Network Working Group M. Stumpf
Internet-Draft S. Hoehne
Expires: October 31, 2005 SpaceNet AG
May 2, 2005
Marking Mail Transfer Agents in Reverse DNS with TXT RRs
draft-stumpf-dns-mtamark-04
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
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/
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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.
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
1.1.1 Reverse Zone vs. Forward Zone . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. PROPOSAL . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1 Defining A Well Known Subdomain for the Reverse DNS
Tree . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2 Service Specific Resource Records . . . . . . . . . . . . 5
2.2.1 Terms and Definitions . . . . . . . . . . . . . . . . 5
2.2.2 Hints for Implementors . . . . . . . . . . . . . . . . 5
2.3 Contact Information . . . . . . . . . . . . . . . . . . . 5
2.3.1 Terms and Definitions . . . . . . . . . . . . . . . . 6
2.3.2 Hints for Implementors . . . . . . . . . . . . . . . . 6
2.4 Example Records . . . . . . . . . . . . . . . . . . . . . 7
3. EFFECTS ON EXISTING MAIL INFRASTRUCTURE . . . . . . . . . . . 7
3.1 Deployment . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Unmarked Addresses . . . . . . . . . . . . . . . . . . . . 7
3.3 Local Mail Clients . . . . . . . . . . . . . . . . . . . . 8
3.4 Roaming Users . . . . . . . . . . . . . . . . . . . . . . 8
3.5 IPv6 Compatibility . . . . . . . . . . . . . . . . . . . . 8
3.6 RFC2317 Style Delegation . . . . . . . . . . . . . . . . . 8
4. EXPANDING THIS PROPOSAL . . . . . . . . . . . . . . . . . . . 8
4.1 Extensibility . . . . . . . . . . . . . . . . . . . . . . 8
4.2 Blacklists, Whitelists and Accreditation Systems . . . . . 8
5. WHY NOT A NEW DNS RR . . . . . . . . . . . . . . . . . . . . . 9
6. IANA CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . 9
7. SECURITY CONSIDERATIONS . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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1. INTRODUCTION
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 broadband technology, 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
meaning
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.
1.1.1 Reverse Zone vs. Forward Zone
One of the big advantages of using the reverse DNS tree instead of
the forward tree is that it is much harder for spammers to get
accreditation records into the reverse tree than to add them to the
forward domain, which is completely under their control.
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].
2. PROPOSAL
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
_srv
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.
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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
_smtp
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 scheme.
Whether SMTP message transmissions should be accepted from that host
is specified by a TXT record within the service subdomain for the
entry
_send
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
accepted.
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
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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].
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 "_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.
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2.4 Example Records
Some examples, how records might look like in BIND syntax:
$ORIGIN 0.0.10.IN-ADDR.ARPA.
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. .
3. EFFECTS ON EXISTING MAIL INFRASTRUCTURE
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 Deployment
Deployment of this method is easy and cheap, even in costs of DNS
records needed. Unlike other methods like SPF (Sender Policy
Framework) or Caller-Id it does not require records to be added for
each and every node in a domain, but only one record per mailserver.
An analysis done by Peter Koch in June 2004 has shown that for the DE
TLD all DNS MX records of the about 7.6 million second level domains
pointed to about 140000 unique IP addresses. About 75% of those had
a working reverse mapping.
3.2 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
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to easily specify a default behavior for unmarked IP addresses.
3.3 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".
3.4 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
authenticate.
3.5 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.
3.6 RFC2317 Style Delegation
Classless IN-ADDR.ARPA delegation [RFC2317] SHOULD be implemented
using DNAMEs. As DNAME is not currently supported by all the
nameservers a parallel set of CNAME records SHOULD be generated.
4. EXPANDING THIS PROPOSAL
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.
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Accreditation systems can use this technique to store multiple
information for an IP address or a network block within one
(sub-)domain.
5. WHY NOT A NEW DNS RR
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.
6. IANA CONSIDERATIONS
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.
7. SECURITY CONSIDERATIONS
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.
8 References
[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,
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"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.
[RFC2317] Eidnes, H., de Groot, G. and P. Vixie, "Classless
IN-ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.
[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
DE
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
DE
Phone: +49 89 32356-0
Fax: +49 89 32356-299
EMail: steff-rfc@space.net
URI: http://www.space.net/
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, Mark Andrews, Arnt
Gulbrandsen, Peter Koch, Scott Nelson, and all the members of the
RIPE Antispam list, the IRTF ASRG 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
xml2rfc.
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