ABOUT BATV-TOOLS

batv-tools implements Bounce Address Tag Validation, a system for filtering backscatter. Briefly, BATV works as follows:

1. The envelope sender of all outgoing mail is rewritten ("signed") to contain a cryptographic token. If the message bounces, it is bounced to this address.

2. The envelope recipient of all incoming bounces is checked for a valid token. If a bounce has a missing or invalid token, it's obviously not in reply to a real message and can be filtered.

BATV is described by an Internet Draft which was last revised in

2008. batv-tools implements the draft standard, but also supports an alternative address meta-syntax based on sub-addressing, described below.

batv-tools provides both a milter (batv-milter) and a collection of standalone tools (batv-sign, batv-validate, batv-sendmail) that do signing and validation. The standalone tools enable individual users to use BATV without the involvement of their system administrators.

HOW BATV-TOOLS WORKS

Outgoing mail is passed through either batv-milter or batv-sign. If a BATV key exists for the envelope sender, the envelope sender is rewritten to a signed BATV address.

Incoming mail is passed through either batv-milter or batv-validate. The envelope recipient is validated and the result of the validation is placed in the X-Batv-Status header. If the envelope recipient contains a valid BATV signature, it is rewritten to its original non-BATV form.

batv-milter can be configured to reject bounces which aren't addressed to a valid BATV signature, but by default, it is the user's responsibility to filter mail based on the X-Batv-Status header.

BUILDING BATV-TOOLS

Run 'make'. To build only the standalone tools (and not the milter), run 'make all-tools'.

GETTING UP AND RUNNING

See doc/quickstart.milter.txt and doc/quickstart.standalone.txt for instructions on getting up-and-running quickly.

DEPENDENCIES

To use the milter, you need:

• libmilter, from Sendmail 8.14.0 or higher
• Postfix 2.6 or higher, Sendmail 8.14.0 or higher, or a MTA with equivalent milter functionality

To build you need a C++ compiler (such as gcc) and development headers for libmilter.

CURRENT STATUS

As of 2021, batv-tools is no longer maintained. The code does not contain any known bugs and worked reliably for many years. However, the author no longer uses it and provides no support for it.

SUB-ADDRESS META-SYNTAX

batv-tools supports an alternative (and highly-recommended) address meta-syntax based on sub-addressing (aka "plus" addressing). Instead of rewriting the entire local part of the email address, the BATV token is appended to the local part after a sub-address delimiter (typically '+' or '-'). For example:

    andrew+prvs=9352e50792@example.com

Since many mail systems support sub-addressing, this syntax permits BATV-signed mail to be routed through mail servers that have no knowledge of BATV. This has a couple advantages:

1. Backup MX servers which accept mail only for known recipients need not support BATV. As long as they support sub-addressing, mail destined to BATV addresses will be accepted. (In contrast, the standard meta-syntax appears to the backup MX server as an unknown recipient, meaning the backup MX server must either accept all mail regardless of recipient (a bad idea), or support BATV (not always feasible).)

2. It lets individual users use BATV with support from their MUA and/or MDA, without the MTA needing to support BATV.

The standalone BATV tools support only the non-standard sub-address syntax (otherwise MTA support would be required). The milter uses the standard syntax by default but may be configured to use sub-address syntax. It is highly recommended that you use the sub-address syntax; batv-tools is most extensively tested with the sub-address syntax, and using the standard syntax may require additional configuration from your MTA that isn't documented here.

DEBIAN PACKAGING

Debian packaging, complete with an init script, can be found in the 'debian' branch of the project Git repository. The package is built using git-buildpackage as follows:

1. Check out the debian branch: git checkout debian
2. Build the package: git-buildpackage -uc -us

FURTHER DOCUMENTATION AND EXAMPLES

See files in the 'doc' sub-directory.

LEGALESE

Copyright (C) 2013 Andrew Ayer

Licensed predominantly under the GPLv3 with OpenSSL Linking Exception. Also contains code licensed under more permissive GPL-compatible licenses.

batv-tools is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

See COPYING for details.