From: ftp://ftp.zcu.cz/pub/doc/internet-drafts/incoming/draft-ietf-marid-core-00.txt Title: MTA Authentication Records in DNS Reference: IETF MARID Working Group, Internet Draft 'draft-ietf-marid-core-00.txt' Date: May 2004 See also v -01: http://xml.coverpages.org/draft-ietf-marid-core-01.txt Topic References: http://xml.coverpages.org/ni2004-06-02-a.html Email Spoofing Targeted in IETF Draft on MTA Authentication Records in DNS ======================================================================== MTA Authentication Records in DNS May 2004 MARID Working Group J. Lyon Internet Draft Microsoft Corp Document: draft-ietf-marid-core-00.txt M. Wong pobox.com Expires: November 2004 May 2004 MTA Authentication Records in DNS Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of [RFC2026]. This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026 except that the right to produce derivative works is not granted. 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://www.ietf.org/ietf/1id-abstracts.txt 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 mechanisms by which a domain owner can publish its set of outgoing MTAs, and 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 - November 2004 [Page 1] MTA Authentication Records in DNS May 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.................................................5 4. Determining the Purported Responsible Address..................6 5. E-mail Policy Document.........................................7 5.1 Elements of the Infoset....................................7 5.1.1 "root" element.......................................7 5.1.2 "ep" element.........................................7 5.1.3 "out" element........................................8 5.1.4 "m" element..........................................9 5.1.5 "exists" element.....................................9 5.1.6 "include" element...................................10 5.1.7 "mx" element........................................10 5.1.8 "r" element.........................................10 5.2 Macro Expansion...........................................11 5.2.1 Macro definitions...................................11 5.2.2 Expansion Examples..................................13 5.3 Determining the EMail Policy Document for a Domain........14 5.3.1 From an XML TXT Record..............................14 5.3.2 From an SPF TXT Record..............................15 6. Actions Based on the Decision.................................16 7. Security Considerations.......................................16 7.1 DNS Attacks...............................................16 7.2 TCP Attacks...............................................17 7.3 Forged Resent-From Attacks................................17 8. Extensibility Considerations..................................17 9. Applicability Statement.......................................18 9.1 Simple EMailers...........................................18 9.2 EMail Forwarders..........................................18 9.3 Mailing List Servers......................................19 9.4 Third-Party Mailers.......................................19 9.5 MTA Implementers..........................................19 9.6 MUA Implementers..........................................19 10. IANA Considerations..........................................20 11. Patent Disclosure............................................20 12. Acknowledgements.............................................20 13. References...................................................20 13.1 Normative References.....................................20 13.2 Informative References...................................21 14. Authors' Addresses...........................................21 15. Full Copyright Statement.....................................22 Appendix A: A Brief Introduction to XML.........................22 Appendix B: XML Schema for urn:ietf:params:xml:schema:marid-1...23 Lyon, Wong Expires - November 2004 [Page 2] MTA Authentication Records in DNS May 2004 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]. << Text contained in double angle brackets describes actions that are yet to be taken and decisions that are yet to be made. No such text should survive in the final version of this draft. >> 1. Introduction Today, a huge majority of the 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 this 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 Lyon, Wong Expires - November 2004 [Page 3] MTA Authentication Records in DNS May 2004 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. 