6651
PROPOSED STANDARD
Extensions to DomainKeys Identified Mail (DKIM) for Failure Reporting
Authors: M. Kucherawy
Date: June 2012
Area: app
Working Group: marf
Stream: IETF
Abstract
This document presents extensions to the DomainKeys Identified Mail (DKIM) specification to allow for detailed reporting of message authentication failures in an on-demand fashion. [STANDARDS-TRACK]
RFC 6651
PROPOSED STANDARD
Internet Engineering Task Force (IETF) M. Kucherawy
Request for Comments: 6651 Cloudmark
Category: Standards Track June 2012
ISSN: 2070-1721
<span class="h1">Extensions to DomainKeys Identified Mail (DKIM) for Failure Reporting</span>
Abstract
This document presents extensions to the DomainKeys Identified Mail
(DKIM) specification to allow for detailed reporting of message
authentication failures in an on-demand fashion.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in <a href="./rfc5741#section-2">Section 2 of RFC 5741</a>.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
<a href="https://www.rfc-editor.org/info/rfc6651">http://www.rfc-editor.org/info/rfc6651</a>.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to <a href="https://www.rfc-editor.org/bcp/bcp78">BCP 78</a> and the IETF Trust's Legal
Provisions Relating to IETF Documents
(<a href="http://trustee.ietf.org/license-info">http://trustee.ietf.org/license-info</a>) 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.
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Table of Contents
<a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
<a href="#section-2">2</a>. Definitions .....................................................<a href="#page-3">3</a>
<a href="#section-2.1">2.1</a>. Key Words ..................................................<a href="#page-3">3</a>
<a href="#section-2.2">2.2</a>. Notation ...................................................<a href="#page-3">3</a>
<a href="#section-2.3">2.3</a>. Imported Definitions .......................................<a href="#page-3">3</a>
<a href="#section-2.4">2.4</a>. Other Definitions ..........................................<a href="#page-3">3</a>
<a href="#section-3">3</a>. Optional Reporting for DKIM .....................................<a href="#page-4">4</a>
<a href="#section-3.1">3.1</a>. Extension DKIM Signature Tag ...............................<a href="#page-4">4</a>
<a href="#section-3.2">3.2</a>. DKIM Reporting TXT Record ..................................<a href="#page-4">4</a>
<a href="#section-3.3">3.3</a>. DKIM Reporting Algorithm ...................................<a href="#page-6">6</a>
<a href="#section-4">4</a>. Optional Reporting Address for DKIM ADSP ........................<a href="#page-8">8</a>
<a href="#section-5">5</a>. Requested Reports ...............................................<a href="#page-9">9</a>
<a href="#section-5.1">5.1</a>. Requested Reports for DKIM Failures .......................<a href="#page-10">10</a>
<a href="#section-5.2">5.2</a>. Requested Reports for DKIM ADSP Failures ..................<a href="#page-10">10</a>
<a href="#section-6">6</a>. Report Generation ..............................................<a href="#page-11">11</a>
<a href="#section-6.1">6.1</a>. Report Format .............................................<a href="#page-11">11</a>
<a href="#section-6.2">6.2</a>. Other Guidance ............................................<a href="#page-11">11</a>
<a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-11">11</a>
<a href="#section-7.1">7.1</a>. DKIM Signature Tag Registration ...........................<a href="#page-11">11</a>
<a href="#section-7.2">7.2</a>. DKIM ADSP Tag Registration ................................<a href="#page-12">12</a>
<a href="#section-7.3">7.3</a>. DKIM Reporting Tag Registry ...............................<a href="#page-12">12</a>
<a href="#section-8">8</a>. Security Considerations ........................................<a href="#page-13">13</a>
<a href="#section-8.1">8.1</a>. Inherited Considerations ..................................<a href="#page-13">13</a>
<a href="#section-8.2">8.2</a>. Report Volume .............................................<a href="#page-13">13</a>
<a href="#section-8.3">8.3</a>. Deliberate Misuse .........................................<a href="#page-13">13</a>
<a href="#section-8.4">8.4</a>. Unreported Fraud ..........................................<a href="#page-14">14</a>
<a href="#section-9">9</a>. References .....................................................<a href="#page-14">14</a>
<a href="#section-9.1">9.1</a>. Normative References ......................................<a href="#page-14">14</a>
<a href="#section-9.2">9.2</a>. Informative References ....................................<a href="#page-15">15</a>
<a href="#appendix-A">Appendix A</a>. Acknowledgements ......................................<a href="#page-16">16</a>
<a href="#appendix-B">Appendix B</a>. Examples ..............................................<a href="#page-16">16</a>
<a href="#appendix-B.1">B.1</a>. Example Use of DKIM Signature Extension Tag ...............<a href="#page-16">16</a>
<a href="#appendix-B.2">B.2</a>. Example DKIM Reporting TXT Record .........................<a href="#page-17">17</a>
<a href="#appendix-B.3">B.3</a>. Example Use of DKIM ADSP Extension Tags ...................<a href="#page-17">17</a>
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<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>. Introduction</span>
DomainKeys Identified Mail [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] introduced a mechanism for message
signing and authentication. It uses digital signing to associate a
domain name with a message in a reliable manner. The verified domain
name can then be evaluated (e.g., checking 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., Author
Domain Signing Practices [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>]) of an ADministrative Management
Domain (ADMD; see [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>]).
