kilter.protocol 0.6.0

Parsers and state machines for the Sendmail milter communications protocol

Kilter Protocol

Kilter is a framework for writing mail filters (known as "milters") compatible with Sendmail and Postfix MTAs. Unlike many previous milter implementations in Python it is not simply bindings to the libmilter library (originally from the Sendmail project). The framework aims to provide Pythonic interfaces for implementing filters, including leveraging coroutines instead of libmilter's callback-style interface.

The kilter.protocol package contains the parsers and filter state machine for the communications protocol used between the Mail Transfer Agents (MTA) and filters.

Users looking for something as simple to use as libmilter should take a look at kilter.service.

What is understood about the wire protocol is documented in Wire Protocol.

Sendmail Filters

The Sendmail filter (milter) API facilitates communication between a Mail Transfer Agent (MTA) and arbitrary filters running as external services. These filters can perform a number of operations on received and outgoing mail, such as: virus scanning; checking senders' reputations; signing outgoing mail; and verifying signatures of incoming mail.

While the protocol was originally for filtering mail through a Sendmail MTA, Postfix has also reverse engineered the protocol and supports most filters made for Sendmail.

libmilter

Historically filters used the libmilter library supplied by the Sendmail project to handle all aspects of communication with an MTA. Filters simply registered callbacks for various events then started the library's main loop. This approach makes implementing simple filters in C easy for users, but makes writing "Pythonic" filters difficult, especially when a user wishes to make use of async/await features.

Use of libmilter to implement filters is almost universal as it is a black-box; the on-the-wire protocol used is undocumented and subject to change between versions, which makes writing a third-party parser difficult.

Usage

Most users will be looking for an asynchronous API using Python coroutines: this is available with the kilter.service package. The protocol handlers provided by this package support asynchronous operation but does not handle any IO, thus have no awaitable entrypoints. Instead a service that handles socket communication with MTAs passes bytes from a socket to a protocol handler, which returns event objects. Some of these events instruct the service to pass bytes back to the socket, others are actionable in ways that are implementation specific. When an implementation wants to communicate with an MTA, it registers messages with the protocol handler, which again returns event objects.