DKIM Working Group M. Kucherawy Internet-Draft Cloudmark Intended status: Informational October 15, 2010 Expires: April 18, 2011 DKIM And Mailing Lists draft-ietf-dkim-mailinglists-04 Abstract DomainKeys Identified Mail (DKIM) allows an administrative mail domain (ADMD) to assume some responsibility for a message. As the industry has now gained some deployment experience, the goal for this document is to explore the use of DKIM for scenarios that include intermediaries, such as Mailing List Managers (MLMs). Status of this Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on April 18, 2011. Copyright Notice Copyright (c) 2010 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. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Kucherawy Expires April 18, 2011 [Page 1] Internet-Draft DKIM and Mailing Lists October 2010 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. MLMs In Infrastructure . . . . . . . . . . . . . . . . . . 4 1.3. Feedback Loops And Other Bi-Lateral Agreements . . . . . . 5 1.4. Document Scope and Goals . . . . . . . . . . . . . . . . . 5 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1. Other Terms . . . . . . . . . . . . . . . . . . . . . . . 7 2.2. DKIM-Specific References . . . . . . . . . . . . . . . . . 7 2.3. 'DKIM-Friendly' . . . . . . . . . . . . . . . . . . . . . 7 2.4. Message Streams . . . . . . . . . . . . . . . . . . . . . 7 3. Mailing Lists and DKIM . . . . . . . . . . . . . . . . . . . . 8 3.1. Roles and Realities . . . . . . . . . . . . . . . . . . . 8 3.2. Types Of Mailing Lists . . . . . . . . . . . . . . . . . . 9 3.3. Current MLM Effects On Signatures . . . . . . . . . . . . 10 4. Non-Participating MLMs . . . . . . . . . . . . . . . . . . . . 12 4.1. Author-Related Signing . . . . . . . . . . . . . . . . . . 12 4.2. Verification Outcomes at Receivers . . . . . . . . . . . . 13 4.3. Handling Choices at Receivers . . . . . . . . . . . . . . 13 4.4. Wrapping A Non-Participating MLM . . . . . . . . . . . . . 13 5. Participating MLMs . . . . . . . . . . . . . . . . . . . . . . 14 5.1. General . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.2. DKIM Author Domain Signing Practices . . . . . . . . . . . 14 5.3. Subscriptions . . . . . . . . . . . . . . . . . . . . . . 15 5.4. Author-Related Signing . . . . . . . . . . . . . . . . . . 15 5.5. Verification Outcomes at MLMs . . . . . . . . . . . . . . 16 5.6. Signature Removal Issues . . . . . . . . . . . . . . . . . 16 5.7. MLM Signatures . . . . . . . . . . . . . . . . . . . . . . 18 5.8. Verification Outcomes at Final Receiving Sites . . . . . . 19 5.9. Use With FBLs . . . . . . . . . . . . . . . . . . . . . . 19 5.10. Handling Choices at Receivers . . . . . . . . . . . . . . 20 6. DKIM Reporting . . . . . . . . . . . . . . . . . . . . . . . . 22 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 8. Security Considerations . . . . . . . . . . . . . . . . . . . 24 8.1. Authentication Results When Relaying . . . . . . . . . . . 24 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 9.1. Normative References . . . . . . . . . . . . . . . . . . . 25 9.2. Informative References . . . . . . . . . . . . . . . . . . 25 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 27 Appendix B. Example Scenarios . . . . . . . . . . . . . . . . . . 28 B.1. MLMs and ADSP . . . . . . . . . . . . . . . . . . . . . . 28 B.2. MLMs and FBLs . . . . . . . . . . . . . . . . . . . . . . 28 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29 Kucherawy Expires April 18, 2011 [Page 2] Internet-Draft DKIM and Mailing Lists October 2010 1. Introduction DomainKeys Identified Mail ([DKIM]) allows an Administrative Mail Domain to take some responsibility for a [MAIL] message. This can be an author's organization, an operational relay (Mail Transfer Agent, or MTA) or one of their agents. Assertion of responsibility is made through a cryptographic signature. Message transit from author to recipient is through relays that typically make no substantive change to the message content and thus preserve the validity of the DKIM signature. In contrast to relays, there are intermediaries, such as mailing list managers (MLMs), that actively take delivery of messages, re-format them, and re-post them, often invalidating DKIM signatures. The goal for this document is to explore the use of DKIM for scenarios that include intermediaries. Questions that will be discussed include: o Under what circumstances is it advisable for an author, or its organization, to apply DKIM to mail sent to mailing lists? o What are the tradeoffs regarding having an MLM verify and use DKIM identifiers? o What are the tradeoffs regarding having an MLM remove existing DKIM signatures prior to re-posting the message? o What are the tradeoffs regarding having an MLM add its own DKIM signature? These and others are open questions for which there may be no definitive answers. However, based on experience since the publication of [DKIM] and its gradual deployment, there are some views that are useful to consider. In general there are, in relation to DKIM, two categories of MLMs: participating and non-participating. As each types has its own issues regarding DKIM-signed messages that are either handled or produced by them (or both), the types are discussed in separate sections. 1.1. Background DKIM signatures permit an agent of the email architecture (see [EMAIL-ARCH]) to make a claim of responsibility for a message by affixing a validated domain-level identifier to the message as it passes through a gateway. Although not the only possibility, this is most commonly done as a message passes through a Mail Transport Agent (MTA) as it departs an Administrative Mail Domain (ADMD) toward the Kucherawy Expires April 18, 2011 [Page 3] Internet-Draft DKIM and Mailing Lists October 2010 general Internet. A DKIM signature will fail to verify if a portion of the message covered by one of its hashes is altered. An MLM commonly alters messages to provide information specific to the mailing list for which it is providing service. Common modifications are enumerated and described in Section 3.3. However, note that MLMs vary widely in behaviour as well as often allowing subscribers to select individual behaviours. Further, this does not consider changes the MTA might make independent of what changes the MLM chooses to apply. The DKIM specification document deliberately refrains from the notion of tying the signing domain (the "d=" tag in a DKIM signature) to any identifier within a message; any ADMD that handles a message could sign it, regardless of its origin or author domain. In particular, DKIM does not define any meaning to the occurrence of a match between the content of a "d=" tag and the value of, for example, a domain name in the RFC5322.From field, nor is there any obvious degraded value to a signature where they do not match. Since any DKIM signature is merely an assertion of "some" responsibility by an ADMD, a DKIM signature added by an MLM has no more, nor less, meaning than a signature with any other "d=" value. 