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 reputation and accreditation services. (These services are not the subject of the present mechanism, but it should enable them.) 2.2 Negative Problem Statement This section describes alternate questions, which this document makes no attempt to solve. Is the host at a particular IP address authorized to act as an SMTP client? [DMP] attempts to answer this question using a reverse DNS lookup. It suffers from the fact that reverse DNS lookups are poorly implemented in major fractions of the IP address space, from the fact that many ISPs refuse to reverse-delegate to small domains, and from the fact that it's all or nothing: authority to act as an SMTP client conveys authority to send absolutely any email, legitimate or otherwise. Is an SMTP client authorized to use a particular domain name in its SMTP EHLO command? [CSV] attempts to answer this question. It suffers from the fact that the EHLO name has a tenuous relationship, at best, with the contents of any mail message. Is an SMTP client authorized to use a particular email address in an SMTP "MAIL FROM:" command? [RMX] and [SPF] attempt to answer this question. These suffer from the fact that the MAIL FROM address really describes where to send NDRs to, and in many third-party and forwarder situations this address is unrelated to the domain that is resending the message. Lyon, Wong Expires - November 2004 [Page 4] MTA Authentication Records in DNS May 2004 Was a message really authored by who it claims to be authored by? [SMIME] and [DK] attempt to answer the question of original authorship. While this is a useful question to answer, these approaches suffer from severe deployment issues. 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 host address authorized to send that email message? 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". << TODO: The below is confusing. Clarify it. >> (3) Find the E-mail Policy Document for the purported responsible domain. Section 5.1 below describes the semantics of an EMail Policy Document. Section 5.2 below describes how to find the document for a domain. The EMail Policy Document contains a description of a "client authorization function". This function that takes four arguments and returns a four-valued result. The arguments are: a. The local-part of an email address. b. A domain name, called the "original domain". c. A domain name, called the "current domain". d. An IP address (either IPv4 or IPv6). The result of the function is one of the four values "unknown", "pass", "fail" or "softFail". (4) Execute the function, passing: the local-part of the Purported Responsible Address, the Purported Responsible Domain (as "original domain"), the Purported Responsible Domain (as "current domain"), and the IP address from which the mail was received. Lyon, Wong Expires - November 2004 [Page 5] MTA Authentication Records in DNS May 2004 A "Pass" result from this check means that the domain has authorized the sending of the message in question. An SMTP server receiving this result SHOULD accept the message. A "Fail" result from this check means that the domain disclaims authorization of the message. An SMTP server receiving this result SHOULD reject the message with a 550 SMTP error code. A "SoftFail" result from this check means that the message did not originate from the domain's primary mail servers, but the domain is unable to definitively state that the message was not authorized. 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. An "Unknown" result from this check means that either the domain is unable to determine whether the message is authorized, or that the domain's EMail policy record is absent, malformed, or otherwise unusable. 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. Note that steps 1, 2 and 4 may fail to complete successfully. In this case, the result of the check is "Unknown". 4. Determining the Purported Responsible Address The purported responsible address of a message is taken to be the first from the following list of items that is present, non-empty, and is a syntactically valid e-mail address: (1) The first Resent-Sender header in the message, unless (per the rules of [RFC2822]) it is preceded by a Resent-From header and one or more Received or Return-Path headers occur after said Resent-From header and before the Resent-Sender header (see section 3.6.6. of RFC2822 for further information on Resent headers). (2) The first mailbox in the first Resent-From header in the message, (3) The first of either the Delivered-To, X-Envelope-To or Envelope- To headers in the message. These headers are added to forwarded messages by some well-known MTAs. Lyon, Wong Expires - November 2004 [Page 6] MTA Authentication Records in DNS May 2004 (4) The Sender header in the message, (5) The first mailbox in the From header in the message, The purported responsible domain of the message is the domain part of the purported responsible address. If a message contains none of the above headers, an MTA SHOULD reject is as hopelessly malformed. 5. E-mail Policy Document An E-mail Policy Document is modeled by an XML infoset that contains, among other things, a definition of the function described in section 3 above. This function can be used to determine whether a domain owner is willing to take responsibility for e-mail that is sent by a particular SMTP client. This document describes those parts of the XML infoset that define the mail acceptance function. The infoset may contain other information relating to e-mail; this other information may be the subject of future IETF consensus processes. Any program that interprets the XML Infoset SHOULD ignore any elements whose tags are not understood by the program. Section 5.1 below contains a detailed definition of the XML infoset. Section 5.2 contains a description of the macro expansion performed on the character data in some of the elements. Section 5.3 below contains the algorithm by which the XML infoset may be obtained. 5.1 Elements of the Infoset The root of the infoset is always a "root" element. 