it in line with the Dynamic
Delegation Discovery System (DDDS) Application specification found in
the document series specified in RFC 3401. It is very important to
note that it is impossible to read and understand this document
without reading the documents discussed in RFC 3401.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 21.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 31.2. Use for these mechanisms for private dialing plans. . . . 31.3. Application of local policy . . . . . . . . . . . . . . . 32. The ENUM Application Specifications . . . . . . . . . . . . . 42.1. Application Unique String . . . . . . . . . . . . . . . . 52.2. First Well Known Rule . . . . . . . . . . . . . . . . . . 52.3. Expected Output . . . . . . . . . . . . . . . . . . . . . 52.4. Valid Databases . . . . . . . . . . . . . . . . . . . . . 52.4.1. Flags. . . . . . . . . . . . . . . . . . . . . . . 62.4.2. Services Parameters. . . . . . . . . . . . . . . . 72.5. What constitutes an 'Enum Resolver'?. . . . . . . . . . . 83. Registration mechanism for Enumservices . . . . . . . . . . . 8
policy which is attached to the zone. One should start looking for
this information 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 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 BCP 14, RFC 2119 .
All other capitalized terms are taken from the vocabulary found in
the DDDS algorithm specification found in RFC 3403 .
1.2. Use for these mechanisms for private dialing plans
This document describes the operation of these mechanisms in the
context of numbers allocated according to the ITU-T recommendation
E.164. The same mechanisms might be used for private dialing plans.
If these mechanisms are re-used, the suffix used for the private
dialing plan MUST NOT be e164.arpa, to avoid conflict with this
specification. Parties to the private dialing plan will need to know
the suffix used by their private dialing plan for correct operation
of these mechanisms. Further, the application unique string used
SHOULD be the full number as specified, but without the leading '+',
and such private use MUST NOT be called "ENUM".
1.3. Application of local policy
The Order field in the NAPTR record specifies in what order the DNS
records are to be interpreted. This is because DNS does not
guarantee the order of records returned in the answer section of a
DNS packet. In most ENUM cases this isn't an issue because the
typical regular expression will be '!^.*$!' since the first query
often results in a terminal Rule.
But there are other cases (non-terminal Rules) where two different
Rules both match the given Application Unique String. As each Rule
is evaluated within the algorithm, one may match a more significant
piece of the AUS than the other. For example, by using a non-
terminal NAPTR a given set of numbers is sent to some private-
dialing-plan-specific zone. Within that zone there are two Rules
that state that if a match is for the entire exchange and the service
is SIP related then the first, SIP-specific rule is used. But the
other Rule matches a longer piece of the AUS, specifying that for
some other service (instant messaging) that the Rule denotes a
departmental level service. If the shorter matching Rule comes
before the longer match, it can 'mask' the other rules. Thus, the
order in which each Rule is tested against the AUS is an important
corner case that many DDDS applications take advantage of.
In the case where the zone authority wishes to state that two Rules
have the same effect or are identical in usage, then the Order for
those records is set to the same value. In that case, the Preference
is used to specify a locally over-ridable suggestion by the zone
authority that one Rule might simply be better than another for some
For ENUM this specifies where a client is allowed to apply local
policy and where it is not. The Order field in the NAPTR is a
request from the holder of the E.164 number that the records be
handled in a specific way. The Preference field is merely a
suggestion from that E.164 holder that one record might be better
than another. A client implementing ENUM MUST adhere to the Order
field but can simply take the Preference value "on advisement" as
part of a client context specific selection method.
2. The ENUM Application Specifications
This template defines the ENUM DDDS Application according to the
rules and requirements found in . The DDDS database used by this
Application is found in  which is the document that defines the
NAPTR DNS Resource Record type.
ENUM is only applicable for E.164 numbers. ENUM compliant
applications MUST only query DNS for what it believes is an E.164
number. Since there are numerous dialing plans which can change over
time, it is probably impossible for a client application to have
perfect knowledge about every valid and dialable E.164 number.
Therefore a client application, doing everything within its power,
can end up with what it thinks is a syntactically correct E.164
number which in reality is not actually valid or dialable. This
implies that applications MAY send DNS queries when, for example, a
user mistypes a number in a user interface. Because of this, there
is the risk that collisions between E.164 numbers and non-E.164
numbers can occur. To mitigate this risk, the E2U portion of the
service field MUST NOT be used for non-E.164 numbers.