1.2. MLMs In Infrastructure The previous section describes some of the things MLMs commonly do that produce broken signatures and thus reducing the perceived value of DKIM. Further, while the advent of standards that are specific to MLM behaviour (e.g. [MAIL], [LIST-ID] and [LIST-URLS]), their adoption has been spotty at best. Hence, efforts to specify the use of DKIM in the context of MLMs needs to be incremental and value-based. MLM behaviors are well-established. Although it can be argued that they frustrate widespread DKIM adoption, it cannot be said that such behaviours are not standards compliant. Thus, the best approach is to provide these best practices to all parties involved, imposing the minimum requirements possible to MLMs themselves. An MLM is an autonomous agent that takes delivery of a message and can re-post it as a new message, or construct a digest of it along with other messages to the members of the list (see [EMAIL-ARCH], Section 5.3). However, the fact that the RFC5322.From field of such a message is typically the same as that of the original message and that recipients perceive the message as "from" the original author rather than the MLM creates confusion about responsibility and autonomy for the re-posted message. This has important implications Kucherawy Expires April 18, 2011 [Page 4] Internet-Draft DKIM and Mailing Lists October 2010 for use of DKIM. A DKIM signature on a message is an expression of some responsibility for the message taken by the signing domain. An open issue, and one this document intends to address, is some idea of how such a signature might be used by a recipient's evaluation module after the message has gone through a mailing list and may or may not have been invalidated, and if it has, where the invalidation may have happened. Note that where in this document there is discussion of an MLM conducting validation of DKIM signatures or ADSP policies, the actual implementation could be one where the validation is done by the MTA or an agent attached to it, and the results of that work are relayed by a trusted channel not specified here. See [AUTH-RESULTS] for a discussion of this. This document does not favour any particular arrangement of these agents over another, but merely talks about the MLM itself doing the work as a matter of simplicity. 1.3. Feedback Loops And Other Bi-Lateral Agreements A Feedback Loop (FBL) is a bi-lateral agreement between two parties to exchange reports of abuse. Typically, a sender registers with a receiving site to receive abuse reports from that site for mail coming from the sender. An FBL reporting address (i.e., an address to which FBL reports are sent) is part of this bi-lateral registration. Some FBLs require DKIM use by the registrant. Messages signed and sent by a registrant through an MLM can therefore result in having abuse reports sent to the original author when the actual problem pertains to the operation of the MLM. However, the original author has no involvement in operation of the MLM, meaning the FBL report is not actionable and thus is undesirable. See Section 6 for additional discussion. FBLs tend to use the ARF ([MARF]) or the IODEF ([IODEF]) format. 1.4. Document Scope and Goals This document provides discussion on the above issues, to improve the handling of possible interactions between DKIM and MLMs. In general, consensus shows a preference toward imposing changes to behaviour at the signer and verifier rather than at the MLM. Wherever possible, MLMs will be conceptually decoupled from MTAs despite the very tight integration that is sometimes observed in implementation. This is done to emphasize the functional Kucherawy Expires April 18, 2011 [Page 5] Internet-Draft DKIM and Mailing Lists October 2010 independence of MLM services and responsibilities from those of an MTA. Parts of this document explore possible changes to common practice by signers, verifiers and MLMs. The suggested enhancements are largely theoretical in nature, taking into account the current email infrastructure, the facilities DKIM can provide as it gains wider deployment, and working group consensus. There is no substantial implementation history upon which these suggestions are based, and the efficacy, performance and security characteristics of them have not yet been fully explored. Kucherawy Expires April 18, 2011 [Page 6] Internet-Draft DKIM and Mailing Lists October 2010 2. Definitions 2.1. Other Terms See [EMAIL-ARCH] for a general description of the current messaging architecture, and for definitions of various terms used in this document. 2.2. DKIM-Specific References Readers are encouraged to become familiar with [DKIM] and [ADSP] which are core specification documents as well as [DKIM-OVERVIEW] and [DKIM-DEPLOYMENT] which are DKIM's primary tutorial documents. 2.3. 'DKIM-Friendly' The term "DKIM-Friendly" is used to describe an email intermediary that, when handling a message, makes no changes to that message which cause valid [DKIM] signatures present on the message on input to fail to verify on output. Various features of MTAs and MLMs seen as helpful to users often have side effects that do render DKIM signatures unverifiable. These would not qualify for this label. 