pre-DKIM E. Allman
Internet-Draft Sendmail, Inc.
Expires: April 26, 2006 M. Delany
Yahoo! Inc
J. Fenton
Cisco Systems, Inc.
October 23, 2005
DKIM Sender Signing Policy
draft-allman-dkim-ssp-01
Status of this Memo
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This Internet-Draft will expire on April 26, 2006.
Copyright Notice
Copyright (C) The Internet Society (2005).
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 [ID-
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DKIM-BASE].
This document describes the policy 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 add new and check existing ABNF.
DKP RR needs to be defined.
Text structure of document needs to be examined; this is a quick
slash-and-burn approach. Stop signs indicate sections that haven't
even been approached yet.
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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 Originator Address . . . . . . . . . . . . . . . . . . . . 5
2.3 Alleged Signer . . . . . . . . . . . . . . . . . . . . . . 5
2.4 Alleged Originator . . . . . . . . . . . . . . . . . . . . 6
2.5 Sender Signing Policy . . . . . . . . . . . . . . . . . . 6
2.6 Suspicious . . . . . . . . . . . . . . . . . . . . . . . . 6
2.7 First Party Signature . . . . . . . . . . . . . . . . . . 6
2.8 Third Party Signature . . . . . . . . . . . . . . . . . . 6
2.9 Verifier Acceptable Third Party Signature . . . . . . . . 6
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . 7
4. Query Mechanism . . . . . . . . . . . . . . . . . . . . . . 8
5. Policy Syntax and Semantics . . . . . . . . . . . . . . . . 9
6. Third Party Signatures and Mailing Lists . . . . . . . . . . 10
6.1 Verifier Actions . . . . . . . . . . . . . . . . . . . . . 10
6.2 Mailing List Manager Actions . . . . . . . . . . . . . . . 11
6.3 Signer Actions . . . . . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
8. Compliance . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . 12
9.1 Fraudulent Sender Address . . . . . . . . . . . . . . . . 12
9.2 DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1 Normative References . . . . . . . . . . . . . . . . . . 13
10.2 Informational References . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 13
A. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 14
A.1 Changes since -00 . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Introduction
DomainKeys Identified Mail (DKIM) defines a simple, low cost, and
effective mechanism by which email messages can be cryptographically
signed, permitting a signing domain to claim responsibility for the
use of a given email address. 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 senders may choose to sign all of their
outgoing mail, for example, to protect their brand name. Such
signers must be able to advertise to verifiers that messages claiming
to be from them that are not signed are forgeries. This is the topic
for sender signing policy.
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 is
participating in DKIM, whether they are testing, and whether it 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 Policy Check.
Sender Signing Policies MAY be expressed on behalf of an entity which
may be a domain or an individual address. Expression of signing
policy on behalf of individual addresses will, of course, entail
additional key server transaction load.
Conceivably, such policy 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 of the need for outside auditing they fall
under the purview of reputation and accreditation.
This document refers to [ID-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 [ID-DKIM-BASE].
Briefly,
o A "Signer" is the agent that signs a message. In normal cases
it will probably 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 Policy 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 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.
2.3 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.
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2.4 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.
2.5 Sender Signing Policy
A "Sender Signing Policy" (or just "policy") is a machine-readable
record published by the domain of the Alleged Originator which
includes information about whether that sender signs all, some, or
none of their email. It must be considered together with the "key"
records, which advertise the public keys associated with the Alleged
Originator.
2.6 Suspicious
Messages that fail an initial signature verification step (either by
an incorrect signature or a lack of signature) and also a further
Sender Signing Policy check 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.
2.7 First Party Signature
A First Party Signature is any valid signature where the signer name
(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. If the
signer name does not include a local-part, then only the domains must
match; otherwise, the two addresses must be identical.
2.8 Third Party Signature
A Third Party Signature is a valid signature where the signer (listed
in the "i=" tag) does not match the address in the "From" header.
2.9 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. "Accept" means that the Verifier
will, on the basis of local policy, take an action based on that
signature, such as matching against a locally maintained allow-list,
doing an external reputation lookup, or any other local action that
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the Verifier deems useful for classifying or otherwise processing the
message.
A Verifier SHOULD accept signatures that correspond with addresses in
the "Sender" header, MAY accept signatures that are for identities
that the Verifier is certain will be displayed to end users, and MAY
accept signatures that pass other tests such as accreditation or
reputation. Verifiers SHOULD NOT accept signatures from identities
that have no known relationship with the message other than their
appearance in the "DKIM-Signature" header.
3. Operation Overview
Sender Signing Policy Checks MUST be based on the Originator Address.
If the message contains a valid signature on behalf of the Originator
Address no Sender Signing Policy Check need be performed: the
verifier SHOULD NOT look up the Sender Signing Policy and the message
SHOULD be considered non-Suspicious.
Verifiers checking messages that do not have at least one valid
signature on behalf of the Originator Address MUST perform a Sender
Signing Policy Check by doing a lookup for the "_policy" record as
described in Section 4 from the domain specified by the Originator
Address.
The result of a Sender Signing Policy Check is one of four possible
policies:
(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 policy.
(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 policy for this domain is expressed at the individual
address level. A second Sender Signing Policy Check should be
performed specifying the individual address
If a message is encountered by a verifier without a valid signature
from the Originator Address, the policy results MUST be interpreted
as follows:
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If the result of the check is policy (1) described above, the
message MUST be considered non-Suspicious.
If the result of the check is policy (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 policy (3), the message MUST be
considered Suspicious.
If the result of the check is policy (4), a second Sender Signing
Policy Check SHOULD be performed based on the entire Originator
Address and interpreted using the above steps. If the result of
that check is policy (4), the signing policy for the originator is
misconfigured, and the message SHOULD be considered non-
Suspicious.
