MTA Authentication Records in DNS July 2004 MARID Working Group J. Lyon Internet Draft Microsoft Corp Document: draft-ietf-marid-core-02.txt M. Wong pobox.com Expires: January 2005 July 2004 Sender ID: Authenticating E-Mail Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. 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 a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract Internet mail suffers from the fact that much unwanted mail is sent using spoofed addresses -- "spoofed" in this case means the address is used without the permission of the domain owner. This document describes the following: mechanisms by which a domain owner can publish its set of outgoing MTAs, mechanisms by which SMTP servers can determine what email address is allegedly responsible for most proximately introducing a message into the Internet mail system, and whether that introduction is authorized by the owner of the domain contained in that email address. The specification is carefully tailored to ensure that the overwhelming majority of legitimate emailers, remailers and mailing list operators are already compliant. Lyon, Wong Expires - January 2005 [Page 1]
MTA Authentication Records in DNS July 2004 Table of Contents 1. Introduction...................................................3 2. Problem Statement..............................................3 2.1 Positive Problem Statement.................................3 2.2 Negative Problem Statement.................................4 3. Decision Model.................................................4 4. Determining the Purported Responsible Address..................5 5. Actions Based on the Decision..................................6 5.1 Neutral or None or PermError...............................6 5.2 Pass.......................................................6 5.3 Fail.......................................................7 5.4 SoftFail...................................................7 5.5 TempError..................................................7 6. Security Considerations........................................7 6.1 DNS Attacks................................................7 6.2 TCP Attacks................................................8 6.3 Forged Resent-From Attacks.................................8 7. Implementation Guidance........................................8 7.1 Simple E-mailers...........................................8 7.2 E-mail Forwarders..........................................9 7.3 Mailing List Servers.......................................9 7.4 Third-Party Mailers........................................9 7.5 MUA Implementers...........................................9 8. IANA Considerations............................................9 9. Acknowledgements..............................................10 10. References...................................................10 10.1 Normative References.....................................10 10.2 Informative References...................................10 11. Authors' Addresses...........................................11 Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. Lyon, Wong Expires - January 2005 [Page 2]
MTA Authentication Records in DNS July 2004 1. Introduction Today, a huge majority of unwanted email contains headers that lie about the origin of the mail. This is true of most spam and substantially all of the virus email that is sent. This document describes a mechanism such that receiving MTAs, MDAs and/or MUAs can recognize mail in the above category and take appropriate action. For example, an MTA might refuse to accept a message, an MDA might discard a message rather than placing it into a mailbox, and an MUA might render that message in some distinctive fashion. In order to avoid further fragmentation of the Internet email system, it is necessary that the Internet community as a whole come to a consensus as to what mail senders should do to make their mail appear non-spoofed, and how mail receivers should determine whether mail is spoofed. On the other hand, it is not necessary to reach a consensus regarding the actions that various parties take once a message has been determined to be spoofed. This can be done unilaterally -- one agent might decide to discard a spoofed message while another decides to add a disclaimer. 2. Problem Statement 2.1 Positive Problem Statement Briefly stated, the mechanisms of this document allow one to answer the following question: When a message is transferred via SMTP between two UNRELATED parties, does the SMTP client host have permission to send mail on behalf of the mailbox that allegedly caused the most recent introduction of the message into the mail delivery system? As seen from the question, this mechanism applies to unrelated parties: it is useful at the point where a message passes across the Internet from one organization to another. It is beyond the scope of this document to describe authentication mechanisms that can be deployed within an organization. The mechanism of this document also seeks to authenticate the mailbox associated with the MOST RECENT introduction of a message into the mail delivery system. In simple cases, this is who the mail is from. However, in the case of a third-party mailer, a forwarder or a mailing list server, the address being authenticated is that of the third party, the forwarder or the mailing list. Lyon, Wong Expires - January 2005 [Page 3]
