Individual Submission E. Allman
Internet-Draft Sendmail, Inc.
Intended status: Standards Track M. Delany
Expires: March 3, 2007 Yahoo! Inc.
J. Fenton
Cisco Systems, Inc.
August 30, 2006
DKIM Sender Signing Practices
draft-allman-dkim-ssp-02
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email using public-key cryptography and
key server technology to permit verification of the source and
contents of messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary DKIM protocol is described in
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[I-D.ietf-dkim-base].
This document describes the records that senders may use to advertise
how they sign their outgoing mail, and how verifiers should access
and interpret those results.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
(Unresolved Issues/To Be Done)
Security Considerations needs further work.
Need to check existing ABNF.
Text structure of document needs to be examined; this is a quick
slash-and-burn approach.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 5
2.1. Terms Imported from DKIM-Base . . . . . . . . . . . . . . 5
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.3. Originator Address . . . . . . . . . . . . . . . . . . . . 5
2.4. Alleged Signer . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Alleged Originator . . . . . . . . . . . . . . . . . . . . 6
2.6. Sender Signing Practices . . . . . . . . . . . . . . . . . 6
2.7. Originator Signature . . . . . . . . . . . . . . . . . . . 6
2.8. Suspicious . . . . . . . . . . . . . . . . . . . . . . . . 6
2.9. Third-Party Signature . . . . . . . . . . . . . . . . . . 6
2.10. Verifier Acceptable Third-Party Signature . . . . . . . . 7
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 7
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 8
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 8
4.2. Record Syntax . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Sender Signing Practices Check Procedure . . . . . . . . . 11
5. Third-Party Signatures and Mailing Lists . . . . . . . . . . . 12
5.1. Mailing List Manager Actions . . . . . . . . . . . . . . . 13
5.2. Signer Actions . . . . . . . . . . . . . . . . . . . . . . 13
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.1. Fraudulent Sender Address . . . . . . . . . . . . . . . . 14
7.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 14
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . . 15
8.2. Informational References . . . . . . . . . . . . . . . . . 15
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 15
A.1. Changes since -01 . . . . . . . . . . . . . . . . . . . . 15
A.2. Changes since -00 . . . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
DomainKeys Identified Mail (DKIM) defines a mechanism by which email
messages can be cryptographically signed, permitting a signing domain
to claim responsibility for the introduction of a message into the
mail stream. Message recipients can verify the signature by querying
the signer's domain directly to retrieve the appropriate public key,
and thereby confirm that the message was attested to by a party in
possession of the private key for the signing domain.
However, the legacy of the Internet is such that not all messages
will be signed, and the absence of a signature on a message is not an
a priori indication of forgery. In fact, during early phases of
deployment it must be expected that most messages will remain
unsigned. However, some domains may choose to sign all of their
outgoing mail, for example, to protect their brand name. It is
highly desirable for such domains to be able to advertise that fact
to verifiers, and that messages claiming to be from them that do not
have a valid signature are likely to be forgeries. This is the topic
for sender signing practices.
In the absence of a valid DKIM signature on behalf of the "From"
address [RFC2822], the verifier of a message MUST determine whether
messages from that sender are expected to be signed, and what
signatures are acceptable. In particular, whether a domain signs all
outbound email must be communicated to the verifier. Without such a
mechanism, the benefit of message signing techniques such as DKIM is
limited since unsigned messages will always need to be considered to
be potentially legitimate. This determination is referred to as a
Sender Signing Practices check.
Sender Signing Practices MAY be expressed on behalf of an entity
which may be a domain or an individual address. Expression of
signing practices on behalf of individual addresses will, of course,
entail additional transaction load.
Conceivably, such expressions might be imagined to be extended in the
future to include information about what hashing algorithms a domain
uses, what kind of messages might be sent (e.g., bulk vs. personal
vs. transactional), etc. Such concerns are out of scope of this
standard, because they can be expressed in the key record
("Selector") with which the signature is verified. Expressions of
signing practice which require outside auditing are out of scope for
this specification because they fall under the purview of reputation
and accreditation.
This document refers extensively to [I-D.ietf-dkim-base], which
should be read as a prerequisite to this document.
