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Versions: 00 01 02 03 04                                                
Network Working Group                                          M. Stumpf
Internet-Draft                                                 S. Hoehne
Expires: August 1, 2004                                      SpaceNet AG
                                                           February 2004

        Marking Mail Transfer Agents in Reverse DNS with TXT RRs

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 August 1, 2004.

Copyright Notice

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


   In contrast to other more extensive approaches to deal with
   unsolicited email, commonly called "spam", this memo discusses a very
   simple authentication scheme. It uses marking of hosts in reverse DNS
   (IN-ADDR.ARPA and IP6.ARPA zones) to allow the receiving mail
   transfer agents to decide whether the connecting (sending) host is a
   designated mail transfer agent (MTA) or not.

   Despite being a weaker scheme than most of the other proposals
   currently discussed, it can reduce the amount of spam and viruses/
   worms significantly and has the advantage that it can be implemented
   based on existing and well-established Internet technology like DNS
   without any changes to that technology.

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   This document is part of the LMAP work of the Anti-Spam Research
   Group (ASRG) of the IRTF.

Table of Contents

   1.    INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1   Motivation . . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.2   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.    PROPOSAL . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.1   Defining A Well Known Subdomain for the Reverse DNS Tree . .  5
   2.2   Service Specific Resource Records  . . . . . . . . . . . . .  5
   2.2.1 Terms and Definitions  . . . . . . . . . . . . . . . . . . .  5
   2.2.2 Hints for Implementors . . . . . . . . . . . . . . . . . . .  6
   2.3   Contact Information  . . . . . . . . . . . . . . . . . . . .  6
   2.3.1 Terms and Definitions  . . . . . . . . . . . . . . . . . . .  6
   2.3.2 Hints for Implementors . . . . . . . . . . . . . . . . . . .  7
   2.4   Example Records  . . . . . . . . . . . . . . . . . . . . . .  7
   3.    EFFECTS ON EXISTING MAIL INFRASTRUCTURE  . . . . . . . . . .  8
   3.1   Unmarked Addresses . . . . . . . . . . . . . . . . . . . . .  8
   3.2   Local Mail Clients . . . . . . . . . . . . . . . . . . . . .  8
   3.3   Roaming Users  . . . . . . . . . . . . . . . . . . . . . . .  8
   3.4   IPv6 Compatibility . . . . . . . . . . . . . . . . . . . . .  8
   4.    EXPANDING THIS PROPOSAL  . . . . . . . . . . . . . . . . . .  9
   4.1   Extensibility  . . . . . . . . . . . . . . . . . . . . . . .  9
   4.2   Blacklists, Whitelists and Accreditation Systems . . . . . .  9
   5.    WHY NOT A NEW DNS RR . . . . . . . . . . . . . . . . . . . . 10
   6.    IANA CONSIDERATIONS  . . . . . . . . . . . . . . . . . . . . 11
   7.    SECURITY CONSIDERATIONS  . . . . . . . . . . . . . . . . . . 12
         References . . . . . . . . . . . . . . . . . . . . . . . . . 13
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
   A.    Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15
         Intellectual Property and Copyright Statements . . . . . . . 16

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1.1 Motivation

   The problem with spam and viruses/worms has increased and changed a
   lot during the last years. In the beginning, distributing unsolicited
   email was accomplished by abusing relay open mail servers [RFC2505].
   Spread of viruses needed humans passing on infected data. The
   situation today shows worms coming packed with their own SMTP
   modules, utilizing address books and scanning documents for new
   addresses and therefore victims. A lot of worms install backdoors and
   (enable) proxy servers. These infected hosts are afterwards abused by
   spammers to send unsolicited email.

   With the growing adoption of DSL techniques, a significant part of
   the Internet hosts shifted to poorly maintained workstations in
   homes. Permanently connected to the Internet, these hosts form an
   easy and "paying" prey for worms and abusers. Not only in homes, also
   in companies the number of poorly maintained hosts is growing.