This document extends [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] and [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>] to add an optional reporting
address and some reporting parameters. Reports are generated using
the format defined in [<a href="#ref-ARF-AUTHFAIL">ARF-AUTHFAIL</a>].
<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>. Definitions</span>
<span class="h3"><a class="selflink" id="section-2.1" href="#section-2.1">2.1</a>. Key Words</span>
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 [<a href="#ref-KEYWORDS" title=""Key words for use in RFCs to Indicate Requirement Levels"">KEYWORDS</a>].
<span class="h3"><a class="selflink" id="section-2.2" href="#section-2.2">2.2</a>. Notation</span>
Certain properties of email messages described in this document are
referenced using notation found in [<a href="#ref-EMAIL-ARCH">EMAIL-ARCH</a>] (e.g.,
"<a href="./rfc5322">RFC5322</a>.From").
<span class="h3"><a class="selflink" id="section-2.3" href="#section-2.3">2.3</a>. Imported Definitions</span>
Numerous DKIM-specific terms used here are defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>].
The definitions of the [<a href="#ref-ABNF" title=""Augmented BNF for Syntax Specifications: ABNF"">ABNF</a>] tokens "domain-name" and
"dkim-quoted-printable" can also be found there.
<span class="h3"><a class="selflink" id="section-2.4" href="#section-2.4">2.4</a>. Other Definitions</span>
report generator: A report generator is an entity that generates and
sends reports. For the scope of this document, the term refers to
Verifiers, as defined in Section 2.2 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>], with the added
capability to generate authentication failure reports according to
this specification.
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<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>. Optional Reporting for DKIM</span>
A domain name owner employing [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] for email signing and
authentication might want to know when signatures that ought to be
verifiable are not successfully verifying. Currently, there is no
such mechanism defined.
This section adds optional "tags" (as defined in [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]) to the
DKIM-Signature header field and the DKIM key record in the DNS, using
the formats defined in that specification.
<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>. Extension DKIM Signature Tag</span>
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:
r= Reporting Requested (plain-text; OPTIONAL; no default). If
present, this tag indicates that the Signer requests that
Verifiers generate a report when verification of the DKIM
signature fails. At present, the only legal value is the single
character "y". A complete description and illustration of how
this is applied can be found in <a href="#section-3.3">Section 3.3</a>.
ABNF:
sig-r-tag = %x72 *WSP "=" *WSP %x79
; "r=y" (lower-case only)
<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>. DKIM Reporting TXT Record</span>
When a Signer wishes to advertise that it wants to receive failed
verification reports, it places in the DNS a TXT Resource Record
(RR). The RR contains a sequence of tag-value objects in a format
similar to DKIM key records (see Section 3.6.1 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]), but it is
entirely independent of those key records and is found at a different
name. The tag-value objects in this case comprise the parameters to
be used when generating the reports. A report generator will request
the content of this record when it sees an "r=" tag in a
DKIM-Signature header field.
Section 3.6.2.2 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] provides guidance with respect to the
handling of a TXT RR that comprises multiple distinct strings
("character-strings" in the parlance of [<a href="#ref-DNS" title=""Domain names - implementation and specification"">DNS</a>]). The same process
MUST be applied here.
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Implementations MUST support all tags defined in this document, and
any other tag found in the content of the record that is not
recognized by an implementation MUST be ignored. See <a href="#section-7.3">Section 7.3</a> for
details about finding or registering extension tags.
