--- 1/draft-ietf-dkim-ssp-09.txt 2009-05-11 23:12:05.000000000 +0200 +++ 2/draft-ietf-dkim-ssp-10.txt 2009-05-11 23:12:05.000000000 +0200 @@ -1,23 +1,23 @@ Network Working Group E. Allman Internet-Draft Sendmail, Inc. Intended status: Standards Track J. Fenton -Expires: August 9, 2009 Cisco Systems, Inc. +Expires: November 12, 2009 Cisco Systems, Inc. M. Delany Yahoo! Inc. J. Levine Taughannock Networks - February 5, 2009 + May 11, 2009 DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP) - draft-ietf-dkim-ssp-09 + draft-ietf-dkim-ssp-10 Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. 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. @@ -26,33 +26,32 @@ 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. - This Internet-Draft will expire on August 9, 2009. + This Internet-Draft will expire on November 12, 2009. Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal - Provisions Relating to IETF Documents - (http://trustee.ietf.org/license-info) in effect on the date of - publication of this document. Please review these documents - carefully, as they describe your rights and restrictions with respect - to this document. + Provisions Relating to IETF Documents in effect on the date of + publication of this document (http://trustee.ietf.org/license-info). + Please review these documents carefully, as they describe your rights + and restrictions with respect to this document. Abstract DomainKeys Identified Mail (DKIM) defines a domain-level authentication framework for email to permit verification of the source and contents of messages. This document specifies an adjunct mechanism to aid in assessing messages that do not contain a DKIM signature for the domain used in the author's address. It defines a record that can advertise whether a domain signs its outgoing mail, and how other hosts can access that record. @@ -60,88 +59,92 @@ Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4 2.1. Terms Imported from DKIM Signatures Specification . . . . 4 2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5 2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5 2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5 2.6. Author Domain Signing Practices . . . . . . . . . . . . . 5 - 2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5 - 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6 + 2.7. Author Domain Signature . . . . . . . . . . . . . . . . . 5 + 3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 5 3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 6 - 3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 7 + 3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6 3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 7 4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7 4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7 4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 8 4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 9 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 10 - 5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 11 - 6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 - 6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 11 - 6.2. DNS Considerations . . . . . . . . . . . . . . . . . . . . 12 - 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12 - 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 - 7.1. References - Normative . . . . . . . . . . . . . . . . . . 13 - 7.2. References - Informative . . . . . . . . . . . . . . . . . 13 - Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 14 - A.1. Domain and ADSP exist . . . . . . . . . . . . . . . . . . 14 - A.2. Domain exists, ADSP does not exist . . . . . . . . . . . . 14 - A.3. Domain does not exist . . . . . . . . . . . . . . . . . . 15 - Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 15 - B.1. Single Location Domains . . . . . . . . . . . . . . . . . 15 - B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 15 - B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 16 - B.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 16 - B.5. Domains with Independent Users and Liberal Use Policies . 17 - B.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 17 - Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 17 - Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 17 - D.1. Changes since -ietf-dkim-08 . . . . . . . . . . . . . . . 17 - D.2. Changes since -ietf-dkim-07 . . . . . . . . . . . . . . . 17 - D.3. Changes since -ietf-dkim-06 . . . . . . . . . . . . . . . 18 - D.4. Changes since -ietf-dkim-05 . . . . . . . . . . . . . . . 18 - D.5. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 18 - D.6. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 18 - D.7. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 19 - D.8. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 20 - D.9. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 21 - D.10. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 21 - D.11. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 22 - D.12. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 22 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 + 5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 10 + 5.3. Authentication-Results Method Registry Update . . . . . . 11 + 5.4. Authentication-Results Result Registry Update . . . . . . 11 + 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13 + 6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 13 + 6.2. DNS Considerations . . . . . . . . . . . . . . . . . . . . 14 + 6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 15 + 6.4. Inappropriate Application of Author Domain Signatures . . 15 + 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 + 7.1. References - Normative . . . . . . . . . . . . . . . . . . 15 + 7.2. References - Informative . . . . . . . . . . . . . . . . . 16 + Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 16 + A.1. Domain and ADSP exist . . . . . . . . . . . . . . . . . . 17 + A.2. Domain exists, ADSP does not exist . . . . . . . . . . . . 17 + A.3. Domain does not exist . . . . . . . . . . . . . . . . . . 17 + Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 17 + B.1. Single Location Domains . . . . . . . . . . . . . . . . . 18 + B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 18 + B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 19 + B.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 19 + B.5. Domains with Independent Users and Liberal Use Policies . 19 + B.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 19 + Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 19 + Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 20 + D.1. Changes since -ietf-dkim-09 . . . . . . . . . . . . . . . 20 + D.2. Changes since -ietf-dkim-08 . . . . . . . . . . . . . . . 20 + D.3. Changes since -ietf-dkim-07 . . . . . . . . . . . . . . . 20 + D.4. Changes since -ietf-dkim-06 . . . . . . . . . . . . . . . 20 + D.5. Changes since -ietf-dkim-05 . . . . . . . . . . . . . . . 21 + D.6. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 21 + D.7. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 21 + D.8. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 22 + D.9. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 22 + D.10. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 23 + D.11. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 24 + D.12. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 24 + D.13. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 25 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 1. Introduction DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a signing domain to claim responsibility for the introduction of a message into the mail stream. Message recipients can verify the signature by querying the signer's domain directly to retrieve the appropriate public key, and thereby confirm that the message was attested to by a party in possession of the private key for the signing domain. However, the legacy of the Internet is such that not all messages will be signed, and the absence of a signature on a message is not an a priori indication of forgery. In fact, during early phases of deployment it is very likely that most messages will remain unsigned. However, some domains might decide to sign all of their outgoing mail, for example, to protect their brand names. It might be desirable for such domains to be able to advertise that fact to other hosts. This is the topic of Author Domain Signing Practices (ADSP). - Hosts implementing this specification can inquire what Author Signing - Practices a domain advertises. This inquiry is called an Author - Signing Practices check. + Hosts implementing this specification can inquire what Author Domain + Signing Practices a domain advertises. This inquiry is called an + Author Domain Signing Practices check. The basic requirements for ADSP are given in [RFC5016]. This document refers extensively to [RFC4871] and assumes the reader is familiar with it. Requirements Notation: 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] @@ -182,42 +185,31 @@ An "Alleged Author" is an Author Address of a message; it is "alleged" because it has not yet been checked. 