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Versions: 00 01                                                         
Internet Draft                        J. Levine, A. DeKok, et al.
Expiration: August 8, 2004     Taughannock Networks and IDT, Inc.
Anti-Spam Research Group                         February 8, 2004

     Lightweight MTA Authentication Protocol (LMAP) Discussion
                           and Comparison

Status of this Memo

   This document is an Internet-Draft and is subject to 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-

   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

   The list of Internet-Draft Shadow Directories can be accessed
   at http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 8, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights


   Lightweight MTA Authentication Protocol (LMAP) is the general
   term for a a family of proposed protocols to help address the
   spam problem by better authenticating mail senders.  This
   document discusses the applicability, and the costs and
   benefits of wide-spread deployment of the protocol, and
   compares the various LMAP proposals.

Table of Contents

   1. Introduction ............................................ 2
     1.1. Summary of the Protocols ............................ 3
     1.2. Interpretation of LMAP Data  ........................ 4

   2. Problem Statement and Scope ............................. 4
     2.1. Types of forgery .................................... 5

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     2.2. Choice of data to authenticate ...................... 5
       2.2.1. IP address ...................................... 6
       2.2.2. HELO/EHLO name .................................. 6
       2.2.3. Return path ..................................... 6
       2.2.4. Message header fields ........................... 6
       2.2.5. Comparison ...................................... 7
     2.3. DNS based attacks ................................... 8

   3. Common Concerns with LMAP ............................... 8
     3.1. LMAP and the end to end nature of the Internet ...... 8
     3.2. Roaming Users and LMAP .............................. 9
       3.2.1. SUBMIT (port 587) ............................... 9
       3.2.2. SMTP relaying to the home provider ............. 10
       3.2.3. Publish LMAP information ....................... 10
       3.2.4. Virtual Private Networks (VPNs) ................ 10
       3.2.5. Other message delivery systems ................. 10
     3.3. Message relaying and forwarding are affected by LMAP 10
     3.4. Kiosk, greeting card, and mail-a-link systems ...... 11

   4. Comparison of Proposals ................................ 11
     4.1. DNS record types ................................... 12
     4.2. Record structure ................................... 12
     4.3. Indirect addressing ................................ 12
     4.4. Address index structure ............................ 13
     4.5. Roaming and forwarding support ..................... 13
     4.6. Number of queries .................................. 13
     4.7. Extensibility ...................................... 14

   5. Privacy Considerations ................................. 14
     5.1. End Users .......................................... 14
     5.2. Network Infrastructure ............................. 15
     5.3. Traffic analysis ................................... 15

   6. Security Considerations ................................ 15

   7. Informative References ................................. 16

   8. Normative References ................................... 16

   9. Authors' Addresses ..................................... 17

1. Introduction

   The Anti Spam Research Group (ASRG) was chartered to address
   the spam problem.  The LMAP family of protocols falls with the
   scope of "changes to existing applications" as described in
   the charter.

   The intended audience for this document includes
   administrators and developers of DNS and SMTP systems.  The
   audience are assumed to be familiar with the workings of SMTP
   [RFC2821] and DNS [RFC1034].

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   The document is organized as follows.  Section 2 describes the
   problem statement and scope of the proposal.  Section 3 deals
   with the most commonly raised concerns associated with LMAP,
   and prior variants thereof.

   This document is intended to evolve, based on comments from
   the Anti-Spam Research Group (ASRG) mailing list.  It is
   certain that the current draft is incomplete and entirely
   possible that it is inaccurate. Hence, comments are eagerly
   sought, preferably in the form of suggested text changes, and
   preferably on the ASRG mailing list, at <asrg@ietf.org>.

   These protocols are experimental and is not suitable for
   widespread deployment or production use.

   We wish to remind readers that implementation and use of LMAP
   is entirely optional and not required to operate SMTP
   services.  This document contains minor updates to the
   semantics of parts of RFC 2821.

   Readers are further reminded that recipients have the right to
   refuse any communication from anyone, for any reason.

1.1. Summary of the Protocols

   LMAP is based on two concepts: publication of authentication
   data by a domain, and application of that data by a recipient
   MTA.  The combination of these concepts permits SMTP
   recipients to establish more reliably whether mail putatively
   from a domain is actually from that domain and that there is a
   responsible contact in case of questions or problems with the
   domain's mail.

