RFC 7001
Message Header Field for Indicating Message Authentication Status
7. Security Considerations
The following security considerations apply when adding or processing the Authentication-Results header field:7.1. Forged Header Fields
An MUA or filter that accesses a mailbox whose messages are handled by a non-conformant MTA, and understands Authentication-Results header fields, could potentially make false conclusions based on forged header fields. A malicious user or agent could forge a header field using the DNS domain of a receiving ADMD as the authserv-id token in the value of the header field and, with the rest of the value, claim that the message was properly authenticated. The non- conformant MTA would fail to strip the forged header field, and the MUA could inappropriately trust it. For this reason, it is best not to have processing of the Authentication-Results header field enabled by default; instead, it should be ignored, at least for the purposes of enacting filtering decisions, unless specifically enabled by the user or administrator after verifying that the border MTA is compliant. It is acceptable to have an MUA aware of this specification but have an explicit list of hostnames whose Authentication-Results header fields are trustworthy; however, this list should initially be empty.
Proposed alternative solutions to this problem were made some time
ago and are listed below. To date, they have not been developed due
to lack of demand but are documented here should the information be
useful at some point in the future:
1. Possibly the simplest is a digital signature protecting the
header field, such as using [DKIM], that can be verified by an
MUA by using a posted public key. Although one of the main
purposes of this document is to relieve the burden of doing
message authentication work at the MUA, this only requires that
the MUA learn a single authentication scheme even if a number of
them are in use at the border MTA. Note that [DKIM] requires
that the From header field be signed, although in this
application, the signing agent (a trusted MTA) likely cannot
authenticate that value, so the fact that it is signed should be
ignored. Where the authserv-id is the ADMD's domain name, the
authserv-id matching this valid internal signature's "d=" DKIM
value is sufficient.
2. Another would be a means to interrogate the MTA that added the
header field to see if it is actually providing any message
authentication services and saw the message in question, but this
isn't especially palatable given the work required to craft and
implement such a scheme.
3. Yet another might be a method to interrogate the internal MTAs
that apparently handled the message (based on Received header
fields) to determine whether any of them conform to Section 5 of
this memo. This, too, has potentially high barriers to entry.
4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to
allow an MUA or filtering agent to acquire the authserv-id in use
within an ADMD, thus allowing it to identify which
Authentication-Results header fields it can trust.
5. On the presumption that internal MTAs are fully compliant with
Section 3.6 of [MAIL] and the compliant internal MTAs are using
their own hostnames or the ADMD's DNS domain name as the
authserv-id token, the header field proposed here should always
appear above a Received header added by a trusted MTA. This can
be used as a test for header field validity.
Support for some of these is being considered for future work.
In any case, a mechanism needs to exist for an MUA or filter to
verify that the host that appears to have added the header field (a)
actually did so and (b) is legitimately adding that header field for
this delivery. Given the variety of messaging environments deployed today, consensus appears to be that specifying a particular mechanism for doing so is not appropriate for this document. Mitigation of the forged header field attack can also be accomplished by moving the authentication results data into metadata associated with the message. In particular, an [SMTP] extension could be established to communicate authentication results from the border MTA to intermediate and delivery MTAs; the latter of these could arrange to store the authentication results as metadata retrieved and rendered along with the message by an [IMAP] client aware of a similar extension in that protocol. The delivery MTA would be told to trust data via this extension only from MTAs it trusts, and border MTAs would not accept data via this extension from any source. There is no vector in such an arrangement for forgery of authentication data by an outside agent.7.2. Misleading Results
Until some form of service for querying the reputation of a sending agent is widely deployed, the existence of this header field indicating a "pass" does not render the message trustworthy. It is possible for an arriving piece of spam or other undesirable mail to pass checks by several of the methods enumerated above (e.g., a piece of spam signed using [DKIM] by the originator of the spam, which might be a spammer or a compromised system). In particular, this issue is not resolved by forged header field removal discussed above. Hence, MUAs and downstream filters must take some care with use of this header even after possibly malicious headers are scrubbed.7.3. Header Field Position
Despite the requirements of [MAIL], header fields can sometimes be reordered en route by intermediate MTAs. The goal of requiring header field addition only at the top of a message is an acknowledgement that some MTAs do reorder header fields, but most do not. Thus, in the general case, there will be some indication of which MTAs (if any) handled the message after the addition of the header field defined here.7.4. Reverse IP Query Denial-of-Service Attacks
Section 5.5 of [SPF] describes a DNS-based denial-of-service attack for verifiers that attempt DNS-based identity verification of arriving client connections. A verifier wishing to do this check and report this information needs to take care not to go to unbounded lengths to resolve "A" and "PTR" queries. MUAs or other filters
making use of an "iprev" result specified by this document need to be aware of the algorithm used by the verifier reporting the result and, especially, its limitations.7.5. Mitigation of Backscatter
Failing to follow the instructions of Section 4.2 can result in a denial-of-service attack caused by the generation of [DSN] messages (or equivalent) to addresses that did not send the messages being rejected.7.6. Internal MTA Lists
