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dkimpyy-milter(8) System Manager's Manual dkimpyy-milter(8)


dkimpy - DKIM signing and verifying filter for MTAs


dkimpy-milter [configfile]


dkimpy-milter implements the DKIM standard for signing and verifying e-mail messages on a per-domain basis.

dkimpy-milter uses the milter interface, originally distributed as part of version 8.11 of sendmail(8), to provide DKIM signing and/or verifying service for mail transiting a milter-aware MTA.


Many of the configuration file parameters will refer to a "dataset" as their values. This refers to a string that either contains the list of desirable values, or to a file that contains them, or a database containing the data.

Some data sets require that the value contain more than one entry. How this is done depends on which data set type is used. Not all these datasets are currently used by dkimpy-milter. See dkimpy-milter.conf(5) for details about specific options and which dataset types they use.

In particular:

If the string begins with "file:", then the remainder of the string is presumed to refer to a flat file that contains elements of the data set, one per line. If a line contains whitespace-separated values, then the line is presumed to define a key and its corresponding value. Blank lines are ignored, and the hash ("#") character denotes the start of a comment. If a value contains multiple entries, the entries should be separated by colons.
If the string begins with "db:" and the program was compiled with Sleepycat DB support, then the remainder of the string is presumed to identify a Sleepycat database containing keys and corresponding values. These may be used only to test for membership in the data set, or for storing keys and corresponding values. If a value contains multiple entries, the entries should be separated by colons. [Not implemented yet]
If the string contains none of these prefixes but ends with ".db", it is presumed to be a Sleepycat DB as described above (if support for same is compiled in). [Not implemented yet]
If the string contains none of these prefixes but starts with a slash ("/") character, it is presumed to be a flat file as described above.
If the string begins with "csl:", the string is treated as a comma-separated list as described in m) below.
If the string begins with "mdb:", it refers to a directory that contains a memory database, as provided by libmdb from OpenLDAP. [Not implemented yet]
In any other case, the string is presumed to be a comma-separated list. Elements in the list are either simple data elements that are part of the set or, in the case of an entry of the form "x=y", are stored as key-value pairs as described above.


dkimpy-milter.conf(5) information about available options. Unlike OpenDKIM, dkimpy-milter does not support command line option switches.

When signing a message, a DKIM-Signature: header will be prepended to the message. The signature is computed using the private key provided. You must be running a version of sendmail(8) recent enough to be able to do header prepend operations (8.13.0 or later).

When verifying a message, an Authentication-Results: header will be prepended to indicate the presence of a signature and whether or not it could be validated against the body of the message using the public key advertised by the sender's nameserver. The value of this header can be used by mail user agents to sort or discard messages that were not signed or could not be verified.


When the filter is started as the superuser and the UserID setting is used, the filter gives up its root privileges by changing to the specified user after the following steps are taken: (1) the configuration file (if any) is loaded; (2) if the KeyFile or KeyFileEd25519 settings are used, the keys are loaded into memory; (3) all data sets in the configuration file are opened, and those that are based on flat files are also read into memory; and (4) if ChangeRootDirectory is set, the process root is changed to that directory. This means on configuration reload, the filter will not be accessing these files or the configuration file as the superuser (and possibly from a different root), and any key files referenced by the KeyTable will also be accessed by the new user.

Thus, keys referenced by the KeyTable must always be accessible for read by the unprivileged user. Also, run-time reloads are not possible if any of the other files will not be readable by the unprivileged user.


The following environment variable(s) can be used to adjust the behaviour of this filter:
The directory to use when creating temporary files. The default is /tmp.


When using DNS timeouts be sure not to use a timeout that is larger than the timeout being used for interaction between sendmail and the filter. Otherwise, the MTA could abort a message while waiting for a reply from the filter, which in turn is still waiting for a DNS reply.

Features that involve specification of IPv4 addresses or CIDR blocks will use the inet_addr(3) function to parse that information. Users should be familiar with the way that function handles the non-trivial cases (for example, "192.0.2/24" and "" are not the same thing).


Filter exit status codes are selected according to sysexits(3).


DKIM is an amalgam of Yahoo!'s DomainKeys proposal, and Cisco's Internet Identified Mail (IIM) proposal.


This man page covers version 0.9.4 of dkimpy-milter.


Copyright (c) 2005-2008, Sendmail, Inc. and its suppliers. All rights reserved.

Copyright (c) 2009-2013, 2015, The Trusted Domain Project. All rights reserved.

Copyright (c) 2018 Scott Kitterman <>


dkimpy-milter.conf(5), sendmail(8)

Sendmail Operations Guide

RFC5321 - Simple Mail Transfer Protocol

RFC5322 - Internet Messages

RFC6376 - DomainKeys Identified Mail

RFC7601 - Message Header Field for Indicating Message Authentication Status

draft-ietf-dcrup-dkim-crypto - A new cryptographic signature method for DKIM