Network Working Group E. Allman
Internet-Draft Sendmail, Inc.
Intended status: Standards Track J. Fenton
Expires: May 26, 2009 Cisco Systems, Inc.
M. Delany
Yahoo! Inc.
J. Levine
Taughannock Networks
November 22, 2008
DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
draft-ietf-dkim-ssp-07
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Abstract
DomainKeys Identified Mail (DKIM) defines a domain-level
authentication framework for email to permit verification of the
source and contents of messages. This document specifies an adjunct
mechanism to aid in assessing messages that do not contain a DKIM
signature for the domain used in the author's address. It defines a
record that can advertise whether a domain signs its outgoing mail,
and how other hosts can access that record.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 4
2.1. Terms Imported from DKIM Signatures Specification . . . . 4
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 5
2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 5
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 5
2.6. Author Domain Signing Practices . . . . . . . . . . . . . 5
2.7. Author Signature . . . . . . . . . . . . . . . . . . . . . 5
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 6
3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 6
3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 7
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 7
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 7
4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 8
4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 10
5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 11
6.2. DNS Attacks . . . . . . . . . . . . . . . . . . . . . . . 12
6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. References - Normative . . . . . . . . . . . . . . . . . . 13
7.2. References - Informative . . . . . . . . . . . . . . . . . 13
Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 13
A.1. Domain and ADSP exist . . . . . . . . . . . . . . . . . . 14
A.2. Domain exists, ADSP does not exist . . . . . . . . . . . . 14
A.3. Domain does not exist . . . . . . . . . . . . . . . . . . 14
Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 15
B.1. Single Location Domains . . . . . . . . . . . . . . . . . 15
B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 15
B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 16
B.4. Third Party Senders . . . . . . . . . . . . . . . . . . . 16
B.5. Domains with Independent Users and Liberal Use Policies . 16
B.6. Non-email Domains . . . . . . . . . . . . . . . . . . . . 16
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 17
Appendix D. Change Log . . . . . . . . . . . . . . . . . . . . . 17
D.1. Changes since -ietf-dkim-06 . . . . . . . . . . . . . . . 17
D.2. Changes since -ietf-dkim-05 . . . . . . . . . . . . . . . 17
D.3. Changes since -ietf-dkim-04 . . . . . . . . . . . . . . . 17
D.4. Changes since -ietf-dkim-03 . . . . . . . . . . . . . . . 18
D.5. Changes since -ietf-dkim-02 . . . . . . . . . . . . . . . 18
D.6. Changes since -ietf-dkim-ssp-01 . . . . . . . . . . . . . 19
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D.7. Changes since -ietf-dkim-ssp-00 . . . . . . . . . . . . . 20
D.8. Changes since -allman-ssp-02 . . . . . . . . . . . . . . . 21
D.9. Changes since -allman-ssp-01 . . . . . . . . . . . . . . . 21
D.10. Changes since -allman-ssp-00 . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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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 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 names. It might be
desirable for such domains to be able to advertise that fact to other
hosts. This is the topic of Author Domain Signing Practices (ADSP).
Hosts implementing this specification can inquire what Author Signing
Practices a domain advertises. This inquiry is called an Author
Signing Practices check.
The basic requirements for 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 [RFC5322].
Briefly,
o A "Signer" is the agent that signs a message, as defined in
section 2.1 of [RFC4871].
o A "Local-part" is the part of an address preceding the @
character, as defined in [RFC5322] and used in [RFC4871].
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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 [RFC5322]. 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 checked.
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=" tag or its default value from the "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
are the same string, and the domains are the same string. Otherwise,
the identities match if the domains are the same string. Following
[RFC5321], Local-part comparisons are case sensitive, but 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
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, it fails to
satisfy ADSP.
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3. Operation Overview
Domain owners can publish 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 ADSP information of the domain(s) specified by
the Author Address(es) as described in Section 4.3. If a message has
multiple Author Addresses the ADSP lookups SHOULD be performed
independently on each address. This document does not address the
process a host might use to combine the lookup results.