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2. Language and Terminology
2.1. Terms Imported from DKIM-Base
Some terminology used herein is derived directly from
[I-D.ietf-dkim-base]. Briefly,
o A "Signer" is the agent that signs a message. In many cases it
will correspond closely with the original author of the message or
an agent working on the author's behalf.
o A "Verifier" is the agent that verifies a message by checking
the actual signature against the message itself and the public key
published by the alleged signer. The Verifier also looks up the
Sender Signing Practices published by the domain of the Originator
Address if the message is not correctly signed by the Alleged
Originator.
o A "Selector" specifies which of the keys published by a signing
domain should be queried. It is essentially a way of subdividing
the address space to allow a single sending domain to publish
multiple keys.
2.2. Valid Signature
A "Valid Signature" is any signature on a message which correctly
verifies using the procedure described in section 6.1 of
[I-D.ietf-dkim-base].
2.3. Originator Address
The "Originator Address" is the email address in the From header
field of a message [RFC2822], or if and only if the From header field
contains multiple addresses, the first address in the From header
field.
NON-NORMATIVE RATIONALE: The alternative option when there are
multiple addresses in the From header field is to use the value of
the Sender header field. This would be closer to the semantics
indicated in [RFC2822] than using the first address in the From
header field. However, the large number of deployed Mail User
Agents that do not display the Sender header field value argues
against that. Multiple addresses in the From header field are
rare in real life.
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2.4. Alleged Signer
An "Alleged Signer" is the identity of the signer claimed in the
DKIM-Signature header field in a message received by a Verifier; it
is "alleged" because it has not yet been verified.
2.5. Alleged Originator
An "Alleged Originator" is the Originator Address of a message
received by a Verifier; it is "alleged" because it has not yet been
verified.
2.6. Sender Signing Practices
"Sender Signing Practices" (or just "practices") consist of a
machine-readable record published by the domain of the Alleged
Originator which includes information about whether or not that
entity signs all of their email, and whether signatures from third
parties are sanctioned by the Alleged Originator.
2.7. Originator Signature
An "Originator Signature" is any Valid Signature where the signing
address (listed in the "i=" tag if present, otherwise the null
address, representing an unknown user, followed by "@", followed by
the value of the "d=" tag) matches the address in the "From" header
field. If the signing address does not include a local-part, then
only the domains must match; otherwise, the two addresses must be
identical.
2.8. Suspicious
Messages that do not contain a valid Originator Signature and which
are inconsistent with a Sender Signing Practices check (e.g., are
received without a Valid Signature and the sender's signing practices
indicate all messages from the entity are signed) are referred to as
"Suspicious". The handling of such messages is at the discretion of
the Verifier or final recipient. "Suspicious" applies only to the
DKIM layer; a Verifier may decide the message should be accepted on
the basis of other information beyond the scope of this document.
Conversely, messages deemed non-Suspicious may be rejected for other
reasons.
2.9. Third-Party Signature
A "Third-Party Signature" is a Valid Signature which is not an
Originator Signature.
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2.10. Verifier Acceptable Third-Party Signature
A Verifier Acceptable Third-Party Signature is a Third-Party
Signature that the Verifier is willing to accept as meaningful for
the message under consideration. The Verifier may use any criteria
it deems appropriate for making this determination.
3. Operation Overview
Sender Signing Practices checks MUST be based on the Originator
Address. If the message contains a valid Originator Signature, no
Sender Signing Practices check need be performed: the Verifier
SHOULD NOT look up the Sender Signing Practices and the message
SHOULD be considered non-Suspicious.
Verifiers checking messages that do not have at least one valid
Originator Signature MUST perform a Sender Signing Practices check on
the domain specified by the Originator Address as described in
Section 4.3.
The result of a Sender Signing Practices check is one of five
possible practices:
(1) Some messages from this entity are not signed; the message
SHOULD be presumed to be legitimate in the absence of a valid
signature. This is the default.
(2) All messages from this entity are signed; all messages from
this entity SHOULD have a Valid Signature, either directly on
behalf of the originator or on behalf of a third party (e.g., a
mailing list or an outsourcing house) handling the message.
(3) All valid messages from this entity are signed, and SHOULD
have a Valid Signature from this entity. Third-Party Signatures
SHOULD not be accepted.
(4) Signing practices for this domain are expressed at the
individual address level. A second Sender Signing Practices check
MUST be performed specifying the individual address.
(5) The domain does not exist.
If a message is encountered by a Verifier without a valid Originator
Signature, the results MUST be interpreted as follows:
If the result of the check is practice (1) described above, the
message MUST be considered non-Suspicious.