   The data published by a domain includes statements as to which
   IP's are permitted to originate mail from the domain in SMTP

   SMTP recipients can look up the authentication data when a
   domain name is used in EHLO/HELO and/or MAIL FROM.  The
   recipient can then choose to apply the requested policy to the
   message.  The result is that messages are delivered only when
   all parties consent to their delivery.  The message originator
   must have the consent of the domain to claim association with
   that domain, and the recipient can verify that that consent
   exists.  After all, if the domain does not consent to an
   originator claiming association with it, there are few reasons
   why a recipient would choose to accept that non-consensual

   The method of establishing whether an IP address can send
   messages for a domain is similar to the usage of Dialup User
   Lists (DUL) such as the MAPS DUL.  With those methods, a
   network provider publishes a list of IP addresses which have
   been assigned to dial-up users.  SMTP recipients may query

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   such lists, and assume that the existence of an IP address on
   the list means that the network provider did not intend SMTP
   traffic to originate from that IP.

   Similarly, methods such as sender callback attempt to discover
   the implied intentions of a domain, by performing certain
   queries to that domain, but such methods only perform a weak
   substitute for authentication.

   This proposal makes all such authentication explicit, through
   the use of published data by a domain.  Publication makes the
   Internet more open.  Less information is hidden, and fewer
   erroneous implications are arrived at.

   Note that recipients should also publish that they use/enforce
   LMAP.  Receiving mail transport agents using protocols with
   the ability to advertise capabilities should advertise a
   capability to the sender that informs the that the sender that
   the receiver will check the incoming IP address with LMAP.  It
   is to the advantage of all parties for a sender that will not
   be able to pass LMAP authentication to be able to discover the
   fact as early as possible and abort the transmission.

   This verification scheme is weaker than cryptographic systems
   but stronger than the current SMTP model.

   These proposals change the semantics of the MAIL FROM command
   as defined in RFC 2821, section 3.3. to imply that the domain
   in the source mailbox is also the responsible party for
   sending the message, and thus must be verified.

1.2. Interpretation of LMAP Data

   Recipient MTAs are free to interpret LMAP data as they wish.
   Possibilities range from rejecting email with a 550 error code
   to using LMAP data as one input to a multi-criterion filter.
   Domains may also optionally use LMAP data to whitelist or give
   higher passing values for email in their filters.

   E-mail from LMAP domains that do not publish LMAP data should
   NOT be rejected since there is no requirement that domains do
   so, and many will not, either for policy reasons or from lack
   of resources.  E-email from non-LMAP domains should be treated
   as e-mail is treated today.

   The local policy decisions remain with the recipient's MTA.
   Readers are reminded that recipients have the right to refuse
   any communication from anyone, for any reason.

2. Problem Statement and Scope

   SMTP, as defined in RFC 821 and RFC 2821, provides no
   authentication at all when transferring mail.  In the early
   years of the Internet, when the net was small and all networks

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   enforced acceptable use policies, that didn't present a
   problem.  Since then, the size of the net has grown
   enormously, the number of networks has also grown, and
   policies vary greatly.  As a result, it has become possible
   and increasingly common for irresponsible users to forge
   addresses without the permission of the legitimate owners of
   those addresses.  The vast majority of mail with forged
   addresses is abusive for reasons beyond the forged addresses.
   Hence, measures to limit address forgery are likely also to
   limit the abuses associated with forgery.

   LMAP attacks the forgery problem by checking that the host
   from which the message was sent is authorized to send mail
   using the a domain in the message's envelope.  While this only
   deters a single category of forgery, the category it attacks
   is a large one, and by ensuring that at least one address is
   valid, recipients have both a reliable channel back to the
   domain's management, and provides a useful criterion for mail
   filtering, as described in the previous section.

2.1. Types of forgery

   Mail forgery is associated with several varieties of abusive

   * Senders of junk email (the largest category of spam), often
     forges return addresses.  It does so to make it harder to
     determine the responsible party, making it harder to tell to
     whom to complain.  It also does so to evade filters, either
     by pretending to be a sender on a recipient's whitelist, or
     to pretend not to be a sender on a recipient's blacklist.

