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Versions: (RFC 2916) 00 01 02 03 04 05 06 07 RFC 3761

ENUM                                                        P. Faltstrom
Internet-Draft                                         Cisco Systems Inc
Obsoletes: 2916 (if approved)                                M. Mealling
Expires: November 10, 2003                                      VeriSign
                                                            May 12, 2003

                The E.164 to URI DDDS Application (ENUM)

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."

   The list of current Internet-Drafts can be accessed at http://

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on November 10, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003). 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.

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Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Use for these mechanisms for private dialing plans . . . . .  3
   1.3   Application of local policy  . . . . . . . . . . . . . . . .  3
   2.    The ENUM Application Specifications  . . . . . . . . . . . .  5
   2.1   Application Unique String  . . . . . . . . . . . . . . . . .  5
   2.2   First Well Known Rule  . . . . . . . . . . . . . . . . . . .  5
   2.3   Expected Output  . . . . . . . . . . . . . . . . . . . . . .  5
   2.4   Valid Databases  . . . . . . . . . . . . . . . . . . . . . .  6
   2.4.1 Flags  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   2.4.2 Services Parameters  . . . . . . . . . . . . . . . . . . . .  7
   2.5   What constitutes an 'Enum Resolver'? . . . . . . . . . . . .  8
   3.    Registration mechanism for Enumservices  . . . . . . . . . . 10
   3.1   Registration Requirements  . . . . . . . . . . . . . . . . . 10
   3.1.1 Functionality Requirement  . . . . . . . . . . . . . . . . . 10
   3.1.2 Naming requirement . . . . . . . . . . . . . . . . . . . . . 10
   3.1.3 Security requirement . . . . . . . . . . . . . . . . . . . . 11
   3.1.4 Publication Requirements . . . . . . . . . . . . . . . . . . 12
   3.2   Registration procedure . . . . . . . . . . . . . . . . . . . 12
   3.2.1 IANA Registration  . . . . . . . . . . . . . . . . . . . . . 12
   3.2.2 Registration Template  . . . . . . . . . . . . . . . . . . . 12
   4.    Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.1   Example  . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   5.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 15
   6.    Security Considerations  . . . . . . . . . . . . . . . . . . 16
   6.1   DNS Security . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.2   Caching Security . . . . . . . . . . . . . . . . . . . . . . 18
   6.3   Call Routing Security  . . . . . . . . . . . . . . . . . . . 18
   6.4   URI Resolution Security  . . . . . . . . . . . . . . . . . . 18
   7.    Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 19
   8.    Changes since RFC 2916 . . . . . . . . . . . . . . . . . . . 20
         Normative References . . . . . . . . . . . . . . . . . . . . 21
         Non-normative references . . . . . . . . . . . . . . . . . . 22
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 22
         Intellectual Property and Copyright Statements . . . . . . . 23

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1. Introduction

   Through transformation of International Public  Telecommunication
   Numbers in the international format [4], called within this document
   E.164 numbers, into DNS names and the use of existing DNS services
   like delegation through NS records and NAPTR records, 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.

   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
   should contact the appropriate zone administrator according to the
   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 world.

1.1 Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

   All other capitalized terms are taken from the vocabulary found in
   the DDDS algorithm specification found in RFC 3403 [1].

1.2 Use for these mechanisms for private dialing plans

   This document specifies how "ENUM" works, that is how to handle
   numbers allocated according to the ITU-T recommendation E.164. But, a
   similar mechanism can be used also for other numbers, such as private
   dialing plans.  To implement that (a) the suffix MUST be selected,
   MUST NOT be e164.arpa, MUST be known for all parties using the same
   dialing plan (b) the application unique string SHOULD be the full
   number as specified but without the leading '+'.

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

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   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.

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2. The ENUM Application Specifications

   This template defines the ENUM DDDS Application according to the
   rules and requirements found in [1]. The DDDS database used by this
   Application is found in [2] 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 "+1-770-923-9595".
   To ensure that no syntactic sugar is allowed into the AUS, all
   non-digits except for "+" are removed, yielding "+17709239595".

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 "+17709239595". Applying the
   First Well Known Rule produces the exact same string, "+17709239595".

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

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   Collected ABNF found in RFC2396 [3].

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) [2] 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
       "+4689761234". This step would simply remove the leading "+",
       producing "4689761234".

   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 [1], Section 4.2 for more information

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   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.

2.4.1 Flags

   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 [3]. See RFC 3404 [2].

   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.

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   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, have
   to be made explicit in the specification for the Enumservice itself.
   A registration of a specific Type also have 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 answer to that depends on
   which of two possible definitions you use for an ENUM Resolver:

   The first type of resolver can be called an 'intelligent' resolver.
   It understands its target application very well. In that case the
   test done at the beginning of Step 4 in the algorithm is a very
   complex one. It can include call backs to the calling thread, GUI
   events, etc. Its at this point that a client can be presented with
   the idea of "multiple Rules" because its at this step that Order is
   understood relative to the other ordered records. In this case the
   ENUM Resolver still returns one Rule to the calling application but
   its smart enough internally that it can apply application specific
   knowledge to the Rule selection test.

