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.

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

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Table of Contents

• 1. Introduction
• 2. Method Results
• 3. IANA Considerations
• 4. Implementation Status
• 5. Security Considerations
• 6. References
• 6.1. Normative References
• 6.2. Informative References
• Appendix A. Example
• Author's Address

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:

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 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 ]
      

[TO BE REMOVED: The registry is currently accessible here:
http://www.iana.org/assignments/email-auth/email-auth.xhtml \
                                           #email-auth-result-names ]
      

   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"