wdiff draft-ietf-dkim-ssp-03.txt draft-ietf-dkim-ssp-04.txt

Network Working Group E. Allman
Internet-Draft Sendmail, Inc.
Intended status: Standards Track J. Fenton
Expires: August 26, 2008 January 3, 2009 Cisco Systems, Inc.
M. Delany
Yahoo! Inc.
J. Levine
Taughannock Networks
February 23,
July 2, 2008

DKIM Author Domain Signing Practices (ASP)
draft-ietf-dkim-ssp-03 (ADSP)
draft-ietf-dkim-ssp-04

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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. This document specifies an adjunct
mechanism to aid in assessing messages by either Mail Transport Agents (MTAs) or Mail
User Agents (MUAs). The primary that do not contain a DKIM protocol is described
signature for the domain used in
[RFC4871]. This document describes the records author's address. It defines a
record that authors' domains can use to advertise their practices for signing whether they sign their outgoing mail, and
how other hosts can access those records.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 3
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4 3
2.1. Terms Imported from DKIM Signatures Specification . . . . 4 3
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5 4
2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5 4
2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5 4
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5 4
2.6. Author Domain Signing Practices . . . . . . . . . . . . . . . . . 5 4
2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5 4
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6 5
3.1. ASP ADSP Applicability . . . . . . . . . . . . . . . . . . . . 5
3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. ASP 5
3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 6
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7 6
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7 6
4.2. Publication of ASP ADSP Records . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . 6
4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 10
5.1. ASP Specification Tag Registry 7
5. IANA Considerations . . . . . . . . . . . . . . 10
5.2. ASP Outbound Signing Practices Registry . . . . . . . 8
5.1. ADSP Specification Tag Registry . . 10
5.3. ASP Flags Registry . . . . . . . . . . . 8
5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 11 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11 9
6.1. ASP ADSP Threat Model . . . . . . . . . . . . . . . . . . . . . 11 9
6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 12 10
6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 11
7.1. References - Normative . . . . . . . . . . . . . . . . . . 13 11
7.2. References - Informative . . . . . . . . . . . . . . . . . 13 11
Appendix A. Usage Examples . . . . . . . . . . . . . . . . . . . 13 11
A.1. Single Location Domains . . . . . . . . . . . . . . . . . 14 12
A.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 14 12
A.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 14 13
A.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 15 13
A.5. Non-email Domains . . . . . . . . . . . . . . . . . . . . 13
Appendix B. Acknowledgements . . . . . . . . . . . . . . . . . . 15 13
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 15 13
C.1. Changes since -ietf-dkim-02 -ietf-dkim-03 . . . . . . . . . . . . . . . 15 14
C.2. Changes since -ietf-dkim-ssp-01 -ietf-dkim-02 . . . . . . . . . . . . . 16 . . 14
C.3. Changes since -ietf-dkim-ssp-00 -ietf-dkim-ssp-01 . . . . . . . . . . . . . 17 15
C.4. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 16
C.5. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 17
C.5. 16
C.6. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 18
C.6. 17
C.7. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 18 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 17
Intellectual Property and Copyright Statements . . . . . . . . . . 20 19

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 is very likely that most messages will remain unsigned.
However, some domains might decide to sign all of their outgoing
mail, for example, to protect their brand name. names. It is desirable for
such domains to be able to advertise that fact to other hosts. This
is the topic of Author Domain Signing Practices (ASP). (ADSP).

Hosts implementing this specification can inquire what Author Signing
Practices a domain advertises. This inquiry is called an Author
Signing Practices check.

The detailed basic requirements for Author Signing Practices ADSP are given in [RFC5016]. This
document refers extensively to [RFC4871] and assumes the reader is
familiar with it.

Requirements Notation: 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]

2. Language and Terminology

2.1. Terms Imported from DKIM Signatures Specification

Some terminology used herein is derived directly from [RFC4871]. In
several cases, references in that document to Sender have been
changed to Author here, to emphasize the relationship to the Author
address(es) in the From: header field described in [RFC2822].
Briefly,

o A "Signer" is the agent that signs a message, as defined in
section 2.1 of [RFC4871].

o A "Selector" specifies which of the keys published by a signing
domain is to be queried, as defined in section 3.1 of [RFC4871].

o A "Local-part" is "Local-part" is the part of an address preceding the @ sign,
character, as defined in [RFC2822] and used in [RFC4871].