   The other type of resolver can be called a 'dumb' or 'driven'
   resolver. It is generic in the sense that there is no internal
   application knowledge. It is run from the 'outside' by a smart
   application that changes the selection criteria that are fed to the
   ENUM Resolver before it starts its resolution task. It returns one
   Rule only and the caller has to determine if that Rule is OK or not.
   If it isn't then it re-runs the resolver with a new set of selection
   criteria. Some might consider the combination of this dumb resolver
   and the application that's driving it as some uber-ENUM-Resolver. In
   that case it can "return more than one Rule" because it can simply
   re-run the algorithm several times to collect an appropriate set of

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   Rules. But that uber-ENUM-Resolver is stretching the definition of an
   ENUM Resolver to the point of being unrecognizable.

   The key point is that the Algorithm only returns one Rule.

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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
   of it.

   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 transfered into the URI resolution
   process itself (LDAP Distinguished Names, transferring of the AUS
   into the resulting URI, etc).

3.1.2 Naming requirement

   The Enumservice MUST be unique in order to be useful as a selection
   criteria. Since the 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,

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   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 associates with this
   Enumservice have not been assessed".

   There is no requirement that Enumservices registered must be secure
   or completely free from risks.  Nevertheless, all known security
   risks must be identified in the registration of a 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
   a 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, a 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.

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3.1.4 Publication Requirements

   Proposals for Enumservices registered must be published as RFCs on
   the Standards Track or as a BCP. IANA will retain copies of all
   Enumservice registration proposals and "publish" them as part of the
   ENUM 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. Location of ENUM Enumservice Registrations

   Enumservice registrations will be published in the IANA repository
   and made available via anonymous FTP at the following URI: "ftp://
   ftp.iana.org/assignments/enum-services/". 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

   Enumservice Type:

   Enumservice Subtype(s):

   URI Scheme(s):

   Functional Specification:

   Security considerations:

   Intended usage: (One of COMMON, LIMITED USE or OBSOLETE)


   Any other information that the author deems interesting:

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   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.

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4. Examples

   The examples below use theoretical services which uses Enumservices
   which might not make sense, but they are still used for educational
   purposes.  For example, the protocol used is in some cases exactly
   the the same string as the URI scheme.  That was the specification in
   RFC 2916, but this default specification of a Enumservice is no
   longer allowed. All Enumservices need to be registered explicitly by
   the procedure specified in section Section 3.

4.1 Example

     IN NAPTR 10 100 "u" "E2U+sip"
"!^.*$!sip:info@example.com!"     .
     IN NAPTR 10 101 "u" "E2U+h323"
"!^.*$!h323:info@example.com!"    .
     IN NAPTR 10 102 "u" "E2U+msg:mailto"
"!^.*$!mailto:info@example.com!"  .

   This describes that the domain is
   preferably contacted by SIP, secondly via H.323 for voice, and
   thirdly by SMTP for messaging. Note that the tokens "sip", "msg",
   "h323", "voice" and "mailto" are Types and Subtypes 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.

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5. 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-T
   Recommendation E.164.  The names allocated should be hierarchic in
   accordance with ITU-T Recommendation E.164, and the codes should
   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 is to create a registry for ENUM Enumservices as specified in
   Section 3. Whenever a new ENUM 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.

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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 [9]:

   Packet Interception
      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 brut 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
      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

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      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 [7] 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 which 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.

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   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 [8]) 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 ENUM
   Enumservice used, as specified in Section 3.1.3.

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7. Acknowledgments

   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.

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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 be

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Normative References

   [1]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Three: The DNS Database", RFC 3403, February 2002.

   [2]  Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
        Four: The URI Resolution Application", RFC 3404, February 2002.

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

   [4]  ITU-T, "The International Public Telecommunication Number Plan",
        Recommendation E.164, May 1997.

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Non-normative references

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

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

   [7]  Eastlake, D., "Domain Name System Security Extensions", RFC
        2535, March 1999.

   [8]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [9]  Atkins, D. and R. Austein, "Threat Analysis Of The Domain Name
        System", draft-ietf-dnsext-dns-threats-01 (work in progress),
        February 2002.

Authors' Addresses

   Patrik Faltstrom
   Cisco Systems Inc
   273 71 Lovestad

   EMail: paf@cisco.com
   URI:   http://www.cisco.com

   Michael Mealling
   21345 Ridgetop Circle
   Sterling, VA  20166

   EMail: michael@neonym.net
   URI:   http://www.verisignlabs.com

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