MTA Authentication Records in DNS July 2004 This document provides means to authenticate the DOMAIN of the appropriate email address; it makes no attempt to authenticate the local-part. A domain owner gets to determine which SMTP clients speak on behalf of addresses within the domain; a responsible domain owner should not authorize SMTP clients that will lie about local parts. In the long run, once the domain of the sender is authenticated, it will be possible to use that domain as part of a mechanism to determine the likelihood that a given message is spam, using, for example, reputation and accreditation services. (These services are not the subject of the present mechanism, but it should enable them.) 2.2 Negative Problem Statement Following are several alternate questions, which this specification makes no attempt to answer: 1. Is the host at a particular IP address authorized to act as an SMTP client? 2. Is an SMTP client authorized to use a particular domain name in its SMTP EHLO command? 3. Is an SMTP client authorized to use a particular email address in an SMTP "MAIL FROM:" command? 4. Was a message really authored by who it claims to be authored by? 3. Decision Model The essence of this specification is: Given an email message, and given an IP address from which it has been (or will be) received, is the SMTP client at that IP address authorized to send that email message? This question will usually be asked by an SMTP server as part of deciding whether to accept an incoming mail message. However, this question could also be asked later by a different party. An MUA, for example, could use the result of this question to determine how to file or present a message. Lyon, Wong Expires - January 2005 [Page 4]
MTA Authentication Records in DNS July 2004 There are four steps to answering this question: (1) From the headers of the email message, extract the "purported responsible address". This is the mailbox that the message claims is responsible for the most recent introduction of the message into the delivery system. This step is described in detail in section 4 below. A separate specification, [Submitter], describes an SMTP extension that allows an SMTP server to perform this check at the time of the SMTP MAIL command instead of the SMTP DATA command. (2) Extract the domain part of the purported responsible address. Call this the "purported responsible domain". (3) Call the check_host function defined in [Protocol], passing the following parameters: a. The IP address (either IPv4 or IPv6) from which the message is being or has been received. b. The purported responsible domain from step (2) above. c. The purported responsible address from step (1) above. The result of the check_host function is one of the values "Neutral", "Pass", "Fail", "SoftFail", "None", "TempError" or "PermError". Section 5 describes how these results are used by MTAs receiving messages. This specification imposes no requirements on parties performing this test in other environments. 4. Determining the Purported Responsible Address The purported responsible address (PRA) of a message is determined by the following algorithm: 1. Locate the first non-empty Resent-Sender header in the message. If no such header is found, continue with step 2. If it is preceded by a non-empty Resent-From header and one or more Received or Return-Path headers occur after said Resent-From header and before the Resent-Sender header, continue with step 2. Otherwise, proceed to step 5. 2. Locate the first non-empty Resent-From header in the message. If a Resent-From header is found, proceed to step 5. Otherwise, continue with step 3. 3. Locate all the non-empty Sender headers in the message. If there are no such headers, continue with step 4. If there is exactly one such header, proceed to step 5. If there is more than one such header, proceed to step 6. Lyon, Wong Expires - January 2005 [Page 5]
MTA Authentication Records in DNS July 2004 4. Locate all the non-empty From headers in the message. If there is exactly one such header, continue with step 5. Otherwise, proceed to step 6. 5. A previous step has selected a single header from the message. If that header is malformed (e.g. it appears to contain multiple mailboxes, or the single mailbox is hopelessly malformed, or the single mailbox does not contain a domain name), continue with step 6. Otherwise, return that single mailbox as the Purported Responsible Address. 6. The message is ill-formed, and it is impossible to determine a Purported Responsible Address. MTAs performing the Sender ID check as part of receiving a message SHOULD reject that message with "550 5.1.7 Missing Purported Responsible Address". Note that what constitutes a hopelessly malformed header or a hopelessly malformed mailbox in step 5 above is a matter for local policy. Such local policy will never cause two implementations to return different PRAs. However it may cause one implementation to return a PRA where another implementation does not. The result is that corner cases may result in messages of questionable deliverability, but they will never result in an MTA doing MARID checks on a different PRA from the address that a PRA-aware MUA chooses to display, even if they make their decisions independently. 5. Actions Based on the Decision When the Sender ID test is used by an SMTP server as part of receiving a message, the server should take the actions described by this section. The check_host function returns one of the following results. See [Protocol] for the meaning of these results. 5.1 Neutral or None or PermError An SMTP server receiving one of these results SHOULD NOT reject the message for this reason alone, but MAY subject the message to heightened scrutiny by other anti-spam measures, and MAY reject the message as a result of this heightened scrutiny. 5.2 Pass An SMTP server receiving this result SHOULD treat the message as authentic. It may accept or reject the message depending on other policies. Lyon, Wong Expires - January 2005 [Page 6]
MTA Authentication Records in DNS July 2004 5.3 Fail An SMTP server receiving this result SHOULD reject the message with a "550 5.7.1 Sender ID xxx - yyy" SMTP error, where "xxx" is replaced with the additional reason returned by the check_host function and "yyy" is replaced with the explanation string returned by the check_host function. 5.4 SoftFail An SMTP server receiving this result SHOULD NOT reject the message for this reason alone, but MAY subject the message to heightened scrutiny by other anti-spam measures, and MAY reject the message as a result of this heightened scrutiny. A message for which the result is "SoftFail" is less likely to be authentic than a message for which the result is "Neutral". 5.5 TempError An SMTP server receiving this result MAY reject the message with a "450 4.4.3 Sender ID check is temporarily unavailable" error code. Alternatively, an SMTP server receiving this result MAY accept a message and optionally subject it to heightened scrutiny by other anti-spam measures. 6. Security Considerations This entire document describes a new mechanism for mitigating spoofed email, which is today a pervasive security problem in the Internet. Assuming that this mechanism is widely deployed, the following sections describe counter-attacks that could be used to defeat this mechanism. 6.1 DNS Attacks The new mechanism is entirely dependent on DNS lookups, and is therefore only as secure as DNS. An attacker bent on spoofing messages could attempt to get his messages accepted by sending forged answers to DNS queries. An MTA could largely defeat such an attack by using a properly paranoid DNS resolver. DNSSEC may ultimately provide a way to completely neutralize this class of attacks. Lyon, Wong Expires - January 2005 [Page 7]
