| MARF Working Group | M.S. Kucherawy |
| Internet-Draft | Cloudmark |
| Intended status: Standards Track | October 18, 2012 |
| Expires: April 19, 2013 |
Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-05
This memo presents extensions to the DomainKeys Identified Mail (DKIM) specification to allow for detailed reporting of message authentication failures in an on-demand fashion.
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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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[DKIM] introduced a mechanism for message signing and authentication. It uses digital signing to associate a domain name with a message in a reliable (i.e. not forgeable) manner. The output is a verified domain name that can then be subjected to some sort of evaluation process (e.g., advertised sender policy, comparison to a known-good list, submission to a reputation service, etc.).
Deployers of message authentication technologies are increasingly seeking visibility into DKIM verification failures and conformance failures involving the published signing practices (e.g., [ADSP]) of an Administrative Management Domain (ADMD; see [EMAIL-ARCH]).
This document extends [DKIM] and [ADSP] to add an optional reporting address and some reporting parameters.
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 [KEYWORDS].
The ABNF token "qp-section" is imported from [MIME].
Numerous DKIM-specific terms used here are defined in [DKIM]. The definition of the ABNF token "domain-name" can also be found there.
A domain name owner employing [DKIM] for email signing and authentication might want to know when signatures in use by specific keys are not successfully verifying. Currently there is no such mechanism defined.
This document adds optional "tags" (as defined in [DKIM]) to the DKIM-Signature header field and the DKIM key record in the DNS, using the formats defined in that specification.
The following tag is added to DKIM-Signature header fields when a Signer wishes to request that reports of failed verifications be generated by a Verifier:
sig-rd-tag = %x72.64 *WSP "=" *WSP domain-name
ABNF:
When a Signer wishes to advertise that it wants to receive failed verification reports, it also places in the DNS a TXT resource record (RR) whose content follows the same general syntax as DKIM key records, as defined in Section 3.6.1 of [DKIM], in that it is made up of a sequence of tag-value objects. In this case, the tags and values comprise the parameters to be used when generating the reports. The Verifier (if it supports this extension) will request the content of this record when it sees an "rd=" tag in a DKIM-Signature header field.
The reassembly rules of Section 3.6.2.2 of [DKIM] also apply here if the reporting TXT record consists of several string fragments.
Any tag found in the content of this record that is not registered with IANA as described in Section 7.3 MUST be ignored.
rep-ra-tag = %x72.61 *WSP "=" *WSP qp-section
rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
rep-rr-type = ( "all" / "s" / "v" / "x" )
rep-rr-tag = %x72 %x6f *WSP "=" *WSP rep-rr-type
*WSP 0* ( ":" *WSP rep-rr-type )
rep-rs-tag = %x72 %x73 *WSP "=" qp-section
The initial list of tags supported for the reporting TXT record is as follows:
In the absence of an "ra=" tag, all other tags listed above MUST be ignored. In other words, an implementation MUST NOT have a default value for the "ra=" tag.
The following algorithm, or one semantically equivalent to it, MUST be applied when an implementation is equipped to generate reports in compliance with this specification and its evaluation of the DKIM-Signature header field (see [DKIM]) on a message fails for some reason. Note that this processing is done as a reporting extension only; the outcome of the specified DKIM evaluation is otherwise unaffected.
This algorithm has the following advantages over previous implementations:
There also exist cases in which a domain name owner employing [ADSP] for announcing signing practises with DKIM may want to know when messages are received without valid author domain signatures. Currently there is no such mechanism defined.
adsp-ra-tag = %x72.61 *WSP "=" qp-section
adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
adsp-rr-type = ( "all" / "s" / "u" )
adsp-rr-tag = %x72 %x6f *WSP "=" *WSP adsp-rr-type
*WSP 0* ( ":" *WSP adsp-rr-type )
adsp-rs-tag = %x72 %x73 *WSP "=" qp-section
This document adds the following optional "tags" (as defined in [ADSP]) to the DKIM ADSP records, using the form defined in that specification:
In the absence of an "r=" tag, all other tags listed above MUST be ignored.
This memo also includes, as the "ro" tags defined above, the means by which the signer can request reports for specific circumstances of interest. Verifiers MUST NOT generate reports for incidents that do not match a requested report, and MUST ignore requests for reports not included in this these lists.
The following report requests are defined for DKIM keys:
The following report requests are defined for ADSP records:
This section describes the process for generating and sending reports in accordance with the request of the signer and/or sender as described above.
All reports generated as a result of requests contained in these extension parameters MUST be generated in compliance with [ARF] and its extension specific to this work, [I-D.MARF-AUTHFAILURE-REPORT].
