IETF A. Vesely Internet-Draft July 15, 2013 Intended status: Informational Expires: January 16, 2014 DNSWL Email Authentication Method Extension draft-vesely-authmethod-dnswl-00 Abstract This document describes a method that can be registered within the Email Authentication Methods IANA registry created by RFC 5451. The method consists in looking up a DNS whitelist. 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 16, 2014. Copyright Notice Copyright (c) 2013 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Vesely Expires January 16, 2014 [Page 1] Internet-Draft DNSWL email-auth-method extension July 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Method Results . . . . . . . . . . . . . . . . . . . . . . . . 3 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 4. Implementation Status . . . . . . . . . . . . . . . . . . . . . 5 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 6.1. Normative References . . . . . . . . . . . . . . . . . . . 6 6.2. Informative References . . . . . . . . . . . . . . . . . . 6 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Vesely Expires January 16, 2014 [Page 2] Internet-Draft DNSWL email-auth-method extension July 2013 1. Introduction One of the many checks that mail servers carry out is to query DNS whitelists (DNSWL, [RFC5782]). The semantics of DNSWL is similar to that of Vouch By Reference (VBR, [RFC5518]); that is, an external organization, trusted by the receiving mail transfer agent (MTA), vouches for the sender. Differently from VBR, the lookup is based on the IP address. That allows it to occur very early in an SMTP transaction, and thus it can be used to counterweight policies that typically occur at those early stages too, such as the Sender Policy Framework (SPF, [RFC4408]). Nevertheless, the result of a DNSWL lookup is likely used at later stages as well; for example, a delivery agent can use it to estimate the spamminess of an email message. Reusing the previously obtained result is more consistent than issuing multiple queries, and saves resources. 2. Method Results In this document, the acronym DNSWL is used both to refer to a generic organization that publishes a whitelist, and to indicate the use of such service, that is the method. The organization defines the DNS zone and the record type(s) to be queried, as well as the meaning of the listings and the procedures to maintain them. Unlike other methods, such as dkim or spf, there is little or no work to be carried out by the receiving MTA on the values retrieved from the DNS query. As explained in Section 2.3 of [RFC5782], DNSWLs encode taxonomical details about the mail sender as bit masks of type A records. The receiving MTA just needs to work out whether the data returned is at all applicable. In that case, it uses that data as an outsourced extension of its local policy. Thus, in a broad sense, the result of the method is the content returned by the query: dns.zone: The name of the DNSWL, which defines the meaning of the result. If the MTA uses a private mirror, this can be different from the zone actually queried. policy.ip: The bit mask value. This entry ("propspec" in the grammar of [I-D.ietf-appsawg-rfc5451bis]) can be repeated if the DNSWL returns multiple A records. policy.txt: The TXT record, or the relevant part of it, if any. This can contain a domain name or an abuse reporting address, which can be used as described in Section 5.3 of [RFC6650]. Vesely Expires January 16, 2014 [Page 3] Internet-Draft DNSWL email-auth-method extension July 2013 In a strict sense, the result of the method states how the query did: pass: The query successfully returned applicable records. fail: The query worked, but no applicable record was found. temperror: There was a transient problem with the query. permerror: There is a setup problem, the query cannot work. 3. IANA Considerations There is a registry of Email Authentication Methods created by RFC5451. The method described in this document is referred by Table 1, it has three ptype.Property values detailed in Table 2. [TO BE REMOVED: The registry is currently accessible here: http://www.iana.org/assignments/email-auth/email-auth.xhtml \ #email-auth-methods ] +--------+--------------+---------+ | Method | Defined | version | +--------+--------------+---------+ | dnswl | [this rfc??] | 1 | +--------+--------------+---------+ Table 1: Method name, definition, and version +--------+----------+---------------------------+--------+ | ptype | Property | Value | Status | +--------+----------+---------------------------+--------+ | dns | zone | The origin of the results | active | | policy | ip | type A result | active | | policy | txt | type TXT result | active | +--------+----------+---------------------------+--------+ Table 2: Method values In addition, this method reuses four of the values already defined in the Email Authentication Result Names associated registry. They are listed in Table 3. [TO BE REMOVED: The registry is currently accessible here: http://www.iana.org/assignments/email-auth/email-auth.xhtml \ #email-auth-result-names ] Vesely Expires January 16, 2014 [Page 4] Internet-Draft DNSWL email-auth-method extension July 2013 +-----------+-----------+--------+ | Code | Meaning | Status | +-----------+-----------+--------+ | pass | Section 2 | active | | fail | Section 2 | active | | temperror | Section 2 | active | | permerror | Section 2 | active | +-----------+-----------+--------+ Table 3: Method results 4. Implementation Status [Note to RFC Editor: please remove this entire section before publication.] This section records the status of a known implementation of the method described in this document at the time of writing, based on a proposal described in [I-D.sheffer-running-code]. See that document for further boilerplate. Courier-MTA is a full-featured, mature mail server, first publicly released in May 2000. A beta release in February 2013 introduced Authentication-Results in combination with DNS-based whitelists. This document is based on that implementation, which made it to production release 0.71, after one month testing. End-user documentation of that feature is available online at http://www.courier-mta.org/couriertcpd.html#idp5867072 In prior releases, only the -block option was present, and the Authentication-Results header field was handled by add-ons, not by the core implementation. The -allow option was added so that black and white lists can be configured using mostly symmetrical syntax. Finally, an option was added to inhibit SPF reject-on-fail for whitelisted senders. It is not possible to know how many installations of Courier-MTA have enabled these new features. However, no questions have been asked about them on the mailing list, yet. Despite the amount of spam, there seems to be little traction for this kind of development. The only DNSWL known to have been used for this purpose is dnswl.org. See http://www.dnswl.org/. With nearly 150K entries, it can make the email messages that get at least one authentication "pass" overreach a critical mass: It seems that subscribing to that list is easier, for some mail admins, than implementing other authentication methods. Vesely Expires January 16, 2014 [Page 5] Internet-Draft DNSWL email-auth-method extension July 2013 5. Security Considerations All of the considerations described in Section 8 of [I-D.ietf-appsawg-rfc5451bis] apply. In addition, the usual caveats apply about importing text from external online sources. Although queried DNSWLs are well known, trusted entities, it is suggested that TXT records be reported only if, upon inspection, their content is deemed actually actionable. If they contain non-ASCII characters, they need to be encoded as appropriate. 6. References 6.1. Normative References [I-D.ietf-appsawg-rfc5451bis] Kucherawy, M., "Message Header Field for Indicating Message Authentication Status", draft-ietf-appsawg-rfc5451bis-10 (work in progress), July 2013. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008. 6.2. Informative References [I-D.sheffer-running-code] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: the Implementation Status Section", draft-sheffer-running-code-06 (work in progress), June 2013. [RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. [RFC5518] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009. [RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, February 2010. [RFC6650] Falk, J. and M. Kucherawy, "Creation and Use of Email Feedback Reports: An Applicability Statement for the Abuse Reporting Format (ARF)", RFC 6650, June 2012. Vesely Expires January 16, 2014 [Page 6] Internet-Draft DNSWL email-auth-method extension July 2013 Appendix A. Example Authentication-Results: mta.example.com; dnswl=pass dns.zone=list.dnswl.example policy.ip=127.0.10.1 policy.txt="example.org http://dnswl.example/s?s=100" Author's Address Alessandro Vesely v. L. Anelli 13 Milano, MI 20122 IT Email: vesely@tana.it Vesely Expires January 16, 2014 [Page 7]