This is a DKIM signing and verification milter.  In theory it works with both
Postfix and Sendmail, but the author has zero experience with Sendmail, so
reports of success/failure with Sendmail and patches are welcom.

The configuration file is designed to be compatible with OpenDKIM, but only
a subset of OpenDKIM options are supported.  If an unsupported option is
specified, an error will be raised.

This package includes a default configuration file and man pages.  For those
to be installed when installing using setup.py, the following incantation is
required because setuptools developers decided not being able to do this by
default is a feature:

python setup.py install --single-version-externally-managed --record=/dev/null

For users of Debian Stable (Debian 9, Codename Squueze), all dependencies are
available in either the main or backports repositories:

[sudo] apt install python-milter python-nacl python-ipaddress python-dnspython
[sudo] apt install -t stretch-backports python-authres python-dkim

The preferred method of installation is from PyPi using pip (if distribution
packages are not available):

[sudo] pip install dkimpy_milter

Using pip will cause required packages to be installed via easy_install if they
have not been previously installed.

The milter will work with either pydns (DNS) or dnspython (dns), preferring
dnspython is both are available.  The dkimpy DKIM module also works with
either.

Both a systemd unit file and a sysv init file are provided.  Both make
assumptions about defaults being used, e.g. if a non-standard pidfile name is
used, they will need to be updated.  The sysv init file is Debian specific and
untested, since the developers are not using sysv init.  Feedback/patches
welcome.

The dkimpy-milter drops priviledges after setup to the user/group specified in
UserID.  During initial setup, this system user needs to be manually created.
As an example, using the default dkimpy-user on Debian, the command would be:

[sudo] adduser --system --no-create-home --quiet --disabled-password \
               --disabled-login --shell /bin/false --group \
               --home /var/run/dkimpy-milter dkimpy-milter

Since /var/run or /run is sometimes on a tempfs, if the PID file directory is
missing, the milter will create it on startup.

To start dkimpy-milter with systemd for the first time, you will need to take
the following steps:

[sudo] systemctl daemon-reload
[sudo] systemctl enable dkimpy-milter
[sudo] systemctl start dkimpy-milter
[sudo] systemctl status dkimpy-milter (to verify it started correctly)

As with all milters, dkimpy-milter needs to be integrated with your MTA of
choice (Sendmail or Postfix).

For Sendmail:

Configuration is very similar to opendkim, but needs some adjustment for
dkimpy-milter. Here's an example configuration line to include in your
sendmail.mc:

INPUT_MAIL_FILTER(`dkimpy-milter', `S=local:/var/run/dkimpy-milter/dkimpy-milter.sock')dnl

Changing the sendmail.mc file requires a Make (to compile it into sendmail.cf)
and a restart of sendmail.  Note that S= needs to match the value of Socket in
the dkimpy-milter configuration file.

Milter support should be present by default in most versions of sendmail
these days, but if not included in your Sendmail build, see:
http://www.elandsys.com/resources/sendmail/milter.html

For Postfix:

Integration of dkimpy-milter into Postfix is like any milter (See Postfix's
README_FILES/MILTER_README).  Here's an example master.cf excerpt the talks to
two dkimpy-milter instances, one configured for signing and one configured for
verification:

smtp       inet  n       -       -       -      -       smtpd
    ...
    -o smtpd_milters=inet:localhost:8892
    ...

submission inet  n       -       -       -      -       smtpd
    ...
    -o smtpd_milters=inet:localhost:8891
    ...

These need to match the Socket value for each dkimpy-milter instance.

Care is required to segregate outbound mail to be signed and inbound mail to
be verified.  The above example uses two instances of dkimpy-milter to do
this.  There are many possible ways.  Here is another example using milter
macros to keep the mail streams segregated:

Postfix master.cf:

smtp       inet  n       -       -       -       -       smtpd
    ...
    -o smtpd_milters=inet:localhost:8891
    -o milter_macro_daemon_name=VERIFYING
    ...

submission inet n       -       -       -       -       smtpd
    -o syslog_name=postfix/submission
    -o smtpd_tls_security_level=encrypt
    -o smtpd_sasl_auth_enable=yes
    ...
    -o milter_macro_daemon_name=ORIGINATING
    -o smtpd_milters=inet:localhost:8891
    ...

Dkimpy-milter.conf:

...
Mode			sv
MacroList		dameon_name|ORIGINATING
MacroListVerify		daemon_name|VERIFYING
...


The python DKIM library, dkimpy, requires the entire message being signed or
verified to be in memory, so dkimpy-milter does not write messages out to a temp
file.  This may impact performance on low-memory systems.

This is an beta grade release to support interoperability testing with Ed25519
signatures sufficient functionality for basic use.  The documented
functionality has been implemented and at least partially tested.  It is free
of known major defects, but is not fully tested in a variety of environments.