2.1. Application Unique String
The Application Unique String is a fully qualified E.164 number minus
any non-digit characters except for the '+' character which appears
at the beginning of the number. The "+" is kept to provide a well
understood anchor for the AUS in order to distinguish it from other
telephone numbers that are not part of the E.164 namespace.
For example, the E.164 number could start out as "+44-116-496-0348".
To ensure that no syntactic sugar is allowed into the AUS, all non-
digits except for "+" are removed, yielding "+441164960348".
2.2. First Well Known Rule
The First Well Known Rule for this Application is the identity rule.
The output of this rule is the same as the input. This is because
the E.164 namespace and this Applications databases are organized in
such a way that it is possible to go directly from the name to the
smallest granularity of the namespace directly from the name itself.
Take the previous example, the AUS is "+441164960348". Applying the
First Well Known Rule produces the exact same string,
2.3. Expected Output
The output of the last DDDS loop is a Uniform Resource Identifier in
its absolute form according to the 'absoluteURI' production in the
Collected ABNF found in RFC2396 .
2.4. Valid Databases
At present only one DDDS Database is specified for this Application.
"Dynamic Delegation Discovery System (DDDS) Part Three: The DNS
Database" (RFC 3403)  specifies a DDDS Database that uses the
NAPTR DNS resource record to contain the rewrite rules. The Keys for
this database are encoded as domain-names.
The output of the First Well Known Rule for the ENUM Application is
the E.164 number minus all non-digit characters except for the +. In
order to convert this to a unique key in this Database the string is
converted into a domain-name according to this algorithm:
1. Remove all characters with the exception of the digits. For
example, the First Well Known Rule produced the Key
"+442079460148". This step would simply remove the leading "+",
2. Put dots (".") between each digit. Example:
3. Reverse the order of the digits. Example:
4. Append the string ".e164.arpa" to the end. Example:
This domain-name is used to request NAPTR records which may contain
the end result or, if the flags field is blank, produces new keys in
the form of domain-names from the DNS.
Some nameserver implementations attempt to be intelligent about items
that are inserted into the additional information section of a given
DNS response. For example, BIND will attempt to determine if it is
authoritative for a domain whenever it encodes one into a packet. If
it is, then it will insert any A records it finds for that domain
into the additional information section of the answer until the
packet reaches the maximum length allowed. It is therefore
potentially useful for a client to check for this additional
information. It is also easy to contemplate an ENUM enhanced
nameserver that understand the actual contents of the NAPTR records
it is serving and inserts more appropriate information into the
additional information section of the response. Thus, DNS servers
MAY interpret Flag values and use that information to include
appropriate resource records in the Additional Information portion of
the DNS packet. Clients are encouraged to check for additional
information but are not required to do so. See the Additional
Information Processing section of RFC 3403 , Section 4.2 for more
information on NAPTR records and the Additional Information section
of a DNS response packet.
The character set used to encode the substitution expression is UTF-
8. The allowed input characters are all those characters that are
allowed anywhere in an E.164 number. The characters allowed to be in
a Key are those that are currently defined for DNS domain-names.
This Database contains a field that contains flags that signal when
the DDDS algorithm has finished. At this time only one flag, "U", is
defined. This means that this Rule is the last one and that the
output of the Rule is a URI . See RFC 3404 .
If a client encounters a record with an unknown flag, it MUST ignore
it and move to the next Rule. This test takes precedence over any
ordering since flags can control the interpretation placed on fields.
A novel flag might change the interpretation of the regexp and/or
replacement fields such that it is impossible to determine if a
record matched a given target.
If this flag is not present then this rule is non-terminal. If a
Rule is non-terminal then clients MUST use the Key produced by this
Rewrite Rule as the new Key in the DDDS loop (i.e., causing the
client to query for new NAPTR records at the domain-name that is the
result of this Rule).
2.4.2. Services Parameters
Service Parameters for this Application take the following form and
are found in the Service field of the NAPTR record.
service-field = "E2U" 1*(servicespec)
servicespec = "+" enumservice
enumservice = type 0*(subtypespec)
subtypespec = ":" subtype
type = 1*32(ALPHA / DIGIT)
subtype = 1*32(ALPHA / DIGIT)
In other words, a non-optional "E2U" (used to denote ENUM only
Rewrite Rules in order to mitigate record collisions) followed by 1
or more or more Enumservices which indicate what class of
functionality a given end point offers. Each Enumservice is
indicated by an initial '+' character.
184.108.40.206. ENUM Services
Enumservice specifications contain the functional specification
(i.e., what it can be used for), the valid protocols, and the URI
schemes that may be returned. Note that there is no implicit mapping
between the textual string "type" or "subtype" in the grammar for the
Enumservice and URI schemes or protocols. The mapping, if any, must
be made explicit in the specification for the Enumservice itself. A
registration of a specific Type also has to specify the Subtypes
The only exception to the registration rule is for Types and Subtypes
used for experimental purposes, and those are to start with the facet
"X-". These elements are unregistered, experimental, and should be
used only with the active agreement of the parties exchanging them.
The registration mechanism is specified in Section 3.
2.5. What constitutes an 'Enum Resolver'?
There has been some confusion over what exactly an ENUM Resolver
returns and what relation that has to the 'Note 1' section in RFC
3402. On first reading it seems as though it might be possible for
an ENUM Resolver to return two Rules.
The ENUM algorithm always returns a single rule. Specific
applications may have application-specific knowledge or facilities
that allow them to present multiple results or speed selection, but
these should never change the operation of the algorithm.
3. Registration mechanism for Enumservices
As specified in the ABNF found in Section 2.4.2, an 'enumservice' is
made up of 'types' and 'subtypes'. For any given 'type', the
allowable 'subtypes' must be specified in the registration. There is
currently no concept of a registered 'subtype' outside the scope of a
given 'type'. Thus the registration process uses the 'type' as its
main key within the IANA Registry. While the combination of each
type and all of its subtypes constitutes the allowed values for the
'enumservice' field, it is not sufficient to simply document those
values. A complete registration will also include the allowed URI
schemes, a functional specification, security considerations,
intended usage, and any other information needed to allow for
interoperability within ENUM. In order to be a registered ENUM
Service, the entire specification, including the template, requires
approval by the IESG and publication of the Enumservice registration
specification as an RFC.
3.1. Registration Requirements
Service registration proposals are all expected to conform to various
requirements laid out in the following sections.
3.1.1. Functionality Requirement
A registered Enumservice must be able to function as a selection
mechanism when choosing one NAPTR resource record from another. That
means that the registration MUST specify what is expected when using
that very NAPTR record, and the URI which is the outcome of the use
Specifically, a registered Enumservice MUST specify the URI scheme(s)
that may be used for the Enumservice, and, when needed, other
information which will have to be transferred into the URI resolution
process itself (LDAP Distinguished Names, transferring of the AUS
into the resulting URI, etc).
3.1.2. Naming requirement
An Enumservice MUST be unique in order to be useful as a selection
criteria. Since an Enumservice is made up of a type and a type-
dependent subtype, it is sufficient to require that the 'type' itself
be unique. The 'type' MUST be unique, conform to the ABNF specified
in Section 2.4.2, and MUST NOT start with the facet "X-" which is
reserved for experimental, private use.
The subtype, being dependent on the type, MUST be unique within a
given 'type'. It must conform to the ABNF specified in Section
2.4.2, and MUST NOT start with the facet "X-" which is reserved for
experimental, private use. The subtype for one type MAY be the same
as a subtype for a different registered type but it is not sufficient
to simply reference another type's subtype. The function of each
subtype must be specified in the context of the type being
3.1.3. Security requirement
An analysis of security issues is required for all registered
Enumservices. (This is in accordance with the basic requirements for
all IETF protocols.)
All descriptions of security issues must be as accurate as possible
regardless of registration tree. In particular, a statement that
there are "no security issues associated with this Enumservice" must
not be confused with "the security issues associated with this
Enumservice have not been assessed".
There is no requirement that an Enumservice must be secure or
completely free from risks. Nevertheless, all known security risks
must be identified in the registration of an Enumservice.
The security considerations section of all registrations is subject
to continuing evaluation and modification.
Some of the issues that should be looked at in a security analysis of
an Enumservice are:
1. Complex Enumservices may include provisions for directives that
institute actions on a user's resources. In many cases provision
can be made to specify arbitrary actions in an unrestricted
fashion which may then have devastating results. Especially if
there is a risk for a new ENUM lookup, and because of that an
infinite loop in the overall resolution process of the E.164.
2. Complex Enumservices may include provisions for directives that
institute actions which, while not directly harmful, may result in
disclosure of information that either facilitates a subsequent
attack or else violates the users privacy in some way.
3. An Enumservice might be targeted for applications that require
some sort of security assurance but do not provide the necessary
security mechanisms themselves. For example, an Enumservice could
be defined for storage of confidential security services
information such as alarm systems or message service passcodes,
which in turn require an external confidentiality service.
3.1.4. Publication Requirements
Proposals for Enumservices registrations MUST be published as one of
the following documents; RFC on the Standards Track, Experimental
RFC, or as a BCP.