2.6. Author Domain Signing Practices "Author Domain Signing Practices" (or just "practices") consist of a machine-readable record published by the domain of an Alleged Author which includes statements about the domain's practices with respect to mail it sends with its domain in the From: line. -2.7. Author Signature - - An "author signature" is a Valid Signature that has the same domain - name in the DKIM signing identity as the domain name in the Author - Address. If the DKIM signing identity has a Local-part, it is be - identical to the Local-part in the Author Address. Following - [RFC5321], Local-part comparisons are case sensitive, but domain - comparisons are case insensitive. +2.7. Author Domain Signature - For example, if a message has a Valid Signature, with the DKIM- - Signature field containing "i=a@domain.example", then domain.example - is asserting that it takes responsibility for the message. If the - message's From: field contains the address "b@domain.example", that - would mean that the message does not have a valid Author Signature. - Even though the message is signed by the same domain, it will not - satisfy ADSP that specifies "dkim=all" or "dkim=discardable". + An "Author Domain Signature" is a Valid Signature in which the domain + name of the DKIM signing entity, the d= tag in the DKIM-Signature + header field, is the same as the domain name in the Author Address. + Following [RFC5321], domain name comparisons are case insensitive. - Note: ADSP is incompatible with valid DKIM usage in which a signer - uses "i=" with values that are not the same as addresses in mail - headers. In that case, a possible workaround could be to add a - second DKIM signature a "d=" value that matches the Author - Address, but no "i=". + For example, if the From: line address is bob@domain.example, and the + message has a Valid Signature, with the DKIM-Signature header field + containing "d=domain.example", then the message has an Author Domain + Signature. 3. Operation Overview Domain owners publish ADSP information via a query mechanism such as the Domain Name System; specific details are given in Section 4.1. Hosts can look up the ADSP information of the domain(s) specified by the Author Address(es) as described in Section 4.3. If a message has multiple Author Addresses the ADSP lookups SHOULD be performed independently on each address. This document does not address the @@ -258,42 +250,44 @@ 3.2. ADSP Usage Depending on the Author Domain(s) and the signatures in a message, a recipient gets varying amounts of useful information from each ADSP lookup. o If a message has no Valid Signature, the ADSP result is directly relevant to the message. - o If a message has an Author Signature, ADSP provides no benefit - relative to that domain since the message is already known to be - compliant with any possible ADSP for that domain. + o If a message has an Author Domain Signature, ADSP provides no + benefit relative to that domain since the message is already known + to be compliant with any possible ADSP for that domain. - o If a message has a Valid Signature other than an Author Signature, - the receiver can use both the Signature and the ADSP result in its - evaluation of the message. + o If a message has a Valid Signature other than an Author Domain + Signature, the receiver can use both the Signature and the ADSP + result in its evaluation of the message. 3.3. ADSP Results An ADSP lookup for an Author Address produces one of four possible results: - o Messages from this domain might or might not have an author + o Messages from this domain might or might not have an author domain signature. This is the default if the domain exists in the DNS but no ADSP record is found. - o All messages from this domain are signed with an Author Signature. + o All messages from this domain are signed with an Author Domain + Signature. - o All messages from this domain are signed with an Author Signature - and discardable, i.e., if a message arrives without a valid Author - Signature, the domain encourages the recipient(s) to discard it. + o All messages from this domain are signed with an Author Domain + Signature and discardable, i.e., if a message arrives without a + valid Author Domain Signature, the domain encourages the + recipient(s) to discard it. o This domain is out of scope, i.e., the domain does not exist in the DNS. An ADSP lookup could terminate without producing any result if a DNS lookup results in a temporary failure. 4. Detailed Description 4.1. DNS Representation @@ -306,24 +300,25 @@ [RFC4871] is used, modified to use WSP rather than FWS. Records not in compliance with that syntax or the syntax of individual tags described in Section 4.3 MUST be ignored (considered equivalent to a NODATA result) for purposes of ADSP, although they MAY cause the logging of warning messages via an appropriate system logging mechanism. If the RDATA contains multiple character strings, the strings are logically concatenated with no delimiters between the strings. Note: ADSP changes the "Tag=Value List" syntax from [RFC4871] to - use WSP rather than FWS in its DNS records. Domains MUST NOT - publish ADSP records with wildcard names. Wildcards within a - domain publishing ADSP records pose a particular problem, as - discussed in more detail in Section 6.3. + use WSP rather than FWS in its DNS records. + + Note: Domains MUST NOT publish ADSP records with wildcard names. + Wildcards within a domain publishing ADSP records pose a + particular problem, as discussed in more detail in Section 6.3. 4.2. Publication of ADSP Records ADSP is intended to apply to all mail sent using the domain name string of an Alleged Author. 4.2.1. Record Syntax ADSP records use the "tag=value" syntax described in section 3.2 of [RFC4871], modified to use WSP rather than FWS. Every ADSP record @@ -333,28 +328,29 @@ Tags used in ADSP records are described below. Unrecognized tags MUST be ignored. In the ABNF below, the WSP token, and the ALPHA and DIGIT tokens are imported from [RFC5234]. dkim= Outbound signing practices for the domain (plain-text; REQUIRED). Possible values are as follows: unknown The domain might sign some or all email. - all All mail from the domain is signed with an Author + all All mail from the domain is signed with an Author Domain Signature. discardable All mail from the domain is signed with an Author - Signature. Furthermore, if a message arrives without a valid - Author Signature due to modification in transit, submission via - a path without access to a signing key, or any other reason, - the domain encourages the recipient(s) to discard it. + Domain Signature. Furthermore, if a message arrives without a + valid Author Domain Signature due to modification in transit, + submission via a path without access to a signing key, or any + other reason, the domain encourages the recipient(s) to discard + it. Any other values are treated as "unknown". ABNF: adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP ("unknown" / "all" / "discardable") 4.3. ADSP Lookup Procedure Hosts doing an ADSP lookup MUST produce a result that is semantically @@ -451,20 +446,131 @@ The initial entries in the registry comprise: +-------------+-----------------+ | TYPE | REFERENCE | +-------------+-----------------+ | unknown | (this document) | | all | (this document) | | discardable | (this document) | +-------------+-----------------+ ADSP Outbound Signing Practices Registry Initial Values +5.3. Authentication-Results Method Registry Update + + IANA is requested to add the following to the Email Authentication + Method Name Registry: + + Method: dkim-adsp + + Defined In: this memo + + ptype: header + + property: from + + value: Contents of the [RFC5322] From: header field, with comments + removed + +5.4. Authentication-Results Result Registry Update + + IANA is requested to add or update the following in the Email + Authentication Result Name Registry: + + Code: none + + Existing/New Code: existing + + Defined In: [RFC5451] + + Auth Method: dkim-adsp (added) + + Meaning: No DKIM author domain signing practises (ADSP) record was + published. + + Code: pass + + Existing/New Code: existing + Defined In: [RFC5451] + + Auth Method: dkim-adsp (added) + + Meaning: This message had an Author Domain Domain Signature that was + validated. (An ADSP check is not strictly required to be + performed for this result, since a valid Author Domain Signature + satisfies all possible ADSP policies.) + + Code: unknown + + Existing/New Code: new + + Defined In: this memo + + Auth Method: dkim-adsp + + Meaning: No valid Author Domain Signature was found on the message + and the published ADSP was "unknown". + + Code: fail + + Existing/New Code: existing + + Defined In: [RFC5451] + + Auth Method: dkim-adsp (added) + + Meaning: No valid Author Domain Signature was found on the message + and the published ADSP was "all". + + Code: discard + + Existing/New Code: new + + Defined In: this memo + + Auth Method: dkim-adsp + + Meaning: No valid Author Domain Signature was found on the message + and the published ADSP was "discardable". + + Code: nxdomain + + Existing/New Code: new + Defined In: this memo + + Auth Method: dkim-adsp + + Meaning: Evaluating the ADSP for the Author's DNS domain indicated + that the Author's DNS domain does not exist. + + Code: temperror + + Existing/New Code: existing + + Defined In: [RFC5451] + + Auth Method: dkim-adsp (added) + + Meaning: An ADSP record could not be retrieved due to some error + that is likely transient in nature, such as a temporary DNS error. + A later attempt may produce a final result. + + Code: permerror + + Existing/New Code: existing + + Defined In: [RFC5451] + + Auth Method: dkim-adsp (added) + + Meaning: An ADSP record could not be retrieved due to some error + that is likely not transient in nature, such as a permanent DNS + error. A later attempt is unlikely to produce a final result. + 6. Security Considerations Security considerations in the ADSP are mostly related to attempts on the part of malicious senders to represent themselves as authors for whom they are not authorized to send mail, often in an attempt to defraud either the recipient or an Alleged Author. Additional security considerations regarding Author Domain Signing Practices are found in the DKIM threat analysis [RFC4686]. @@ -523,29 +629,46 @@ 6.3. DNS Wildcards DNS wildcards (described in [RFC4592]) that exist in the DNS hierarchy at or above the domain being checked interfere with the