5.1.1 "root" element Attributes: none Child elements: A single "ep" element. The "root" element carries no semantic information -- it's an artifact of the manner in which the infoset is constructed. 5.1.2 "ep" element Attributes: "testing" (Boolean) Lyon, Wong Expires - November 2004 [Page 7] MTA Authentication Records in DNS May 2004 Child elements: A single optional "out" element. The "ep" element is intended to describe a domain's EMail policy. At the present writing, this is limited to describing the client authorization function, but other information may be added in the future. If the "testing" attribute is "true", then programs SHOULD behave as if the EMail policy is absent, except by prior arrangement with the owner of the domain. If the "out" element is present, it describes the client authorization function. If "out" is absent, the client authorization function always returns "unknown". 5.1.3 "out" element Attributes: "default" (one of "pass", "fail", "softFail", "unknown") "redirect" (a domain name with macro expansion) Child elements: A sequence of zero or more "m" elements. The "out" element describes the client authorization function. To evaluate this function, evaluate each of the "m" elements in sequence as described below, until one of them returns a definite result. That result is the result of the client authorization function. If no child "m" element returns a definite result, then: 1. If the "redirect" attribute is present, extract the domain name from it with macro expansion, obtain that domain's EMail policy document, and return the result of evaluating that document's client authorization function, passing: the local-part that was passed to this function, the original domain name that was passed to this function, the domain name from the "redirect" attribute (after macro expansion), and the IP address that was passed to this function. 2. Otherwise, if the "default" attribute is present, the result of the client authorization function is the value of this attribute. 3. Otherwise, the result of the client authorization function is "fail". Lyon, Wong Expires - November 2004 [Page 8] MTA Authentication Records in DNS May 2004 5.1.4 "m" element Attributes: "result" (one of "pass", "fail", "softFail", "unknown") Child Elements: A sequence of zero or more "a", "exists", "include", "mx" and "r" elements. Each "m" element describes a portion of the client authorization function. To evaluate the client authorization function, evaluate each of the child elements in sequence, until one of them returns a match. If one of them returns a match, the result of the client authorization function is the value of the "result" attribute. If no child element returns a match, evaluation will continue with the next "m" element. "a" element Attributes: None Child Elements: None Character Data: An optional domain name or IPv4 or IPv6 address, with macro expansion. If the character data is empty, replace it with "%D". If an "a" element contains an IPv4 or IPv6 address, it matches iff that address exactly equals the IP address passed to the client authorization function. Otherwise, obtain the A or AAAA records for the given domain name (after macro expansion). The "a" element matches if the IP address in any of these records exactly equals the IP address that was passed to the client authorization function. If the specified name does not exist in DNS, or if no A or AAAA records (as appropriate) exist, the "a" element does not match. If any other error occurs during lookup, the result of the client authorization function is "unknown". 5.1.5 "exists" element Attributes: None Child Elements: None Character Data: A domain name, with macro expansion. This element matches iff the domain given by the character data (after macro expansion) exists in DNS. If the DNS lookup fails for any reason other than nonexistence, the client authorization function returns "unknown". Lyon, Wong Expires - November 2004 [Page 9] MTA Authentication Records in DNS May 2004 5.1.6 "include" element Attributes: None Child Elements: None Character Data: A domain name, with macro expansion. In order to determine whether this element matches, obtain the EMail policy record for the specified domain (after macro expansion), and evaluate its client authorization function, passing the original local-part the original domain-name the domain name from the character data (after macro expansion), the original IP address. The "include" element matches iff that domain's client authorization function returns "pass". If any error occurs in obtaining the domain's EMail policy record, the current client authorization function evaluation returns "unknown". 5.1.7 "mx" element Attributes: None Child Elements: None Character Data: An optional domain name, with macro expansion. If the character data is empty, replace it with "%D". This element matches iff the IP address is listed as a mail server for the specified domain. This can occur for either of two reasons: 1. One or more MX records exists for the domain, and one of the MX records references a domain that contains an A or AAAA record that exactly matches the input IP address. 2. No MX records exist for the domain, and an A or AAAA record for the domain exactly matches the input IP address. If any DNS lookup error other than domain not found occurs, the client authorization function returns "unknown". 