2.4. Message Streams This document makes reference to the concept of "message streams". The idea is to identify groups of messages originating from within an ADMD that are distinct in intent, origin and/or use, and partition them somehow (i.e., via changing the value in the "d=" tag value in the context of DKIM) so as to keep them associated to users yet distinct in terms of their evaluation and handling by verifiers or receivers. A good example might be user mail generated by a company's employees, versus operational or transactional mail that comes from automated sources, versus marketing or sales campaigns. Each of these could have different security policies imposed against them, or there might be a desire to insulate one from the other (e.g., a marketing campaign that gets reported by many spam filters could cause the marketing stream's reputation to degrade without automatically punishing the transactional or user streams). Kucherawy Expires April 18, 2011 [Page 7] Internet-Draft DKIM and Mailing Lists October 2010 3. Mailing Lists and DKIM It is important to make some distinctions among different MLM-like agents, their typical implementations, and the impacts they have in a DKIM-aware environment. 3.1. Roles and Realities In DKIM parlance, there are several key roles in the transit of a message. Most of these are defined in [EMAIL-ARCH]. author: The agent that provided the content of the message being sent through the system, and performed the initial submission. This can be a human using an MUA or a common system utility such as "cron", etc. originator: The agent that accepts a message from the author, ensures it conforms to the relevant standards such as [MAIL], and then relays it toward its destination(s). This is often referred to as the Mail Submission Agent (MSA). signer: Any agent that affixes one or more DKIM signature(s) to a message on its way toward its ultimate destination. There is typically a signer running at the MTA that sits between the author's ADMD and the general Internet. The originator and/or author might also be a signer. verifier: Any agent that conducts DKIM signature analysis. One typically running at the MTA that sits between the general Internet and the receiver's ADMD. Note that any agent that handles a signed message could conduct verification; this document only considers that action and its outcomes either at an MLM or at the receiver. Filtering decisions could be made by this agent based on verificaiton results. receiver: The agent that is the final transit relay for the message prior to being delivered to the recipient(s) of the message. Filtering decisions based on results made by the verifier could be applied by the receiver. The verifier and the receiver could be the same agent. In the case of simple user-to-user mail, these roles are fairly straightforward. However, when one is sending mail to a list, which then gets relayed to all of that list's subscribers, the roles are often less clear to the general user as particular agents may hold multiple important but separable roles. The above definitions are intended to enable more precise discussion of the mechanisms involved. Kucherawy Expires April 18, 2011 [Page 8] Internet-Draft DKIM and Mailing Lists October 2010 3.2. Types Of Mailing Lists There are four common MLM implementation modes: aliasing: An aliasing MLM (see Section 5.1 of [EMAIL-ARCH]) is one that makes no changes to a message as it redistributes; any modifications are constrained to changes to the [SMTP] envelope recipient list (RCPT commands) only. There are no changes to the message body at all and only [MAIL] trace header fields are added. The output of such an MLM is considered to be a continuation of the author's original message. An example of such an MLM is an address that expands directly in the MTA, such as a list of local system administrators used for relaying operational or other internal-only messages. See also Section 3.9.2 of [SMTP]. resending: A resending MLM (see Sections 5.2 and 5.3 of [EMAIL-ARCH]) is one that may make changes to a message. The output of such an MLM is considered to be a new message; delivery of the original has been completed prior to distribution of the re-posted message. Such messages are often re-formatted, such as with list-specific header fields or other properties, to facilitate discussion among list subscribers. authoring: An authoring MLM is one that creates the content being sent as well as initiating its transport, rather than basing it on one or more messages received earlier. This is a special case of the MLM paradigm, one that generates its own content and does not act as an intermediary. Typically replies are not generated, or if they are, they go to a specific recipient and not back to the list's full set of recipients. Examples include newsletters and bulk marketing mail. digesting: A special case of the resending MLM is one that sends a single message comprising an aggregation of recent MLM submissons, which might be a message of [MIME] type "multipart/digest" (see [MIME-TYPES]). This is obviously a new message but it may contain a sequence of original messages that may themselves have been DKIM-signed. In the remainder of this document we distinguish two relevant steps, corresponding to the following SMTP transactions: MLM Input: Originating user is author; originating ADMD is originator and signer; MLM's ADMD is verifier; MLM's input function is receiver. Kucherawy Expires April 18, 2011 [Page 9] Internet-Draft DKIM and Mailing Lists October 2010 MLM Output: MLM (sending its reconstructed copy of the originating user's message) is author; MLM's ADMD is originator and signer; the ADMD of each subscriber of the list is a verifier; each subscriber is a receiver. Much of this document focuses on the resending class of MLM as it has the most direct conflict operationally with DKIM. The dissection of the overall MLM operation into these two distinct steps allows the DKIM-specific issues with respect to MLMs to be isolated and handled in a logical way. The main issue is that the repackaging and reposting of a message by an MLM is actually the construction of a completely new message, and as such the MLM is introducing new content into the email ecosystem, consuming the author's copy of the message and creating its own. When considered in this way, the dual role of the MLM and its ADMD becomes clear. Some issues about these activities are discussed in Section 3.6.4 of [MAIL] and in Section 3.4.1 of [EMAIL-ARCH]. 