MTA Authentication Records in DNS July 2004 6.2 TCP Attacks This mechanism is designed to be used in conjunction with SMTP over TCP. A sufficiently resourceful attacker might be able to send TCP packets with forged from-addresses, and thus execute an entire SMTP session that appears to come from somewhere other than its true origin. Such an attack requires guessing what TCP sequence numbers an SMTP server will use. It also requires transmitting completely in the blind - the attack will be unable hear any of the server's side of the conversation. Attacks of this sort can be ameliorated if IP gateways refuse to forward packets when the source address is clearly bogus. 6.3 Forged Resent-From Attacks This mechanism chooses a purported responsible address from one of a number of message headers, and then uses that address for validation. A message with a true Resent-From header (for example), but a forged From header will be accepted. Since many MUAs do not display all of the headers of received messages, the message will appear to be forged when displayed. In order to avoid this attack, MUAs will need to start displaying at least the header that was verified. 7. Implementation Guidance This section describes the actions that certain members of the Internet email ecosystem must take to be compliant with this specification. 7.1 Simple E-mailers A domain that injects original email into the Internet, using its own name in From headers, need do nothing to be compliant. However, such domains SHOULD publish e-mail policy records in DNS. Lyon, Wong Expires - January 2005 [Page 8]
MTA Authentication Records in DNS July 2004 7.2 E-mail Forwarders A program that forwards received mail to other addresses MUST add an appropriate header that contains an email address that it is authorized to use. Such programs SHOULD use the Resent-From header for this purpose. Some of today's forwarders already add an appropriate header (although many of them use Sender rather than Resent-From.) 7.3 Mailing List Servers A mailing list server MUST add an appropriate header that contains an email address that it is authorized to use. Such programs SHOULD use the Resent-From header for this purpose. Most of today's mailing list software already adds an appropriate header (although most of them use Sender rather than Resent-From). 7.4 Third-Party Mailers A program that sends mail on behalf of another user MUST add an appropriate header than contains an email address that it is authorized to use. Such programs SHOULD use the Sender header for this purpose. Many, but not all, of today's third-party mailers are already compliant. 7.5 MUA Implementers When displaying a received message, an MUA SHOULD display the purported responsible address as defined by this document whenever that address differs from the RFC 2822 From address. This display SHOULD be in addition to the RFC 2822 From address. When a received message contains multiple headers that might be used for the purported responsible address determination, an MUA should consider displaying all of them. That is, if a message contains several Resent-From's, a Sender and a From, an MUA should consider displaying all of them. 8. IANA Considerations This document contains no actions for IANA. Lyon, Wong Expires - January 2005 [Page 9]
MTA Authentication Records in DNS July 2004 9. Acknowledgements Variations on the idea of using a DNS record to check the legitimacy of an email address have occurred multiple times. The earliest known work is [Vixie]; others include [RMX], [SPF] and [CallerID]. The current document borrows heavily from each of the above, and incorporates ideas proposed by many members of the MARID working group. The contributions of each of the above are gratefully acknowledged. 10. References 10.1 Normative References [Protocol] M. Wong and M. Lentczner, "The SPF Record Format and Test Protocol", draft-ietf-marid-protocol-00. Work in progress. [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119. 10.2 Informative References [CallerID] Microsoft Corporation, Caller ID for E-Mail Technical Specification, http://www.microsoft.com/mscorp/twc/privacy/spam_callerid .mspx. [RMX] H. Danisch, "The RMX DNS RR and method for lightweight SMTP sender authorization", draft-danisch-dns-rr-smtp-04. Work in progress. [SPF] M. Lentczner and M. Wong, "Sender Policy Framework (SPF): A Convention to Describe Hosts Authorized to Send SMTP Traffic", draft-mengwong-spf-01. Work in progress. [Submitter] E. Allman and H. Katz, "SMTP Service Extension for Indicating the Responsible Submitter of an E-mail Message", draft-ietf-marid-submitter-00. Work in progress. [Vixie] Paul Vixie, "Repudiating Mail-From", http://ops.ietf.org/lists/namedroppers/namedroppers.2002/ msg00658.html Lyon, Wong Expires - January 2005 [Page 10]
MTA Authentication Records in DNS July 2004 11. Authors' Addresses Jim Lyon Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA jimlyon@microsoft.com Meng Weng Wong Singapore mengwong@dumbo.pobox.com Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf- ipr@ietf.org. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE Lyon, Wong Expires - January 2005 [Page 11]
MTA Authentication Records in DNS July 2004 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Lyon, Wong Expires - January 2005 [Page 12]