IANA will retain copies of all Enumservice registration proposals and
"publish" them as part of the Enumservice Registration tree itself.
3.2. Registration procedure
3.2.1. IANA Registration
Provided that the Enumservice has obtained the necessary approval,
and the RFC is published, IANA will register the Enumservice and make
the Enumservice registration available to the community in addition
to the RFC publication itself.
220.127.116.11. Location of Enumservice Registrations
Enumservice registrations will be published in the IANA repository
and made available via anonymous FTP at the following URI:
18.104.22.168. Change Control
Change control of Enumservices stay with the IETF via the RFC
publication process. Especially, Enumservice registrations may not
be deleted; Enumservices which are no longer believed appropriate for
use can be declared OBSOLETE by publication of a new RFC and a change
to their "intended use" field; such Enumservice will be clearly
marked in the lists published by IANA.
3.2.2. Registration Template
Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)
Any other information that the author deems interesting:
Note: In the case where a particular field has no value, that field
is left completely blank, especially in the case where a given type
has no subtypes.
The examples below use theoretical services that contain Enumservices
which might not make sense, but that are still used for educational
purposes. For example, the protocol used is in some cases exactly
the same string as the URI scheme. That was the specification in RFC
2916, but this 'default' specification of an Enumservice is no longer
allowed. All Enumservices need to be registered explicitly by the
procedure specified in section Section 3.
NAPTR 10 100 "u" "E2U+sip" "!^.*$!sip:email@example.com!" .
NAPTR 10 101 "u" "E2U+h323" "!^.*$!h323:firstname.lastname@example.org!" .
NAPTR 10 102 "u" "E2U+msg" "!^.*$!mailto:email@example.com!" .
This describes that the domain 22.214.171.124.126.96.36.199.188.8.131.52.e164.arpa. is
preferably contacted by SIP, secondly via H.323 for voice, and
thirdly by SMTP for messaging. Note that the tokens "sip", "h323",
and "msg" are Types registered with IANA, and they have no implicit
connection with the protocols or URI schemes with the same names.
In all cases, the next step in the resolution process is to use the
resolution mechanism for each of the protocols, (specified by the URI
schemes sip, h323 and mailto) to know what node to contact for each.
5. IANA Considerations
RFC 2916 (which this document replaces) requested IANA to delegate
the E164.ARPA domain following instructions to be provided by the
IAB. The domain was delegated according to those instructions.
Names within this zone are to be delegated to parties according to
the ITU-T Recommendation E.164. The names allocated should be
hierarchic in accordance with ITU-T Recommendation E.164, and the
codes should be assigned in accordance with that Recommendation.
IAB is to coordinate with ITU-T TSB if the technical contact for the
domain e164.arpa is to change, as ITU-T TSB has an operational
working relationship with this technical contact which needs to be
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.
IANA has created a registry for Enumservices as specified in Section
3. Whenever a new Enumservice is registered by the RFC process in
the IETF, IANA is at the time of publication of the RFC to register
the Enumservice and add a pointer to the RFC itself.
6. Security Considerations
6.1. DNS Security
As ENUM uses DNS, which in its current form is an insecure protocol,
there is no mechanism for ensuring that the data one gets back is
authentic. As ENUM is deployed on the global Internet, it is
expected to be a popular target for various kind of attacks, and
attacking the underlying DNS infrastructure is one way of attacking
the ENUM service itself.
There are multiple types of attacks that can happen against DNS that
ENUM implementations should be aware of. The following threats are
taken from Threat Analysis Of The Domain Name System :
Some of the simplest threats against DNS are various forms of
packet interception: monkey-in-the-middle attacks, eavesdropping
on requests combined with spoofed responses that beat the real
response back to the resolver, and so forth. In any of these
scenarios, the attacker can simply tell either party (usually the
resolver) whatever it wants that party to believe. While packet
interception attacks are far from unique to DNS, DNS's usual
behavior of sending an entire query or response in a single
unsigned, unencrypted UDP packet makes these attacks particularly
easy for any bad guy with the ability to intercept packets on a
shared or transit network.
ID Guessing and Query Prediction
Since the ID field in the DNS header is only a 16-bit field and
the server UDP port associated with DNS is a well-known value,
there are only 2**32 possible combinations of ID and client UDP
port for a given client and server. Thus it is possible for a
reasonable brute force attack to allow an attacker to masquerade
as a trusted server. In most respects, this attack is similar to
a packet interception attack except that it does not require the
attacker to be on a transit or shared network.