3.1. 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 checker implementation
to return an appropriate error result for Author Domains outside the
scope of ADSP.
ADSP applies to specific domains, not domain subtrees. If, for
example, an Author Address were user@domain.example, the Author
Domain would be domain.example, and the applicable ADSP record would
be at _adsp._domainkey.domain.example. An Author Address in a
subdomain such as user@sub.domain.example would have a different ADSP
record at _adsp._domainkey.sub.domain.example. ADSP makes no
connection between a domain and its parent or child domains.
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 checker 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 ADSP
lookup.
o If a message has no Valid Signature, the ADSP result is directly
relevant to the message.
o If a message has a Valid Signature from an Author Domain, ADSP
provides no benefit relative to that domain since the message is
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already known to be compliant with any possible 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
ADSP result in its evaluation of the message.
3.3. ADSP Results
An 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 ADSP record is found.
o All messages from this domain are signed.
o All messages from this domain are signed and discardable, i.e., if
a message arrives without a valid Author Signature, the domain
encourages the recipient(s) to discard it.
o This domain is out of scope, i.e., the domain does not exist in
the DNS.
An ADSP lookup could terminate without producing any result if a DNS
lookup results in a temporary failure.
4. Detailed Description
4.1. DNS Representation
ADSP records are published using the DNS TXT resource record type.
The RDATA for ADSP resource records is textual in format, with
specific syntax and semantics relating to their role in describing
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 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.
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Note: ADSP changes the "Tag=Value List" syntax from [RFC4871] to
use WSP rather than FWS in its DNS records. Domains MUST NOT
publish ADSP records with wildcard names. Wildcards within a
domain publishing ADSP records pose a particular problem, as
discussed in more detail in Section 6.3.
4.2. Publication of ADSP Records
ADSP is intended to apply to all mail sent using the domain name
string of an Alleged Author.
4.2.1. Record Syntax
ADSP records use the "tag=value" syntax described in section 3.2 of
[RFC4871]. Every ADSP record MUST start with an outbound signing
practices tag, so the first four characters of the record are lower
case "dkim", followed by optional whitespace and "=". .
Tags used in ADSP records are described below. Unrecognized tags
MUST be ignored. In the ABNF below, the WSP token, and 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 any other reason,
the domain encourages the recipient(s) to discard it.
Any other values are treated as "unknown".
ABNF:
adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP
("unknown" / "all" / "discardable")
4.3. ADSP Lookup Procedure
Hosts doing an ADSP lookup MUST produce a result that is semantically
equivalent to applying the following steps in the order listed below.
In practice, these steps can be performed in parallel in order to
improve performance. However, implementations SHOULD avoid doing
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unnecessary DNS lookups.
For the purposes of this section a "valid ADSP record" is one that is
both syntactically and semantically correct; in particular, it
matches the ABNF for a "tag-list" and starts with a valid "dkim" tag.
Check Domain Scope: An ADSP checker implementation MUST determine
whether a given Author Domain is within scope for ADSP. Given the
background in Section 3.1 the checker MUST decide which degree of
approximation is acceptable. The checker MUST return an
appropriate error result for Author Domains that are outside the
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 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, rcode=3 in [RFC1035]), the algorithm MUST
terminate with an error indicating that the domain is out of
scope. Note that a result with rcode=0 but no records (often
called "NODATA") is not the same as NXDOMAIN.
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 because this record type is thought to
be the most common for domains used in e-mail, and will
therefore produce a result which can be more readily cached
than a negative result.
If the domain does exist, the checker MAY make more extensive
checks to verify the existence of the domain, such as the ones
described in Section 5 of [RFC5321]. 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 checker 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
corresponding to the Author Domain prefixed by "_adsp._domainkey."
(note the trailing dot).
If the result of this query is a "NOERROR" response (rcode=0 in
[RFC1035]) with an answer which is a single record that is a valid
ADSP record, use that record, and the algorithm terminates.
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If the result of the query is NXDOMAIN or NOERROR with zero
records, there is no ADSP record. If the result of the query
contains more than one record, or a record that is not a valid
ADSP record, the ADSP result is undefined.