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 [RFC4871].

2.3. Author Address

An "Author Address" is an email address in the From header field of a
message [RFC2822]. If the From header field contains multiple
addresses, the message has multiple Author Addresses.

2.4. Author Domain

An "Author Domain" is everything to the right of the "@" in an Author
Address (excluding the "@" itself).

2.5. Alleged Author

An "Alleged Author" is an Author Address of a message; it is
"alleged" because it has not yet been verified.

2.6. Author Domain Signing Practices

"Author Domain Signing Practices" (or just "practices") consist of a
machine-readable record published by the domain of an Alleged Author
which includes statements about the domain's practices with respect
to mail it sends with its domain in the From: line.

2.7. Author Signature

An "Author Signature" is any Valid Signature where the identity of
the user or agent on behalf of which the message is signed (listed in
the ""i="" "i=" tag or its default value from the ""d="" "d=" tag) matches an
Author Address in the message. When the identity of the user or
agent includes a Local-part, the identities match if the Local-parts
match
are the same string, and the domains match. are the same string. Otherwise,
the identities match if the domains match. are the same string. Following
[RFC2821], Local-part comparisons are case sensitive, domain
comparisons are case insensitive.

For example, if a message has a Valid Signature, with the DKIM-
Signature field containing "i=a@domain.example", then domain.example
is asserting that it takes responsibility for the message. If the
message's From: field contains the address "b@domain.example" and an
ASP
ADSP query produces a "dkim=all" or "dkim=discardable" result, that
would mean that the message does not have a valid Author Signature.
Even though the message is signed by the same domain, its failure it fails to
satisfy ASP could be problematic. ADSP.

3. Operation Overview

Domain owners can publish Author Signing Practices ADSP information via a query mechanism such
as the Domain Name System; specific details are given in Section 4.1.

Hosts can look up the Author Signing Practices ADSP information of the domain(s) specified by
the Author Address(es) as described in Section 4.2.2. 4.3. If a message has
multiple Author Addresses the ASP ADSP lookups SHOULD be performed
independently on each address. This standard does not address the
process a host might use to combine the lookup results.

3.1. ASP ADSP Applicability

ADSP as defined in this document is bound to DNS. For this reason,
ADSP is applicable only to Author Domains with appropriate DNS
records (see Note below). The handling of other Author Domains is
outside the scope of this document. However, attackers may use such
domain names in a deliberate attempt to sidestep an organization's
ADSP policy statements. It is up to the ADSP verifier implementation
to return an appropriate error result for Author Domains outside the
scope of ADSP.

Note: The results from DNS queries that are intended to validate a
domain name unavoidably approximate the set of Author Domains that
can appear in legitimate email. For example, a DNS A record could
belong to a device that does not even have an email
implementation. It is up to the verifier to decide what degree of
approximation is acceptable.

3.2. ADSP Usage

Depending on the Author Domain(s) and the signatures in a message, a
recipient gets varying amounts of useful information from each ASP ADSP
lookup.

o If a message has no Valid Signature, the ASP ADSP result is directly
relevant to the message.

o If a message has a Valid Signature from an Author Domain, ASP ADSP
provides no benefit relative to that domain since the message is
already known to be compliant with any possible ASP ADSP for that
domain.

o If a message has a Valid Signature from a domain other than an
Author Domain, the receiver can use both the Signature and the ASP
ADSP result in its evaluation of the message.

3.2. ASP

3.3. ADSP Results

An Author Signing Practices ADSP lookup for an Author Address produces one of four possible
results:

o Messages from this domain might or might not have an author
signature. This is the default if the domain exists in the DNS
but no record is found.

o All messages from this domain are signed.

o All messages from this domain are signed and discardable.

o The domain does is not exist. a valid mail domain.

4. Detailed Description

4.1. DNS Representation

Author Signing Practices

ADSP records are published using the DNS TXT resource record type.