Name-based attacks use the actual DNS caching behavior as a tool
to insert bad data into a victim's cache, thus potentially
subverting subsequent decisions based on DNS names. Most examples
occur with CNAME, NS and DNAME Resource Records as they redirect a
victim's query to another location. The common thread in all of
these attacks is that response messages allow the attacker to
introduce arbitrary DNS names of the attacker's choosing and
provide further information that the attacker claims is associated
with those names; unless the victim has better knowledge of the
data associated with those names, the victim is going to have a
hard time defending against this class of attacks.
Betrayal By A Trusted Server
Another variation on the packet interception attack is the trusted
server that turns out not to be so trustworthy, whether by
accident or by intent. Many client machines are only configured
with stub resolvers, and use trusted servers to perform all of
their DNS queries on their behalf. In many cases the trusted
server is furnished by the user's ISP and advertised to the client
via DHCP or PPP options. Besides accidental betrayal of this
trust relationship (via server bugs, successful server break-ins,
etc), the server itself may be configured to give back answers
that are not what the user would expect (whether in an honest
attempt to help the user or to further some other goal such as
furthering a business partnership between the ISP and some third
Denial of Service
As with any network service (or, indeed, almost any service of any
kind in any domain of discourse), DNS is vulnerable to denial of
service attacks. DNS servers are also at risk of being used as
denial of service amplifiers, since DNS response packets tend to
be significantly longer than DNS query packets.
Authenticated Denial of Domain Names
The existence of RR types whose absence causes an action other
than immediate failure (such as missing MX and SRV RRs, which fail
over to A RRs) constitutes a real threat. In the specific case of
ENUM, even the immediate failure of a missing RR can be considered
a problem as a method for changing call routing policy.
Because of these threats, a deployed ENUM service SHOULD include
mechanisms which ameliorate these threats. Most of these threats can
be solved by verifying the authenticity of the data via mechanisms
such as DNSSEC  once it is deployed. Others, such and Denial Of
Service attacks, cannot be solved by data authentication. It is
important to remember that these threats include not only the NAPTR
lookups themselves, but also the various records needed for the
services to be useful (for example NS, MX, SRV and A records).
Even if DNSSEC is deployed, a service that uses ENUM for address
translation should not blindly trust that the peer is the intended
party as all kind of attacks against DNS can not be protected against
with DNSSEC. A service should always authenticate the peers as part
of the setup process for the service itself and never blindly trust
any kind of addressing mechanism.
Finally, as an ENUM service will be implementing some type of
security mechanism, software which implements ENUM MUST be prepared
to receive DNSSEC and other standardized DNS security responses,
including large responses, EDNS0 signaling, unknown RRs, etc.
6.2. Caching Security
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 ) must
therefore be done very carefully.
6.3. Call Routing Security
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.
6.4. URI Resolution Security
A large amount of Security Issues have to do with the resolution
process itself, and use of the URIs produced by the DDDS mechanism.
Those have to be specified in the registration of the Enumservice
used, as specified in Section 3.1.3.
Support and ideas leading to RFC 2916 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 arrived from
ITU-T SG2, Working Party 1/2 (Numbering, Routing, Global Mobility and
Enumservice Definition), the ENUM working group in the IETF, John
Klensin and Leif Sunnegardh.
This update of RFC 2916 is created with specific input from: Randy
Bush, David Conrad, Richard Hill, Jon Peterson, Jim Reid, Joakim
Stralmark, Robert Walter and James Yu.
8. Changes since RFC 2916
Part from clarifications in the text in this document, the major
changes are two:
The document uses an explicit DDDS algorithm, and not only NAPTR
resource records in an "ad-hoc" mode. In reality this doesn't imply
any changes in deployed base of applications, as the algorithm used
for ENUM resolution is exactly the same.
The format of the service field has changed. The old format was of
the form "example+E2U", while the new format is "E2U+example".
Reason for this change have to with the added subtypes in the
enumservice, the ability to support more than one enumservice per
NAPTR RR, and a general agreement in the IETF that the main selector
between different NAPTR with the same owner (E2U in this case) should
9.1. Normative References
 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403, October
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Four: The Uniform Resource Identifiers (URI) Resolution
Application", RFC 3404, October 2002.
 Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
 ITU-T, "The International Public Telecommunication Number Plan",
Recommendation E.164, May 1997.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
One: The Comprehensive DDDS", RFC 3401, October 2002.
 Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Two: The Algorithm", RFC 3402, October 2002.
9.2. Informative References
 Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
 Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
 Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
System", Work in Progress, April 2004.
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