Network Working Group J. Levine Internet-Draft Taughannock Networks Intended status: Informational July 31, 2013 Expires: February 01, 2014 Publishing Organization Boundaries in the DNS draft-levine-orgboundary-02 Abstract Often, the organization that manages a subtree in the DNS is different from the one that manages the tree above it. Rather than describing a particular design, we describe an architecture to publish in the DNS the boundaries between organizations that can be adapted to various policy models. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. 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Levine Expires February 01, 2014 [Page 1] Internet-Draft Org Boundaries July 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Design Issues . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Lookup Process . . . . . . . . . . . . . . . . . . . . . . . 3 4. Record Syntax . . . . . . . . . . . . . . . . . . . . . . . . 5 5. DNS Records . . . . . . . . . . . . . . . . . . . . . . . . . 5 6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. Normative References . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 8 A.1. Changes from -01 to -02 . . . . . . . . . . . . . . . . . 8 A.2. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 8 1. Introduction Often, the organization that manages a subtree in the DNS is different from the one that manages the tree above it. Many applications use information about such boundaries to implement security policies. For example, web browsers use them to limit the names where web cookies can be set, and Secure Socket Layer (SSL) certificate services use them to determine the party responsible for the domain in a signing request. Some mail security applications such as Domain-based Messaging Authetnication, Reporting and Conformance (DMARC) use them to locate an organization's policy records in the DNS. The most widely used source of boundary data is the Public Suffix List, maintained by the Mozilla Foundation. It is distributed as a text file available at a well known location, with some libraries available to parse and search it. While it has worked adequately, it has a variety of issues: 1. Processing the file is relatively slow, so adequate performance requires that each application cache a copy of the file. 2. The file is in a unique format that requires a specialized parser and lookup routines. 3. The file is managed by a third party rather than directly by the domain owners, meaning the likelihood of stale data is a concern. Levine Expires February 01, 2014 [Page 2] Internet-Draft Org Boundaries July 2013 2. Design Issues Organization boundaries can be assigned on what one could call an opt-in or opt-out basis. "Opt-in" means that two names are only managed by the same organization if both actively assert that they are related. "Opt-out" means that if there is any boundary information at all for a DNS subtree, each name is assumed to be under the same management as its parent unless there is a boundary assertion to the contrary. This design describes an opt-out model. Within the opt-out model, this design can adapt to a variety of scenarios: o Policies can be published by the domains themselves, or by a third party. In the former case, each domain might assert its own boundary policies. In the latter case, the third party makes the assertions, which may or may not agree with what the domains themselves would want. o Multiple levels of delegation may be implemented, which is different from irregular boundaries. For example, "ca", "on.ca", and "toronto.on.ca" are irregular boundaries, because they're all handled by the Canadian Internet Registration Authority (CIRA). CentralNIC's "uk.com" would be a second level of delegation below Verisign's com. o Different sets of boundary rules can be published for different applications. For example, the boundaries for SSL certificates might be different from the boundaries for e-mail policies, or for web cookie setting policies. In the lookup process below, the boundary point data is stored in TXT records. While it wouldn't be harmful to use a new RRTYPE, it would also provide little benefit: All of the DNS names used include a distinctive prefixed name component, so there should not be any resource records at those names other than the TXT record for the boundary. To avoid confusion due to top-level wildcard records that might inadvertently cover the subtree of boundary records, the TXT strings contain a prefix tag. 3. Lookup Process The DNS protocol is defined in [RFC1034] and [RFC1035]. The query is for RRTYPE 15 (TXT). Each collection of boundary data has a "base name" and a "prefix tag". Levine Expires February 01, 2014 [Page 3] Internet-Draft Org Boundaries July 2013 The base name is a location in the DNS tree where the organizational boundary data is rooted. In particular, the "official" boundary data has a base name of "." (the DNS root), while using some other base name implies the owner of the base name is the party making organizational boundary assertions. The prefix tag is the string "_ob" and is intended to be unique. If there are different kinds of boundary data, the tag can be further prefixed, e.g., "_cookie._ob" for conventional HTTP cookies, or "_dmarc._ob" for organizational boundaries intended to be used only within the context of the DMARC protocol. [[TBD: Add a DMARC reference.]]. Note that due to the way wildcard matches, a query for a specific boundary type will match the general boundary type if there is no specific boundary. [[ I think this also falls out correctly even when there are also non-wildcard records, but I need to check that, ]] A valid policy record starts with a fixed string "ob=1 ", meaning organization boundary version 1. If a lookup returns no TXT records that start with this string, or more than one record that starts with the string, the lookup fails. In general, the lookup process takes as input a domain name and returns the name of the node in the DNS closest to the root that also belongs to the same organization. This may be the domain itself or a parent. If there is no policy for the domain the lookup fails; there are no defaults, and the DNS root is not within any organization boundary. (Applications may apply defaults of their own, but that is beyond the scope of this specification.) For the first lookup, the client extracts the top level component (i.e., the rightmost label, as "label" is defined in Section 3 of [RFC1034])) of the domain name from the subcomponents, if any, and inserts the prefix in front of that component, after other components if any. For example, if the domain to be checked is "example.com" or "www.example.com", the client issues a DNS query for "example._ob.com" or "www.example._ob.com". If the domain is simply "example", the client looks up "_ob.example". If the base name is not null, it is appended to the name to