   * In account fraud, also known as ``phishing'', a sender poses
     as a person or organization with whom the recipient has a
     business relationship, or would like to have a business
     relationship.  The sender does so in order to trick the
     recipient into revealing personal information that the
     sender can use for identity theft or related crimes.

   * In a ``joe job,'' a sender sends out abusive mail and forges
     the address of an unrelated party.  The goal is to discredit
     the party whose address is forged.

   * Viruses, trojans, worms, and related automated malware use
     forged return addresses to trick recipients into accepting
     or opening messages with hostile active content.  The goal
     is to get the recipients to run the active content, thereby
     propagating the malware.

2.2. Choice of data to authenticate

   When a message is sent via SMTP, the recipient MTA has a
   variety of items that it could use to authenticate the mail
   sender.  In the order that they are available to the recipient

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   MTA they are:

2.2.1 IP address

   The MTA can easily determine the IP address from which a mail
   message is sent.  Many parties publish DNS blacklists and DNS
   whitelists (DNSBLs and DNSWLs) with lists of IP addresses from
   which the publishers recommend that recipient MTAs reject or
   accept incoming mail.  DNSBLs are reasonably effective at
   identifying IP address ranges of networks with a history of
   sending abusive mail, of IP addresses of specific hosts with a
   history of sending abusive mail, typically because they are
   controlled by a trojan or other hostile software, and ranges
   of IP addresses assigned hosts whose users generally send mail
   via their ISP's mail servers and are not expected to send mail

   The MTA Mark[N3] and Selective Sender[N5] proposals are an
   extremely simple form of LMAP in which the owner of a range of
   IP addresses (or more precisely, the party that controls a
   range's reverse DNS, which should be the same as the network
   owner but is not always in practice) can identify which hosts
   in the network should be sending e-mail and which shouldn't.
   MTA Mark provides a further facility to publish a contact e-
   mail address or URL that recipients can use in case of
   questions or complaints about mail from a specific address.

2.2.2 HELO/EHLO name

   Section of RFC 2821 defines the HELO and EHLO commands
   which SMTP clients should use at the beginning of each SMTP
   session.  The argument to HELO or EHLO is a domain or a domain
   literal that should identify the sending host.

2.2.3 Return path

   Section of RFC 2821 defines the MAIL command which
   SMTP clients use to provide the reverse path for a message.
   The reverse path is usually an e-mail address, but in the case
   of a bounce message (or a message pretending to be a bounce
   message), the reverse path can be empty.

2.2.4 Message header fields

   RFC 2822 defines a list of message header fields including
   From:, Sender:, Resent-From:. and Resent-Sender: that include
   the e-mail addresses of the party or parties responsible for a
   particular message.  The SMTP server receives the message
   headers and message body in a block after accepting the DATA
   command defined by section of RFC 2821.  SMTP makes no
   provision for sending just the headers or a partial message;
   if an SMTP server accepts any of the headers at all, it must

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   be prepared to accept the entire message.

2.2.5 Comparison

   All of these items, IP address, HELO/EHLO argument, return
   path, and message headers, can be used for various kinds of

   The IP address is the cheapest to use, since it can be
   determined even before the SMTP session begins, but reveals
   the least information about the mail sender.  There is no
   connection between an IP address and a responsible domain or
   e-mail address, unless there is MTA Mark contact information
   for the IP.  Even if an IP address is a legitimate mail
   sending host, the IP address provides no basis to distinguish
   between mail sent by the host's legitimate users and mail sent
   by hostile users or malware with access to that host.  The IP
   address gives no information about what domains have
   authorized mail to be sent from that host.

   The HELO/EHLO string, if it is a domain name, can be validated
   using an LMAP-like scheme such as DRIP [N2] to check that the
   host at the connecting IP is authorized to use the domain name
   it is offering.  Like IP validation, HELO/EHLO validation
   doesn't provide a contact e-mail address.  The HELO/EHLO
   address should be the host's name, such as MAIL.EXAMPLE.COM,
   even if the mail it sends all has addresses in the domain
   EXAMPLE.COM.  Although the address Postmaster@EXAMPLE.COM
   would exist, the address Postmaster@MAIL.EXAMPLE.COM may well
   not exist if no legitimate mail is sent with MAIL.EXAMPLE.COM
   return addresses.  If the HELO/EHLO string is a domain
   literal, it can't be verified other than by IP address
   verification, and it's unlikely to be useful for contact e-
   mail since few hosts accept mail to domain literal addresses.
   RFC 2821 permits the SMTP server to validate a domain address
   provided to EHLO or HELO by doing a DNS lookup to see if the
   IP address matches, but does not permit the server to reject
   mail if the validation fails.  HELO/EHLO-based LMAP would
   modify RFC2821 by allowing the server to reject mail based on
   HELO/EHLO validation failure.