The initial list of tags supported for the reporting TXT record is as
follows:
ra= Reporting Address (plain-text; OPTIONAL). A
dkim-quoted-printable string (see Section 2.11 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>])
containing the local-part of an email address to which a report
SHOULD be sent when mail fails DKIM verification for one of the
reasons enumerated below. The value MUST be interpreted as a
local-part only. To construct the actual address to which the
report is sent, the Verifier simply appends to this value an "@"
followed by the domain name found in the "d=" tag of the
DKIM-Signature header field. Therefore, a Signer making use of
this specification MUST ensure that an email address thus
constructed can receive reports generated as described in
<a href="#section-6">Section 6</a>.
ABNF:
rep-ra-tag = %x72.61 *WSP "=" *WSP dkim-quoted-printable
; "ra=..." (lower-case only for the tag name)
rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is an integer from 0 to 100 inclusive that
indicates what percentage of incidents of signature
authentication failures, selected at random, are to cause
reports to be generated. The report generator SHOULD NOT issue
reports for more than the requested percentage of incidents.
Report generators MAY make use of the "Incidents:" field in
[<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] to indicate that there are more reportable incidents than
there are reports.
ABNF:
rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
; "rp=..." (lower-case only)
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rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing
those conditions under which a report is desired. See
<a href="#section-5.1">Section 5.1</a> for a list of valid tokens.
ABNF:
rep-rr-type = ( "all" / "d" / "o" / "p" / "s" / "u" / "v" / "x" )
rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type
*WSP *( ":" *WSP rep-rr-type )
; "rr=..." (lower-case only for the tag name)
rs= Requested SMTP Error String (plain-text; OPTIONAL; no default).
The value is a dkim-quoted-printable string that the publishing
ADMD requests be included in [<a href="#ref-SMTP" title=""Simple Mail Transfer Protocol"">SMTP</a>] error strings if messages
are rejected during the delivery SMTP session.
ABNF:
rep-rs-tag = %x72.73 *WSP "=" dkim-quoted-printable
; "rs=..." (lower-case only for the tag name)
In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be
ignored, and the report generator MUST NOT issue a report.
<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>. DKIM Reporting Algorithm</span>
Report generators MUST apply the following algorithm, or one
semantically equivalent to it, for each DKIM-Signature header field
whose verification fails for some reason. Note that this processing
is done as a reporting extension only; the outcome of the specified
DKIM evaluation MUST be otherwise unaffected.
1. If the DKIM-Signature field did not contain a valid "r=" tag,
terminate.
2. Issue a [<a href="#ref-DNS" title=""Domain names - implementation and specification"">DNS</a>] TXT query to the name that results from appending
the value of the "d=" tag in the DKIM-Signature field to the
string "_report._domainkey.". For example, if the DKIM-Signature
header field contains "d=example.com", issue a DNS TXT query to
"_report._domainkey.example.com".
3. If the DNS query returns anything other than RCODE 0 (NOERROR),
or if multiple TXT records are returned, terminate.
4. If the resultant TXT is in several string fragments, concatenate
them as described in Section 3.6.2.2 of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>].
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5. If the TXT content is syntactically invalid (see <a href="#section-3.2">Section 3.2</a>),
terminate.
6. If the reason for the signature evaluation failure does not match
one of the report requests found in the "rr=" tag (or its default
value), terminate.
7. If a report percentage ("rp=") tag was present, select a random
number between 0 and 99, inclusive; if the selected number is not
lower than the tag's value, terminate.
8. If no "ra=" tag was present, skip this step and the next one.
Otherwise, determine the reporting address by extracting the
value of the "ra=" tag and appending to it an "@" followed by the
domain name found in the "d=" tag of the DKIM-Signature header
field.
9. Construct and send a report in compliance with <a href="#section-6">Section 6</a> of this
document that includes as its intended recipient the address
constructed in the previous step.
10. If the [<a href="#ref-SMTP" title=""Simple Mail Transfer Protocol"">SMTP</a>] session during which the DKIM signature was
evaluated is still active and the SMTP server has not already
given its response to the DATA command that relayed the message,
and an "rs=" tag was present in the TXT record, the SMTP server
SHOULD include the decoded string found in the "rs=" tag in its
SMTP reply to the DATA command.
In order to thwart attacks that seek to convert report generators
into unwitting denial-of-service attack participants, a report
generator SHOULD NOT issue more than one report to any given domain
as a result of a single message. Further, a report generator SHOULD
establish an upper bound on the number of reports a single message
can generate overall. For example, a message with three invalid
signatures, two from example.com and one from example.net, would
generate at most one report to each of those domains.
This algorithm has the following advantages over previous
pre-standardization implementations, such as early versions of
[<a href="#ref-OPENDKIM" title=""OpenDKIM -- Open Source DKIM Library and Filter"">OPENDKIM</a>]:
a. If the DKIM signature fails to verify, no additional DNS check is
made to see if reporting is requested; the request is active in
that it is included in the DKIM-Signature header field.
(Previous implementations included the reporting address in the
DKIM key record, which is not queried for certain failure cases.
This meant, for full reporting, that the key record had to be
retrieved even when it was not otherwise necessary.)
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b. The request is confirmed by the presence of a corresponding TXT
record in the DNS, since the Signer thus provides the parameters
required to construct and send the report. This means a
malicious Signer cannot falsely assert that someone else wants
failure reports and cause unwanted mail to be generated. It can
cause additional DNS traffic against the domain listed in the
"d=" signature tag, but negative caching of the requested DNS
record will help to mitigate this issue.
c. It is not possible for a Signer to direct reports to an email
address outside of its own domain, preventing distributed email-
based denial-of-service attacks.
See <a href="#section-8.4">Section 8.4</a> for some considerations regarding limitations of this
mechanism.
<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>. Optional Reporting Address for DKIM ADSP</span>
A domain name owner employing Author Domain Signing Practices [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>]
may also want to know when messages are received without valid author
domain signatures. Currently, there is no such mechanism defined.
This section adds the following optional "tags" (as defined in
[<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>]) to the DKIM ADSP records, using the form defined in that
specification:
ra= Reporting Address (plain-text; OPTIONAL; no default). The value
MUST be a dkim-quoted-printable string containing the local-part
of an email address to which a report SHOULD be sent when mail
claiming to be from this domain failed the verification
algorithm described in [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>], in particular because a message
arrived without a signature that validates, which contradicts
what the ADSP record claims. The value MUST be interpreted as a
local-part only. To construct the actual address to which the
report is sent, the Verifier simply appends to this value an "@"
followed by the domain whose policy was queried in order to
evaluate the sender's ADSP, i.e., the <a href="./rfc5322">RFC5322</a>.From domain of the
message under evaluation. Therefore, a Signer making use of
this extension tag MUST ensure that an email address thus
constructed can receive reports generated as described in
<a href="#section-6">Section 6</a>.
ABNF:
adsp-ra-tag = %x72.61 *WSP "=" dkim-quoted-printable
; "ra=..." (lower-case only for the tag name)
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rp= Requested Report Percentage (plain-text; OPTIONAL; default is
"100"). The value is a single integer from 0 to 100 inclusive
that indicates what percentage of incidents of ADSP evaluation
failures, selected at random, are to cause reports to be
generated. The report generator SHOULD NOT issue reports for
more than the requested percentage of incidents. An exception
to this might be some out-of-band arrangement between two
parties to override it with some mutually agreed value. Report
generators MAY make use of the "Incidents:" field in [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] to
indicate that there are more reportable incidents than there are
reports.
ABNF:
adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
; "rp=..." (lower-case only)
rr= Requested Reports (plain-text; OPTIONAL; default is "all"). The
value MUST be a colon-separated list of tokens representing
those conditions under which a report is desired. See
<a href="#section-5.2">Section 5.2</a> for a list of valid tokens.
ABNF:
adsp-rr-type = ( "all" / "o" / "p" / "s" / "u" )
adsp-rr-tag = %x72.72 *WSP "=" *WSP adsp-rr-type
*WSP *( ":" *WSP adsp-rr-type )
; "rr=..." (lower-case only for the tag name)
rs= Requested SMTP Error String (plain-text; OPTIONAL; no default).
The value is a string the signing domain requests be included in
[<a href="#ref-SMTP" title=""Simple Mail Transfer Protocol"">SMTP</a>] error strings when messages are rejected during a single
SMTP session.
ABNF:
adsp-rs-tag = %x72.73 *WSP "=" dkim-quoted-printable
; "rs=..." (lower-case only for the tag name)
In the absence of an "ra=" tag, the "rp=" and "rr=" tags MUST be
ignored, and the report generator MUST NOT issue a report.