   History and viruses like VBS/LoveLet class or worms like CodeRed and
   Nimda and the zillions of open proxy servers show, we cannot count on
   users or administrators to get the problems fixed.

   However, what the administrators can decide proactively is whether a
   certain host, represented by its IP address, is meant to be a MTA
   that should have the ability to talk to other MTAs across the
   Internet. Most - if not all - of the proxy servers or workstations do
   not need to have this ability.

   We suggest a mechanism to enable the administrator to mark IP
   addresses in the Domain Name System [RFC1034], [RFC1035] with labels

   o  "This IP address is assigned to a MTA that is intended to send
      messages across the Internet"

   o  "This IP address does not host a MTA, it is not recommended to
      accept unauthorized message transmission from that IP address."

   and therefore give receiving MTAs a hint as whether to accept
   messages from the sending MTA or not.

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   This document describes such a mechanism that

   o  is easy, fast and cheap to deploy and implement,

   o  uses existing Internet technology without modification and without
      breaking it or the need for workarounds

   While this document specializes on SMTP the technique used in this
   proposal is not limited to SMTP, but can be adopted by any service
   and is easily extensible.

1.2 Terminology

   The keywords "MUST", "MUST NOT", "SHOULD", "SHOULD NOT",
   "RECOMMENDED", "NOT RECOMMENDED", and "MAY" in this document are to
   be interpreted as described in RFC2119 [RFC2119].

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2.1 Defining A Well Known Subdomain for the Reverse DNS Tree

   Storing arbitrary string attributes in the Domain Name System
   [RFC1464] is a technique described and used at least since 1993. One
   solution that we took into consideration has been to store string
   attributes like "MTA=1" or "MTA=0" at the same level as PTR records.

   However this method does not support specific queries and has a high
   overhead for parsing the responses, is prone to naming collisions and
   will trigger errors and problems in old implementations of DNS
   servers with the 512 byte size limit.

   Thus we propose expanding the reverse DNS tree with a subdomain with
   the well known name


   This subdomain MAY be inserted at any level in the DNS tree for IPv4
   IN-ADDR.ARPA reverse zones. For IPv6, to limit the number of DNS
   queries, _srv is only queried at the /128 (host), /64 (subnet) and /
   32 (site) level. That way it can either provide information for a
   specific IP address or for a whole network block. More specific
   information takes precedence over information found closer to the top
   of the tree.

2.2 Service Specific Resource Records

2.2.1 Terms and Definitions

   Within the above "_srv" subdomain there will be another subdomain
   named after the service for which the specific records will be
   defined. For SMTP the name of the subdomain will be


   The symbolic name of the desired service is the same as defined in
   Assigned Numbers [RFC3232] or locally. An underscore (_) is prepended
   to the service identifier to avoid collisions with DNS labels that
   occur in nature. The service name is case insensitive. Readers
   familiar with RFC2782 [RFC2782] are already accustomed to that naming

   Whether SMTP message transmissions should be accepted from that host
   is specified by a TXT record within the service subdomain for the

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   The name "_send" is case insensitive.

   The value of the TXT record will be either "1" or "0":

   1 -  (MTA=yes) indicates that the connection is originating from an
      IP address that is intended to be a MTA talking to other MTAs
      across the public Internet and that the message SHOULD be

   0 -  (MTA=no) indicates that the IP address of the sending
      communication partner is NOT meant to be an accredited sending MTA
      and that messages SHOULD NOT be accepted from that connection,
      unless successful authentification via other methods (e.g. ODMR
      [RFC2645]) advise the contrary.

2.2.2 Hints for Implementors

   o  The "_send" record for a given service MUST be unique for a given
      IP address within the "_smtp" subdomain. In the case of multiple
      (contradictory) records implementors are free which record to
      choose. However we recommend using the first record found.

   o  If the value of the resource record is other than "1" (MTA=yes) or
      "0" (MTA=no) the value MUST be treated as "0" (MTA=no).