be looked up, so if the name "example.com" were to be checked in a collection with the base name "cookies.example", the full queried name would be "example._ob.com.cookies.example." If a valid policy record is returned, the contents of the record after the required prefix string comprise the domain name at or just below the policy boundary. For example, a check for a boundary between "com" and "example.com" would be issued at "example._ob.com", and the expected response would be "ob=1 com". Levine Expires February 01, 2014 [Page 4] Internet-Draft Org Boundaries July 2013 If there are no boundaries below the queried point, the policy record contains "ob=1 ." indicating the root. For example, if all subdomains of the "example" top-level domain (TLD) are under the same management as the TLD itself, checks for "_ob.example" or "www._ob.example" would return "ob=1 .". If the client process only handles a single level of delegation, the process stops after a single lookup. If it allows multiple levels of delegation, the client inserts the prefix tag into the name just below (i.e., to the left of) the name at the largest matching boundary indicated in the lookup result, and repeats the lookup. For example: o When evaluating "www.foo.example.com", the first query would be to "www.foo.example._ob.com". If the reply to this is "ob=1 com", then the second query would go to "www.foo._ob.example.com". o When evaluating "www.example.on.ca", the first query would be to "www.example.on_ob.ca". If the reply to this is "ob=1 on.ca", the next lookup would be to "www._ob.example.on.ca". This process repeats until a DNS lookup returns NXDOMAIN (DNS RRCODE 3), at which point the result of this algorithm is the domain name that last returned a successful query with the first label ("_ob") removed. The total number of DNS lookups is the number of levels of boundary delegation, which is unlikely to be more than 1 or 2 in most realistic scenarios. 4. Record Syntax Expressed in ABNF ([RFC5234]), the syntax of a reply is as follows: ob-reply = "ob=1 " ("." | Domain ) "Domain" is defined in Section 4.1.2 of [RFC5321]. 5. DNS Records The publishing entity uses wildcards and prefixed names that parallel the regular names under a TLD to cover the relevant name space. If there is a boundary below a given name, an entry in the TLD record covers the names below it. For example, if there is a boundary immediately below ".TEST", a suitable record would be: Levine Expires February 01, 2014 [Page 5] Internet-Draft Org Boundaries July 2013 *._ob.test IN TXT "ob=1 test" If the boundary is at the TEST domain, i.e., TEST is under the same management as FOO.TEST, the record would indicate no boundaries, and an additional non-wildcard record is needed to cover TEST itself: *._ob.test txt "ob=1 ." _ob.test txt "ob=1 ." In domains with irregular policy boundaries, multiple records in the record describe the boundary points. For example, in the CA (Canada) TLD, for national organizations there might be a boundary directly below the national TLD; for provincial organizations there might be a boundary below a provincial subdomain such as "on.ca"; and for local (e.g., municipal) organizations, a boundary below a municipal subdomain such as "toronto.on.ca" might exist. A suitable set of of records covers this structure. The closest encloser rule in RFC 4592 [RFC4592] makes the wildcards match the appropriate names. *._ob.ca txt "ob=1 ca" *.on._ob.ca txt "ob=1 on.ca" *.toronto.on._ob.ca txt "ob=1 toronto.on.ca" For any set of policy boundaries in a tree of DNS names, a suitable set of policy records can describe the boundaries, so a client can find the boundary for any name in the tree with a single policy lookup per level of delegation. Since the delegation structure is unlikely to change with any frequency, very long time-to-live (TTL) values in the organizational boundary records are appropriate. 6. Discussion Some domains have very irregular boundaries. This may require a large number of records to describe all the boundaries, perhaps several hundred, but it doesn't seem like a number that would challenge modern DNS servers. The wildcard lookup means that each time an application looks up the boundaries for a hostname, the lookup results use DNS cache entries that will not be reused other than for subsequent lookups for the identical hostname. This might cause cache churn, but it seems at worst no more than we already tolerate from DNSBL lookups. Levine Expires February 01, 2014 [Page 6] Internet-Draft Org Boundaries July 2013 The record format described above uses dots as reserved characters to indicate the boundaries between labels in names. This is adequate if policies only apply to host names, but if becomes important to allow arbitrary names, it would be easy to encode them in TXT records. Keeping in mind that a TXT record contains a sequence of strings, each preceded by a count byte, each component of the name is represented as a string. So the last record above would be represented as: *.toronto.on._ob.ca IN TXT "ob=1" "toronto" "on" "ca" 7. Security Considerations The purpose of publishing organization boundaries is to provide advice to third parties that wish to know whether two names are managed by the same organization, allowing those names to be treated "as the same" in some sense. Clients that rely on published boundaries are outsourcing some part of their own security policy to the publisher, so their own security depends on the publisher's boundaries being accurate. Although in some sense domains are always in control of their subdomains, there are many situations in which parent domains are not expected to influence subdomains. For example, the Internet Corporation for Assigned Names and Numers (ICANN) contracted global TLDs (gTLDs) and registers second level domains. Since there is no technical bar to a parent publishing records that shadow part or all of the boundary record namespace for delegated subdomains, correct operation depends on the parent and subdomains agreeing about who publishes what. 8. Normative References [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, November 1987. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name System", RFC 4592, July 2006. [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, October 2008. Levine Expires February 01, 2014 [Page 7] Internet-Draft Org Boundaries July 2013 Appendix A. Change Log *NOTE TO RFC EDITOR: This section may be removed upon publication of this document as an RFC.* A.1. Changes from -01 to -02 Add ABNF. MSK overhaul of the middle part. Put the wildcards back. A.2. Changes from -00 to -01 Take out wildcards and put everything in one record. Add DNS nits. Author's Address John Levine Taughannock Networks PO Box 727 Trumansburg, NY 14886 Phone: +1 831 480 2300 Email: standards@taugh.com URI: http://jl.ly Levine Expires February 01, 2014 [Page 8]