If the Sender Signing Policy record does not 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. Query Mechanism
Signing policy records for a domain are published in key servers as
the "_policy" selector. Signing policy records for individual
addresses are published as the "<user>._policy" selector.
NON-NORMATIVE RATIONALE: Use of a synthetic selector allows non-
DNS based access for signer policies.
Initially, the query mechanism defined uses DNS to look up the key
"<selector>._domainkey.<domain>", where <selector> is either
"_policy" for the initial lookup or "<user>._policy" for per-user
lookups, and <domain> is the domain of the Originator Address. When
represented in DNS, signing policy checks MUST search for a DKSSP
(DomainKey Sender Signing Policy) RR type first. If no DKSSP RR is
found, signing policy checks MUST search for a TXT RR type. The
DKSSP record MUST override the TXT record.
To avoid a Denial-of-Service attack, signer policy searches for
signing policy checks of very deeply nested domains MUST strip off
all but the last five components of a domain name. If a policy
record is not found, the verifier MUST repeat the request to
successively higher levels of the domain hierarchy until the root is
reached. This allows subdomains to inherit the signing policy of
their parent domains without allowing attackers to specify extremely
deep subdomains such as
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"a.b.c.d.e.f.g.h.i.j.k.l.m.n.o.p.q.r.s.t.u.v.w.x.y.z.example.com".
If presented with such a signing domain in a DKIM-Signature header
field, the search for a policy record would start at
"x.y.z.example.com" and proceed upwards. Verifiers MUST stop
searching at the first policy record they encounter.
NON-NORMATIVE DISCUSSION: It seems like this limitation should be
part of the DNS binding rather than a general restriction.
5. Policy Syntax and Semantics
Signing policy records follow the tag-value syntax described in [ID-
DKIM-BASE]. Tags used in signing policy records are as follows:
n= Human readable notes regarding the record (quoted-printable with
semicolon encoded in addition to the standard characters;
OPTIONAL, default is no notes).
o= Outbound signing policy for the entity (plain-text; OPTIONAL,
default is "~"). Possible values are as follows:
~ The entity signs some but not all email.
- All mail from the entity is signed; unsigned email MUST NOT be
accepted, but email signed with a Verifier Acceptable Third
Party Signature SHOULD be accepted.
! All mail from the entity is signed; Third-Party signatures
SHOULD NOT be accepted
. This entity never sends email. The "." policy can be used to
"short circuit" searches from subdomains; for example, the
"ad.jp" domain might use this. If an initial policy search
receives this policy then the email SHOULD NOT be accepted; if
found while searching parent domains then the search should
terminate as though no policy record was found.
^ Repeat query at user level. This value MUST NOT be used in
user-level policy records. A Verifier MUST look up the
selector for "<user>._policy" where <user> is the local-part of
the Originator Address (i.e., the portion of the address before
the "@" character).
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r= Email address for reports and inquiries regarding the signing
policy for this entity (plain-text; OPTIONAL, default is no
contact address available).
t= A vertical-bar separated list of flags (plain-text; OPTIONAL,
default is that no flags are set). Flag values are:
y The entity is testing signing policy, and the verifier SHOULD
NOT consider a message suspicious based on the record.
u= Reserved for future reference to a URI to provide more detailed
policy information.
6. 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 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
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
discussion. However, the third case is problematic. The remainder
of this session applies only to that case.
6.1 Verifier Actions
Generally speaking, a Verifier SHOULD treat messages as Suspicious
unless they have a First Party Signature or Verifier Acceptable Third
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Party Signature. Since Verifiers SHOULD accept signatures for
identities listed in the Sender header field, adding such a field is
the recommended approach for Mailing List Managers.
6.2 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.
In particular, a DKIM-aware MLM MUST NOT sign an Authentication-
Results header field that it can not personally verify, whether
that header field be added locally or remotely.
NON-NORMATIVE RATIONALE: An attacker might send a message
through a DKIM-aware Mailing List Manager that included an
existing Authentication-Results header that claims that the MLM
verified the signature when the signature was not valid in an
attempt to gain creditability.
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 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 address. The Mailing List Manager SHOULD NOT sign
messages for which they are unwilling to accept responsibility.
Mailing List Managers MAY:
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o Reject messages with signatures that do not verify or otherwise
satisfy their policy.
6.3 Signer Actions
All Signers SHOULD:
o Include the From header as a signed header field (i.e., in the
"h=" tag) under all circumstances.
o Include any existing Sender header in the signed header field
list, if the Sender header exists.
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 a Sender Signing Policy that does not permit the use of
Third Party Signatures
7. IANA Considerations
Use of the _policy prefix in DNS records will require registration by
IANA.
The DKSSP RR type must be registered by IANA.
8. Compliance
[[To be done.]]
9. Security Considerations
Security considerations in the Sender Signing Policy 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.
9.1 Fraudulent Sender Address
[[Assuming 3rd party signature is based on Sender header]] If the
Sender Signing Policy permits third party signing, an attacker can
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create a message with a From header of an arbitrary sender and a
legitimately signed Sender header
9.2 DNS Attacks
An attacker might attack the DNS infrastructure in an attempt to
impersonate policy 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].
10. References
10.1 Normative References
[ID-DKIM-BASE]
Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)",
draft-allman-dkim-base-00 (work in progress), July 2005.
[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.
10.2 Informational References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
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:
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Mark Delany
Yahoo! Inc
701 First Avenue
Sunnyvale, CA 95087
USA
Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com
URI:
Jim Fenton
Cisco Systems, Inc.
MS SJ-24/2
170 W. Tasman Drive
San Jose, CA 95134-1706
USA
Phone: +1 408 526 5914
Email: fenton@cisco.com
URI:
Appendix A. Change Log
A.1 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.
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