3.3. Current MLM Effects On Signatures As described above, an aliasing MLM does not affect any existing signature, and an authoring MLM is always creating new content and thus there is never an existing signature. However, the changes a resending MLM can make typically affect the RFC5322.Subject header field, addition of some list-specific header fields, and/or modification of the message body. The impacts of each of these on DKIM verification are discussed below. Subject tags: A popular feature of MLMs is the "tagging" of an RFC5322.Subject field by prefixing the field's contents with the name of the list, such as "[example]" for a list called "example". Altering the RFC5322.Subject field on new submissions by adding a list-specific prefix or suffix will invalidate the signer's signature if that header field was included when creating that signature. [DKIM] lists RFC5322.Subject as one that should be covered, so this is expected to be an issue for any list that makes such changes. List-specific header fields: Some lists will add header fields specific to list administrative functions such as those defined in [LIST-ID] and [LIST-URLS], or the "Resent-" fields defined in [MAIL]. It is unlikely that a typical MUA would include such fields in an original message, and DKIM is resilient to the addition of header fields in general (see notes about the "h=" tag in Section 3.5 of [DKIM]). Therefore this is seen as less of a concern. Kucherawy Expires April 18, 2011 [Page 10] Internet-Draft DKIM and Mailing Lists October 2010 Other header fields: Some lists will add or replace header fields such as "Reply-To" or "Sender" in order to establish that the message is being sent in the context of the mailing list, so that the list is identified ("Sender") and any user replies go to the list ("Reply-To"). If these fields were included in the original message, it is possible that one or more of them may have been signed, and those signatures will thus be broken. Minor body changes: Some lists prepend or append a few lines to each message to remind subscribers of an administrative URL for subscription issues, or of list policy, etc. Changes to the body will alter the body hash computed at the DKIM verifier, so these will render any existing signatures unverifiable. Major body changes: There are some MLMs that make more substantial changes to message bodies when preparing them for re-distribution, such as adding, deleting, reordering, or reformatting [MIME] parts, "flatten" HTML messages into plain text, or insert headers or footers within HTML messages. Most or all of these changes will invalidate a DKIM signature. MIME part removal: Some MLMs that are MIME-aware will remove large MIME parts from submissions and replace them with URLs to reduce the size of the distributed form of the message and to prevent inadvertent automated malware delivery. Except in cases where a body length limit is applied in generation of the DKIM signature, the signature will be broken. There reportedly still exist a few scattered mailing lists in operation that are actually run manually by a human list manager, whose workings in preparing a message for distribution could include the above or even some other changes. In general, absent a general movement by MLM developers and operators toward more DKIM-friendly practices, an MLM subscriber cannot expect signatures applied before the message was processed by the MLM to be valid on delivery to a receiver. Such an evolution is not expected in the short term due to general development and deployment inertia. Moreover, even if an MLM currently passes messages unmodified such that author signatures validate, it is possible that a configuration change or software upgrade to that MLM will cause that no longer to be true. Kucherawy Expires April 18, 2011 [Page 11] Internet-Draft DKIM and Mailing Lists October 2010 4. Non-Participating MLMs This section contains a discussion of issues regarding sending DKIM- signed mail to or through an MLM that is not DKIM-aware. Specifically, the header fields introduced by [DKIM] and [AUTH-RESULTS] carry no special meaning to such an MLM. 4.1. Author-Related Signing If an author knows that the MLM to which a message is being sent is a non-participating resending MLM, the author is advised to be cautious when deciding whether or not to send to the list when that mail would be signed. The MLM could make a change that would invalidate the author's signature but not remove it prior to re-distribution. Hence, list recipients would receive a message purportedly from the author but bearing a DKIM signature that would not verify. There exist DKIM modules that incorrectly penalize messages with signatures that do not validate, so this may have detrimental effects outside of the author's control. (Additional discussion of this is below.) This problem could be compounded if there were receivers that applied signing policies (e.g., [ADSP]) and the author published any kind of strict policy. For domains that do publish strict ADSP policies, the originating site can consider using a separate message stream, such as a sub- domain, for the "personal" mail -- a subdomain that is different from domain(s) used for other mail streams. This allows each to develop an independent reputation, and more stringent policies (including ADSP) can be applied to the mail stream(s) that do not go through mailing lists or perhaps do not get signed at all. However, all of this presupposes a level of infrastructure understanding that is not expected to be common. Thus, it will be incumbent upon site administrators to consider how support of users wishing to participate in mailing lists might be accomplished as DKIM achieves wider adoption. In general, the more strict practices and policies are likely to be successful only for the mail streams subject to the most end-to-end control by the originating organization. That typically excludes mail going through MLMs. Therefore, authors whose ADSP is published as "discardable" are advised not to send mail to MLMs, as it is likely to be rejected by ADSP-aware verifiers or recipients. (This is