Network Working Group M. Thomas
Request for Comments: 5016 Cisco Systems
Category: Informational October 2007
Requirements for a
DomainKeys Identified Mail (DKIM) Signing Practices Protocol
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
DomainKeys Identified Mail (DKIM) provides a cryptographic mechanism
for domains to assert responsibility for the messages they handle. A
related mechanism will allow an administrator to publish various
statements about their DKIM signing practices. This document defines
requirements for this mechanism, distinguishing between those that
must be satisfied (MUST), and those that are highly desirable
(SHOULD).
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Table of Contents
1. Introduction ....................................................2
2. Definitions and Requirements Language ...........................3
3. SSP Problem Scenarios ...........................................4
3.1. Problem Scenario 1: Is All Mail Signed with DKIM? ..........4
3.2. Problem Scenario 2: Illegitimate Domain Name Use ...........5
4. SSP Deployment Considerations ...................................6
4.1. Deployment Consideration 1: Outsourced Signing .............6
4.2. Deployment Consideration 2: Subdomain Coverage .............6
4.3. Deployment Consideration 3: Resent Original Mail ...........7
4.4. Deployment Consideration 4: Incremental Deployment
of Signing .................................................7
4.5. Deployment Consideration 5: Performance and Caching ........8
4.6. Deployment Consideration 6: Human Legibility of Practices ..8
4.7. Deployment Consideration 7: Extensibility ..................8
4.8. Deployment Consideration 8: Security .......................8
5. Requirements ....................................................9
5.1. Discovery Requirements .....................................9
5.2. SSP Transport Requirements ................................10
5.3. Practice and Expectation Requirements .....................10
5.4. Extensibility and Forward Compatibility Requirements ......13
6. Requirements for SSP Security ..................................13
7. Security Considerations ........................................13
8. Acknowledgments ................................................13
9. References .....................................................14
9.1. Normative References ......................................14
1. Introduction
DomainKeys Identified Mail [RFC4871] defines a message level signing
and verification mechanism for email. While a DKIM signed message
speaks for itself, there is ambiguity if a message doesn't have a
valid first party signature (i.e., on behalf of the [RFC2822].From
address): is this to be expected or not? For email, this is an
especially difficult problem since there is no expectation of a
priori knowledge of a sending domain's practices. This ambiguity can
be used to mount a bid down attack that is inherent with systems like
email that allow optional authentication: if a receiver doesn't know
otherwise, it should not assume that the lack of a valid signature is
exceptional without other information. Thus, an attacker can take
advantage of the ambiguity and simply not sign messages. If a
protocol could be developed for a domain to publish its DKIM signing
practices, a message verifier could take that into account when it
receives an unsigned piece of email.
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This document defines the requirements for a mechanism that permits
the publication of Sender Signing Practices (SSP). The document is
organized into two main sections: first, a Problem and Deployment
Scenario section that describes the problems that SSP is intended to
address as well as the deployment issues surrounding the base
problems, and the second section is the Requirements that arise
because of those scenarios.
2. Definitions and Requirements Language
o Domain Holder: the entity that controls the contents of the DNS
subtree starting at the domain, either directly or by delegation
via NS records it controls.
o First Party Address: for DKIM, a first party address is defined to
be the [RFC2822].From address in the message header; a first party
address is also known as an Author address.
o First Party Signature: a first party signature is a valid
signature where the signing identity (the d= tag or the more
specific identity i= tag) matches the first party address.
"Matches" in this context is defined in [RFC4871].
o Third Party Signature: a third party signature is a valid
signature that does not qualify as a first party signature. Note
that a DKIM third party signature is not required to correspond to
a header field address such as the contents of Sender or List-Id,
etc.
o Practice: a statement according to the [RFC2822].From domain
holder of externally verifiable behavior in the email messages it
sends.
o Expectation: an expectation combines with a practice to convey
what the domain holder considers the likely survivability of the
practice for a receiver, in particular receivers that may be more
than one SMTP hop away.
o DKIM Signing Complete: a practice where the domain holder asserts
that all legitimate mail will be sent with a valid first party
signature.
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 RFC 2119 [RFC2119].
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3. SSP Problem Scenarios
The email world is a diverse place with many deployment
considerations. This section outlines expected usage scenarios where
DKIM signing/verifying will take place, and how a new protocol might
help to clarify the relevance of DKIM-signed mail.