5.1.8 "r" element Attributes: None Child Elements: None Lyon, Wong Expires - November 2004 [Page 10] MTA Authentication Records in DNS May 2004 Character Data: IP address '/' count This element matches iff the IP address in the character data is of the same form as the input IP address (IPv4 or IPv6), and the two addresses are equal when comparing only the most significant 'count' bits. 5.2 Macro Expansion This section defines the macro expansion that occurs on many of the domain names in the EMail policy document. 5.2.1 Macro definitions macro-string = *( macro-char / VCHAR ) macro-char = ( "%{" ALPHA transformer *delimiter "}" ) / "%%" / "%_" / "%-" transformer = [ *DIGIT ] [ "r" ] delimiter = "." / "-" / "+" / "," / "/" / "_" / "=" A literal "%" is expressed by "%%". %_ expands to a single " " space. %- expands to a URL-encoded space, viz. "%20". The following macro letters are expanded in directive arguments: l = local-part passed to the client authorization function. s = Same as "%{l}@%{o}". o = original domain passed to the client authorization function. d = current domain passed to the client authorization function. i = SMTP client IP (nibble format when an IPv6 address). p = SMTP client domain name v = client IP version string: "in-addr" for ipv4 or "ip6" for ipv6 r = domain name of the SMTP server. The uppercase versions of all these macros are URL-encoded. A '%' character not followed by a '{', '%', '-', or '_' character MUST be interpreted as a literal. SPF publishers SHOULD NOT rely on this feature; they MUST escape % literals. Legal optional transformers are: *DIGIT ; zero or more digits 'r' ; reverse value, splitting on dots by default If transformers or delimiters are provided, the macro strings are split into parts. After performing any reversal operation or removal Lyon, Wong Expires - November 2004 [Page 11] MTA Authentication Records in DNS May 2004 of left-hand parts, the parts are rejoined using "." and not the original splitting characters. By default, strings are split on "." (dots). Macros may specify delimiter characters which are used instead of ".". Delimiters MUST be one or more of the characters: "." / "-" / "+" / "," / "/" / "_" / "=" The 'r' transformer indicates a reversal operation: if the client IP address were 192.0.2.1, the macro %{i} would expand to "192.0.2.1" and the macro %{ir} would expand to "1.2.0.192". The DIGIT transformer indicates the number of right-hand parts to use after optional reversal. If a DIGIT is specified, it MUST be nonzero. If no DIGITs are specified, or if the value specifies more parts than are available, all the available parts are used. If the DIGIT was 5, and only 3 parts were available, the macro interpreter would pretend the DIGIT was 3. Implementations MAY limit the number, but MUST support at least a value of 9. For IPv4 addresses, both the "i" and "c" macros expand to the standard dotted-quad format. For IPv6 addresses, the "i" macro expands to dot-format address; it is intended for use in %{ir}. The "c" macro may expand to any of the hexadecimal colon-format addresses specified in [RFC3513] section 2.2. It is intended for humans to read. Use of the "t" macro in DNS lookups would greatly reduce the effectiveness of DNS caching. The "t" macro is only allowed in explanation records. The value of the "t" macro SHOULD NOT change during the evaluation of a given SPF record. The "p" macro expands to the validated domain name of the SMTP client. The validation procedure is described in section 4.6. If there are no validated domain names, the word "unknown" is substituted. If multiple validated domain names exist, the first one returned in the PTR result is chosen. The "r" macro expands to the name of the receiving MTA. This SHOULD be a fully qualified domain name, but if one does not exist (as when the checking is done by a script) or if policy restrictions dictate otherwise, the word "unknown" SHOULD be substituted. The domain name MAY be different than the name found in the MX record that the client MTA used to locate the receiving MTA. The "s" macro expands to the sender email address: a localpart, an @ sign, and a domain. The "o" macro is the domain part of the "s". Lyon, Wong Expires - November 2004 [Page 12] MTA Authentication Records in DNS May 2004 They remain the same during a recursive "include" or "redirect" subquery. When the result of macro expansion is used in a domain name query, if the expanded domain name exceeds 255 characters (the maximum length of a domain name), the left side is truncated to fit, by removing successive subdomains until the total length falls below 255 characters. Uppercased macros are URL escaped. URL encoding is described in [RFC2396]. 5.2.2 Expansion Examples The is strong-bad@email.example.com. The IPv4 SMTP client IP is 192.0.2.3. The IPv6 SMTP client IP is 5f05:2000:80ad:5800::1. The PTR domain name of the client IP is mx.example.org. macro expansion ------------------------------- %{s} strong-bad@email.example.com %{o} email.example.com %{d} email.example.com %{d4} email.example.com %{d3} email.example.com %{d2} example.com %{d1} com %{p} mx.example.org %{p2} example.org %{dr} com.example.email %{d2r} example.email %{l} strong-bad %{l-} strong.bad %{lr} strong-bad %{lr-} bad.strong %{l1r-} strong macro-string expansion --------------------------------------------------------------------- %{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com %{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com %{lr-}.lp.