Section 5 describes a procedure for scrubbing header fields that may contain forged authentication results about a message. A compliant installation will have to include, at each MTA, a list of other MTAs known to be compliant and trustworthy. Failing to keep this list current as internal infrastructure changes may expose an ADMD to attack.7.7. Attacks against Authentication Methods
If an attack becomes known against an authentication method, clearly then the agent verifying that method can be fooled into thinking an inauthentic message is authentic, and thus the value of this header field can be misleading. It follows that any attack against the authentication methods supported by this document is also a security consideration here.7.8. Intentionally Malformed Header Fields
It is possible for an attacker to add an Authentication-Results header field that is extraordinarily large or otherwise malformed in an attempt to discover or exploit weaknesses in header field parsing code. Implementers must thoroughly verify all such header fields received from MTAs and be robust against intentionally as well as unintentionally malformed header fields.7.9. Compromised Internal Hosts
An internal MUA or MTA that has been compromised could generate mail with a forged From header field and a forged Authentication-Results header field that endorses it. Although it is clearly a larger concern to have compromised internal machines than it is to prove the value of this header field, this risk can be mitigated by arranging that internal MTAs will remove this header field if it claims to have been added by a trusted border MTA (as described above), yet the [SMTP] connection is not coming from an internal machine known to be
running an authorized MTA. However, in such a configuration, legitimate MTAs will have to add this header field when legitimate internal-only messages are generated. This is also covered in Section 5.7.10. Encapsulated Instances
MIME messages can contain attachments of type "message/rfc822", which contain other messages. Such an encapsulated message can also contain an Authentication-Results header field. Although the processing of these is outside of the intended scope of this document (see Section 1.3), some early guidance to MUA developers is appropriate here. Since MTAs are unlikely to strip Authentication-Results header fields after mailbox delivery, MUAs are advised in Section 4.1 to ignore such instances within MIME attachments. Moreover, when extracting a message digest to separate mail store messages or other media, such header fields should be removed so that they will never be interpreted improperly by MUAs that might later consume them.7.11. Reverse Mapping
Although Section 3 of this memo includes explicit support for the "iprev" method, its value as an authentication mechanism is limited. Implementers of both this proposal and agents that use the data it relays are encouraged to become familiar with the issues raised by [DNSOP-REVERSE] when deciding whether or not to include support for "iprev".8. References
8.1. Normative References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [IANA-HEADERS] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, September 2004. [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, October 2008.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008.8.2. Informative References
[ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, "DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)", RFC 5617, August 2009. [AR-VBR] Kucherawy, M., "Authentication-Results Registration for Vouch by Reference Results", RFC 6212, April 2011. [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) Authorized Third-Party Signatures", RFC 6541, February 2012. [AUTH] Siemborski, R. and A. Melnikov, "SMTP Service Extension for Authentication", RFC 4954, July 2007. [DKIM] Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures", STD 76, RFC 6376, September 2011. [DNS] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, "DNS Extensions to Support IP Version 6", RFC 3596, October 2003. [DNSOP-REVERSE] Senie, D. and A. Sullivan, "Considerations for the use of DNS Reverse Mapping", Work in Progress, March 2008. [DOMAINKEYS] Delany, M., "Domain-Based Email Authentication Using Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, May 2007. [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications", RFC 3464, January 2003.
[EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, July 2009. [IANA-CONSIDERATIONS] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", STD 53, RFC 1939, May 1996. [RFC5451] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", RFC 5451, April 2009. [SECURITY] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003. [SENDERID] Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", RFC 4406, April 2006. [SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009.
Appendix A. Acknowledgements
The author wishes to acknowledge the following individuals for their review and constructive criticism of this document: Dave Cridland, Dave Crocker, Bjoern Hoehrmann, Scott Kitterman, John Levine, Alexey Melnikov, S. Moonesamy, and Alessandro Vesely.Appendix B. Legacy MUAs
Implementers of this protocol should be aware that many MUAs are unlikely to be retrofitted to support the new header field and its semantics. In the interests of convenience and quicker adoption, a delivery MTA might want to consider adding things that are processed by existing MUAs in addition to the Authentication-Results header field. One suggestion is to include a Priority header field, on messages that don't already have such a header field, containing a value that reflects the strength of the authentication that was accomplished, e.g., "low" for weak or no authentication, "normal" or "high" for good or strong authentication. Some modern MUAs can already filter based on the content of this header field. However, there is keen interest in having MUAs make some kind of graphical representation of this header field's meaning to end users. Until this capability is added, other interim means of conveying authentication results may be necessary while this proposal and its successors are adopted.Appendix C. Authentication-Results Examples
This section presents some examples of the use of this header field to indicate authentication results.