ability to verify the scope of the ADSP check described in Section 4.3. For example, a wildcard record for *.domain.example makes all subdomains such as foo.domain.example exist in the DNS. Domains that intend to make active use of ADSP by publishing a practice other than Unknown are advised to avoid the use of wildcards - elsewhere in their hierarchy. + in their hierarchy. If a domain contains wildcards, then any name that matches the wildcard can appear to be a valid mail domain eligible for ADSP. But the "_adsp._domainkey." prefix on ADSP records does not allow publication of wildcard records that cover ADSP records without also - covering non-ADSP records, nor of wildcard records that cover non- - ADSP records without also covering ADSP records. Hence a domain MUST - NOT publish wildcard ADSP records. + covering non-ADSP records, nor publication of wildcard records that + cover non-ADSP records without also covering ADSP records. A domain + that uses ADSP practices other than unknown SHOULD NOT publish + wildcard records. + +6.4. Inappropriate Application of Author Domain Signatures + + In one model of DKIM usage, a domain signs messages that are in + transit through their system. Since any signature whose domain + matches the Author Domain is by definition an Author Domain + Signature, it would be unwise to sign mail whose Author Domain is the + signer's domain if the mail is not known to meet the domain's + standards for an Author Domain Signature. + + One such use case is where a domain might apply such a signature is + following application of an Authentication-Results header field as + described in Section 7.1 of [RFC5451]. This problem can be easily + avoided either by not applying a signature that might be confused + with an Author Domain Signature or by applying a signature from some + other domain, such as a subdomain of the Author Domain. 7. References 7.1. References - Normative [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. @@ -564,20 +688,23 @@ [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008. + [RFC5451] Kucherawy, M., "Message Header Field for Indicating + Message Authentication Status", RFC 5451, April 2009. + 7.2. References - Informative [RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys Identified Mail (DKIM)", RFC 4686, September 2006. [RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices Protocol", RFC 5016, October 2007. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, @@ -615,21 +742,22 @@ A mail message contains this From: header line: From: alice@bbb.example (Old-fashioned Alice) The ADSP Lookup first identifies the Author Address alice@bbb.example and the Author Domain bbb.example. It does an MX DNS query for bbb.example, and gets back record (3). Since that query didn't return an error, it then proceeds to a TXT DNS query for _adsp._domainkey.bbb.example, which returns NXDOMAIN. Since the domain exists but there is no ADSP record, ADSP returns the default - unknown result: messages may or may not have an author signature. + unknown result: messages may or may not have an author domain + signature. A.3. Domain does not exist A mail message contains this From: header line: From: frank@ccc.example (Unreliable Frank) The ADSP Lookup first identifies the Author Address frank@ccc.example and the Author Domain ccc.example. It does an MX DNS query for ccc.example, and gets back an NXDOMAIN result since there are no @@ -641,78 +769,78 @@ These examples are intended to illustrate typical uses of ADSP. They are not intended to be exhaustive, nor to apply to every domain's or mail system's individual situation. Domain managers are advised to consider the ways that mail processing can modify messages in ways that will invalidate an existing DKIM signature, such as mailing lists, courtesy forwarders, and other paths that could add or modify headers, or modify the message body. In that case, if the modifications invalidate the DKIM signature, - recipient hosts will consider the mail not to have an Author + recipient hosts will consider the mail not to have an Author Domain Signature, even though the signature was present when the mail was originally sent. B.1. Single Location Domains A common mail system configuration handles all of a domain's users' incoming and outgoing mail through a single MTA or group of MTAs. In that case, the MTA(s) can be configured to sign outgoing mail with an - Author Signature. + Author Domain Signature. In this situation it might be appropriate to publish an ADSP record for the domain containing "all", depending on whether the users also - send mail through other paths that do not apply an Author Signature. - Such paths could include MTAs at hotels or hotspot networks used by - travelling users, web sites that provide "mail an article" features, - user messages sent through mailing lists, or third party mail clients - that support multiple user identities. + send