%{ir}.%{v}._spf.%{d2} bad.strong.lp.3.2.0.192.in-addr._spf.example.com %{ir}.%{v}.%{l1r-}.lp._spf.%{d2} 3.2.0.192.in-addr.strong.lp._spf.example.com Lyon, Wong Expires - November 2004 [Page 13] MTA Authentication Records in DNS May 2004 %{p2}.trusted-domains.example.net example.org.trusted-domains.example.net IPv6: %{ir}.example.org 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8. 5.d.a.0.8.0.0.0.2.5.0.f.5.example.org 5.3 Determining the EMail Policy Document for a Domain In order to find the EMail policy document for a domain, the following algorithm (or an algorithm that always yields the same result) MUST be used. In this algorithm, "$DOMAIN" stands for the domain whose EMail policy document is desired. 1. From DNS, obtain the set of TXT records for domain "_ep.$DOMAIN". If the set contains exactly one TXT record, apply the transformations in section 5.2.1 to that record. If the set is empty, or if the domain "_ep.$DOMAIN" does not exist, continue to step 2. If the set contains more than one record, or if any other error occurs on the DNS lookup, the EMail policy document cannot be obtained. 2. From DNS, obtain the set of TXT records for domain "$DOMAIN". If the set contains exactly one record whose first string starts with the characters "v=spf1 ", apply the transformations in section 5.2.2 to that record to obtain the EMail policy document. Otherwise, if domain "$DOMAIN" does not exist, continue to step 3. Otherwise, the EMail policy document cannot be obtained. 3. Any domain that does not exist is defined to have an EMail policy document containing the XML Infoset described by the following XML text: 5.3.1 From an XML TXT Record Given a DNS TXT record from the "_ep" subdomain of the desired domain, the EMail policy document is obtained as follows: 1. Append a CRLF to each string in the DNS record. Lyon, Wong Expires - November 2004 [Page 14] MTA Authentication Records in DNS May 2004 2. Concatenate all of the resulting strings together, in order. 3. Prefix the resulting string with the following: 4. Suffix the resulting string with the following: 5. If the resulting string is a well-formed and valid XML document, the Infoset corresponding to that string is the domain's EMail policy document. 6. If the resulting string is either not well-formed or not valid, the EMail policy document for the domain cannot be determined. XML namespace "urn:ietf:params:xml:schema:marid-1" is defined by this specification, the "...:marid-2" through "...:marid-5" namespaces are reserved for definition by future IETF consensus processes, and the namespaces "http://www.w3.org/2001/XMLSchema" and "http://www.w3.org/2000/09/xmldsig#" are defined by [XMLSchema] and [RFC3275], respectively. 5.3.2 From an SPF TXT Record << This text really needs to be rewritten. What follows gives the concept, but isn't legalistic enough. It probably needs to stand alone instead of referencing the SPF spec. >> Given a TXT record from the desired domain that starts with the characters "v=spf1 ", the EMail policy document is obtained via the following algorithm. The namespace of all elements and attributes in the resulting XML infoset is "urn:ietf:params:xml:schema:marid-1". 1. Start with the infoset described by the following XML text: Lyon, Wong Expires - November 2004 [Page 15] MTA Authentication Records in DNS May 2004 2. For each SPF mechanism in the input, create a new "m" node under the "out" node. The "result" attribute of the "m" node is "pass", "fail", "softFail" or "unknown", depending on whether the mechanism's prefix is "+", "-", "~", or "?". 3. Unless the mechanism name is "all", create a new element under the new "m" node, of a type as follows: mechanism element --------- ------- a a exists exists include include ip4 without "/" a ip4 with "/" r ip6 without "/" a ip6 with "/" r mx mx ptr ptr Place everything after the ":" (if any) into the character data of the new element. 4. Treat any unknown mechanism as if it said "?all". 5. If there's a "redirect" modifier, set the "redirect" attribute of the "out" node. The resulting infoset is now complete. 6. Actions Based on the Decision << To be written. >> 7. 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. 7.1 DNS Attacks Lyon, Wong Expires - November 2004 [Page 16] MTA Authentication Records in DNS May 2004 The new mechanism is entirely dependent of 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 attack. 7.2 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. 7.3 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. 