<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>. Requested Reports</span>
The "rr" tags defined above allow a Signer to specify the types of
errors about which it is interested in receiving reports. This
section defines the error types and corresponding token values.
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Verifiers MUST NOT generate reports for incidents that do not match a
requested report and MUST ignore requests for reports not included in
this list.
<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>. Requested Reports for DKIM Failures</span>
The following report requests are defined for DKIM keys:
all All reports are requested.
d Reports are requested for signature evaluation errors that
resulted from DNS issues (e.g., key retrieval problems).
o Reports are requested for any reason related to DKIM signature
evaluation not covered by other report requests listed here.
p Reports are requested for signatures that are rejected for local
policy reasons at the Verifier that are related to DKIM
signature evaluation.
s Reports are requested for signature or key syntax errors.
u Reports are requested for signatures that include unknown tags
in the signature field.
v Reports are requested for signature verification failures or
body hash mismatches.
x Reports are requested for signatures rejected by the Verifier
because the expiration time has passed.
<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>. Requested Reports for DKIM ADSP Failures</span>
The following report requests are defined for ADSP records:
all All reports are requested.
o Reports are requested for any [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>]-related failure reason not
covered by other report requests listed here.
p Reports are requested for messages that are rejected for local
policy reasons at the Verifier that are related to [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>].
s Reports are requested for messages that have a valid [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
signature but do not match the published [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>] policy.
u Reports are requested for messages that have no valid [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
signature and do not match the published [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>] policy.
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<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>. Report Generation</span>
This section describes the process for generating and sending reports
in accordance with the request of the Signer and/or sender as
described above.
<span class="h3"><a class="selflink" id="section-6.1" href="#section-6.1">6.1</a>. Report Format</span>
All reports generated as a result of requests contained in these
extension parameters MUST be generated in compliance with [<a href="#ref-ARF" title=""An Extensible Format for Email Feedback Reports"">ARF</a>] and
its extension specific to this work, [<a href="#ref-ARF-AUTHFAIL">ARF-AUTHFAIL</a>]. Moreover,
because abuse reports from unverified sources might be handled with
some skepticism, report generators are strongly advised to use [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>]
to sign reports they generate.
<span class="h3"><a class="selflink" id="section-6.2" href="#section-6.2">6.2</a>. Other Guidance</span>
Additional guidance about the generation of these reports can be
found in [<a href="#ref-ARF-AS" title=""Creation and Use of Email Feedback Reports: An Applicability Statement for the Abuse Reporting Format (ARF)"">ARF-AS</a>], especially in <a href="#section-6">Section 6</a>.
<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>. IANA Considerations</span>
As required by [<a href="#ref-IANA-CONS">IANA-CONS</a>], this section contains registry
information for the new [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] signature tags and for the new [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>]
tags. It also creates a DKIM reporting tag registry.
<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>. DKIM Signature Tag Registration</span>
IANA has added the following item to the DKIM Signature Tag
Specifications registry:
+------+-----------------+--------+
| TYPE | REFERENCE | STATUS |
+------+-----------------+--------+
| r | (this document) | active |
+------+-----------------+--------+
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<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>. DKIM ADSP Tag Registration</span>
IANA has added the following items to the DKIM ADSP Specification
Tags registry:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| ra | (this document) |
| rp | (this document) |
| rr | (this document) |
| rs | (this document) |
+------+-----------------+
<span class="h3"><a class="selflink" id="section-7.3" href="#section-7.3">7.3</a>. DKIM Reporting Tag Registry</span>
IANA has created a sub-registry of the DKIM Parameters registry
called "DKIM Reporting Tag Registry". Additions to this registry
follow the "Specification Required" rules, with the following columns
required for all registrations:
Tag: The name of the tag being used in reporting records
Reference: The document that specifies the tag being defined
Status: The status of the tag's current use -- either "active"
indicating active use, or "historic" indicating discontinued or
deprecated use
The initial registry entries are as follows:
+-----+-----------------+--------+
| TAG | REFERENCE | STATUS |
+-----+-----------------+--------+
| ra | (this document) | active |
| rp | (this document) | active |
| rr | (this document) | active |
| rs | (this document) | active |
+-----+-----------------+--------+
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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>. Security Considerations</span>
Security issues with respect to these reports are similar to those
found in [<a href="#ref-DSN" title=""An Extensible Message Format for Delivery Status Notifications"">DSN</a>].