NON-NORMATIVE DISCUSSION [to be removed before publication]: There
has been considerable discussion on the DKIM WG mailing list
regarding the relative advantages of TXT and a new resource record
(RR) type. Read the archive for details.

The RDATA for ASP ADSP resource records is textual in format, with
specific syntax and semantics relating to their role in describing
Author Signing Practices.
ADSP. The "Tag=Value List" syntax described in section 3.2 of
[RFC4871] is used. Records not in compliance with that syntax or the
syntax of individual tags described in Section 4.3 MUST be ignored
(considered equivalent to a NODATA result) for purposes of ASP, ADSP,
although they MAY cause the logging of warning messages via an
appropriate system logging mechanism. If the RDATA contains multiple
character strings, the strings are logically concatenated with no
delimiters between the strings.

The ASP ADSP record for a domain is published at a location in the
domain's DNS hierarchy prefixed by _asp._domainkey.; _adsp._domainkey.; e.g., the ASP ADSP
record for example.com would be a TXT record that is published at
"_asp._domainkey.example.com".
"_adsp._domainkey.example.com". A domain MUST NOT publish more than
one ASP ADSP record; the semantics of an ASP ADSP lookup that returns
multiple
ASP ADSP records for a single domain are undefined. (Note that
example.com and mail.example.com are different domains.)

4.2. Publication of ASP ADSP Records

Author Signing Practices are

ADSP is intended to apply to all mail sent from using the domain name
string of an Alleged Author. In order to ensure that ASP applies
to any hosts

Wildcards within that a domain (e.g., www.example.com,
ftp.example.com.) the ASP lookup algorithm looks up one level publishing ADSP records pose a particular
problem. This is discussed in more detail in the
domain tree. For example, mail signed by www.example.com could be
covered by the ASP record for example.com. This avoids the need to
include an ASP record for every name within a given domain.

Normally, a domain expressing Author Signing Practices will want to
do so for both itself and all of its "descendants" (child domains at
all lower levels). Domains wishing to do so MUST publish ASP records
for the domain itself and any subdomains.

Wildcards within a domain publishing ASP records pose a particular
problem. This is discussed in more detail in Section 6.3.

4.2.1. Record Syntax

ASP records use Section 6.3.

4.2.1. Record Syntax

ADSP records use the "tag=value" syntax described in section 3.2 of
[RFC4871].

Tags used in ASP ADSP records are described below. Unrecognized tags
MUST be ignored. In the ABNF below, the WSP FWS token is imported from
[RFC2822].
[RFC4871]. The ALPHA and DIGIT tokens are imported from [RFC5234].

dkim= Outbound signing practices for the domain (plain-text;
REQUIRED). Possible values are as follows:

unknown The domain might sign some or all email.

all All mail from the domain is signed with an Author
Signature.

discardable All mail from the domain is signed with an Author
Signature. Furthermore, if a message arrives without a valid
Author Signature due to modification in transit, submission via
a path without access to a signing key, or other reason, the
domain encourages the recipient(s) to discard it.

ABNF:

asp-dkim-tag
adsp-dkim-tag = %x64.6b.69.6d *WSP *FWS "="
*WSP *FWS
("unknown" / "all" / "discardable")

t= Flags, represented as a colon-separated list of names (plain-text;
OPTIONAL, default is that no flags are set). Flag values are:

s The signing practices apply only to the named domain, and not
to subdomains.

ABNF:

asp-t-tag = %x74 *WSP "=" *WSP { asp-t-tag-flag
0*( *WSP ":" *WSP asp-t-tag-flag )
asp-t-tag-flag = "s" / hyphenated-word
; for future extension
hyphenated-word = ALPHA [ *(ALPHA / DIGIT / "-")
(ALPHA / DIGIT) ]

Unrecognized flags MUST be ignored.

4.2.2. Author Signing Practices

4.3. ADSP Lookup Procedure

Hosts doing an ASP ADSP lookup MUST produce a result that is semantically
equivalent to applying the following steps in the order listed below.
In practice, several of these steps can be performed in parallel in order to
improve performance. However, implementations SHOULD avoid doing
unnecessary DNS lookups.