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If the result of the check is practice (2), and any verifiable
signature is present from some signer other than the Originator
Address in the message, the message SHOULD be considered non-
Suspicious.
If the result of the check is practice (3) or (5), the message
MUST be considered Suspicious.
If the result of the check is practice (4), a second Sender
Signing Practices check SHOULD be performed based on the entire
Originator Address and interpreted using the above steps. If no
signing practices are published at the user level, the signing
practices of the domain should be used instead and interpreted as
described above.
If the Sender Signing Practices record for the domain does not exist
but the domain does exist, Verifier systems MUST assume that some
messages from this entity are not signed and the message SHOULD NOT
be considered to be Suspicious.
4. Detailed Description
4.1. DNS Representation
Sender Signing Practices records are published using the "DKIMP" DNS
resource record type. The numeric record type for DKIMP is [[TBD]].
The RDATA for DKIMP resource records is textual in format, like that
of TXT records but with specific syntax and semantics relating to
their role in describing sender signing practices. The "Tag=Value
List" syntax described in section 3.2 of [I-D.ietf-dkim-base] is
used. Records not in compliance with that syntax or the syntax of
individual tags described in Section 4.2 MUST be ignored (considered
equivalent to a NODATA result) for purposes of message disposition,
although they MAY cause the logging of warning messages via an
appropriate system logging mechanism.
SSP records for a domain are published at the domain's location
(e.g., example.com) in the DNS hierarchy. SSP records for individual
addresses are published at "<user>._user.<domain>" (e.g.,
jdoe._user.example.com), with any characters not permitted in DNS
labels removed and with the resulting label truncated, if necessary,
at 63 characters per section 2.3.1 of [RFC1035].
NON-NORMATIVE RATIONALE: Use of a separate resource record allows
the Verifier to determine whether the domain exists in addition to
the existence of an SSP record with a single query. Use of the
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"_user" separator in the user-level query prevents the publication
of practices for the subdomain jdoe.example.com from conflicting
with user-level practices for <jdoe@example.com>.
Since the Domain Name System returns a NODATA, rather than an
NXDOMAIN (nonexistent domain) response for any record, such as a
hostname, for which there is a record of any resource record type,
a query for the signing practices of such a name would indicate
that there are no signing practices for the address, which might
be undesirable. Conversely, the burden of publishing SSP records
for all such names could be considerable.
To address this problem, when a domain-level SSP query returns a
NODATA response, the Verifier MUST repeat the query to the next
higher level of the domain hierarchy unless the query is already at
the top-level. This allows hostnames and other identifiers that may
be used in Originator Addresses to inherit the signing practices of
their parent domain. Bona fide subdomains SHOULD publish separate
SSP records; otherwise, hostnames and other identifiers in subdomains
will result in the Verifier not finding the SSP record.
4.2. Record Syntax
Signing practices records follow the tag-value syntax described in
section 3.2 of [I-D.ietf-dkim-base]. Tags used in SSP records are as
follows. Unrecognized tags and tags with illegal values MUST be
ignored. In the ABNF below, the FWS token is inherited from
[RFC2822] with the exclusion of obs-FWS. The ALPHA and DIGIT tokens
are imported from [RFC4234].
p= Outbound signing practices for the entity (plain-text; OPTIONAL,
default is "unknown"). Possible values are as follows:
unknown The entity may sign some or all email.
all All mail from the entity is signed; unsigned email MUST be
considered Suspicious. The entity may send messages through
agents that may modify and re-sign messages, so email signed
with a Verifier Acceptable Third-Party Signature SHOULD be
considered non-Suspicious.
strict All mail from the entity is signed; messages lacking a
valid Originator Signature MUST be considered Suspicious. The
entity does not expect to send messages through agents that may
modify and re-sign messages.
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NON-NORMATIVE RATIONALE: Strict practices may be used by
entities which send only transactional email to individual
addresses and which are willing to accept the consequence of
having some mail which is re-signed appear suspicious in
return for additional control over their addresses. Strict
practices may also be used by entities which do not send
(and therefore do not sign) any email.
ABNF:
ssp-p-tag = %x70 [FWS] "=" [FWS] "unknown" / "all" / "strict"
u= User-level signing practices for the entity (plain-text; OPTIONAL,
default is "no"). This tag MUST NOT be present in user-level SSP
records. Possible values are as follows:
yes Repeat query at user level. The Verifier MUST look up the
user-level Signing Practices by querying for a DKIMP record at
"<user>._user.<domain>" where <user> is the local-part of the
Originator Address (i.e., the portion of the address before the
"@" character) with any characters not permitted in DNS labels
removed and with the resulting label truncated, if necessary,
at 63 characters per section 2.3.1 of [RFC1035] and <domain> is
the domain-part of the Originator Address (i.e., the portion of
the address after the "@" character). If no user-level SSP
record is found (either a NODATA or NXDOMAIN response is
received), the practices described in this record should be
used.
no Do not repeat the query at user level; use the practices
described in this record.
ABNF:
ssp-u-tag = %x75 [FWS] "=" [FWS] "yes" / "no"
t= Flags, represented as a colon-separated list of names (plain-text;
OPTIONAL, default is that no flags are set). Flag values are:
y The entity is testing signing practices, and the Verifier
SHOULD NOT consider a message suspicious based on the record.
s The signing practices apply only to the named domain, and not
to subdomains.
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ABNF:
ssp-t-tag = %x75 [FWS] "=" [FWS] ssp-t-tag-flag
0*( [FWS] ":" [FWS] ssp-t-tag-flag )
ssp-t-tag-flag = "y" / "s" / hyphenated-word ; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-") (ALPHA / DIGIT) ]
Unrecognized flags MUST be ignored.
4.3. Sender Signing Practices Check Procedure
The Sender Signing Practices check SHOULD be performed after DKIM
signature(s), including any where the Alleged Signer is the Alleged
Originator, have been verified. Verifiers MUST produce a result that
is semantically equivalent to applying the following steps in the
order listed. In practice, several of these steps can be performed
in parallel in order to improve performance.
1. If a valid Originator Signature exists, the message is non-
Suspicious, and the algorithm terminates.
2. The Verifier MUST query DNS for a DKIMP record corresponding to
the domain part of the Originator Address. If the result of
this query is a NODATA response, proceed to step 10. If the
result of this query is a NXDOMAIN response, the message is
Suspicious and the algorithm terminates. Otherwise, proceed to
the following steps using the record retrieved by the query.
3. If the SSP "t" tag exists and any of the flags is "y"
(indicating testing), the message is non-Suspicious and the
algorithm terminates.
4. If the SSP "u" tag exists and the value is "yes", retain the
value of the "p" tag; otherwise proceed to step 7.
5. The Verifier MUST query DNS for a user-level DKIMP record at the
location defined in Section 4.1. If the result of this query is
a NODATA or NXDOMAIN response, then a user-level SSP record was
not found; go to step 7 and proceed using the retained value of
the "p" tag from the domain-level practices.
6. Proceed using the value of the "p" tag from the user-level
query.
7. If the value of the SSP "p" tag is "unknown", the message is
non-Suspicious and the algorithm terminates.
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8. If the value of the SSP "p" tag is "all", and one or more Valid
Signatures are present on the message, the message is Not
Suspicious and the algorithm terminates.
9. The message is Suspicious and the algorithm terminates.
10. (check for parent domain policy) If the domain of the Originator
Address is a top-level domain (e.g., a country code), then an
SSP record was not found and the message is Not Suspicious.
11. The Verifier MUST query DNS for a DKIMP record corresponding to
the immediate parent of the Originator Address. If the result
of this query is a NODATA response, then an SSP record was not
found and the message is non-Suspicious.
12. If the SSP "t" tag exists and any of the flags is "y"
(indicating testing) or "s" (indicating that the record should
not be used apply to a subdomain), the message is non-Suspicious
and the algorithm terminates. Otherwise proceed to step 4.
5. Third-Party Signatures and Mailing Lists
There are several forms of mailing lists, which interact with signing
in different ways.
o "Verbatim" mailing lists send messages without modification
whatsoever. They are often implemented as MTA-based aliases.
Since they do not modify the message, signatures are unaffected
and will continue to verify. There is no reason for the forwarder
to re-sign the message.
o "Digesting" mailing lists collect together one or more postings
and then retransmit them, often on a nightly basis, to the
subscription list. These are essentially entirely new messages
which must be independently authored (that is, they will have a
"From" header field referring to the list, not the submitters) and
signed by the Mailing List Manager itself, if they are signed at
all.
o "Resending" mailing lists receive a message, modify it (often to
add "unsubscribe" information or advertising), and immediately
resend that message to the subscription list. They are
problematic because they usually do not change the "From" header
field of the message, but they do invalidate the signature in the
process of modifying the message.