In the case of transmitted reports in the form of a new message (versus rejections during an [SMTP] session), it is necessary to construct the message so as to avoid amplification attacks, deliberate or otherwise. Thus, per Section 2 of [DSN], the envelope sender address of the report SHOULD be chosen to ensure that no delivery status reports will be issued in response to the report itself, and MUST be chosen so that these reports will not generate mail loops. Whenever an [SMTP] transaction is used to send a report, the MAIL FROM command MUST use a NULL return address, i.e. "MAIL FROM:<>".
As required by [IANA-CONSIDERATIONS], this section contains registry information for the new [DKIM] signature tags, and the new [ADSP] tags. It also creates a DKIM reporting tag registry.
IANA is requested to update the DKIM Signature Tag Specification Registry to include the following new items:
+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| rd | (this document) | active |
+------+-----------------+--------+
IANA is requested to update the DKIM ADSP Specification Tag Registry to include the following new items:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| ra | (this document) |
| rp | (this document) |
| rr | (this document) |
| rs | (this document) |
+------+-----------------+
IANA is requested to create a sub-registry of the DKIM Parameters registry called "DKIM Reporting Tags". Additions to this registry follow the "Specification Required" rules, with the following columns required for all registrations:
+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| ra | (this document) | active |
| rp | (this document) | active |
| rr | (this document) | active |
| rs | (this document) | active |
+------+-----------------+--------+
The initial registry entries are as follows:
Security issues with respect to these reports are similar to those found in [DSN].
Implementers are advised to consider the Security Considerations sections of [DKIM] and [ADSP].
Some threats caused by deliberate misuse of this mechanism are discussed in Section 3.3.
These reports may be forged as easily as ordinary Internet electronic mail. User agents and automatic mail handling facilities (such as mail distribution list exploders) that wish to make automatic use of reports of any kind should take appropriate precautions to minimize the potential damage from denial-of-service attacks.
Security threats related to forged reports include the sending of:
Perhaps the simplest means of mitigating this threat is to assert that these reports should themselves be signed with something like DKIM. On the other hand, if there's a problem with the DKIM infrastructure at the verifier, signing DKIM failure reports may produce reports that aren't trusted or even accepted by their intended recipients.
Failure to compile with the normative statements in Section 6.2 can lead to amplification denial-of-service attacks. See that section for details.
The mechanisms described in this memo are primarily intended for use in generating reports to aid implementers of [DKIM] and [ADSP] and other related protocols in development and debugging. Therefore, they are not designed for prolonged forensic use. However, such uses are possible by ADMDs that want to keep a close watch for fraud or infrastructure problems.
Automatic generation of these reports by verifying agents can cause a denial-of-service attack when a large volume of email is sent that causes authentication failures for whatever reason.
Limiting the rate of generation of these messages may be appropriate but threatens to inhibit the distribution of important and possibly time-sensitive information.
In general ARF feedback loop terms, it is often suggested that report generators only create these (or any) ARF reports after an out-of-band arrangement has been made between two parties. This mechanism then becomes a way to adjust parameters of an authorized abuse report feedback loop that is configured and activated by private agreement rather than starting to send them automatically based solely on data found in the DKIM signatures, which could have been fraudulently inserted.
If it is known that a particular host generates abuse reports upon certain incidents, an attacker could forge a high volume of messages that will trigger such a report. The recipient of the report could then be innundated with reports. This could easily be extended to a distributed denial-of-service attack by finding a number of report-generating servers.
The incident count referenced in [ARF] provides a limited form of mitigation. The host generating reports may elect to send reports only periodically, with each report representing a number of identical or near-identical incidents. One might even do something inverse-exponentially, sending reports for each of the first ten incidents, then every tenth incident up to 100, then every 100th incident up to 1000, etc. until some period of relative quiet after which the limitation resets.
The use of this for "near-identical" incidents in particular causes a degradation in reporting quality, however. If for example a large number of pieces of spam arrive from one attacker, a reporting agent may decide only to send a report about a fraction of those messages. While this averts a flood of reports to a system administrator, the precise details of each incident are similarly not sent.
| [DSN] | Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications ", RFC 3464, January 2003. |
The authors wish to acknowledge the following for their review and constructive criticism of this proposal: Steve Atkins, Monica Chew, Dave Crocker, Tim Draegen, Frank Ellermann, JD Falk, and John Levine.
This section contains examples of the use of each of the extensions defined by this memo.
A DKIM-Signature field including use of the extensions defined by this memo:
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
d=example.com; s=jan2012;
r=dkim-errors; ro=v:x;
h=from:to:subject:date:message-id;
bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8
Example 1: DKIM-Signature field using these extensions
This example DKIM-Signature field contains the following data in addition to the basic DKIM signature data:
A DKIM ADSP record including use of the extensions defined by this memo:
dkim=all; r=dkim-adsp-errors; ro=u
Example 2: DKIM ADSP record using these extensions
This example ADSP record makes the following assertions: