Internet DRAFT - draft-hallambaker-dkimpolicy


          Internet Draft                                       P. Hallam-Baker 
          Document: draft-hallambaker-dkimpolicy-00.txt          VeriSign Inc. 
          Expires: December 2006                                     June 2006 
                       Considerations for DKIM Policy Language 
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       If we are going to change the current DKIM policy language as 
       deployed in any way including definition of a new DNS RR we should do 
       the job properly and define a layered infrastructure that is capable 
       of other uses. 

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                      Considerations for DKIM Policy Language      June 2006 

       Considerations for DKIM Policy Language.............................1 
         Status of this Memo...............................................1 
       1. Conventions used in this document................................2 
       2. Is change necessary?.............................................2 
       3. What is a Layered Infrastructure?................................3 
         3.1 What a Layered Architecture is Not............................4 
         3.2 Reusable Escape Mechanism.....................................4 
         3.3 Reusable Cryptography Attributes..............................5 
         3.4 Reusable Discovery Strategy...................................5 
       4. Next Steps.......................................................8 
         4.1 Attempt a Layered Definition of SSP...........................8 
         4.2 New RR Definitions Should Support Infrastructure, not DKIM....8 
       Authors’ Addresses..................................................9 
         Conventions used in this document 
       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 [RRC 2119]. 
         Is change necessary? 
       Sender Security Policy (SSP) appears to be an adequate basis for a 
       security policy to support the immediate needs of DKIM. It appears 
       that the email sender has sufficient scope to describe the 
       configuration of their outbound email authentication using the DKIM 
       message format and it is is clearly capable of extension to support 
       additional attributes that may be found to be necessary. 
       An entirely reasonable approach to supporting the policy needs of 
       DKIM is to simply adopt the SSP framework as is maintaining strict 
       backwards compatibility but making whatever minor adjustments as may 
       be found necessary. 
       The adoption of a new DNS Resource Record is not supported in any 
       existing DKIM infrastructure and thus represents an incompatible 
       If however backwards compatibility is to be lost for any reason the 
       working group should reject the current SSP document which is 

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                      Considerations for DKIM Policy Language      June 2006 

       exclusively focused on DKIM and instead adopt a layered approach in 
       which the working group first defines a framework for expressing 
       general security policy and then defines the DKIM policy mechanism as 
       an instance of that framework. 
       DKIM is an important resource for managing email security. The lack 
       of robust, practical, ubiquitous email authentication is clearly the 
       most significant defect in the present email system. It is not 
       however the only security defect in the email system that needs to be 
       fixed. For many years large numbers of email servers have been 
       capable of supporting email exchange over TLS, without policy layer 
       support however this infrastructure is vulnerable to a downgrade 
       attack. The use of domain level S/MIME and PGP face the same problem. 
       The need for a policy layer integrated into the signaling system 
       represents a clear deficiency in the Internet infrastructure and is 
       one of the principal causes for many of the limitations that are felt 
       when using or administering Internet protocols. Most protocols 
       provide support for version numbers but as is widely acknowledged a 
       version number does little more than avoid unintended consequences 
       when a client attempts to connect to a server using an incompatible 
       protocol version. Administration of a transition from one protocol 
       version to another or from one protocol to a different one requires a 
       policy infrastructure. 
       The Internet is gradually acquiring a policy infrastructure and a 
       decision by the DKIM working group to decide to adopt or reject a 
       layered approach will have little impact on this process. A policy 
       infrastructure is already being developed to support Web Services, 
       the need to extend this framework to support REST or AJAX based 
       applications is inevitable. 
       Adopting a layered approach in DKIM will not change the fact of 
       policy infrastructure deployment but it may impact the consistency 
       and comprehensiveness of the infrastructure that evolves. A similar 
       effect was seen in the deployment of MX and SRV. The need to provide 
       fault tolerant service was felt first in email. Later it was realized 
       that this need was inherent in every Internet protocol and the SRV 
       record was defined. 
         What is a Layered Infrastructure? 
       The principle of layered abstraction is a basic tool of network 
       design yet providing a definition of a layered abstraction is almost 
       challenging as designing one. 

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       The statements we want to make for DKIM are instances of more general 
            "Every email from has a DKIM signature with 
       characteristics {x, y, z}" 
            "The email server always offers STARTTLS with 
       charateristics {p, q}" 
            " supports HTTP/2.0" 
       Instead of defining statements and syntax that are specific to DKIM 
       we should attempt to define policy statements in such a way that 
       encourages reuse whenever possible. 
          What a Layered Architecture is Not 
       Equally important is the definition of what a layered architecture is 
       not. The key to the design of a layered architecture is the 
       specification of a series of well defined abstractions and the 
       definition of the communication between those abstractions. 
       If the definition of the policy language requires constant recourse 
       to make changes to the DNS protocol the policy language framework is 
       The DNS is a large deployed infrastructure with a very specific task. 
       Changes to the DNS infrastructure to support deployment of a policy 
       infrastructure should be avoided if at all possible but are 
       acceptable for the purposes of deploying a policy infrastructure that 
       will serve multiple protocols. 
       What is not acceptable is a ‘policy infrastructure’ where each policy 
       definition to support a new protocol requires changes to be made to 
       the DNS infrastructure. 
          Reusable Escape Mechanism 
       Clearly there is a strict limit to the detail that is practical 
       within the context of DNS publication of service configuration data, 
       clearly we do not want people entering war and peace into the DNS to 
       configure an email service. It is bad enough the fact that airline 
       check in assistants have to write novels while checking people in for 
       a flight from Boston to Washington without network administrators 
       attempting to write complex security policies into DNS configuration 
       files to be emitted as 500 byte DNS response messages.  
       Writing security policy should not be like writing a haiku: fitting a 
       complex idea into a tightly restricted form of expression. 

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       There are two means of extension that might be employed. First the 
       policy mechanism might extend within the DNS itself using some form 
       of include directive similar to that defined in SenderID/SPF 
       framework. Alternatively the extension might be by means of an 
       arbitrary URL from which the full policy may be obtained. 
       If the extension mechanism is to be to an arbitrary URL it is 
       probably most appropriate to allow use of a richer, more expressive 
       syntax such as XML than the compact tag value encoding forms 
       generally considered more appropriate for use in the DNS. 
          Reusable Cryptography Attributes 
       A security policy is at root a statement that says that a certain 
       network principle shall always offer a certain minimum level of 
       security when making a communication.  
       For example  
            ‘ will always sign outgoing emails using a 
                 minimum of SHA-1 and RSA1024’ 
            ‘ will always offer STARTTLS in SMTP 
                 transactions with a certificate validated via a cert 
                 chain whose root cert has a SHA1 fingerprint of 
                 Q282hd9213h23ey23== and offer a minimum of RSA 1024 
                 and RC4’ 
       It is clear that many of the attributes referenced in the above 
       policies will be generally applicable. For example IANA maintains a 
       registry of consistent names for cryptographic algorithms. 
       Note that the above policy statements need not exhaustively enumerate 
       every cryptographic algorithm that might be offered. If the email 
       server in the second example offers AES 128 it need not mention that 
       it also supports 3DES and AES 256.  
       It is however essential that each of the algorithms described be 
       offered in the form described. Otherwise there would be the 
       possibility of a downgrade through unacceptable upgrade attack where 
       a man in the middle inserts an offer of RSA 32768 knowing that it 
       will be rejected even though it is stronger than RSA 1024. 
          Reusable Discovery Strategy 

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       The principle area in which the current SSP specification is 
       unsatisfactory is in the area of policy discovery. It appears that 
       unless a new RR is defined specifically for DKIM policy records that 
       it will be impossible to support the form of wildcarding we require 
       without resort to heuristics or exhaustive search strategies that may 
       facilitate denial of service attacks. 
       Definition of a new RR should be avoided if at all possible. 
       According to source code samples demonstrated by one of the leading 
       providers of the deployed base of DNS servers their product is not 
       capable of saving data associated with unknown RRs. Any configuration 
       changes made to the server by mechanisms such as the dynamic DNS 
       component which does support use of new RRs will thus be lost 
       whenever the service is stopped and restarted.  
       In the absence of proof that the DNS server vendor concerned engaged 
       in a deliberate lie it is prudent to assume that any deployment of 
       new DNS RRs has a significant infrastructure cost and should be 
       avoided unless absolutely necessary. 
       As we know the DNS wildcard scheme is not ideal for our purposes: 
            1: There is no way to specify a wildcard of the form 
            2: The prefix * does not match if 
            there is any record defined for that node 
       The solution recommended by the DNSEXT Working Group members to the 
       first problem is to define a new DNS RR. 
       The second problem exists whether or not a new record is defined. The 
       only way to address this problem within the existing DNSSEC framework 
       is to add support at the DNS server level for synthetic wildcards. So 
       the admin enters a policy for ? which is expanded out to 
       create records for every host in that does exist and does 
       not have a policy record otherwise defined. 
       The first problem is the hard one, one solution is to define a new 
       resource record. This is not sustainable as an infrastructure. Every 
       internet protocol will need a DNS policy record associated with it so 
       we would need to define 10,000 new RRs just to support existing 
       A better solution is to define a record to act as the wildcard 
       record. The use of the PTR record is currently undefined for the 
       forward DNS and allows the needed information to be defined. 

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       Alternatively a new record could be specified, but this would then 
       make implementations dependent on the deployment of DNS extensions. 
       The policy discovery strategy then becomes: 
       To discover the policy for DKIM at 
            1) policy = lookup (TXT, "") 
                 IF policy <> NULL THEN RETURN policy 
            2) pointer = lookup (PTR, "") 
                 IF pointer == NULL THEN RETURN NULL 
            3) policy = lookup (TXT, "_dkim." + pointer) 
                 return policy 
       This strategy is guaranteed to always return in three steps without 
       exception and is 100% compatible with the deployed DNS infrastructure 
       with no known exceptions.  
       The strategy is can be applied to an arbitrary protocol and allows 
       for much simplified administration. In most networks the majority of 
       the host configurations are essentially identical. Only a few 
       machines that perform special roles such as mail handling or file 
       server support will require custom configuration. 
       The redirect strategy allows the administrator to define standard 
       network policy configurations, for example in an enterprise there 
       would probably be defined network policy configs for standard 
       desktops, standard laptops and developer machines. If necessary the 
       standard config may be overriden for individual domain names. 
       The redirect stategy does not depend on walking up the chain, finding 
       a zone cut or anything similar, it also overcomes a frequent 
       objection to such attempts, it is does not allow an unauthorized 
       intermediate DNS server to override policy definitions made by a 
       subordinate zone - except of course to the extent that it can do this 
       by changing the delegation and hijacking the entire zone. 
       A possible objection to the strategy is that it imposes new semantics 
       on the PTR record, a record which to date has only been defined for 
       use in the reverse DNS. Fortunately however the lack of defined 
       semantics outside the reverse DNS means that the only problem that 
       may occur in the forward DNS is that this use may collide with other 
       attempts to redefine PTR. 

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       The issue of the reverse DNS is more complex. The most prudent course 
       might be to prohibit the use of PTR for policy redirection, yet the 
       semantics that are achieved through use of PTR appear to be exactly 
       what we might want such semantics to be. It is probably prudent to 
       simply note this apparent oddity and note that since email is not 
       sent from the reverse DNS zone it is unlikely to impact DKIM. 
         Next Steps 
          Attempt a Layered Definition of SSP 
       We recommend that the DKIKM working group examine the possibility of 
       redefining the current SSP specification in terms of a layered model 
       where the policy infrastructure framework is separated from the 
       instance to support DKIM. 
          New RR Definitions Should Support Infrastructure, not DKIM 
       A previously noted a policy infrastructure should separate the 
       specific needs of DKIM from needs that are general to multiple 
       If it is the case that a new RR definition is essential and it is not 
       possible to deploy DKIM in a manner that meets all the objectives of 
       DKIM within the capabilities of the legacy DNS then whatever RRs are 
       defined should be designed to support a policy infrastructure rather 
       than the specific DKIM protocol. 
       It is simply not acceptable or sustainable for all attempts to define 
       protocol policy records to be gated on a single IETF working group, 
       particularly one that will in any case be wound up as soon as the DNS 
       Security deployment is complete. There are many forums that define 
       Internet protocols and every protocol will require policy. 
       There are already far more unofficial definitions of DNS SRV entries 
       than there are official ones. Unless a realistic approach is adopted 
       that supports the principal of independent innovation once considered 
       the founding principle of the IETF as an institution. 
       Copyright (C) The Internet Society (2006). 
       This document is subject to the rights, licenses and restrictions 
       contained in BCP 78, and except as set forth therein, the authors 
       retain all their rights. 

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                      Considerations for DKIM Policy Language      June 2006 

       This document and the information contained herein are provided on an 
        [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate 
            Requirement Levels", BCP 14, RFC 2119, March 1997 
       The author would like to acknowledge the contribution of Stephen 
       Farrell without whose insistence on writing it this document would 
       never have been written. 
       Authors’ Addresses 
       Phillip Hallam-Baker 
       VeriSign Inc. 

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