<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>. Inherited Considerations</span>
Implementers are advised to consider the Security Considerations
sections of [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>], [<a href="#ref-ADSP" title=""DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)"">ADSP</a>], [<a href="#ref-ARF-AS" title=""Creation and Use of Email Feedback Reports: An Applicability Statement for the Abuse Reporting Format (ARF)"">ARF-AS</a>], and [<a href="#ref-ARF-AUTHFAIL">ARF-AUTHFAIL</a>]. Many
security issues related to this document are already covered in those
documents.
<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>. Report Volume</span>
It is impossible to predict the volume of reports this facility will
generate when enabled by a report receiver. An implementer ought to
anticipate substantial volume, since the amount of abuse occurring at
receivers cannot be known ahead of time, and may vary rapidly and
unpredictably.
<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>. Deliberate Misuse</span>
Some threats caused by deliberate misuse of this error-reporting
mechanism are discussed in <a href="#section-3.3">Section 3.3</a>, but they warrant further
discussion here.
The presence of the DNS record that indicates willingness to accept
reports opens the recipient to abuse. In particular, it is possible
for an attacker to attempt to cause a flood of reports toward the
domain identified in a signature's "d=" tag in one of these ways:
1. Alter existing DKIM-Signature header fields by adding an "r=y"
tag (and possibly altering the "d=" tag to point at the target
domain);
2. Add a new but bogus signature bearing an "r=y" tag and a "d=" tag
pointing at the target domain;
3. Generate a completely new message bearing an "r=y" tag and a "d="
tag pointing at the target domain.
Consider, for example, the situation where an attacker sends out a
multi-million-message spam run and includes in the messages a fake
DKIM signature containing "d=example.com; r=y". It won't matter that
those signatures couldn't possibly be real: each will fail
verification, and any implementations that support this specification
will report those failures, in the millions and in short order, to
example.com.
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Implementers are therefore strongly advised not to advertise the DNS
record specified in this document except when failure reports are
desired. Upon doing so, unexpected traffic volumes and attacks
should be anticipated.
Negative caching offers some protection against this pattern of
abuse, although it will work only as long as the negative time-to-
live on the relevant SOA record in the DNS.
Positive caching of this DNS reply also means that turning off the
flow of reports by removing the record is not likely to have an
immediate effect. A low time-to-live on the record needs to be
considered.
<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>. Unreported Fraud</span>
An attacker can craft fraudulent DKIM-Signature fields on messages,
without using "r=" tags, and avoid having these reported. The
procedure described in <a href="#section-3.3">Section 3.3</a> does not permit the detection and
reporting of such cases.
It might be useful to some Signers to receive such reports, but the
mechanism does not support it. To offer such support, a Verifier
would have to violate the first step in the procedure and continue
even in the absence of an "r=" tag. Although that would enable the
desired report, it would also create a possible denial-of-service
attack: such Verifiers would always look for the reporting TXT
record, so a generator of fraudulent messages could simply send a
large volume of messages without an "r=" tag to a number of
destinations. To avoid that outcome, reports of fraudulent
DKIM-Signature header fields are not possible using the published
mechanism.
<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>. References</span>
<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>. Normative References</span>
[<a id="ref-ABNF">ABNF</a>] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", <a href="./rfc5234">RFC 5234</a>, January 2008.
[<a id="ref-ADSP">ADSP</a>] Allman, E., Fenton, J., Delany, M., and J. Levine,
"DomainKeys Identified Mail (DKIM) Author Domain Signing
Practices (ADSP)", <a href="./rfc5617">RFC 5617</a>, August 2009.
[<a id="ref-ARF">ARF</a>] Shafranovich, Y., Levine, J., and M. Kucherawy, "An
Extensible Format for Email Feedback Reports", <a href="./rfc5965">RFC 5965</a>,
August 2010.
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[<a id="ref-ARF-AS">ARF-AS</a>] Falk, J. and M. Kucherawy, Ed., "Creation and Use of Email
Feedback Reports: An Applicability Statement for the Abuse
Reporting Format (ARF)", <a href="./rfc6650">RFC 6650</a>, June 2012.
[<a id="ref-ARF-AUTHFAIL">ARF-AUTHFAIL</a>]
Fontana, H., "Authentication Failure Reporting Using the
Abuse Reporting Format", <a href="./rfc6591">RFC 6591</a>, April 2012.