If a query results in a "SERVFAIL" error response (rcode=2 in
[RFC1035]), the algorithm terminates without returning a result;
possible actions include queuing the message or returning an SMTP
error indicating a temporary failure.
See Appendix A for examples of ADSP Lookup.
5. IANA Considerations
ADSP adds the following namespaces to the IANA registry. In all
cases, new values are assigned only for values that have been
documented in a published RFC after IETF Review as specified in
[RFC5226].
5.1. ADSP Specification Tag Registry
An ADSP record provides for a list of specification tags. IANA has
established the ADSP Specification Tag Registry for specification
tags that can be used in ADSP fields.
The initial entry in the registry is:
+------+-----------------+
| TYPE | REFERENCE |
+------+-----------------+
| dkim | (this document) |
+------+-----------------+
ADSP Specification Tag Registry Initial Values
5.2. 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 ADSP
Outbound Signing Practices Registry for Outbound Signing Practices.
The initial entries in the registry comprise:
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+-------------+-----------------+
| TYPE | REFERENCE |
+-------------+-----------------+
| unknown | (this document) |
| all | (this document) |
| discardable | (this document) |
+-------------+-----------------+
ADSP Outbound Signing Practices Registry Initial Values
6. Security Considerations
Security considerations in the 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. 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 a legitimate email author's name-
recognition among recipients, by using 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.
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
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showing some indicator to the end recipient. Such a system is out of
scope for this specification.
ADSP checkers may perform multiple DNS lookups per Alleged Author
Domain. Since these lookups are driven by domain names in email
message headers of possibly fraudulent email, legitimate ADSP
checkers can become participants in traffic multiplication attacks on
domains that appear in fraudulent email.
6.2. DNS Attacks
An attacker might attack the DNS infrastructure in an attempt to
impersonate 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 ADSP operates within the framework of the legacy e-mail
system, the default result in the absence of an ADSP record is that
the domain does not sign all of its messages. It is therefore
important that the ADSP clients distinguish a DNS failure such as
"SERVFAIL" from other DNS errors so that appropriate actions can be
taken.
6.3. DNS Wildcards
DNS wildcards (described in [RFC4592]) that exist in the DNS
hierarchy at or above the domain being checked interfere with the
ability to verify the scope of the ADSP check described in
Section 4.3. For example, a wildcard record for *.domain.example
makes all subdomains such as foo.domain.example exist in the DNS.
Domains that intend to make active use of ADSP by publishing a
practice other than Unknown are advised to avoid the use of wildcards
elsewhere in their hierarchy.
If a domain contains wildcards, then any name that matches the
wildcard can appear to be a valid mail domain eligible for ADSP. But
the "_adsp._domainkey." prefix on ADSP records does not allow
publication of wildcard records that cover ADSP records without also
covering non-ADSP records, nor of wildcard records that cover non-
ADSP records without also covering ADSP records. Hence a domain MUST
NOT publish wildcard ADSP records.
7. References
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7.1. References - Normative
[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.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, July 2006.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008.
7.2. References - Informative
[RFC4686] Fenton, J., "Analysis of Threats Motivating DomainKeys
Identified Mail (DKIM)", RFC 4686, September 2006.
[RFC5016] Thomas, M., "Requirements for a DomainKeys Identified Mail
(DKIM) Signing Practices Protocol", RFC 5016,
October 2007.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
October 2008.
Appendix A. Lookup Examples
Assume the example domain publishes these DNS records: (In these
examples, the numbers in parentheses are comments to help identify
the records, not part of the records themselves.)
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aaa.example A 192.0.2.1 (1)
_adsp._domainkey.aaa.example TXT "dkim=all" (2)
bbb.example MX 10 mail.bbb.example (3)
mail.bbb.example A 192.0.2.2 (4)
A.1. Domain and ADSP exist
A mail message contains this From: header line:
From: bob@aaa.example (Bob the Author)
The ADSP Lookup first identifies the Author Address bob@aaa.example
and the Author Domain aaa.example. It does an MX DNS query for
aaa.example, and gets back a NOERROR result with no DNS records.
