Network Working Group E. Allman
Request for Comments: 5617 Sendmail, Inc.
Category: Standards Track J. Fenton
Cisco Systems, Inc.
M. Delany
Yahoo! Inc.
J. Levine
Taughannock Networks
August 2009
DomainKeys Identified Mail (DKIM) Author Domain Signing Practices (ADSP)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
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 as
well as how other hosts can access that record.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Language and Terminology . . . . . . . . . . . . . . . . . . . 3
2.1. Terms Imported from the DKIM Signatures Specification . . 3
2.2. Valid Signature . . . . . . . . . . . . . . . . . . . . . 4
2.3. Author Address . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Author Domain . . . . . . . . . . . . . . . . . . . . . . 4
2.5. Alleged Author . . . . . . . . . . . . . . . . . . . . . . 4
2.6. Author Domain Signing Practices . . . . . . . . . . . . . 4
2.7. Author Domain Signature . . . . . . . . . . . . . . . . . 4
3. Operation Overview . . . . . . . . . . . . . . . . . . . . . . 4
3.1. ADSP Applicability . . . . . . . . . . . . . . . . . . . . 5
3.2. ADSP Usage . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. ADSP Results . . . . . . . . . . . . . . . . . . . . . . . 6
4. Detailed Description . . . . . . . . . . . . . . . . . . . . . 6
4.1. DNS Representation . . . . . . . . . . . . . . . . . . . . 6
4.2. Publication of ADSP Records . . . . . . . . . . . . . . . 7
4.3. ADSP Lookup Procedure . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 10
5.1. ADSP Specification Tag Registry . . . . . . . . . . . . . 9 10
5.2. ADSP Outbound Signing Practices Registry . . . . . . . . . 10
5.3. Authentication-Results Method Registry Update . . . . . . 10 11
5.4. Authentication-Results Result Registry Update . . . . . . 10 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12 13
6.1. ADSP Threat Model . . . . . . . . . . . . . . . . . . . . 13
6.2. DNS Considerations . . . . . . . . . . . . . . . . . . . . 13 14
6.3. DNS Wildcards . . . . . . . . . . . . . . . . . . . . . . 14 15
6.4. Inappropriate Application of Author Domain Signatures . . 14 15
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15 16
Appendix A. Lookup Examples . . . . . . . . . . . . . . . . . . . 15 16
A.1. Domain and ADSP Exist . . . . . . . . . . . . . . . . . . 16 17
A.2. Domain Exists, ADSP Does Not Exist . . . . . . . . . . . . 16 17
A.3. Domain Does Not Exist . . . . . . . . . . . . . . . . . . 16 17
Appendix B. Usage Examples . . . . . . . . . . . . . . . . . . . 17
B.1. Single Location Domains . . . . . . . . . . . . . . . . . 17 18
B.2. Bulk Mailing Domains . . . . . . . . . . . . . . . . . . . 17 18
B.3. Bulk Mailing Domains with Discardable Mail . . . . . . . . 18 19
B.4. Third-Party Senders . . . . . . . . . . . . . . . . . . . 18 19
B.5. Domains with Independent Users and Liberal Use Policies . 18 19
B.6. Non-Email Domains . . . . . . . . . . . . . . . . . . . . 18 19
Appendix C. Acknowledgements . . . . . . . . . . . . . . . . . . 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 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 Domain
Signing Practices a domain advertises. This inquiry is called an
Author Domain 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 the 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].
2.2. Valid Signature
A "Valid Signature" is any signature on a message that 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
that includes statements about the domain's practices with respect to
mail it sends with its domain in the From: line.
2.7. Author Domain Signature
An "Author Domain Signature" is a Valid Signature in which the domain
name of the DKIM signing entity, i.e., the d= tag in the DKIM-
Signature header field, is the same as the domain name in the Author
Address. Following [RFC5321], domain name comparisons are case
insensitive.
For example, if the From: line address is bob@domain.example, and the
message has a Valid Signature with the DKIM-Signature header field
containing "d=domain.example", then the message has an Author Domain
Signature.
3. Operation Overview
Domain owners 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 (i.e., A, AAAA, and/or MX) indicating the possible use of the
domain for email. 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: If an organization wants to publish Author Domain Signing
Practices for all the subdomains it uses, such as host names
of servers within the domain, it does so by creating ADSP
records for every _adsp._domainkey.<sub>.domain.example.
Wildcards cannot be used (see Section 6.3.); however,
suitable DNS management tools could automate creation of the
ADSP records.
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 an Author Domain Signature, ADSP provides no
benefit relative to that domain since the message is already known
to be compliant with any possible ADSP for that domain.
o If a message has a Valid Signature other than an Author Domain
Signature, 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 Domain
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 with an Author Domain
Signature.
o All messages from this domain are signed with an Author Domain
Signature and are discardable, i.e., if a message arrives without
a valid Author Domain 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, modified to use whitespace (WSP) rather than
folding whitespace (FWS). 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.
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], modified to use WSP rather than FWS. Every ADSP record
MUST start with an outbound signing-practices tag, so the first four
characters of the record are lowercase "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 is 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
Domain Signature.
discardable
All mail from the domain is signed with an Author
Domain Signature. Furthermore, if a message
arrives without a valid Author Domain 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:
; Copyright (c) 2009 IETF Trust and the persons identified as
; authors of the code. All rights reserved.
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided that the following conditions
; are met:
; - Redistributions of source code must retain the above copyright
; notice, this list of conditions and the following disclaimer.