The first two cases act in obvious ways and do not require further
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discussion. However, the third case is problematic. The remainder
of this session applies only to that case.
5.1. Mailing List Manager Actions
Mailing List Managers should make every effort to ensure that
messages that they relay and which have Valid Signatures upon receipt
also have Valid Signatures upon retransmission. In particular,
Mailing List Managers that modify the message in ways that break
existing signatures SHOULD:
o Verify any existing DKIM Signatures. A DKIM-aware Mailing List
Manager MUST NOT re-sign an improperly signed message in such a
way that would imply that the existing signature is acceptable.
o Apply regular anti-spam policies. A Mailing List Manager SHOULD
apply message content security policy just as they would messages
destined for an individual user's mailbox. In fact, a Mailing
List Manager might apply a higher standard to messages destined to
a mailing list than would normally be applied to individual
messages.
NON-NORMATIVE RATIONALE: Since reputation will accrue to
signers, Mailing List Managers should verify the source and
content of messages before they are willing to sign lest their
reputation be sullied by nefarious parties.
o Add a Sender header field using a valid address pointing back to
the Mailing List Administrator or an appropriate agent (such as an
"owner-" or a "-request" address).
o Sign the resulting message with a signature that is valid for the
Sender header field address. The Mailing List Manager SHOULD NOT
sign messages for which they are unwilling to accept
responsibility.
Mailing List Managers MAY:
o Reject messages with signatures that do not verify or are
otherwise Suspicious.
5.2. Signer Actions
All Signers SHOULD:
o Include any existing Sender header field in the signed header
field list, if the Sender header field exists.
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Signers wishing to avoid the use of Third-Party Signatures SHOULD do
everything listed above, and also:
o Include the Sender header field name in the header field list
("h=" tag) under all circumstances, even if the Sender header
field does not exist in the header block. This prevents another
entity from adding a Sender header field.
o Publish Sender Signing Practices that does not sanction the use of
Third-Party Signatures
6. IANA Considerations
Use of the _user prefix in DKIMP DNS records will require
registration by IANA.
The DKIMP RR type must be registered by IANA.
7. Security Considerations
Security considerations in the Sender Signing Practices are mostly
related to attempts on the part of malicious senders to represent
themselves as other senders, often in an attempt to defraud either
the recipient or the Alleged Originator.
7.1. Fraudulent Sender Address
[[Assuming 3rd party signature is based on Sender header field]] If
the Sender Signing Practices sanction third-party signing, an
attacker can create a message with a From header field of an
arbitrary sender and a legitimately signed Sender header field
7.2. DNS Attacks
An attacker might attack the DNS infrastructure in an attempt to
impersonate SSP records. However, such an attacker is more likely to
attack at a higher level, e.g., redirecting A or MX record lookups in
order to capture traffic that was legitimately intended for the
target domain. Domains concerned about this should use DNSSEC
[RFC4033].
8. References
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8.1. Normative References
[I-D.ietf-dkim-base]
Allman, E., "DomainKeys Identified Mail (DKIM)
Signatures", draft-ietf-dkim-base-05 (work in progress),
August 2006.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
8.2. Informational References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
Appendix A. Change Log
A.1. Changes since -01
o Changed term "Sender Signing Policy" to "Sender Signing
Practices".
o Changed query methodology to use a separate DNS resource record
type, DKIMP.
o Changed tag values from SPF-like symbols to words.
o User level policies now default to that of the domain if not
specified.
o Removed the "Compliance" section since we're still not clear on
what goes here.
o Changed the "parent domain" policy to only search up one level
(assumes that subdomains will publish SSP records if appropriate).
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o Added detailed description of SSP check procedure.
A.2. Changes since -00
From a "diff" perspective, the changes are extensive. Semantically,
the changes are:
o Added section on "Third-Party Signatures and Mailing Lists"
o Added "Compliance" (transferred from -base document). I'm not
clear on what needs to be done here.
o Extensive restructuring.
Authors' Addresses
Eric Allman
Sendmail, Inc.
6425 Christie Ave, Suite 400
Emeryville, CA 94608
USA
Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org
URI:
Mark Delany
Yahoo! Inc.
701 First Avenue
Sunnyvale, CA 94089
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Allman, et al. Expires March 3, 2007 [Page 16]
Internet-Draft DKIM SSP August 2006
Jim Fenton
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
Allman, et al. Expires March 3, 2007 [Page 17]
Internet-Draft DKIM SSP August 2006
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