(There's no MX record, but since record (1) exists, the name exists
in the DNS.) Since that query didn't return an error, the Lookup
proceeds to a TXT DNS query for _adsp._domainkey.aaa.example, which
returns record (2). Since this is a valid DKIM record, the result is
that all messages from this domain are signed.
A.2. Domain exists, ADSP does not exist
A mail message contains this From: header line:
From: alice@bbb.example (Old-fashioned Alice)
The ADSP Lookup first identifies the Author Address alice@bbb.example
and the Author Domain bbb.example. It does an MX DNS query for
bbb.example, and gets back record (3). Since that query didn't
return an error, it then proceeds to a TXT DNS query for
_adsp._domainkey.bbb.example, which returns NXDOMAIN. Since the
domain exists but there is no ADSP record, ADSP returns the default
unknown result: messages may or may not have an author signature.
A.3. Domain does not exist
A mail message contains this From: header line:
From: frank@ccc.example (Unreliable Frank)
The ADSP Lookup first identifies the Author Address frank@ccc.example
and the Author Domain ccc.example. It does an MX DNS query for
ccc.example, and gets back an NXDOMAIN result since there are no
records at all for ccc.example. The lookup terminates with the
result that the domain does not exist in the DNS and so is out of
scope.
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Appendix B. Usage Examples
These examples are intended to illustrate typical uses of 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.
B.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 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, web sites that provide "mail an article" features,
user messages sent through mailing lists, or third party mail clients
that support multiple user identities.
B.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
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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".
B.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. (In the case of
domains with tightly controlled outgoing mail, this latter kind of
mail is sometimes loosely called "forgeries".) In that case, it
might be appropriate to publish an 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.
B.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 RFC5322 From: or other headers. Due to the many and varied
complexities of such agreements, third party signing is not addressed
in this specification.
B.5. Domains with Independent Users and Liberal Use Policies
When a domain has independent users and its usage policy does not
explicitly restrict them to sending mail only from designated mail
servers (e.g. many ISP domains and even some corporate domains), then
it is only appropriate to publish an ADSP record containing
"unknown". Publishing either "all" or "discardable" will likely
result in significant breakage because independent users are likely
to send mail from the external paths enumerated in Appendix B.1.
B.6. 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.
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Appendix C. 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 D. Change Log
*NOTE TO RFC EDITOR: This section may be removed upon publication of
this document as an RFC.*
D.1. Changes since -ietf-dkim-06
Minor editorial changes suggested by AD:
o expand DKIM in title
o clarify that there's no subdomain matching in Section 3.1
o ADSP lookup can terminate without a result if the DNS lookup fails
o random dkim= values are treated as unknown
o in 4.2 note WSP not FWS
o in 4.3 note that NODATA is not NXDOMAIN
o add new Appendix A with lookup examples
Also address Tony's nits in
http://mipassoc.org/pipermail/ietf-dkim/2008q3/010720.html. Make the
examples consistently use the .example domain.
D.2. Changes since -ietf-dkim-05
Minor editorial nits: define NOERROR, SERVFAIL, NXDOMAIN as rfc1035
rcodes, change some punctuation, IANA section change IETF Consensus
to the new IETF Review.
D.3. Changes since -ietf-dkim-04
o Require dkim at the front of each record.
o Disparage wildcard records.
o Changed ABNF use of whitespace from FWS back to WSP, dkim-base is
wrong.
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o RFC 2434 -> 5226, make ref to 4686 informational since it's not
standards track.
o Improve examples with material from Ellen.
D.4. 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.
D.5. Changes since -ietf-dkim-02
o Merge in more text from ADSP draft.
o Phrase actions as host's rather than checker.
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o Explanatory description of i= matching.
o Lookup procedure consistently refers to one 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
D.6. 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.
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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).
D.7. 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.
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D.8. 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.
D.9. 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.
D.10. 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.
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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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Allman, et al. Expires May 26, 2009 [Page 23]
