IETF A. Vesely Internet-Draft August 1, 2013 Intended status: Informational Expires: February 2, 2014 DNSxL Email Authentication Method Extension draft-vesely-authmethod-dnswl-01 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 white or black list, and interpreting any returned data. 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 February 2, 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 February 2, 2014 [Page 1] Internet-Draft DNSxL email-auth-method extension August 2013 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Method Results . . . . . . . . . . . . . . . . . . . . . . . . 3 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4 4. Implementation Status . . . . . . . . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6.1. Normative References . . . . . . . . . . . . . . . . . . . 7 6.2. Informative References . . . . . . . . . . . . . . . . . . 7 Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Vesely Expires February 2, 2014 [Page 2] Internet-Draft DNSxL email-auth-method extension August 2013 1. Introduction One of the many checks that mail servers carry out is to query DNS white and black lists (DNSxL, [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. The semantics of DNSBL often implies blocking any attempt to send mail, or even outright blocking any routing or peering from listed addresses. However, in some cases mail is accepted despite a positive blacklist lookup. In such cases, it is useful to record the result of the lookup, which can be done symmetrically, using the method described in this document. 2. Method Results In this document, the acronyms DNSWL and DNSBL are used both to refer to a generic organization that publishes a DNS list, and to indicate the list itself. 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. DNSxL is used to indicate either or both those two, while dnsxl (lowercase) is the name of the method being defined here. As explained in "Combined IP Address DNSxL" (Section 2.3 of [RFC5782]), DNSxLs encode taxonomical details about the mail sender as bit masks of type A records. The receiving MTA needs to determine whether the data returned is applicable. In that case, it uses a uniformized rendering of that data as an outsourced extension of its local policy. The properties, uniform across DNSxLs, that this document provides for are listed below: Vesely Expires February 2, 2014 [Page 3] Internet-Draft DNSxL email-auth-method extension August 2013 policy.dnszone: The name of the DNSxL, where the result is obtained from. policy.score: This is a number in the range [-100, 100]. Zero (0.0) is for neutral, non-listed senders. Positive values are for black lists. Negative values are for white lists, with meaning roughly defined as: -0.1 only avoid outright blocking (e.g. free mail providers), -1 reduce chance of false positives, -10 make sure to avoid false positives but allow override for clear cases, -100 highly trusted sender, avoid override. policy.contact: This is either a domain name or an abuse reporting address, which can be used as described in "Where to Send Reports" (Section 5.3 of [RFC6650]). The result proper of the dnsxl method is defined as follows: pass: A query to a DNSWL completed, and a reply was returned, meaning the IP address is whitelisted. fail: A query to a DNSBL completed, and a reply was returned, meaning the IP is blacklisted. neutral: A query to a DNSBL completed, and a reply containing no answers was returned, meaning the IP is not listed. temperror: The DNS evaluation could not be completed due to some error that is likely transient in nature, such as a temporary DNS error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or other error condition resulted. A later attempt might produce a final result. permerror: The DNS evaluation could not be completed because of some kind of misconfiguration, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN. A later attempt is unlikely to produce a final result. 3. IANA Considerations There is a registry of Email Authentication Methods created by RFC5451. The method described in this document is referred by Vesely Expires February 2, 2014 [Page 4] Internet-Draft DNSxL email-auth-method extension August 2013 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 | +--------+------------+---------+ | dnsxl | [this rfc] | 1 | +--------+------------+---------+ Table 1: Method name, definition, and version +--------+----------+---------------------------+--------+ | ptype | Property | Value | Status | +--------+----------+---------------------------+--------+ | policy | dnszone | The origin of the results | active | | policy | score | sender trustworthiness | active | | policy | contact | abuse reporting | active | +--------+----------+---------------------------+--------+ Table 2: Method values In addition, this method reuses five 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 ] +-----------+-----------+--------+ | Code | Meaning | Status | +-----------+-----------+--------+ | pass | Section 2 | active | | fail | Section 2 | active | | neutral | Section 2 | active | | temperror | Section 2 | active | | permerror | Section 2 | active | +-----------+-----------+--------+ Table 3: Method results Finally, if at all possible, this document reserves the name dnswl, as detailed in Table 4. Vesely Expires February 2, 2014 [Page 5] Internet-Draft DNSxL email-auth-method extension August 2013 +--------+------------+---------+----------+ | Method | Defined | version | Status | +--------+------------+---------+----------+ | dnswl | [this rfc] | 1 | reserved | +--------+------------+---------+----------+ Table 4: Reserved method name 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 "Improving Awareness of Running Code: The Implementation Status Section" ([RFC6982]). See that document for further boilerplate that should have been copied here. OpenDKIM has optional DNSxL query support, and plans to implement this. 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. It made it to production in release 0.71, after one month testing, using the reserved method name. 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 February 2, 2014 [Page 6] Internet-Draft DNSxL email-auth-method extension August 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. 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 [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. [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, July 2013. Vesely Expires February 2, 2014 [Page 7] Internet-Draft DNSxL email-auth-method extension August 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 February 2, 2014 [Page 8]