2.3 Contact Information

2.3.1 Terms and Definitions

   The contact information provides automatic notification of
   administrators, if hosts within their responsibility get abused or
   infected by viruses.

   Currently there is no easy way to get information about contacts for
   a given IP address. There are a lot of different sources, where the
   best are probably the whois databases of the various (Regional
   Internet Registries (RIR) like ARIN, RIPE, APNIC or LACNIC. However,
   there is no common agreed upon format for abuse contacts, and for
   some allocations, referrals have to be followed to other registries
   like BRNIC or KRNIC, that again use different record formats.

   An easy way to specify contact information for a given IP address is
   to use the Responsible Person (RP) resource record as defined in RFC
   1183 [RFC1183].

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   Another use of an email address provided with the contact information
   is the possibility for a MTA to customize the error message
   [RFC2821], [RFC1893] like in

       550-5.7.1 Message rejected. Sender is not labeled a sending MTA.
       550 5.7.1 Please contact <abuse@example.com>.

   where "abuse@example.com" is derived from the information stored in
   the RP records.

   The RP resource records SHOULD be inserted into the IN-ADDR.ARPA and
   IP6.ARPA zone at the same level as the "_srv" records.

   However, there MAY be more than one contact address for various
   services involved, so service specific contact information MAY also
   be provided at the service subdomain level.

2.3.2 Hints for Implementors

   o  Programs utilizing these records should first query for RP records
      along with the service subdomain and if that fails try again and
      query for RP records at the &quote;_srv" level.

   o  More than one RP resource record may be specified. It is up to the
      reporting program or person to choose a random contact to notify
      or send notification to all of them.

2.4 Example Records

   Some examples, how records might look like in BIND syntax:


   1                       IN  PTR  mail.example.com.
   _send._smtp._srv.1      IN  TXT  "1"
   _smtp._srv.1            IN  RP   abuse.example.com. .

   2                       IN  PTR  www.example.com.
   2                       IN  RP   abuse.example.com. .
   _send._smtp._srv.2      IN  TXT  "0"
   _smtp._srv.2            IN  RP   spam.example.com. .

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   One of the main goals of this proposal has been to limit the impact
   on existing Internet infrastructure as much as possible.

   Putting this proposal in effect will not break existing
   infrastructure or widely used mechanisms like gatewaying, forwarding
   and (authenticated) relaying of emails

3.1 Unmarked Addresses

   Each receiving MTA is free to decide how to classify connections from
   IP addresses without the marks as defined in this document.

   However, as a general guideline, we propose a grace period of six
   months after publication of this document, where missing marks are to
   be treated with a default of "MTA=yes" and after the grace period
   missing marks are to be treated as "MTA=no".

   Implementors are asked to provide a mechanism for the administrator
   to easily specify a default behavior for unmarked IP addresses.

3.2 Local Mail Clients

   MTAs implementing the policy defined in this document should take
   care to provide mechanisms for the administrators to easily specify a
   list of "local addresses" which use the receiving MTA as an outgoing
   relay. The MTA will accept messages from those IP addresses despite
   them being marked as "MTA=no".

3.3 Roaming Users

   Typically, roaming users or local users from dialin/dynamic IP
   addresses have "MTA=no" set on the connection to the receiving MTA.
   The ODMR [RFC2645] extension to SMTP [RFC2821] specifies a way for
   roaming users to authenticate themselves to the receiving MTA and
   validate the connection.

   Connections MUST NOT be rejected solely based on a "MTA=no" label
   before the initiator of the connection had the chance to

3.4 IPv6 Compatibility

   This proposal is fully compatible with IPv6. The same TXT and RP RRs
   and lookup mechanisms can be applied to the "IP6.ARPA" zone as well.