   The return path, if its domain is verified by LMAP, is likely
   to be a valid contact address for the message.  Since LMAP
   only looks at the domain, not the full address, it's still
   possible that the mailbox isn't valid, in which case the
   contact address might be abuse@(domain) or
   Postmaster@(domain).  Return paths can legitimately be empty,
   in which case the server would either have to skip LMAP
   validation, or use HELO/EHLO or message header data instead.

   A message header address, if verified by LMAP, is also likely
   to be a valid contact address for the message.  Any valid
   message has at least one address in a From: or Sender: line.

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   Although any or all of these items could be used for message
   validation, the LMAP proposals use the return path as a
   compromise between the best quality data and efficiency of
   operation.  A verified return path, unlike IP or HELO/EHLO
   data, is very likely to provide a specific responsible address
   and responsible domain for a message.  Existing SMTP
   implementations tend to preserve the return path throughout
   the delivery process so it can be used at any stage that a
   contact address is needed.  The HELO/EHLO domain, on the other
   hand, is generally preserved only in a Received: header, and
   although RFC 2821 specifies the way that the domain is stored
   in that header, MTAs do not all conform to it.

   A verified message header address is also high-quality data,
   but the cost of extracting the header addresses is much higher
   than for using return path or EHLO/HELO domains because of the
   cost of receiving the entire message before deciding whether
   it passes LMAP validation and of parsing the headers, which is
   otherwise not needed at SMTP time.  If an SMTP server is going
   to receive the entire message anyway, it might be appropriate
   to apply other more powerful cryptographic signature
   verification instead of or in addition to LMAP.

2.3. DNS based attacks

   All versions of LMAP use the DNS to distribute the data
   against which mail is authenticated.  This makes the DNS the
   critical resource required by all of these proposals.
   Insecurities in the DNS, as described in [7], could allow
   hostile parties to page forged authentication information into
   the DNS.  Packet floods and other denial of service attacks
   against DNS servers could make it impossible for LMAP clients
   to obtain LMAP authentication data.

3. Common Concerns with LMAP

   This section describes the most common concerns raised about
   LMAP, and responds in detail to those concerns.

3.1. LMAP and the end to end nature of the Internet

   Concerns have been raised that this proposal negatively
   affects the "end to end" nature of the Internet.  LMAP does
   not change SMTP, except for changing the semantics of the
   mailbox used in MAIL FROM command.  The end to end nature of
   SMTP is therefore unchanged.  What this proposal does offer is
   a way to hold the originating end of an SMTP session
   accountable for any association it alleges it has with a
   domain.  Claims that this accountability is an unwarranted
   restriction on the "end to end" nature of the Internet should

     1) SMTP originators who wish to be unaccountable for their

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     behavior are called "spammers".  The intention of this
     proposal is to address the problem of spam, not to condone

     2) SMTP originators who wish to force their messages onto
     recipients, despite the recipients desire not to receive
     them, are also called "spammers".  If a recipient chooses to
     request that a sender be publicly accountable for his
     behavior and the sender refuses, then the recipient is free
     to reject or discard any messages from the sender.

   The lack of accountability is a major technical reason why
   spam is such a problem.

   This proposal extends the end-to-end principles on which the
   Internet was built, because it allows each end to publish its
   policy, and to discover the others policy.  Sharing of
   information enhances trust, and permits the discovery of
   problems related to Man in the Middle (MITM) issues.

3.2. Roaming Users and LMAP

   Another concern raised about LMAP is that it will negatively
   affect roaming users, that is, users not connected to their
   usual or home network.  The main concern of roaming users is
   that the deployment of LMAP will break the "end to end" nature
   of the Internet.

   The response to those concerns can best be summarized as

     Stopping mail forgery requires every one of them to give up

   The practices of roaming users currently require that the SMTP
   recipient do significant amounts of work to authenticate or
   filter their messages.  Recipients in return request that the
   roaming user (and the alleged originating domain) share some
   of that work.  This response serves as the foundation for the
   design of LMAP.