3.1. Problem Scenario 1: Is All Mail Signed with DKIM?
After auditing their outgoing mail and deploying DKIM signing for all
of their legitimate outgoing mail, a domain could be said to be DKIM
signing complete. That is, the domain has, to the best of its
ability, ensured that all legitimate mail purporting to have come
from that domain contains a valid DKIM signature.
A receiver in the general case doesn't know what the practices are
for a given domain. Thus, the receiver is at a disadvantage in not
knowing whether or not it should expect all mail to be signed from a
given domain. This knowledge gap leads to a trivially exploitable
bid-down attack where the attacker merely sends unsigned mail; since
the receiver doesn't know the practices of the signing domain, it
cannot treat the message any more harshly for lack of a valid
signature.
An information service that allows a receiver to query for the
practices and expectations of the first party domain when no valid
first party signature is found could be useful in closing this gap.
A receiver could use this information to treat such questionable mail
with varying degrees of prejudice.
Note that, for the foreseeable future, unrestricted use patterns of
mail (e.g., where users may be members of mailing lists, etc.) will
likely suffer occasional, non-malicious signature failure in transit.
While probably not a large percentage of total traffic, this kind of
breakage may be a significant concern for those usage patterns. This
scenario defines where the sender cannot set any expectation as to
whether an individual message will arrive intact.
Even without that expectation, a receiver may be able to take
advantage of the knowledge that the domain's practice is to sign all
mail and bias its filters against unsigned or damaged in transit
mail. This information should not be expected to be used in a binary
yes/no fashion, but instead as a data point among others in a
filtering system.
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The following exchange illustrates problem scenario 1.
1. Mail with a [RFC2822].From domain Alice is sent to domain Bob
with a missing or broken DKIM first party signature from Alice.
2. Domain Bob would like to know whether that is an expected state
of affairs.
3. Domain Alice provides information that it signs all outgoing
mail, but places no expectation on whether it will arrive with an
intact first party signature.
4. Domain Bob could use this information to bias its filters to
examine the message with some suspicion.
3.2. Problem Scenario 2: Illegitimate Domain Name Use
A class of mail typified by transactional mail from high-value
domains is currently the target of phishing attacks. In particular,
many phishing scams forge the [RFC2822].From address in addition to
spoofing much of the content to trick unsuspecting users into
revealing sensitive information. Domain holders sending this mail
would like the ability to give an enhanced guarantee that mail sent
with their domain name should always arrive with the proof that the
domain holder consented to its transmission. That is, the message
should contain a valid first party signature as defined above.
From a receiver's standpoint, knowing that a domain not only signs
all of its mail, but places a very high value on the receipt of a
valid first party signature from that domain is helpful. Hence, a
receiver knows that the sending domain signs all its mail, and that
the sending domain considers mail from this domain particularly
sensitive in some sense, and is asking the receiver to be more
suspicious than it otherwise might be of a broken or missing first-
party signature. A receiver with the knowledge of the sender's
expectations in hand might choose to process messages not conforming
to the published practices in a special manner. Note that the
ability to state an enhanced guarantee of a valid signature means
that senders should expect mail that traverses modifying
intermediaries (e.g., mailing lists, etc.) will likely be quarantined
or deleted; thus, this scenario is more narrow than problem scenario
1.
Informative Note: a receiving filter may choose to treat scenario
2 much more harshly than scenario 1; where scenario 1 looks odd,
scenario 2 looks like something is very wrong.
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1. Mail with a [RFC2822].From domain Alice is sent to domain Bob
with a missing or broken first party DKIM signature from domain
Alice.
2. Domain Bob would like to know whether that is an expected state
of affairs.
3. Domain Alice provides information that it signs all outgoing
mail, and furthermore places an expectation that it should arrive
with an intact first party signature, and that the receiver
should be much more wary if it does not.
4. Domain Bob could use this information to bias its filters such
that it examines the message with great suspicion.
4. SSP Deployment Considerations
Given the problems enumerated above for which we'd like SSP to
provide information to recipients, there are a number of scenarios
that are not related to the problems that are to be solved, per se,
but the actual mechanics of implementing/deploying the information
service that SSP would provide.
4.1. Deployment Consideration 1: Outsourced Signing
Many domains do not run their own mail infrastructure, or may
outsource parts of it to third parties. It is desirable for a domain
holder to have the ability to delegate to other entities the ability
to sign for the domain holder. One obvious use scenario is a domain
holder from a small domain that needs to have the ability for their
outgoing ISP to sign all of their mail on behalf of the domain
holder. Other use scenarios include outsourced bulk mail for
marketing campaigns, as well as outsourcing various business
functions, such as insurance benefits, etc.
4.2. Deployment Consideration 2: Subdomain Coverage
An SSP client will perform lookups on incoming mail streams to
provide the information as proposed in the problem scenarios. The
domain part of the first address of the [RFC2822].From will form the
basis to fetch the published information. A trivial attack to
circumvent finding the published information can be mounted by simply
using a subdomain of the parent domain that doesn't have published
information. This attack is called the subdomain attack: that is, a
domain wants to not only publish a policy for a given DNS label it
controls, but it would also like to protect all subdomains of that
label as well. If this characteristic is not met, an attacker would
need only create a possibly fictitious subdomain that was not covered
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by the SSP's information service. Thus, it would be advantageous for
SSP to not only cover a given domain, but all subdomains of that
domain as well.
4.3. Deployment Consideration 3: Resent Original Mail
Resent mail is a common occurrence in many scenarios in the email
world of today. For example, domain Alice sends a DKIM-signed
message with a published practice of signing all messages to domain
Bob's mailing list. Bob, being a good net citizen, wants to be able
to take his part of the responsibility of the message in question, so
he DKIM signs the message, perhaps corresponding to the Sender
address.
Note that this scenario is completely orthogonal to whether Alice's
signature survived Bob's mailing list: Bob merely wants to assert his
part in the chain of accountability for the benefit of the ultimate
receivers. It would be useful for this practice to be encouraged as
it gives a more accurate view of who handled the message. It also
has the side benefit that remailers that break DKIM first party
signatures can be potentially assessed by the receiver based on the
receiver's opinion of the signing domains that actually survived.
4.4. Deployment Consideration 4: Incremental Deployment of Signing
As a practical matter, it may be difficult for a domain to roll out
DKIM signing such that they can publish the DKIM Signing Complete
practice given the complexities of the user population, the
outsourced vendors sending on its behalf, etc. This leaves open an
exploit that high-value mail, such as in Problem Scenario 2, must be
classified to the least common denominator of the published
practices. It would be desirable to allow a domain holder to publish
a list of exceptions that would have a more restrictive practices
statement. NB: this consideration has been deemed met by the
mechanisms provided by the base DKIM signing mechanism; it is merely
documented here as having been an issue.
For example, bigbank.example.com might be ready to say that
statements@bigbank.example.com is always signed, but the rest of the
domain, say, is not. Another situation is that the practices of some
address local parts in a given domain are not the same as practices
of other local parts. Using the same example of
statements@bigbank.example.com being a transactional kind of email
that would like to publish very strong practices, mixed in with the
rest of the user population local parts, which may go through mailing
lists, etc., for which a less strong statement is appropriate.
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It should be said that DKIM, through the use of subdomains, can
already support this kind of differentiation. That is, in order to
publish a strong practice, one only has to segregate those cases into
different subdomains. For example: accounts.bigbank.example.com
would publish constrained practices, while
corporateusers.bigbank.example.com might publish more permissive
practices.
4.5. Deployment Consideration 5: Performance and Caching
Email service provides an any-any mesh of potential connections: all
that is required is the publication of an MX record and an SMTP
listener to receive mail. Thus, the use of SSP is likely to fall
into two main scenarios, the first of which are large, well-known
domains that are in constant contact with one another. In this case,
caching of records is essential for performance, including the
caching of the non-existence of records (i.e., negative caching).
The second main scenario is when a domain exchanges mail with a much
smaller volume domain. This scenario can be both perfectly normal as
with the case of vanity domains, and unfortunately, a vector for
those sending mail for anti-social reasons. In this case, we'd like
the message exchange burden to SSP querier to be low, since many of
the lookups will not provide a useful answer. Likewise, it would be
advantageous to have upstream caching here as well so that, say, a
mailing list exploder on a small domain does not result in an
explosion of queries back at the root and authoritative server for
the small domain.
4.6. Deployment Consideration 6: Human Legibility of Practices
While SSP records are likely to be primarily consumed by an
automaton, for the foreseeable future they are also likely to be
inspected by hand. It would be nice to have the practices stated in
a fashion that is also intuitive to the human inspectors.
4.7. Deployment Consideration 7: Extensibility
While this document pertains only to requirements surrounding DKIM
signing practices, it would be beneficial for the protocol to be able
to extend to other protocols.
4.8. Deployment Consideration 8: Security
SSP must be able to withstand life in a hostile, open-Internet
environment. These include DoS attacks, and especially DoS attacks
that leverage themselves through amplification inherent in the
protocol. In addition, while a useful protocol may be built without
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strong source authentication provided by the information service, a
path to strong source authentication should be provided by the
protocol, or underlying protocols.
5. Requirements
This section defines the requirements for SSP. As with most
requirements documents, these requirements define the MINIMUM
requirements that a candidate protocol must provide. It should also
be noted that SSP must fulfill all of the requirements.
5.1. Discovery Requirements
Receivers need a means of obtaining information about a sender's DKIM
practices. This requires a means of discovering where the
information is and what it contains.
1. The author is the first-party sender of a message, as specified
in the [RFC2822].From field. SSP's information is associated
with the author's domain name, and is published subordinate to
that domain name.
2. In order to limit the cost of its use, any query service
supplying SSP's information MUST provide a definitive response
within a small, deterministic number of message exchanges under
normal operational conditions.
Informative Note: this, for all intents and purposes is a
prohibition on anything that might produce loops or result in
extended delays and overhead; also though "deterministic"
doesn't specify how many exchanges, the expectation is "few".
Refs: Deployment Considerations, Sections 4.2 and 4.5.
3. SSP's publishing mechanism MUST be defined such that it does not
lead to multiple resource records of the same type for different
protocols residing at the same location.
Informative note: an example is multiple resource record of the
same type within a common DNS leaf. Hence, uniquely defined
leaf names or uniquely defined resource record types will
ensure unambiguous referencing.
Refs: Deployment Consideration, Section 4.2.
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4. SSP retrieval SHOULD provide coverage for not only a given domain
but all of its subdomains as well. It is recognized that there
is some reasonable doubt about the feasibility of a widely
accepted solution to this requirement. If the working group does
not achieve rough consensus on a solution, it MUST document the
relevant security considerations in the protocol specification.
Refs: Deployment Considerations, Sections 4.2 and 4.5.
5.2. SSP Transport Requirements
The publication and query mechanism will operate as an internet-based
message exchange. There are multiple requirements for this lower-
layer service:
1. The exchange SHOULD have existing widespread deployment of the
transport layer, especially if riding on top of a true transport
layer (e.g., TCP, UDP).
Refs: Deployment Considerations, Sections 4.5 and 4.7.
2. The query/response in terms of latency time and the number of
messages involved MUST be low (less than three message exchanges
not counting retransmissions or other exceptional conditions).
Refs: Deployment Consideration, Section 4.5.
3. If the infrastructure doesn't provide caching (a la DNS), the
records retrieved MUST provide initiators the ability to maintain
their own cache. The existing caching infrastructure is,
however, highly desirable.
Refs: Deployment Consideration, Section 4.5.
4. Multiple geographically and topologically diverse servers MUST be
supported for high availability.
Refs: Deployment Considerations, Sections 4.5 and 4.7.
5.3. Practice and Expectation Requirements
As stated in the definitions section, a practice is a statement
according to the [RFC2822].From domain holder of externally
verifiable behavior in the email messages it sends. As an example, a
practice might be defined such that all email messages will contain a
DKIM signature corresponding to the [RFC2822].From address. Since
there is a possibility of alteration between what a sender sends and
a receiver examines, an expectation combines with a practice to
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convey what the [RFC2822].From domain considers the likely outcome of
the survivability of the practice at a receiver. For example, a
practice that a valid DKIM for the [RFC2822].From address is present
when it is sent from the domain, and an expectation that it will
remain present and valid for all receivers whether topologically
adjacent or not.
1. SSP MUST be able to make practices and expectation assertions
about the domain part of a [RFC2822].From address in the context
of DKIM. SSP will not make assertions about other addresses for
DKIM at this time.
Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.
2. SSP MUST provide a concise linkage between the [RFC2822].From and
the identity in the DKIM i= tag, or its default if it is missing
in the signature. That is, SSP MUST precisely define the
semantics of what qualifies as a first party signature.
Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.
3. SSP MUST be able to publish a practice that the domain's signing
behavior is "DKIM Signing Complete". That is, all messages were
transmitted with a valid first party signature.
Refs: Problem Scenario 1, Section 3.1.
4. SSP MUST be able to publish an expectation that a verifiable
first party DKIM signature should be expected on receipt of a
message.
Refs: Problem Scenario 2, Section 3.2.
5. Practices and expectations MUST be presented in SSP syntax using
as intuitive a descriptor as possible. For example, p=? would be
better represented as p=unknown.
Refs: Deployment Consideration, Section 4.6.
6. Because DKIM uses DNS to store selectors, there is always the
ability for a domain holder to delegate all or parts of the
_domainkey subdomain to an affiliated party of the domain
holder's choosing. That is, the domain holder may set an NS
record for _domainkey.example.com to delegate to an email
provider who manages the entire namespace. There is also the
ability for the domain holder to partition its namespace into
subdomains to further constrain third parties. For example, a
domain holder could delegate only _domainkey.benefits.example.com
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to a third party to constrain the third party to only be able to
produce valid signatures in the benefits.example.com subdomain.
Last, a domain holder can even use CNAME's to delegate individual
leaf nodes. Given the above considerations, SSP need not invent
a different means of allowing affiliated parties to sign on a
domain's behalf at this time.
Refs: Deployment Consideration, Section 4.4.
7. Practices and expectations MUST be presented as an information
service from the signing domain to be consumed as an added factor
to the receiver's local policy. In particular, a practice or
expectation MUST NOT mandate any disposition stance on the
receiver.
Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.
8. There is no requirement that SSP publish practices of any/all
third parties that MUST NOT sign on the domain holder's behalf.
This should be considered out of scope.
INFORMATIVE NOTE: this is essentially saying that the protocol
doesn't have to concern itself with being a blacklist
repository.
Refs: Problem Scenarios 1 and 2, Sections 3.1 and 3.2.
9. SSP MUST NOT be required to be invoked if a valid first party
signature is found.
Refs: Deployment Consideration, Section 4.2.
10. SSP MUST NOT provide a mechanism that impugns the existence of
non-first party signatures in a message. A corollary of this
requirement is that the protocol MUST NOT link practices of first
party signers with the practices of third party signers.
INFORMATIVE NOTE: the main thrust of this requirement is that
practices should only be published for that which the publisher
has control, and should not meddle in what is ultimately the
local policy of the receiver.
Refs: Deployment Consideration, Section 4.3.
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5.4. Extensibility and Forward Compatibility Requirements
1. SSP MUST NOT extend to any protocol other than DKIM for email at
this time. SSP SHOULD be extensible for protocols other than
DKIM.
Refs: Deployment Consideration, Section 4.7.
2. SSP MUST be able to add new practices and expectations within the
existing discovery/transport/practices in a backward compatible
fashion.
Refs: Deployment Consideration, Section 4.7.
6. Requirements for SSP Security
1. SSP for a high-value domain is potentially a high-value DoS
target, especially since the unavailability of SSP's record could
make unsigned messages less suspicious.
2. SSP MUST NOT make highly leveraged amplification or make-work
attacks possible. In particular, the work and message exchanges
involved MUST be order of a constant.
Refs: Deployment Consideration, Section 4.8.
3. SSP MUST have the ability for a domain holder to provide SSP's
data such that a receiver can determine that it is authentically
from the domain holder with a large degree of certainty. SSP may
provide means that provide less certainty in trade off for ease
of deployment.
Refs: Deployment Consideration, Section 4.8.
7. Security Considerations
This document defines requirements for a new protocol and the
security related requirements are defined above. Since it is
expected that the new protocol will use the DNS as a basis for the
published SSP information, most if not all of the threats described
in [RFC4686] will be applicable.
8. Acknowledgments
Dave Crocker and Jim Fenton provided substantial review of this
document. Thanks also to Vijay Gurbani and David Harrington for
their helpful last call reviews.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
April 2001.
[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.
Author's Address
Michael Thomas
Cisco Systems
606 Sanchez St
San Francisco, California 94114
USA
Phone: +1-408-525-5386
Fax: +1-408-525-5386
EMail: mat@cisco.com
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