mail through other paths that do not apply an Author Domain + Signature. Such paths could include MTAs at hotels or hotspot + networks used by travelling users, web sites that provide "mail an + article" features, user messages sent through mailing lists, or third + party mail clients that support multiple user identities. B.2. Bulk Mailing Domains Another common configuration uses a domain solely for bulk or broadcast mail, with no individual human users, again typically sending all the mail through a single MTA or group of MTAs that can - apply an Author Signature. In this case, the domain's management can - be confident that all of its outgoing mail will be sent through the - signing MTA. Lacking individual users, the domain is unlikely to - participate in mailing lists, but could still send mail through other - paths that might invalidate signatures. + apply an Author Domain Signature. In this case, the domain's + management can be confident that all of its outgoing mail will be + sent through the signing MTA. Lacking individual users, the domain + is unlikely to participate in mailing lists, but could still send + mail through other paths that might invalidate signatures. Domain owners often use specialist mailing providers to send their bulk mail. In that case, the mailing provider needs access to a - suitable signing key in order to apply an Author Signature. One - possible route would be for the domain owner to generate the key and - give it to the mailing provider. Another would be for the domain to - delegate a subdomain to the mailing provider, for example, + suitable signing key in order to apply an Author Domain Signature. + One possible route would be for the domain owner to generate the key + and give it to the mailing provider. Another would be for the domain + to delegate a subdomain to the mailing provider, for example, bigbank.example might delegate email.bigbank.example to such a provider. In that case, the provider can generate the keys and DKIM DNS records itself and use the subdomain in the Author address in the mail. Regardless of the DNS and key management strategy chosen, whoever maintains the DKIM records for the domain could also install an ADSP record containing "all". B.3. Bulk Mailing Domains with Discardable Mail In some cases, a domain might sign all of its outgoing mail with an - Author Signature, but prefer that recipient systems discard mail - without a valid Author Signature to avoid confusion from mail sent - from sources that do not apply an Author Signature. (In the case of - domains with tightly controlled outgoing mail, this latter kind of - mail is sometimes loosely called "forgeries".) In that case, it - might be appropriate to publish an ADSP record containing - "discardable". Note that a domain SHOULD NOT publish a "discardable" - record if it wishes to maximize the likelihood that mail from the - domain is delivered, since it could cause some fraction of the mail - the domain sends to be discarded. + Author Domain Signature, but prefer that recipient systems discard + mail without a valid Author Domain Signature to avoid confusion from + mail sent from sources that do not apply an Author Domain Signature. + (In the case of domains with tightly controlled outgoing mail, this + latter kind of mail is sometimes loosely called "forgeries".) In + that case, it might be appropriate to publish an ADSP record + containing "discardable". Note that a domain SHOULD NOT publish a + "discardable" record if it wishes to maximize the likelihood that + mail from the domain is delivered, since it could cause some fraction + of the mail the domain sends to be discarded. B.4. Third Party Senders Another common use case is for a third party to enter into an agreement whereby that third party will send bulk or other mail on behalf of a designated author or author domain, using that domain in the RFC5322 From: or other headers. Due to the many and varied complexities of such agreements, third party signing is not addressed in this specification. @@ -736,38 +864,48 @@ This document greatly benefited from comments by Steve Atkins, Jon Callas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, Ellen Siegel, Michael Thomas, and Wietse Venema. Appendix D. Change Log *NOTE TO RFC EDITOR: This section may be removed upon publication of this document as an RFC.