; - Redistributions in binary form must reproduce the above copyright
; notice, this list of conditions and the following disclaimer in
; the documentation and/or other materials provided with the
; distribution.
; - Neither the name of Internet Society, IETF or IETF Trust, nor the
; names of specific contributors, may be used to endorse or promote
; products derived from this software without specific prior
; written permission.
; THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
; 'AS IS' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
; LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
; A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
; OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
; SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
; LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
; DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
; THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
; (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
; OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
adsp-dkim-tag = %x64.6b.69.6d *WSP "=" *WSP
("unknown" / "all" / "discardable" /
x-adsp-dkim-tag)
x-adsp-dkim-tag = hyphenated-word ; for future extension
; hyphenated-word is defined in RFC 4871
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
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 the 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 email, and will
therefore produce a result that 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 that is a single record that is a valid
ADSP record, use that record, and the algorithm terminates.
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 | (RFC 5617) |
+------+-----------------+
ADSP Specification Tag Registry Initial Values
5.2. ADSP Outbound Signing Practices Registry
The "dkim=" tag specification, 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:
+-------------+-----------------+
| TYPE | REFERENCE |
+-------------+-----------------+
| unknown | (RFC 5617) |
| all | (RFC 5617) |
| discardable | (RFC 5617) |
+-------------+-----------------+
ADSP Outbound Signing Practices Registry Initial Values
5.3. Authentication-Results Method Registry Update
IANA has added the following to the Email Authentication Method Name
Registry:
Method: dkim-adsp
Defined In: RFC 5617
ptype: header
property: from
value: contents of the [RFC5322] From: header field, with comments
removed
5.4. Authentication-Results Result Registry Update
IANA has added the following in the Email Authentication Result Name
Registry:
Code: none
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: No DKIM Author Domain Signing Practices (ADSP) record was
published.
Code: pass
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: This message had an Author Domain Signature that was
validated. (An ADSP check is not strictly required to be
performed for this result since a valid Author Domain
Signature satisfies all possible ADSP policies.)
Code: unknown
Existing/New Code: new
Defined In: RFC 5617
Auth Method: dkim-adsp
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "unknown".
Code: fail
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "all".
Code: discard
Existing/New Code: new
Defined In: RFC 5617
Auth Method: dkim-adsp
Meaning: No valid Author Domain Signature was found on the message
and the published ADSP was "discardable".
Code: nxdomain
Existing/New Code: new
Defined In: RFC 5617
Auth Method: dkim-adsp
Meaning: Evaluating the ADSP for the Author's DNS domain indicated
that the Author's DNS domain does not exist.
Code: temperror
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: An ADSP record could not be retrieved due to some error
that is likely transient in nature, such as a temporary DNS
error. A later attempt may produce a final result.
Code: permerror
Existing/New Code: existing
Defined In: [RFC5451]
Auth Method: dkim-adsp (added)
Meaning: An ADSP record could not be retrieved due to some error
that is likely not transient in nature, such as a permanent
DNS error. A later attempt is unlikely to produce a final
result.
6. Security Considerations
Security considerations in the ADSP are mostly related to attempts on
the part of malicious Senders 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 Mail User Agents
(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 potential 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
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 Considerations
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 email
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
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 does it allow publication of wildcard
records that cover non-ADSP records without also covering ADSP
records. A domain that uses ADSP practices other than unknown SHOULD
NOT publish wildcard records.
6.4. Inappropriate Application of Author Domain Signatures
In one model of DKIM usage, a domain signs messages that are in
transit through their system. Since any signature whose domain
matches the Author Domain is, by definition, an Author Domain
Signature, it would be unwise to sign mail whose Author Domain is the
Signer's domain if the mail is not known to meet the domain's
standards for an Author Domain Signature.
One such use case is where a domain might apply such a signature
following application of an Authentication-Results header field as
described in Section 7.1 of [RFC5451]. This problem can be easily
avoided either by not applying a signature that might be confused
with an Author Domain Signature or by applying a signature from some
other domain, such as a subdomain of the Author Domain.
7. References
7.1. Normative References
[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.
[RFC5451] Kucherawy, M., "Message Header Field for Indicating
Message Authentication Status", RFC 5451, April 2009.
7.2. Informative References
[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):
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 ADSP 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 domain
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.
Appendix B. Usage Examples
These examples are intended to illustrate typical uses of ADSP. They
are not intended to be exhaustive or 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.
If the modifications invalidate the DKIM signature, recipient hosts
will consider the mail not to have an Author Domain Signature, even
though the signature was present when the mail was originally sent.
B.1. Single Location Domains
One common mail system configuration handles all of a domain's users'
incoming and outgoing mail through a single Mail Transport Agent
(MTA) or group of MTAs. With this configuration, the MTA(s) can be
configured to sign outgoing mail with an Author Domain 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 Domain
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 Domain Signature. In this case, the domain's
management can be confident that all of its outgoing mail will be
sent through the signing MTA(s). 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 this case, the mailing provider needs access to a
suitable signing key in order to apply an Author Domain 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 this 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 Domain Signature and prefer that recipient systems discard
mail without a valid Author Domain Signature in order to avoid having
its mail confused with mail sent from sources that do not apply an
Author Domain Signature. (In the case of domains with tightly
controlled outgoing mail, this latter kind of mail is sometimes
loosely called "forgeries".) In such cases, 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.
Appendix C. Acknowledgements
This document greatly benefited from comments by Steve Atkins, Jon
Callas, Dave Crocker, Pasi Eronen, JD Falk, Arvel Hathcock, Ellen
Siegel, Michael Thomas, and Wietse Venema.
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