The Sendmail Content Management API (Milter) provides an interface for third-party software to validate and modify messages as they pass through the mail transport system. Filters can process messages' connection (IP) information, envelope protocol elements, message headers, and/or message body contents, and modify a message's recipients, headers, and body. The MTA configuration file specifies which filters are to be applied, and in what order, allowing an administrator to combine multiple independently-developed filters.

We expect to see both vendor-supplied, configurable mail filtering applications and a multiplicity of script-like filters designed by and for MTA administrators. A certain degree of coding sophistication and domain knowledge on the part of the filter provider is assumed. This allows filters to exercise fine-grained control at the SMTP level. However, as will be seen in the example, many filtering applications can be written with relatively little protocol knowledge.

Given these expectations, the API is designed to achieve the following goals:

  1. Safety/security. Filter processes should not need to run as root (of course, they can if required, but that is a local issue); this will simplify coding and limit the impact of security flaws in the filter program.

  2. Reliability. Coding failures in a Milter process that cause that process to hang or core-dump should not stop mail delivery. Faced with such a failure, sendmail should use a default mechanism, either behaving as if the filter were not present or as if a required resource were unavailable. The latter failure mode will generally have sendmail return a 4xx SMTP code (although in later phases of the SMTP protocol it may cause the mail to be queued for later processing).

  3. Simplicity. The API should make implementation of a new filter no more difficult than absolutely necessary. Subgoals include:

  4. Performance. Simple filters should not seriously impact overall MTA performance.

Implementing Filtering Policies

Milter is designed to allow a server administrator to combine third-party filters to implement a desired mail filtering policy. For example, if a site wished to scan incoming mail for viruses on several platforms, eliminate unsolicited commercial email, and append a mandated footer to selected incoming messages, the administrator could configure the MTA to filter messages first through a server based anti-virus engine, then via a large-scale spam-catching service, and finally append the desired footer if the message still met requisite criteria. Any of these filters could be added or changed independently.

Thus the site administrator, not the filter writer, controls the overall mail filtering environment. In particular, he/she must decide which filters are run, in what order they are run, and how they communicate with the MTA. These parameters, as well as the actions to be taken if a filter becomes unavailable, are selectable during MTA configuration. Further details are available later in this document.

MTA - Filter communication

Filters run as separate processes, outside of the sendmail address space. The benefits of this are threefold:
  1. The filter need not run with "root" permissions, thereby avoiding a large family of potential security problems.
  2. Failures in a particular filter will not affect the MTA or other filters.
  3. The filter can potentially have higher performance because of the parallelism inherent in multiple processes.

Each filter may communicate with multiple MTAs at the same time over local or remote connections, using multiple threads of execution. Figure 1 illustrates a possible network of communication channels between a site's filters, its MTAs, and other MTAs on the network:

Figure 1: A set of MTA's interacting with a set of filters.

The Milter library (libmilter) implements the communication protocol. It accepts connections from various MTAs, passes the relevant data to the filter through callbacks, then makes appropriate responses based on return codes. A filter may also send data to the MTA as a result of library calls. Figure 2 shows a single filter process processing messages from two MTAs:

Figure 2: A filter handling simultaneous requests from two MTA's.

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