For the purposes of this section a "valid ASP ADSP record" is one that is
both syntactically and semantically correct; in particular, it
matches the ABNF for a "tag-list" and includes a defined "dkim=" tag.

1. _Fetch Named ASP Record._ The host

Verify Domain Scope: An ADSP verifier implementation MUST query DNS for determine
whether a TXT
record corresponding to the given Author Domain prefixed by
"_asp._domainkey." (note is within scope for ADSP. Given the trailing dot). If
background in Section 3.1 the result verifier MUST decide which degree of
this query
over-approximation is a "NOERROR" response with acceptable. The verifier MUST return an answer which is a
valid ASP record, use
appropriate error result for Author Domains that record; otherwise, continue to are outside the
next step.

2. _Verify Domain Exists._
scope of ADSP.

The host MUST perform a DNS query for a record corresponding to
the Author Domain (with no prefix). The type of the query can be
of any type, since this step is only to determine if the domain
itself exists in DNS. This query MAY be done in parallel with the
query made in step 2. to fetch the Named ADSP Record. If the result of this query
is that the Author domain does not exist in the DNS (often called
an "NXDOMAIN" error, error), the algorithm MUST terminate with an appropriate error. error
indicating that the domain is out of scope.

NON-NORMATIVE DISCUSSION: Any resource record type could be
used for this query since the existence of a resource record of
any type will prevent an "NXDOMAIN" error. MX is a reasonable
choice for this purpose is because this record type is thought to
be the most common for likely domains, domains used in e-mail, and will
therefore result in produce a result which can be more readily cached
than a negative result.

3. _Try Parent Domain._

If the domain does exist, the verifier MAY make more extensive
checks to verify the existence of the domain, such as the ones
described in Section 5 of [RFC2821]. If those checks indicate
that the Author domain does not exist for mail, e.g., the domain
has no MX, A, or AAAA record, the verifier SHOULD terminate with
an error indicating that the domain is out of scope.

Fetch Named ADSP Record: The host MUST query DNS for a TXT record for
corresponding to the immediate parent domain, Author Domain prefixed with "_asp._domainkey." by "_adsp._domainkey."
(note the trailing dot).

If the result of this query is anything other than a "NOERROR" response with an answer
which is a valid ASP ADSP record, the algorithm terminates with a
result indicating use that no ASP record was present. If the ASP "t"
tag exists in the response record, and any of the flags is "s"
(indicating it does not apply to a subdomain), the algorithm also
terminates without finding an ASP record. Otherwise, use that
record.
terminates.

If any of the queries involved in the Author Signing Practices Check
result a query results in a "SERVFAIL" error response, the algorithm
terminates without returning a result; possible actions include
queuing the message or returning an SMTP error indicating a
temporary failure.

5. IANA Considerations

ASP introduces some new

ADSP adds the following namespaces that have been registered with
IANA. to the IANA registry. In all
cases, new values are assigned only for values that have been
documented in a published RFC that has IETF Consensus [RFC2434].

INFORMATIVE NOTE [ to be removed before publication ]: RFC 4871
defines a selector as a sub-domain, importing the term from RFC 2822.
A sub-domain starts with a letter or digit, hence names such as _asp
that start with an underscore cannot collide with valid selectors.

5.1. ASP ADSP Specification Tag Registry

An ASP ADSP record provides for a list of specification tags. IANA has
established the ASP ADSP Specification Tag Registry for specification
tags that can be used in ASP ADSP fields.

ASP
ADSP Specification Tag Registry Initial Values

5.2. ASP ADSP Outbound Signing Practices Registry

The "dkim=" tag spec, defined in Section 4.2.1, provides for a value
specifying Outbound Signing Practices. IANA has established the ASP ADSP
Outbound Signing Practices Registry for Outbound Signing Practices.

ASP
ADSP Outbound Signing Practices Registry Initial Values

5.3. ASP Flags Registry

The "t=" tag spec, defined in Section 4.2.1, provides for a value
specifying Flags. IANA has established the ASP Flags Registry for
ASP Flags.