discussed further in Section 5.6 below.) Kucherawy Expires April 18, 2011 [Page 12] Internet-Draft DKIM and Mailing Lists October 2010 4.2. Verification Outcomes at Receivers There does not appear to be a reliable way to determine that a piece of mail arrived via a non-participating MLM. Sites whose users subscribe to non-participating MLMs should be prepared for legitimate mail to arrive with no valid signature, just as it always has in the absence of DKIM. 4.3. Handling Choices at Receivers A receiver's ADMD would have to have some way to register such non- participating lists to exempt them from the expectation of signed mail as discussed in Section 4.1. This is, however, probably not a scalable solution as it imposes a burden on the receiver that is predicated on sender behaviour. Note that the [DKIM] specification explicitly directs verifiers and receivers to treat a verification failure as though the message was not signed in the first place. In the absence of specific ADSP direction, any treatment of a verification failure as having special meaning is either outside the scope of DKIM or is in violation of it. Use of restrictive domain policies such as [ADSP] "discardable" presents an additional challenge. In that case, when a message is unsigned or the signature can no longer be verified, discarding of the message is requested. There is no exception in the policy for a message that may have been altered by an MLM, nor is there a reliable way to identify such mail. Participants are thus advised to honor the policy and disallow the message. 4.4. Wrapping A Non-Participating MLM One approach to adding DKIM support to an otherwise non-participating MLM is to "wrap" it, or in essence place it between other DKIM-aware components (such as MTAs) that provide some DKIM services. For example, the ADMD operating a non-participating MLM could have a DKIM verifier act on submissions, enforcing some of the features and recommendations of Section 5 on behalf of the MLM, and the MTA or MSA receiving the MLM Output could also add a DKIM signature for the MLM's domain. Kucherawy Expires April 18, 2011 [Page 13] Internet-Draft DKIM and Mailing Lists October 2010 5. Participating MLMs This section contains a discussion of issues regarding sending DKIM- signed mail to or through an MLM that is DKIM-aware, and may also be ADSP-aware. 5.1. General As DKIM becomes more widely deployed, it is highly desirable that MLM software adopt more DKIM-friendly processing. Changes that merely add new header fields, such as those specified by [LIST-ID], [LIST-URLS] and [MAIL] are generally the most friendly to a DKIM-participating email infrastructure, in that their addition by an MLM will not affect any existing DKIM signatures unless those fields were already present and covered by a signature's hash or a signature was created specifically to disallow their addition (see the note about "h=" in Section 3.5 of [DKIM]). However, the practice of applying headers and footers to message bodies is common and not expected to fade regardless of what documents this or any standards body might produce. This sort of change will invalidate the signature on a message where the body hash covers the entire message. Thus, the following sections also discuss and suggest other processing alternatives. A possible mitigation to this incompatibility is use of the "l=" tag to bound the portion of the body covered by the DKIM body hash, but this is not workable for [MIME] messages; moreover, it has security considerations (see Section 3.5 of [DKIM]). Its use is therefore discouraged. MLM operators often arrange to affix to outgoing messages expressions of list-specific policy and related information (e.g., rules for participation, small advertisements, etc.). There is currently no header field proposed for relaying such general operational MLM details apart from what [LIST-URLS] already supports. This sort of information is what is commonly included in appended footer text or prepended header text. The working group recommends periodic, automatic mailings to the list to remind subscribers of list policy. These will be repetitive, of course, but by being generally the same each time they can be easily filtered if desired. 5.2. DKIM Author Domain Signing Practices [ADSP] presents a particular challenge. An author domain posting a policy of "discardable" imposes a very tight restriction on the use of mailing lists, essentially constraining that domain's users to Kucherawy Expires April 18, 2011 [Page 14] Internet-Draft DKIM and Mailing Lists October 2010 lists operated by aliasing MLMs only; any MLM that alters a message from such a domain or removes its signature subjects the message to severe action by verifiers or receivers. It is the consensus of the working group that a resending MLM is advised to reject outright any mail from an author whose domain posts such a policy as it is likely to be rejected by any ADSP-aware recipients, and might also be well advised to discourage such subscribers when they first sign up to the list. Further discussion of this appears in Section 5.3. Where the above practice is not observed and "discardable" mail arrives via a list to a verifier that applies ADSP checks, the message can either be discarded (i.e. accept the message at the [SMTP] level but discard it without delivery) or the message can be rejected by returning a 5xx error code. In the latter case, some advice for how to conduct the rejection in a potentially meaningful way can be found in Section 5.10. See also Appendix B.5 of [ADSP] for further discussion. 5.3. Subscriptions At subscription time, an ADSP-aware MLM could check for a published ADSP record for the new subscriber's domain. If the policy specifies "discardable", the MLM might disallow the subscription or present a warning that the subscriber's submissions to the mailing list might not be deliverable to some recipients because subscriber's ADMD's published policy. Of course, such a policy record could be applied after subscription, so this is not a universal solution. An MLM implementation could do periodic checks of its subscribers and issue warnings where such a policy is detected, or simply check upon each submission. 5.4. Author-Related Signing An important consideration is that authors rarely have any direct influence over the management of an MLM. As such, a signed message from an author will in essence go to a set of unexpected places, sometimes coupled with other messages from other sources. In the future, as DKIM signature outputs (e.g. the AUID or SDID of [DKIM-UPDATE]) are used as inputs to reputation modules, there may be a desire to insulate one's reputation from influence by the unknown results of sending mail through an MLM. In that case, authors may be well-advised to create a mail stream specifically used for generating signatures when sending traffic to MLMs. This suggestion can be made more general. Mail that is of a transactional or generally end-to-end nature, and not likely to be Kucherawy Expires April 18, 2011 [Page 15] Internet-Draft DKIM and Mailing Lists October 2010 forwarded around either by MLMs or users, should come from a different mail stream than a stream that serves more varied uses. 5.5. Verification Outcomes at MLMs MLMs typically attempt to authenticate messages posted through them. They usually do this through the trivial (and insecure) means of verifying the RFC5322.From field email address (or, less frequently, the RFC5321.MailFrom parameter) against a list registry. DKIM enables a stronger form of authentication, although this is not yet formally documented: It can require that messages using a given RFC5322.From address also have a DKIM signature with a corresponding "d=" domain. This feature would be somewhat similar to using ADSP, except that the requirement for it would be imposed by the MLM and not the author's organization. As described, the MLM might conduct DKIM verification of a signed message to attempt to confirm the identity of the author. Although it is a common and intuitive conclusion, not all signed mail will include an author signature (see [ADSP]). MLM implementors are advised to accomodate such in their configurations. For example, an MLM might be designed to accomodate a list of possible signing domains (the "d=" portion of a DKIM signature) for a given author, and determine at verification time if any of those are present. A message that cannot be thus authenticated could be held for moderation or rejected outright. This logic could apply to any list operation, not just list submission. In particular, this improved authentication could apply to subscription, unsubscription, and/or changes to subscriber options that are sent via email rather than through an authenticated, interactive channel such as the web. In the case of verification of signatures on submissions, MLMs are advised to add an [AUTH-RESULTS] header field to indicate the signature(s) observed on the submission as it arrived at the MLM and what the outcome of the evaluation was. Downstream agents may or may not trust the content of that header field depending on their own a priori knowledge of the operation of the ADMD generating (and, preferably, signing) that header field. See [AUTH-RESULTS] for further discussion. 5.6. Signature Removal Issues A message that arrives signed with DKIM means some domain prior to MLM Input has made a claim of some responsibility for the message. An obvious benefit to leaving the input-side signatures intact, then, Kucherawy Expires April 18, 2011 [Page 16] Internet-Draft DKIM and Mailing Lists October 2010 is to preserve that chain of responsibility of the message so that the receivers of the final message have an opportunity to evaluate the message with that information available to them. However, if the MLM is configured to make changes to the message prior to re-posting that would invalidate the original signature(s), further action is recommended to prevent invalidated signatures from arriving at final recipients, possibly triggering unwarranted filter actions. (Note, however, that such filtering actions are plainly wrong; [DKIM] stipulates that an invalid signature is to be treated as no signature at all.) A possible solution would be to: 1. Attempt verification of all DKIM signatures present on the input message; 2. Apply local policy to authenticate the identity of the author; 3. Add an [AUTH-RESULTS] header field to the message to indicate the results of the above; 4. Remove all previously-evaluated DKIM signatures; 5. Affix a new signature that covers the Authentication-Results header field just added (see Section 5.7). Removing the original signature(s) seems particularly appropriate when the MLM knows it is likely to invalidate any or all of them due to the nature of the reformatting it will do. This avoids false negatives at the list's subscribers in their roles as receivers of the message; although [DKIM] stipulates that an invalid signature is the same as no signature, it is anticipated that there will be some implementations that ignore this advice. The MLM could re-evaluate existing signatures after making its message changes to determine whether or not any of them have been invalidated. The cost of this is reduced by the fact that, presumably, the necessary public keys have already been downloaded and one or both of the message hashes could be reused. Per the discussion in [AUTH-RESULTS], there is no a priori reason for the final receivers to put any faith in the veracity of that header field when added by the MLM. Thus, the final recipients of the message have no way to verify on their own the authenticity of the author's identity on that message. However, should that field be the only one on the message when the verifier gets it, and the verifier explicitly trusts the signer (in this case, the MLM), the verifier is Kucherawy Expires April 18, 2011 [Page 17] Internet-Draft DKIM and Mailing Lists October 2010 in a position to believe that a valid author signature was present on the message. Since an aliasing MLM makes no substantive changes to a message, it need not consider the issue of signature removal as the original signatures should arrive at least to the next MTA unmodified. It is possible that future domain-based reputations would prefer a more rich data set on receipt of a message, and in that case signature removal would be undesirable. An authoring MLM is closed to outside submitters, thus much of this discussion does not apply in that case. 5.7. MLM Signatures DKIM-aware resending MLMs and authoring MLMs are encouraged to affix their own signatures when distributing messages. The MLM is responsible for the alterations it makes to the original messages it is re-sending, and should express this via a signature. This is also helpful for getting feedback from any FBLs that might be set up so that undesired list mail can generate appropriate action. MLM signatures will likely be used by recipient systems to recognize list mail, and they give the MLM's ADMD an opportunity to develop a good reputation for the list itself. A signing MLM is, as any other MLM, free to omit redistribution of a message if that message was not signed in accordance with its own local configuration or policy. It could also redistribute but not sign such mail. However, selective signing is discouraged; essentially that would create two message streams from the MLM, one signed and one not, which can confuse DKIM-aware verifiers and receivers. As is typical with DKIM signing, the MLM signature must be generated immediately prior to sending, only after all other processing the MLM wishes to apply has been completed. Failing to do so generates a signature that can not be expected to validate. A signing MLM could add a List-Post: header field (see [LIST-URLS]) using a DNS domain matching what will be used in the "d=" tag of the DKIM signature it will add to the new message. This could be used by verifiers or receivers to identify the DKIM signature that was added by the MLM. This is not required, however; it is believed the reputation of the signer will be a more critical data point rather than this suggested binding. Furthermore, this is not a binding recognized by any current specification document. Kucherawy Expires April 18, 2011 [Page 18] Internet-Draft DKIM and Mailing Lists October 2010 Such MLMs are advised to ensure the signature's header hash will cover: o Any [AUTH-RESULTS] fields added by the MLM; o Any [LIST-ID] or [LIST-URLS] fields added by the MLM; o Any [MAIL] fields, especially Sender and Reply-To, added or replaced by the MLM. A DKIM-aware resending MLM is encouraged to sign the entire message after being prepared for distribution (i.e. the "MLM Output" from Section 3.2), including any original signatures. DKIM-aware authoring MLMs are advised to sign the mail they send according to the regular signing guidelines given in [DKIM]. One concern is that having an MLM apply its signature to unsigned mail might cause some verifiers or receivers to interpret the signature as conferring more authority or authenticity to the message content than is defined by [DKIM]. This is an issue beyond MLMs and primarily entails receive-side processing outside of the scope of [DKIM]. It is nevertheless worth noting here. In the case of MLMs, the presence of an MLM signature is best treated as similar to MLM handling that affixes an RFC5322.Subject tag or similar information. It therefore does not introduce any new concerns. 5.8. Verification Outcomes at Final Receiving Sites In general, verifiers and receivers can treat a signed message from an MLM like any other signed message; indeed, it would be difficult to discern any difference since specifications such as [LIST-URLS] and [LIST-ID] are not universally deployed and can be trivially spoofed. However, because the author domain will commonly be different from the MLM's signing domain, there may be a conflict with [ADSP] as discussed in Section 4.3 and Section 5.6, especially where an ADMD has misused ADSP. 5.9. Use With FBLs An FBL operator may wish to act on a complaint from a user about a posting to a list. Some FBLs could choose to generate feedback reports based on DKIM verifications in the subject message. Such operators are advised to send a report to each domain with a valid signature that has an FBL agreement established, as DKIM signatures are claims of some responsibility for that message. Because authors Kucherawy Expires April 18, 2011 [Page 19] Internet-Draft DKIM and Mailing Lists October 2010 generally have limited control over the operation of a list, this point makes MLM signing all the more important. Where the FBL wishes to be more specific, it could act solely on a DKIM signature where the signing domain matches the DNS domain found in a List-Post: header field (or similar). Use of FBLs in this way should be made explicit to list subscribers. For example, if it is the policy of the MLM's ADMD to handle an FBL item by unsubscribing the user that was the apparent sender of the offending message, advising subscribers of this in advance would help to avoid surprises later. 5.10. Handling Choices at Receivers A recipient that explicitly trusts signatures from a particular MLM may wish to extend that trust to an [AUTH-RESULTS] header field signed by that MLM. The recipient may then do additional processing of the message, using the results recorded in the Authentication- Results header field instead of the original author's DKIM signature. This includes possibly processing the message as per ADSP requirements. Receivers are advised to ignore or remove all unsigned externally- applied Authentication-Results header fields, and those not signed by an ADMD that can be trusted by the receiver. See Section 5 and Section 7 of [AUTH-RESULTS] for further discussion. Upon DKIM and ADSP evaluation during an SMTP session (a common implementation), an agent might decide to reject a message during an SMTP session. If this is done, use of an [SMTP] failure code not normally used for "user unknown" (550) is suggested; 554 seems an appropriate candidate. If the rejecting SMTP server supports [ENHANCED] status codes, is advised to make a distinction between messages rejected deliberately due to policy decisions rather than those rejected because of other deliverability issues. In particular, a policy rejection is advised to be relayed using a 5.7.1 enhanced status code and some appropriate wording in the text part of the reply, in contrast to a code of 5.1.1 indicating the user does not exist. Those MLMs that automatically attempt to remove users with prolonged delivery problems (such as account