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4.1 Extensibility

   This proposal concentrates on labeling SMTP servers. However the
   principle is generic and can be used for other services, too.

   Other entries in the service subdomain like e.g. "_key" can be used
   to store the public key the MTA at that IP address uses for
   authentication or signing of messages.

4.2 Blacklists, Whitelists and Accreditation Systems

   While this document specifies the mechanisms for the reverse DNS tree
   the same labeling scheme can also be used within other domains.
   Accreditation systems can use this technique to store multiple
   information for an IP address or a network block within one

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   The problem with a new DNS RR (and one reason why we try to avoid it)
   is the resulting need to modify all kinds of DNS software. DNS
   servers, DNS resolvers and - probably the strongest argument against
   - ISP management software. Internet Service Providers do not edit
   zone files with an editor. They have a database and a GUI of some
   sort that is capable handling all kinds of "well known" RRs.

   We had quick, easy and cheap adoption in mind and if all ISP
   management software has to be changed to make use of the new RR, it
   will either take a long time or will never happen. TXT and RP records
   are well understood for years.

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   The IANA already maintains the registry for service names [RFC3232]
   that are used to name the service subdomain proposed. No other IANA
   services are required by this document.

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   The authors believe that this specification does not cause any new
   security problems.

   The same security issues apply as to other DNS based services.

   Probably the worst case scenario is hijacking of a part of the
   reverse DNS zone and modification of the special TXT record defined
   in this document to "MTA=no" to block email sending capabilities for
   hosts with that IP addresses.

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   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1101]  Mockapetris, P., "DNS encoding of network names and other
              types", RFC 1101, April 1989.

   [RFC1183]  Everhart, C., Mamakos, L., Ullmann, R. and P. Mockapetris,
              "New DNS RR Definitions", RFC 1183, October 1990.

   [RFC1464]  Rosenbaum, R., "Using the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464, May 1993.

   [RFC1893]  Vaudreuil, G., "Enhanced Mail System Status Codes", RFC
              1893, January 1996.

   [RFC1912]  Barr, D., "Common DNS Operational and Configuration
              Errors", RFC 1912, February 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2505]  Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs",
              BCP 30, RFC 2505, February 1999.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

   [RFC2645]  Gellens, R., "ON-DEMAND MAIL RELAY (ODMR) SMTP with
              Dynamic IP Addresses", RFC 2645, August 1999.

   [RFC2782]  Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC2821]  Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
              April 2001.

   [RFC3232]  Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
              an On-line Database", RFC 3232, January 2002.

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Authors' Addresses

   Markus Stumpf
   SpaceNet AG
   Joseph-Dollinger-Bogen 14
   Muenchen,   80807

   Phone: +49 89 32356-0
   Fax:   +49 89 32356-299
   EMail: maex-rfc@space.net
   URI:   http://www.space.net/

   Steff Hoehne
   SpaceNet AG
   Joseph-Dollinger-Bogen 14
   Muenchen,   80807

   Phone: +49 89 32356-0
   Fax:   +49 89 32356-299
   EMail: steff-rfc@space.net
   URI:   http://www.space.net/

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Appendix A. Acknowledgements

   The authors gratefully acknowledge the contributions of: Christian
   Brunner, Gert Doering and Sebastian von Bomhard, Elmar Bartel also
   for some good hints that should plate our English, Arnt Gulbrandsen,
   Phillip Hallam-Baker, Scott Nelson and all the members of the RIPE
   Antispam list, the IRTF ASRG, IETF MARID and a lot of our net.friends
   for their comments and input.

   Our sincere thanks go to Yakov Shafranovich, one of the chairs of the
   IRTF ASRG, who always was willing to help. His dedication formed the
   IRTF ASRG into a productive group and set the stage for successfully
   addressing the spam problem.

   A big 'Thank You' goes also to Marshall T. Rose for the wonderful

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Intellectual Property Statement

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   This document and the information contained herein is provided on an

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   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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