C.1. Trivial Case; Header Field Not Present
The trivial case: Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by server.example.org (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.org Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 1: Trivial Case The Authentication-Results header field is completely absent. The MUA may make no conclusion about the validity of the message. This could be the case because the message authentication services were not available at the time of delivery, or no service is provided, or the MTA is not in compliance with this specification.C.2. Nearly Trivial Case; Service Provided, but No Authentication Done
A message that was delivered by an MTA that conforms to this specification but provides no actual message authentication service: Authentication-Results: example.org 1; none Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by server.example.org (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.org Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 2: Header Present but No Authentication Done
The Authentication-Results header field is present, showing that the delivering MTA conforms to this specification. It used its DNS domain name as the authserv-id. The presence of "none" (and the absence of any method and result tokens) indicates that no message authentication was done. The version number of the specification to which the field's content conforms is explicitly provided.C.3. Service Provided, Authentication Done
A message that was delivered by an MTA that conforms to this specification and applied some message authentication: Authentication-Results: example.com; spf=pass smtp.mailfrom=example.net Received: from dialup-1-2-3-4.example.net (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: sender@example.net Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.com Message-Id: <12345.abc@example.net> Subject: here's a sample Hello! Goodbye! Example 3: Header Reporting Results The Authentication-Results header field is present, indicating that the border MTA conforms to this specification. The authserv-id is once again the DNS domain name. Furthermore, the message was authenticated by that MTA via the method specified in [SPF]. Note that since that method cannot authenticate the local-part, it has been omitted from the result's value. The MUA could extract and relay this extra information if desired.
C.4. Service Provided, Several Authentications Done, Single MTA
A message that was relayed inbound via a single MTA that conforms to this specification and applied three different message authentication checks: Authentication-Results: example.com; auth=pass (cram-md5) smtp.auth=sender@example.net; spf=pass smtp.mailfrom=example.net Authentication-Results: example.com; sender-id=pass header.from=example.net Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.com From: sender@example.net Message-Id: <12345.abc@example.net> Subject: here's a sample Hello! Goodbye! Example 4: Headers Reporting Results from One MTA The Authentication-Results header field is present, indicating that the delivering MTA conforms to this specification. Once again, the receiving DNS domain name is used as the authserv-id. Furthermore, the sender authenticated herself/himself to the MTA via a method specified in [AUTH], and both SPF and Sender ID checks were done and passed. The MUA could extract and relay this extra information if desired. Two Authentication-Results header fields are not required since the same host did all of the checking. The authenticating agent could have consolidated all the results into one header field. This example illustrates a scenario in which a remote user on a dialup connection (example.net) sends mail to a border MTA (example.com) using SMTP authentication to prove identity. The dialup provider has been explicitly authorized to relay mail as example.com resulting in passes by the SPF and Sender ID checks.
C.5. Service Provided, Several Authentications Done, Different MTAs
A message that was relayed inbound by two different MTAs that conform to this specification and applied multiple message authentication checks: Authentication-Results: example.com; sender-id=fail header.from=example.com; dkim=pass (good signature) header.d=example.com Received: from mail-router.example.com (mail-router.example.com [192.0.2.1]) by auth-checker.example.com (8.11.6/8.11.6) with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; t=1188964191; c=simple/simple; h=From:Date:To:Subject: Message-Id:Authentication-Results; bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= Authentication-Results: example.com; auth=pass (cram-md5) smtp.auth=sender@example.com; spf=fail smtp.mailfrom=example.com Received: from dialup-1-2-3-4.example.net (dialup-1-2-3-4.example.net [192.0.2.200]) by mail-router.example.com (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 From: sender@example.com Date: Fri, Feb 15 2002 16:54:30 -0800 To: receiver@example.com Message-Id: <12345.abc@example.com> Subject: here's a sample Hello! Goodbye! Example 5: Headers Reporting Results from Multiple MTAs The Authentication-Results header field is present, indicating conformance to this specification. Once again, the authserv-id used is the recipient's DNS domain name. The header field is present twice because two different MTAs in the chain of delivery did authentication tests. The first MTA, mail-router.example.com, reports that SMTP AUTH and SPF were both used and that the former passed while the latter failed. In the SMTP AUTH case, additional information is provided in the comment field, which the MUA can choose to render if desired.
