IETF A. Vesely
Internet-Draft August 01, 2013
Intended status: Informational
Expires: February 02, 2014

DNSxL Email Authentication Method Extension


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.

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This Internet-Draft will expire on February 02, 2014.

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Table of Contents

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:

The name of the DNSxL, where the result is obtained from.
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:
only avoid outright blocking (e.g. free mail providers),
reduce chance of false positives,
make sure to avoid false positives but allow override for clear cases,
highly trusted sender, avoid override.
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:

A query to a DNSWL completed, and a reply was returned, meaning the IP address is whitelisted.
A query to a DNSBL completed, and a reply was returned, meaning the IP is blacklisted.
A query to a DNSBL completed, and a reply containing no answers was returned, meaning the IP is not listed.
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.
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 Table 1, it has three ptype.Property values detailed in Table 2.

[TO BE REMOVED: The registry is currently accessible here: \
                                                #email-auth-methods ]

Method name, definition, and version
Method Defined version
dnsxl [this rfc] 1
Method values
ptype Property Value Status
policy dnszone The origin of the results active
policy score sender trustworthiness active
policy contact abuse reporting active

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: \
                                           #email-auth-result-names ]

Method results
Code Meaning Status
pass Section 2 active
fail Section 2 active
neutral Section 2 active
temperror Section 2 active
permerror Section 2 active

Finally, if at all possible, this document reserves the name dnswl, as detailed in Table 4.

Reserved method name
Method Defined version Status
dnswl [this rfc] 1 reserved

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

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 See 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.

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", Internet-Draft draft-ietf-appsawg-rfc5451bis-10, 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

[RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, July 2013.
[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.
[RFC5782] Levine, J., "DNS Blacklists and Whitelists", RFC 5782, February 2010.
[RFC5518] Hoffman, P., Levine, J. and A. Hathcock, "Vouch By Reference", RFC 5518, April 2009.
[RFC4408] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006.

Appendix A. Example

       policy.txt=" http://dnswl.example/s?s=100"

Author's Address

Alessandro Vesely v. L. Anelli 13 Milano, MI 20122 IT EMail: