|Network Working Group||M.A. Thomas|
|Internet Draft||Cisco Systems|
|Intended status: Informational|
|Expires: February 2008|
Requirements for a DKIM Signing Practices Protocol
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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 satisified (MUST), and those that are highly desirable (SHOULD).
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].
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 (ie, 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.
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: a Problem and Deployment Scenario section which describes the problems that SSP is intended to address as well as the deployment issues surrounding the base problems. The second section is the Requirements that arise because of those 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.
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 it should expect all mail to be signed from a given domain or not. 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 which 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 (eg 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, the 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.
The following exchange illustrates problem scenario 1.
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 can know that the domain not only signs all of its mail, but also feels it essential that legitimate mail must have its first party signatures survive transit. 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 (eg, mailing lists, etc) will be likely be quarantined or deleted, thus this scenario is more narrow than problem scenario 1.
The following exchange illustrates problem scenario 2:
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.
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 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.
A 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 which 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 by 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.
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 are not friendly to DKIM first party signatures (ie, break them) can be potentially assessed by the receiver based on the receiver's opinion of the signing domains that actually survived.
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, 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 which 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 firstname.lastname@example.org 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 email@example.com being a transactional kind of email which 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.
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.
Email service provides an any-any mesh of potential connections: all that is required is the publication of an MX record and a 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 who 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 (ie, 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.
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 which is also intuitive to the human inspectors.
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.
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 strong source authentication provided by the information service, a path to strong source authentication should be provided by the protocol, or underlying protocols.
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.
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.
The publication and query mechanism will operate as an an internet-based message exchange. There are multiple requirements for this lower layer service:
As stated in the definitions 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 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 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.
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an RFC.
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.
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.
|[RFC2119]||Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.|
|[RFC2822]||Resnick, P., “Internet Message Format”, RFC 2822, April 2001.|
|[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.|
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