The second MTA, auth-checker.example.com, reports that it did a Sender ID test (which failed) and a DKIM test (which passed). Again, additional data about one of the tests is provided as a comment, which the MUA may choose to render. Also noteworthy here is the fact that there is a DKIM signature added by example.com that assured the integrity of the lower Authentication-Results field. Since different hosts did the two sets of authentication checks, the header fields cannot be consolidated in this example. This example illustrates more typical transmission of mail into example.com from a user on a dialup connection example.net. The user appears to be legitimate as he/she had a valid password allowing authentication at the border MTA using SMTP AUTH. The SPF and Sender ID tests failed since example.com has not granted example.net authority to relay mail on its behalf. However, the DKIM test passed because the sending user had a private key matching one of example.com's published public keys and used it to sign the message.C.6. Service Provided, Multi-Tiered Authentication Done
A message that had authentication done at various stages, one of which was outside the receiving ADMD: Authentication-Results: example.com; dkim=pass reason="good signature" header.i=@mail-router.example.net; dkim=fail reason="bad signature" header.i=@newyork.example.com Received: from mail-router.example.net (mail-router.example.net [192.0.2.250]) by chicago.example.com (8.11.6/8.11.6) for <recipient@chicago.example.com> with ESMTP id i7PK0sH7021929; Fri, Feb 15 2002 17:19:22 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=furble; d=mail-router.example.net; t=1188964198; c=relaxed/simple; h=From:Date:To:Message-Id:Subject:Authentication-Results; bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= Authentication-Results: example.net; dkim=pass (good signature) header.i=@newyork.example.com Received: from smtp.newyork.example.com (smtp.newyork.example.com [192.0.2.220]) by mail-router.example.net (8.11.6/8.11.6) with ESMTP id g1G0r1kA003489; Fri, Feb 15 2002 17:19:07 -0800 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby;
d=newyork.example.com;
t=1188964191; c=simple/simple;
h=From:Date:To:Message-Id:Subject;
bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=;
b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM=
From: sender@newyork.example.com
Date: Fri, Feb 15 2002 16:54:30 -0800
To: meetings@example.net
Message-Id: <12345.abc@newyork.example.com>
Subject: here's a sample
Example 6: Headers Reporting Results from Multiple MTAs
in Different ADMDs
In this example, we see multi-tiered authentication with an extended
trust boundary.
The message was sent from someone at example.com's New York office
(newyork.example.com) to a mailing list managed at an intermediary.
The message was signed at the origin using DKIM.
The message was sent to a mailing list service provider called
example.net, which is used by example.com. There,
meetings@example.net is expanded to a long list of recipients, one of
whom is at the Chicago office. In this example, we will assume that
the trust boundary for chicago.example.com includes the mailing list
server at example.net.
The mailing list server there first authenticated the message and
affixed an Authentication-Results header field indicating such using
its DNS domain name for the authserv-id. It then altered the message
by affixing some footer text to the body, including some
administrivia such as unsubscription instructions. Finally, the
mailing list server affixes a second DKIM signature and begins
distribution of the message.
The border MTA for chicago.example.com explicitly trusts results from
mail-router.example.net, so that header field is not removed. It
performs evaluation of both signatures and determines that the first
(most recent) is a "pass" but, because of the aforementioned
modifications, the second is a "fail". However, the first signature
included the Authentication-Results header added at mail-
router.example.net that validated the second signature. Thus,
indirectly, it can be determined that the authentications claimed by
both signatures are indeed valid.
Note that two styles of presenting metadata about the result are in use here. In one case, the "reason=" clause is present, which is intended for easy extraction by parsers; in the other case, the CFWS production of the ABNF is used to include such data as a header field comment. The latter can be harder for parsers to extract given the varied supported syntaxes of mail header fields.C.7. Comment-Heavy Example
The formal syntax permits comments within the content in a number of places. For the sake of illustration, this example is also legal: Authentication-Results: foo.example.net (foobar) 1 (baz); dkim (Because I like it) / 1 (One yay) = (wait for it) fail policy (A dot can go here) . (like that) expired (this surprised me) = (as I wasn't expecting it) 1362471462 Example 7: A Very Comment-Heavy but Perfectly Legal ExampleAppendix D. Operational Considerations about Message Authentication
This protocol is predicated on the idea that authentication (and presumably in the future, reputation) work is typically done by border MTAs rather than MUAs or intermediate MTAs; the latter merely make use of the results determined by the former. Certainly this is not mandatory for participation in electronic mail or message authentication, but this protocol and its deployment to date are based on that model. The assumption satisfies several common ADMD requirements: 1. Service operators prefer to resolve the handling of problem messages as close to the border of the ADMD as possible. This enables, for example, rejection of messages at the SMTP level rather than generating a DSN internally. Thus, doing any of the authentication or reputation work exclusively at the MUA or intermediate MTA renders this desire unattainable. 2. Border MTAs are more likely to have direct access to external sources of authentication or reputation information since modern MUAs are more likely to be heavily firewalled. Thus, some MUAs might not even be able to complete the task of performing authentication or reputation evaluations without complex proxy configurations or similar burdens.