8. Extensibility Considerations Many of the XML elements are defined to allow any attribute and any child element. A program MUST ignore any element or attribute whose meaning it does not understand. It will be easy to use new elements and attributes to describe other pieces of email policy that are not currently specified. For example, it may become appropriate to define elements that allow a domain to specify how to complain about spam from that domain, which accreditation agencies can vouch for the Lyon, Wong Expires - November 2004 [Page 17] MTA Authentication Records in DNS May 2004 domain, whether the domain is interested in answering challenges in challenge / response systems, and so forth. While it will be easy to add new kinds of information to the EMail policy document, future implementers must proceed with extreme care if they attempt to modify the semantics of the client authorization function. Code that is not aware of the new semantics will completely ignore any newly invented elements. It is anticipated that extensions will only be defined for the XML version of the DNS record, not for the SPF version. With the final draft of this document, the "urn:ietf:params:xml:schema:marid-1" namespace schema becomes frozen for all time. Future extensions will need to use other namespaces. In order to avoid avoid having to write very long namespaces in DNS TXT records, the namespace prefixes "m2" through "m5" are defined at this time, even though their referents will only be defined in the future. Their referents, the namespaces "urn:ietf:params:xml:schema:marid-2" through "...-5" are of the form that they can only be defined by a future IETF consensus process. 9. Applicability Statement This section describes the actions that certain members of the Internet email ecosystem must take to be compliant with this specification. << Be more precise and prescriptive about what header to insert where. >> 9.1 Simple EMailers 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 EMail policy records in DNS. 9.2 EMail 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. Lyon, Wong Expires - November 2004 [Page 18] MTA Authentication Records in DNS May 2004 Most of today's forwarders already add an appropriate header (although most of them use Delivered-To or some other nonstandard header rather than Resent-From.) 9.3 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). 9.4 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. 9.5 MTA Implementers MTAs that are acting as SMTP servers SHOULD implement the checks described in this document. An MTA SHOULD limit the number of DNS lookups (or the time spent performing the lookup) that it will perform in the process of checking a message. Such a limit SHOULD permit at least 20 DNS lookups. 9.6 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 From address. This display SHOULD be in addition to the 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. Lyon, Wong Expires - November 2004 [Page 19] MTA Authentication Records in DNS May 2004 10. IANA Considerations The IANA is requested to register the following URI: URI: urn:ietf:params:xml:schema:marid-1 Registrant Contact: IESG. XML: The contents of Appendix B of this document 11. Patent Disclosure << Write a statement to the effect that Microsoft has pending patents whose claims cover some of this. Get appropriate wording about licensing from the lawyers. The current intent is to retain the commitment to RAND/RF (reasonable and nondiscriminatory, royalty- free) licensing. >> 12. 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 [RMX]; others include [SPF} and [CallerID]. The current document borrows heavily from each of the above, and incorporates many ideas proposed by many members of the MARID working group. 13. References 13.1 Normative References [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119. [RFC2026] S. Bradner, "The Internet Standards Process - - Revision 3", RFC 2026. [RFC3275] D. Eastlake et al, "XML-Signature Syntax and Processing", RFC 3275. [XMLSchema] "XML Schema Part 1: Structures", W3C Recommendation 2 May 2001, http://www.w3c.org/TR/2001/REC-xmlschema-1- 20010502/ Lyon, Wong Expires - November 2004 [Page 20] MTA Authentication Records in DNS May 2004 13.2 Informative References [CallerID] Microsoft Corporation, Caller ID for E-Mail Technical Specification, http://www.microsoft.com/mscorp/twc/privacy/spam_callerid .mspx. [CSV] D. Crocker, "Client SMTP Validation (CSV)", draft- crocker-marid-smtp-validate-01. Work in progress. [DK] M. Delany, "Domain-based Email Authentication Using Public-Keys Advertised in the DNS (DomainKeys)", draft- delany-domainkeys-base-00.txt. Work in progress. [DMP] G. Feyck, "Designated Mailers Protocol (DMP)", draft- fecyk-dmp-01.txt. Work in progress. [RMX] H. Danisch, "The RMX DNS RR and method for lightweight SMTP sender authorization", draft-danisch-dns-rr-smtp- 04.txt. Work in progress. [SMIME] B. Ramsdell (editor), "S/MIME Version 3 Message Specification", RFC 2633. [SPF] M. Lentczner and M. Weng, "Sender Policy Framework (SPF): A Convention to Describe Hosts Authorized to Send SMTP Traffic", draft-mengwong-spf-01.txt. 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.txt. Work in progress. 14. Authors' Addresses Jim Lyon Microsoft Corporation One Microsoft Way Redmond, WA 98052 USA jimlyon@microsoft.com Meng Weng Wong Singapore mengwong@dumbo.pobox.com Lyon, Wong Expires - November 2004 [Page 21] MTA Authentication Records in DNS May 2004 15. Full Copyright Statement Copyright (C) The Internet Society (2004). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Appendix A: A Brief Introduction to XML << To be written. >> Lyon, Wong Expires - November 2004 [Page 22] MTA Authentication Records in DNS May 2004 Appendix B: XML Schema for urn:ietf:params:xml:schema:marid-1 Lyon, Wong Expires - November 2004 [Page 24]