   If the roaming user is unwilling to share the work of
   demonstrating accountability, then the recipient is as always
   free to reject any communication with that roaming user.

   Existing practice includes a variety of methods through which
   roaming users may send email messages in circumstances where
   LMAP is widely deployed.

3.2.1 SUBMIT (port 587)

   That is, SMTP on another port [RFC 2476].  Roaming users can
   use SUBMIT to send messages to their home provider, which then
   sends those messages to the final recipients via SMTP.  This

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   method has all of the benefits of SMTP, with none of the
   drawbacks of recipients being responsible for authenticating
   roaming end-users.

   The primary cost or delay associated with this method is
   deployment in mail client software.

3.2.2 SMTP relaying to the home provider

   Since many network providers currently block outgoing SMTP
   traffic (on port 25), this option is not universally
   available.  The roaming users are then in the awkward
   circumstance of having their attempt to behave like spammers
   blocked, by an attempt to prevent spam.

3.2.3 Publish LMAP information

   Roaming users may update LMAP information for their domain
   through Dynamic DNS (DDNS).  Any messages they send will then
   pass LMAP criteria, subject to DNS propagation delays.

   Roaming users can also publish a policy though LMAP that any
   IP address on the Internet is permitted to claim association
   with their domain.  Administrators who publish such
   information for their domain should be aware that this
   practice will open them up to spammers claiming association
   with their domain.  For this reason, we do not recommend such

3.2.4 Virtual Private Networks (VPNs)

   With this solution, roaming users allow their home provider to
   authenticate them, and any SMTP traffic is sent through a
   secure tunnel.  That traffic then appears to issue from the
   network of the home provider, where LMAP information may
   easily be published and maintained.

3.2.5 Other message delivery systems

   Bi-directional POP, IMAP, Webmail, etc. all exist, and are
   sub-optimal.  But they work.

   They also have the added benefit that they are not required to
   scale with the Internet.  Rather, they scale with the number
   of users at a domain.  So it is not the problem of the rest of
   the Internet to deal with those issues, but instead the domain
   with roaming users.

3.3. Message relaying and forwarding are affected by LMAP

   Mail forwarders have traditionally left the sender envelope
   untouched.  "Forwarding" is used in the sense of Unix user
   .forward and forwarding services such as those provided by
   pobox.com and ieee.org.

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   Let us examine a situation where an LMAP conformant domain A
   sends a message to address B which forwards the message to
   LMAP conformant recipient C using the original sender address
   from A.  If the B->C forward had been set up without the
   consent of the recipient C, A's LMAP records would be checked
   by C's LMAP client, and the message would be correctly

   If the recipient C did desire the B->C forwarding, possible
   changes to work with LMAP include:

     1) B's MTA could rewrite the sender address to to one in B's

     2) the user B could alter the .forward to apply a return
     path in B's domain

     3) the recipient C could provide a whitelist to C's MTA
     indicating that forwarded messages are expected to arrive
     for C from B.

   LMAP conformant SMTP forwarders could implement a sender
   rewriting scheme or its equivalent.  The technical details of
   doing so appear simple in most popular mail systems.

3.4. Kiosk, greeting card, and mail-a-link systems

   Many web sites offer a facility to mail content from the web
   site to a third party, with the web user's return address.
   The content may be a greeting card, a magazine article, or a
   message entered by the user.  Few of these sites do any
   validation of the sender's address, although they tend to be
   rate limited or inherently slow enough that they're not useful
   for sending out spam, but since users can enter any return
   address, the mail they send is technically indistinguishable
   from mail with forged return addresses.

   The most straightforward way to make such systems comply with
   LMAP would be for them to use their own domain in the return
   address, while using the user's entered address on the From:

4. Comparison of Proposals

   Several different varieties of LMAP have been proposed in
   recent months.  They include:

   RMX (Hadmut Danisch) [N5]

   DMP (Designated Mailers Protocol, Gordon Fecyk) [N1]

   SPF (Sender Permitted From, Mark Lentczner and Meng Weng Wong)

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   FSV (Flexible Sender Validation, John Levine) [N3]

   Two simpler but similar proposals are:

   MM (MTA Mark, Markus Stumpf and Steff Hoehne) [N4]

   SS (Selective Sender, John Levine) [N6]

   They both identify IP addresses as mail senders or not,
   without asserting anything about what e-mail addresses should
   originate mail from what address.