The initial entries in the registry comprise:

+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| s | (this document) |
+------+-----------------+

ASP Flags Registry Initial Values

6. Security Considerations

Security considerations in the Author Signing Practices ADSP are mostly related to attempts on
the part of malicious senders to represent themselves as authors for
whom they are not authorized to send mail, often in an attempt to
defraud either the recipient or an Alleged Author.

Additional security considerations regarding Author Domain Signing
Practices are found in the DKIM threat analysis [RFC4686].

6.1. ASP ADSP Threat Model

Email recipients often have a core set of content authors that they
already trust. Common examples include financial institutions with
which they have an existing relationship and Internet web transaction
sites with which they conduct business.

Email abuse often seeks to exploit the name-recognition that
recipients will have, for a legitimate email author, author's name-
recognition among recipients, by using its the author's domain name in
the From: header field. Especially since many popular MUAs do not
display the author's email address, there is no empirical evidence of
the extent that this particular unauthorized use of a domain name
contributes to recipient deception or that eliminating it will have
significant effect.

However, closing this exploit could facilitate some types of
optimized processing by receive-side message filtering engines, since
it could permit them to maintain higher-confidence assertions about
From: header field uses of a domain, when the occurrence is
authorized.

Unauthorized uses of domain names occur elsewhere in messages, as do
unauthorized uses of organizations' names. These attacks are outside
the scope of this specification.

ASP

ADSP does not provide any benefit--nor, indeed, have any effect at
all--unless an external system acts upon the verdict, either by
treating the message differently during the delivery process or by
showing some indicator to the end recipient. Such a system is out of
scope for this specification.

ASP Checkers

ADSP checkers may perform up to three multiple DNS lookups per Alleged Author
Domain. Since these lookups are driven by domain names in email
message headers of possibly fraudulent email, legitimate ASP Checkers ADSP
checkers can become participants in traffic multiplication attacks.

6.2. DNS Attacks

An attacker might attack the DNS infrastructure in an attempt to
impersonate ASP records, in an attempt ADSP records to influence a receiver's decision on how it
will handle mail. 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. These DNS security issues are addressed by DNSSEC [RFC4033].

Because ASP ADSP operates within the framework of the legacy e-mail
system, the default result in the absence of an ASP ADSP record is that
the domain does not sign all of its messages. It is therefore
important that the ASP ADSP clients distinguish a DNS failure such as
"SERVFAIL" from other DNS errors so that appropriate actions can be
taken.

6.3. DNS Wildcards

Wildcards within

If a domain publishing ASP records, including but not
limited to contains wildcards, then any name that matches the
wildcard MX records, pose according to [RFC4592] is potentially a particular problem. While
referencing the immediate parent valid mail domain allows the discovery of an
ASP record corresponding
eligible for ADSP. It is possible to an unintended immediate-child subdomain, add a wildcard records apply at multiple levels. For example, if there is TXT record
alongside a wildcard MX record that will provide suitable ADSP records for "example.com", the
any domain
"foo.bar.example.com" can receive mail through the named mail
exchanger. Conversely, the existence of the record makes it
impossible to tell whether "foo.bar.example.com" is a legitimate name chosen by an attacker, since a query for that name if the wildcard synthesizes
chosen-name.example.com IN MX, it will not return then also synthesize
_adsp._domainkey.chosen-name.example.com IN TXT. However multiple
wildcard TXT records produce an "NXDOMAIN" error. For
that reason, ASP coverage undefined ADSP result, which means
you cannot also publish both ADSP records and records for any other
TXT-using protocol (such as SPF) for subdomains of domains containing a wildcard record is incomplete.

NON-NORMATIVE NOTE: Complete ASP coverage of domains containing (or
where any parent contains) wildcards generally cannot be provided by
standard DNS servers. domain.

7. References

7.1. References - Normative

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.

[RFC2822] Resnick, P., "Internet Message Format", RFC 2822,
April 2001.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.

[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.

[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.

[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

7.2. References - Informative

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.

[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016,
October 2007.

Appendix A. Usage Examples

These examples are intended to illustrate typical uses of ASP. ADSP. They
are not intended to be exhaustive, nor to apply to every domain's or
mail system's individual situation.