[<a id="ref-DKIM">DKIM</a>] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", <a href="./rfc6376">RFC 6376</a>,
September 2011.
[<a id="ref-DNS">DNS</a>] Mockapetris, P., "Domain names - implementation and
specification", STD 13, <a href="./rfc1035">RFC 1035</a>, November 1987.
[<a id="ref-EMAIL-ARCH">EMAIL-ARCH</a>]
Crocker, D., "Internet Mail Architecture", <a href="./rfc5598">RFC 5598</a>,
July 2009.
[<a id="ref-IANA-CONS">IANA-CONS</a>]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc5226">RFC 5226</a>,
May 2008.
[<a id="ref-KEYWORDS">KEYWORDS</a>] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.
[<a id="ref-SMTP">SMTP</a>] Klensin, J., "Simple Mail Transfer Protocol", <a href="./rfc5321">RFC 5321</a>,
October 2008.
<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>. Informative References</span>
[<a id="ref-DSN">DSN</a>] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", <a href="./rfc3464">RFC 3464</a>,
January 2003.
[<a id="ref-OPENDKIM">OPENDKIM</a>] Kucherawy, M., "OpenDKIM -- Open Source DKIM Library and
Filter", August 2009, <<a href="http://www.opendkim.org">http://www.opendkim.org</a>>.
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<span class="h2"><a class="selflink" id="appendix-A" href="#appendix-A">Appendix A</a>. Acknowledgements</span>
The author wishes to acknowledge the following for their review and
constructive criticism of this proposal: Steve Atkins, Monica Chew,
Dave Crocker, Tim Draegen, Frank Ellermann, J.D. Falk, John Levine,
Scott Kitterman, and Andrew Sullivan.
<span class="h2"><a class="selflink" id="appendix-B" href="#appendix-B">Appendix B</a>. Examples</span>
This section contains examples of the use of each of the extensions
defined by this document.
<span class="h3"><a class="selflink" id="appendix-B.1" href="#appendix-B.1">B.1</a>. Example Use of DKIM Signature Extension Tag</span>
This example shows a DKIM-Signature field using the extension tag
defined by this document:
DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
d=example.com; s=jan2012; r=y;
h=from:to:subject:date:message-id;
bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8
Example 1: DKIM-Signature Field Using This Extension
This example DKIM-Signature field contains the "r=" tag that
indicates reports are requested on verification failure.
Assuming the public key retrieved from the DNS and processed
according to [<a href="#ref-DKIM" title=""DomainKeys Identified Mail (DKIM) Signatures"">DKIM</a>] would determine that the signature is invalid, a
TXT query will be sent to "_report._domainkey.example.com" to
retrieve a reporting address and other report parameters as described
in <a href="#section-3.3">Section 3.3</a>.
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<span class="h3"><a class="selflink" id="appendix-B.2" href="#appendix-B.2">B.2</a>. Example DKIM Reporting TXT Record</span>
An example DKIM Reporting TXT record as defined by this document is
as follows:
ra=dkim-errors; rp=100; rr=v:x
Example 2: Example DKIM Reporting TXT Record
This example, continuing from the previous one, shows a message that
might be found at "_report._domainkey.example.com" in a TXT record.
It makes the following requests:
o Reports about signature evaluation failures should be sent to the
address "dkim-errors" at the Signer's domain;
o All incidents (100%) should be reported;
o Only reports about signature verification failures and expired
signatures should be generated.
<span class="h3"><a class="selflink" id="appendix-B.3" href="#appendix-B.3">B.3</a>. Example Use of DKIM ADSP Extension Tags</span>
This example shows a DKIM ADSP record using the extensions defined by
this document:
dkim=all; ra=dkim-adsp-errors; rr=u
Example 3: DKIM ADSP Record Using These Extensions
This example ADSP record makes the following assertions:
o The sending domain (i.e., the one that is advertising this policy)
signs all mail it sends;
o Reports about ADSP evaluation failures should be sent to the
address "dkim-adsp-errors" at the Author's domain;
o Only reports about unsigned messages should be generated.
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Author's Address
Murray S. Kucherawy
Cloudmark
128 King St., 2nd Floor
San Francisco, CA 94107
US
Phone: +1 415 946 3800
EMail: [email protected]
Kucherawy Standards Track [Page 18]
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