   Each of these proposals is a work in progress, hence the
   reader must refer to the latest defining document for each to
   learn the exact details of each.  These comparisons are not
   intended to recommend one proposal over another, but rather to
   highlight the differences among them.

4.1. DNS record types

   RMX: defines two new record types, RMX and APL

   SPF: uses standard TXT records

   DMP: uses standard TXT records

   FSV: uses standard TXT and A records

   MM: uses standard TXT records

   SS: uses standard TXT records

4.2. Record structure

   RMX: RMX records have multiple subtypes, APL records are a
   list of CIDR ranges

   SPF: TXT records contain syntax that require parsing

   DMP: TXT records contain fixed strings

   FSV: TXT records contain ASCII CIDR ranges

   MM and SS: TXT records contain fixed strings

4.3. Indirect addressing

   RMX: RMX records can refer to RMX and APL records in other

   SPF: syntax includes indirect references to SPF data in other

   DMP: none other than CNAMES

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   FSV: none other than CNAMES

   MM and SS: none other than CNAMES

4.4. Address index structure

   (That is, how is the set of authorized IP addresses for a
   domain stored.)

   RMX: Aggregated data for a domain are contained in APL
   records, or implicitly via host or MX records

   SPF: Similar to RMX, with all data encoded in TXT records

   DMP: Individual records per IP per domain

   FSV: Both individual A records per IP per domain, and
   aggregate data in TXT record for a domain

   DMP: Individual records per IP per domain

   MM and SS: Individual records per IP address

4.5. Roaming and forwarding support

   RMX: None

   SPF: Domain's data can permit mail from anywhere in addition
   to listed IPs

   DMP: HELO/EHLO name can be validated instead of envelope

   FSV: same as DMP

   MM and SS: Not an issue, doesn't attempt to validate domains

4.6. Number of queries

   RMX: Data for domain retrieved in one or more queries, can be
   cached for future mail from the same domain

   SPF: Similar to RMX with multiple levels of indirection

   DMP: Single domain+IP query to validate a message.  In case of
   failure, possible second query to see if domain announces DMP

   FSV: Single query to fetch all of domain's data, or single
   domain+IP to validate a single message, with possible second
   query to see if domain announces FSV data.

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   MM and SS: One per IP address

4.7. Extensibility

   RMX: not addressed

   SPF: version number in data and extensible record syntax allow
   for additional definitions

   DMP: not addressed, new fixed strings could be defined

   FSV: not addressed, new fixed strings could be defined

   MM and SS: not addressed, new fixed strings could be defined

5. Privacy Considerations

   This proposal does not examine message contents, or user
   identities in MAIL FROM.  It therefore has no privacy
   considerations which affect those fields.

   The largest effects of this proposal on privacy are in three
   areas: end users, publication of network infrastructure, and
   traffic analysis.

5.1. End Users

   This proposal affects the privacy of end users in two ways.
   First, it permits recipients to associate user identities or
   message contents with a domain that is accountable for the
   message.  Second, it prevents users from sending mail
   pseudonymously, by using an address in another domain, or in a
   non-existent domain.

   The first effect on privacy has little impact.  The user
   sending the message is already claiming association with a
   domain, so there is no loss of privacy if that association is
   verified by the message recipient.  Also, as noted previously,
   this proposal does not prevent users from fraudulently
   claiming to be another user within a domain.

   The second effect on privacy may be considered undesirable by
   some observers.  True anonymity through the practice of forged
   association with domains, forged email addresses, and by
   sending email through hijacked or trojaned systems will become
   more difficult.  This sort of anonymity is highly correlated
   with spam, however, and is precisely the kind of abuse that
   this proposal attempts to prevent.

   Users who wish anonymity may gain it through accountable
   mechanisms.  Throw-away accounts at reputable network
   providers may be created and paid for in cash under an assumed
   name, for example.  Other organizations will guarantee a
   degree of anonymity (more realistically called shelter from

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   others), if certain requirements are met.