Domain managers are advised to consider the ways that mail processing
can modify messages in ways that will invalidate an existing DKIM
signature, such as mailing lists, courtesy forwarders, and other
paths that could add or modify headers, or modify the message body.
In that case, if the modifications invalidate the DKIM signature,
recipient hosts will consider the mail not to have an Author
Signature, even though the signature was present when the mail was
originally sent.

A.1. Single Location Domains

A common mail system configuration handles all of a domain's users'
incoming and outgoing mail through a single MTA or group of MTAs. In
that case, the MTA(s) can be configured to sign outgoing mail with an
Author Signature.

In this situation it might be appropriate to publish an ASP ADSP record
for the domain containing "all", depending on whether the users also
send mail through other paths that do not apply an Author Signature.
Such paths could include MTAs at hotels or hotspot networks used by
travelling users, or web sites that provide "mail an article"
features.

A.2. Bulk Mailing Domains

Another common configuration uses a domain solely for bulk or
broadcast mail, with no individual human users, again typically
sending all the mail through a single MTA or group of MTAs that can
apply an Author Signature. In this case, the domain's management can
be confident that all of its outgoing mail will be sent through the
signing MTA. Lacking individual users, the domain is unlikely to
participate in mailing lists, but could still send mail through other
paths that might invalidate signatures.

Domain owners often use specialist mailing providers to send their
bulk mail. In that case, the mailing provider needs access to a
suitable signing key in order to apply an Author Signature. One
possible route would be for the domain owner to generate the key and
give it to the mailing provider. Another would be for the domain to
delegate a subdomain to the mailing provider, for example,
bigbank.example might delegate email.bigbank.example to such a
provider. In that case, the provider can generate the keys and DKIM
DNS records itself and use the subdomain in the Author address in the
mail.

Regardless of the DNS and key management strategy chosen, whoever
maintains the DKIM records for the domain could also install an ADSP
record containing "all".

A.3. Bulk Mailing Domains with Discardable Mail

In some cases, a domain might sign all of its outgoing mail with an
Author Signature, but prefer that recipient systems discard mail
without a valid Author Signature to avoid confusion from mail sent
from sources that do not apply an Author Signature. (This latter
kind of mail is sometimes loosely called "forgeries".) In that case,
it might be appropriate to publish an ASP ADSP record containing
"discardable". Note that a domain SHOULD NOT publish a "discardable"
record if it wishes to maximize the likelihood that mail from the
domain is delivered, since it could cause some fraction of the mail
the domain sends to be discarded.

As a special case, if a domain sends no mail at all, it can safely
publish a "discardable" ASP record, since any mail with an author
address in the domain is a forgery.

A.4. Third Party Senders

Another common use case is for a third party to enter into an
agreement whereby that third party will send bulk or other mail on
behalf of a designated author or author domain, using that domain in
the RFC2822 From: or other headers. Due to the many and varied
complexities of such agreements, third party signing is not addressed
in this specification.

A.5. Non-email Domains

If a domain sends no mail at all, it can safely publish a
"discardable" ADSP record, since any mail with an author address in
the domain is a forgery.

Appendix B. Acknowledgements

This document greatly benefited from comments by Steve Atkins, Jon
Callas, Dave Crocker, JD Falk, Arvel Hathcock, Ellen Siegel, Michael
Thomas, and Wietse Venema.

Appendix C. Change Log

*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*

C.1. Changes since -ietf-dkim-03

o Name change for title and filename, to be ADSP

o String changes throughout, to author Domain signing practices and
to aDsp.

o Added some keywords.

o Clarified comparison of local part and domain in Author Address.

o Streamlined the Abstract.

o Revised text of last bullet in Results list.

o Removed definitions not used in the document.

o Removed all specification details pertaining to sub-domains.

o Moved Lookup Procedure up one document level.

o Revised domain validity specification. Part in ADSP Usage in
Operations section, and part as it as first step in Lookup.

o Fixed xml for figures, including labeling ABNF with new xml2rfc
construct.

o Revised wildcard text.

o Removed 't' tag.

o Removed ADSP Flags Registry section.

o Changed ABNF use of whitespace from WSP back to FWS, for
consistency with dkim-base.