* -D.1. Changes since -ietf-dkim-08 +D.1. Changes since -ietf-dkim-09 + + o Change author signature to use d=. + + o Change all "author signature" to "author domain signature". + + o Add authentication results for IANA per Murray K. + + o Minor editorial clarifications. + +D.2. Changes since -ietf-dkim-08 o Simplify and clarify interpretation of d= and i=. o Add note pointing out that i= usage conflicts with normal usage, and suggest workaround. o Add note pointing out that you can mechanically add ADSP records for all the subdomains you use. o in Section 3.2 fix text to say that only an Author Signature counts. -D.2. Changes since -ietf-dkim-07 +D.3. Changes since -ietf-dkim-07 Clarify that ADSP records use WSP rather than FWS in 4.1 and 4.2.1. -D.3. Changes since -ietf-dkim-06 +D.4. Changes since -ietf-dkim-06 Minor editorial changes suggested by AD: o expand DKIM in title o clarify that there's no subdomain matching in Section 3.1 o ADSP lookup can terminate without a result if the DNS lookup fails o random dkim= values are treated as unknown @@ -775,41 +912,41 @@ o in 4.2 note WSP not FWS o in 4.3 note that NODATA is not NXDOMAIN o add new Appendix A with lookup examples Also address Tony's nits in http://mipassoc.org/pipermail/ietf-dkim/2008q3/010720.html. Make the examples consistently use the .example domain. -D.4. Changes since -ietf-dkim-05 +D.5. Changes since -ietf-dkim-05 Minor editorial nits: define NOERROR, SERVFAIL, NXDOMAIN as rfc1035 rcodes, change some punctuation, IANA section change IETF Consensus to the new IETF Review. -D.5. Changes since -ietf-dkim-04 +D.6. Changes since -ietf-dkim-04 o Require dkim at the front of each record. o Disparage wildcard records. o Changed ABNF use of whitespace from FWS back to WSP, dkim-base is wrong. o RFC 2434 -> 5226, make ref to 4686 informational since it's not standards track. o Improve examples with material from Ellen. -D.6. Changes since -ietf-dkim-03 +D.7. Changes since -ietf-dkim-03 o Name change for title and filename, to be ADSP o String changes throughout, to author Domain signing practices and to aDsp. o Added some keywords. o Clarified comparison of local part and domain in Author Address. @@ -831,21 +968,21 @@ o Revised wildcard text. o Removed 't' tag. o Removed ADSP Flags Registry section. o Changed ABNF use of whitespace from WSP back to FWS, for consistency with dkim-base. -D.7. Changes since -ietf-dkim-02 +D.8. Changes since -ietf-dkim-02 o Merge in more text from ADSP draft. o Phrase actions as host's rather than checker. o Explanatory description of i= matching. o Lookup procedure consistently refers to one ADSP record per lookup. @@ -853,21 +990,21 @@ o Simplify imports of terms from other RFCs, add Local-part, 4234 -> 5234. o Add usage example appendix. o Add IANA considerations. o Update authors list -D.8. Changes since -ietf-dkim-ssp-01 +D.9. Changes since -ietf-dkim-ssp-01 o Reworded introduction for clarity. o Various definition clarifications. o Changed names of practices to unknown, all, and discardable. o Removed normative language mandating use of SSP in particular situations (issue 1538). @@ -901,21 +1038,21 @@ o Introduced the concepts of "SSP Checker" and "Evaluator". o Multiple author case now handled my separate invocations of SSP checker by Evaluator (issue 1525). o Removed check to avoid querying top-level domains. o Changed ABNF use of whitespace from [FWS] to *WSP (partially addresses issue 1543). -D.9. Changes since -ietf-dkim-ssp-00 +D.10. Changes since -ietf-dkim-ssp-00 o Clarified Operation Overview and eliminated use of Legitimate as the counterpart of Suspicious since the words have different meanings. o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT records in DNS vs. a new RR type. o Clarified publication rules for multilevel names. @@ -930,56 +1067,56 @@ o Added "handling" tag to express alleged sending domain's preference about handling of Suspicious messages. o Clarified handling of SERVFAIL error in SSP check. o Replaced "entity" with "domain", since with the removal of user- granularity SSP, the only entities having sender signing policies are domains. -D.10. Changes since -allman-ssp-02 +D.11. Changes since -allman-ssp-02 o Removed user-granularity SSP and u= tag. o Replaced DKIMP resource record with a TXT record. o Changed name of the primary tag from "p" to "dkim". o Replaced lookup algorithm with one which traverses upward at most one level. o Added description of records to be published, and effect of wildcard records within the domain, on SSP. -D.11. Changes since -allman-ssp-01 +D.12. Changes since -allman-ssp-01 o Changed term "Sender Signing Policy" to "Sender Signing Practices". o Changed query methodology to use a separate DNS resource record type, DKIMP. o Changed tag values from SPF-like symbols to words. o User level policies now default to that of the domain if not specified. o Removed the "Compliance" section since we're still not clear on what goes here. o Changed the "parent domain" policy to only search up one level (assumes that subdomains will publish SSP records if appropriate). o Added detailed description of SSP check procedure. -D.12. Changes since -allman-ssp-00 +D.13. Changes since -allman-ssp-00 From a "diff" perspective, the changes are extensive. Semantically, the changes are: o Added section on "Third-Party Signatures and Mailing Lists" o Added "Compliance" (transferred from -base document). I'm not clear on what needs to be done here. o Extensive restructuring.