   We suggest that accountable methods of creating anonymity be
   used, rather than unaccountable methods.  One individual's
   desire for anonymity does not, and should not, require the
   rest of the Internet to accept large volumes of spam.

5.2. Network Infrastructure

   Publication of LMAP information results in a readily available
   list of IP addresses of hosts authorized to send messages
   associated with a domain.  These lists yield information about
   the network structure, and business relationships, and
   presents hostile parties with a list of targets to attempt to

   However, such information is often already publicly accessible
   through other means.  Anyone communicating with individuals at
   a domain may readily obtain this information, and share it
   with anyone else.  Business relationships have been
   discovered, for example, prior to "official" public
   announcement, by examining DNS records.  Nearly all such
   "private" information about network structure and
   relationships may therefore be described as already being
   readily available.

   If such information is to be kept secret, it is the users
   responsibility to send messages in such a way as to keep that
   information private.

5.3. Traffic analysis

   Any LMAP aware MTA and DNS server requires additional network
   traffic beyond that required by SMTP.  This traffic may be
   analyzed in order to verify that two parties are
   communicating, or that a particular message has been received.
   The additional traffic may still be analyzed in this manner,
   even if the SMTP session is encrypted.

   However, many MTAs already query MX and A records of a domain
   after receiving a MAIL FROM command, so the threat of this new
   traffic is minimal.

6. Security Considerations

   This document describes common uses of LMAP, attacks on it,
   and defenses which may be implemented.  While much of this
   document deals with security issues, it does not propose any
   standard, and therefore does not have any direct security

   However, implementors and administrators of systems using LMAP

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   should be aware of the issues raised herein.

7. Informative References

   [1] Bradner, S., "The Internet Standards Process -- Revision
   3", BCP9, RFC 2026, October 1996.

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

   [3] Crocker, D. "Technical Considerations for Spam Control
   Mechanisms", work in progress,

   [4] Hoffman, P. "SMTP Service Extension for Secure SMTP over
   TLS", RFC 2487, January, 1999

   [5] Myers, J. "SMTP Service Extension for Authentication", RFC
   2554, March, 1999.

   [6] Klensin, J. (Ed) "Simple Mail Transfer Protocol", RFC
   2821, April 2001.

   [7] D. Atkins and R. Austein, "Threat Analysis Of The Domain
   Name System," Internet Draft internet-drafts/draft-ietf-
   dnsext-dns-threats-05.txt, Nov 2003.

8. Normative References

   [N1] G. Feyck, "Designated Mailers Protocol (DMP)", Internet
   draft draft-fecyk-dmp-01.txt, December 2003.

   [N2] R. S. Brand and L. Sherzer, "Designated Relays Inquiry
   Protocol (DRIP)," http://www.sherzer.net/draft-brand-
   drip-02.txt, October 2003,

   [N3] J. Levine, "Flexible Sender Validation (FSV)",
   http://www.taugh.com/draft-levine-fsv-00.txt, February 2004.

   [N4] M. Stumpf and S. Hoehne. "MTA Mark",
   September 2003.

   [N5] H. Danisch, "The RMX DNS RR and method for lightweight
   SMTP sender authorization ", IETF draft draft-danisch-dns-rr-
   smtp-03.txt, October 2003.

   [N6] J. Levine, Selective Sender,
   http://www.taugh.com/mp/ss.html, January 2004.

   [N7] M. Lentczner and M. W. Wong, "Sender Authentication with
   Sender Permitted From (SPF) A Convention to Describe Hosts
   Authorized to Send SMTP Traffic", http://spf.pobox.com/draft-

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   mengwong-spf.02.9.6.txt, January 2004.

9. Authors' Addresses

   John R. Levine
   Taughannock Networks
   PO Box 727
   Trumansburg NY 14886 USA
   E-mail: johnl@taugh.com
   Phone: +1 607 330 5711

   Alan DeKok
   IDT Canada, Inc.
   1575 Carling Ave.
   Ottawa, ON K1G 0T3 Canada
   Email: alan.dekok@idt.com
   Phone: +1 613 724 6004 ext. 231

Full Copyright Statement

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   The limited permissions granted above are perpetual and will
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   This document and the information contained herein is provided

   $Id: draft-irtf-asrg-lmap-discussion-00.n,v 1.5 2004/02/08
   21:19:27 johnl Exp $

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