C.2. Changes since -ietf-dkim-02

o Merge in more text from ASP ADSP draft.

o Phrase actions as host's rather than checker.

o Explanatory description of i= matching.

o Lookup procedure consistently refers to one ASP ADSP record per
lookup.

o Update security section w/ language from W. Venema

o Simplify imports of terms from other RFCs, add Local-part, 4234 ->
5234.

o Add usage example appendix.

o Add IANA considerations.

o Update authors list

C.2.

C.3. Changes since -ietf-dkim-ssp-01

o Reworded introduction for clarity.

o Various definition clarifications.

o Changed names of practices to unknown, all, and discardable.

o Removed normative language mandating use of SSP in particular
situations (issue 1538).

o Clarified possible confusion over handling of syntax errors.

o Removed normative language from Introduction (issue 1538).

o Changed "Originator" to "Author" throughout (issue 1529).

o Removed all references to Third-Party Signatures (issues 1512,
1521).

o Removed all mention of "Suspicious" (issues 1528, 1530).

o Removed "t=y" (testing) flag (issue 1540).

o Removed "handling" tag (issue 1513).

o Broke up the "Sender Signing Practices Check Procedure" into two
algorithms: fetching the SSP record and interpretation thereof
(issues 1531, 1535; partially addresses issue 1520).
Interpretation is now the responsibility of the Evaluator.

o Document restructuring for better flow and remove redundancies
(some may address issue 1523, but I'm not sure I understand that
issue completely; also issues 1532, 1537).

o Removed all mention of how this interacts with users, even though
it makes parts of the document harder to understand (issue 1526).

o Introduced the concepts of "SSP Checker" and "Evaluator".

o Multiple author case now handled my separate invocations of SSP
checker by Evaluator (issue 1525).

o Removed check to avoid querying top-level domains.

o Changed ABNF use of whitespace from [FWS] to *WSP (partially
addresses issue 1543).

C.3.

C.4. Changes since -ietf-dkim-ssp-00

o Clarified Operation Overview and eliminated use of Legitimate as
the counterpart of Suspicious since the words have different
meanings.

o Improved discussion (courtesy of Arvel Hathcock) of the use of TXT
records in DNS vs. a new RR type.

o Clarified publication rules for multilevel names.

o Better description of overall record syntax, in particular that
records with unknown tags are considered syntactically correct.

o Clarified Sender Signing Practices Check Procedure, primarily by
use of new term Author Domain.

o Eliminated section "Third-Party Signatures and Mailing Lists" that
is better included in the DKIM overview document.

o Added "handling" tag to express alleged sending domain's
preference about handling of Suspicious messages.

o Clarified handling of SERVFAIL error in SSP check.

o Replaced "entity" with "domain", since with the removal of user-
granularity SSP, the only entities having sender signing policies
are domains.

C.4.

C.5. Changes since -allman-ssp-02

o Removed user-granularity SSP and u= tag.

o Replaced DKIMP resource record with a TXT record.

o Changed name of the primary tag from "p" to "dkim".

o Replaced lookup algorithm with one which traverses upward at most
one level.

o Added description of records to be published, and effect of
wildcard records within the domain, on SSP.

C.5.

C.6. Changes since -allman-ssp-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).

o Added detailed description of SSP check procedure.

C.6.

C.7. Changes since -allman-ssp-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.
6475 Christie Ave, Suite 350
Emeryville, CA 94608

Phone: +1 510 594 5501
Email: eric+dkim@sendmail.org

Jim Fenton
Cisco Systems, Inc.
MS SJ-9/2
170 W. Tasman Drive
San Jose, CA 95134-1706

Phone: +1 408 526 5914
Email: fenton@cisco.com

Mark Delany
Yahoo! Inc.
701 First Avenue
Sunnyvale, CA 94089

Phone: +1 408 349 6831
Email: markd+dkim@yahoo-inc.com

John Levine
Taughannock Networks
PO Box 727
Trumansburg, NY 14886

Phone: +1 831 480 2300
Email: standards@taugh.com
URI: http://www.taugh.com

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