3. MUAs rely upon the upstream MTAs within their trust boundaries to
make correct (as much as is possible) evaluations about the
message's envelope, header, and content. Thus, MUAs don't need
to know how to do the work that upstream MTAs do; they only need
the results of that work.
4. Evaluations about the quality of a message, from simple token
matching (e.g., a list of preferred DNS domains) to cryptanalysis
(e.g., public/private key work), are at least a little bit
expensive and thus need to be minimized. To that end, performing
those tests at the border MTA is far preferred to doing that work
at each MUA that handles a message. If an ADMD's environment
adheres to common messaging protocols, a reputation query or an
authentication check performed by a border MTA would return the
same result as the same query performed by an MUA. By contrast,
in an environment where the MUA does the work, a message arriving
for multiple recipients would thus cause authentication or
reputation evaluation to be done more than once for the same
message (i.e., at each MUA), causing needless amplification of
resource use and creating a possible denial-of-service attack
vector.
5. Minimizing change is good. As new authentication and reputation
methods emerge, the list of methods supported by this header
field would presumably be extended. If MUAs simply consume the
contents of this header field rather than actually attempt to do
authentication and/or reputation work, then MUAs only need to
learn to parse this header field once; emergence of new methods
requires only a configuration change at the MUAs and software
changes at the MTAs (which are presumably fewer in number). When
choosing to implement these functions in MTAs vs. MUAs, the
issues of individual flexibility, infrastructure inertia, and
scale of effort must be considered. It is typically easier to
change a single MUA than an MTA because the modification affects
fewer users and can be pursued with less care. However, changing
many MUAs is more effort than changing a smaller number of MTAs.
6. For decisions affecting message delivery and display, assessment
based on authentication and reputation is best performed close to
the time of message transit, as a message makes its journey
toward a user's inbox, not afterwards. DKIM keys and IP address
reputations, etc., can change over time or even become invalid,
and users can take a long time to read a message once delivered.
The value of this work thus degrades, perhaps quickly, once the
delivery process has completed. This seriously diminishes the
value of this work when done other than at MTAs.
Many operational choices are possible within an ADMD, including the venue for performing authentication and/or reputation assessment. The current specification does not dictate any of those choices. Rather, it facilitates those cases in which information produced by one stage of analysis needs to be transported with the message to the next stage.Appendix E. Changes since RFC 5451
o Erratum #2617 was addressed in RFC 6577 and was incorporated here. o Requested Internet Standard status. o Changed IANA rules from "IETF Review" to "designated expert". o Updated existing IANA registries from the old RFC to this one. o Added references to ADSP, ATPS, and VBR. o Removed all the "X-" stuff, per BCP 178. o Adjusted language to indicate that this header field was already defined and that we're just refreshing and revising. o In a few places, RFC 2119 language had been used in lowercase terms; fixed here. o Erratum #2818 addressed. o Erratum #3195 addressed. o Performed some minor wordsmithing and removed odd prose. o ABNF: changed "dot-atom" to "Keyword" since "dot-atom" allows "=", which leads to ambiguous productions. o ABNF: the authserv-id can be a "value", not a "dot-atom". o ABNF: separated the spec version from the method version; they're syntactically the same but semantically different. Added a section discussing them. o Called out the SMTP verb exceptions ("mailfrom" and "rcptto"); the previous RFC didn't do this, leading to interoperability problems. o Rather than deleting suspect header fields, they could also be renamed to something harmless; there is at least one implementation of this.
o Updated IANA "Email Authentication Methods" registry to include
version numbers.
o Rather than repeating what RFC 4408 says the SPF results are, just
referred to those documents.
o To avoid confusing consumers, constrained inclusion of unnecessary
properties.
o Reviewed usage of "should" vs. "SHOULD".
o Updated prose around authserv-id (Section 2.4).
Author's Address
Murray S. Kucherawy
270 Upland Drive
San Francisco, CA 94127
US
EMail: superuser@gmail.com