deletion) will thus be able to tell the difference between policy rejection and other delivery failures, and act accordingly. SMTP servers doing so are also advised to use appropriate wording in the text portion of the reply, perhaps explicitly using the string "ADSP" to facilitate searching of relevant data in logs. The preceding paragraph does not apply to an [ADSP] policy of Kucherawy Expires April 18, 2011 [Page 20] Internet-Draft DKIM and Mailing Lists October 2010 "discardable". In such cases where the submission fails that test, the receiver or verifier is strongly advised to discard the message but return an SMTP success code, i.e. accept the message but drop it without delivery. An SMTP rejection of such mail instead of the requested discard action causes more harm than good. Kucherawy Expires April 18, 2011 [Page 21] Internet-Draft DKIM and Mailing Lists October 2010 6. DKIM Reporting The MARF working group is developing mechanisms for reporting forensic details about DKIM verification failures. At the time of this writing, this is still a work in progress. MLMs are encouraged to apply these or other DKIM failure reporting mechanisms as a method for providing feedback to signers about issues with DKIM infrastructure. This is especially important for MLMs that implement DKIM verification as a mechanism for authentication of list configuration commands and submissions from subscribers. Kucherawy Expires April 18, 2011 [Page 22] Internet-Draft DKIM and Mailing Lists October 2010 7. IANA Considerations This document includes no IANA actions. Kucherawy Expires April 18, 2011 [Page 23] Internet-Draft DKIM and Mailing Lists October 2010 8. Security Considerations This document provides suggested or best current practices for use with DKIM, and as such does not introduce any new technologies for consideration. However, the following security issues should be considered when implementing the above practices. 8.1. Authentication Results When Relaying Section 5 advocates addition of an [AUTH-RESULTS] header field to indicate authentication status of a message received as MLM Input. Per Section 7.2 of [AUTH-RESULTS], receivers generally should not trust such data without a good reason to do so, such as an a priori agreement with the MLM's ADMD. Such agreements are strongly advised to include a requirement that those header fields be covered by a [DKIM] signature added by the MLM's ADMD. Kucherawy Expires April 18, 2011 [Page 24] Internet-Draft DKIM and Mailing Lists October 2010 9. References 9.1. Normative References [ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM Sender Signing Practises", RFC 5617, August 2009. [AUTH-RESULTS] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. [DKIM] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [MAIL] Resnick, P., "Internet Message Format", RFC 5322, October 2008. 9.2. Informative References [DKIM-DEPLOYMENT] Hansen, T., Siegel, E., Hallam-Baker, P., and D. Crocker, "DomainKeys Identified Mail (DKIM) Development, Deployment and Operations", I-D DRAFT-IETF-DKIM-DEPLOYMENT, January 2010. [DKIM-OVERVIEW] Hansen, T., Crocker, D., and P. Hallam-Baker, "DomainKeys Identified Mail (DKIM) Service Overview", RFC 5585, July 2009. [DKIM-UPDATE] Crocker, D., "RFC 4871 DomainKeys Identified Mail (DKIM) Signatures -- Update", RFC 5672, August 2009. [ENHANCED] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003. [IODEF] Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident Object Description Exchange Format", RFC 5070, December 2007. [LIST-ID] Chandhok, R. and G. Wenger, "List-Id: A Structured Field Kucherawy Expires April 18, 2011 [Page 25] Internet-Draft DKIM and Mailing Lists October 2010 and Namespace for the Identification of Mailing Lists", RFC 2919, March 2001. [LIST-URLS] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax for Core Mail List Commands and their Transport through Message Header Fields", RFC 2369, July 1998. [MARF] Shafranovich, Y., Levine, J., and M. Kucherawy, "An Extensible Format for Email Feedback Reports", RFC 5965, August 2010. [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [MIME-TYPES] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types", RFC 2046, November 1996. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. Kucherawy Expires April 18, 2011 [Page 26] Internet-Draft DKIM and Mailing Lists October 2010 Appendix A. Acknowledgements The author wishes to acknowledge the following for their review and constructive criticism of this document: Serge Aumont, Daniel Black, Dave Crocker, JD Falk, Tony Hansen, Eliot Lear, Charles Lindsey, John Levine, Jeff Macdonald, S. Moonesamy, Rolf E. Sonneveld, and Alessandro Vesely. Kucherawy Expires April 18, 2011 [Page 27] Internet-Draft DKIM and Mailing Lists October 2010 Appendix B. Example Scenarios This section describes a few MLM-related DKIM scenarios that were part of the impetus for this work, and the recommended resolutions for each. B.1. MLMs and ADSP Problem: o author ADMD advertises an ADSP policy of "dkim=discardable" o author sends DKIM-signed mail to a non-participating MLM, which invalidates the signature o receiver MTA checks DKIM and ADSP at SMTP time, and is configured to reject ADSP failures, so rejects this message o process repeats a few times, after which the MLM unsubscribes the receiver Solution: MLMs should refuse mail from domains advertising ADSP policies of "discardable" unless the MLMs are certain they make no changes that invalidate DKIM signatures. B.2. MLMs and FBLs Problem: o subscriber sends signed mail to a non-participating MLM that does not invalidate the signature o a recipient reports the message as spam o FBL at recipient ADMD sends report to contributor rather than list manager Solution: MLMs should sign mail they send and might also strip existing signatures; FBLs should report to list operators instead of subscribers where such can be distinguished, otherwise to all parties with valid signatures. Kucherawy Expires April 18, 2011 [Page 28] Internet-Draft DKIM and Mailing Lists October 2010 Author's Address Murray S. Kucherawy Cloudmark 128 King St., 2nd Floor San Francisco, CA 94107 US Phone: +1 415 946 3800 Email: msk@cloudmark.com Kucherawy Expires April 18, 2011 [Page 29]