OVERVIEW
========
This is a DKIM signing and verification milter. It has been tested with both
Postfix and Sendmail.
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.
INSTALLATION
===========
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:
python3 setup.py install --single-version-externally-managed --record=/dev/null
For users of Debian Stable (Debian 9, Codename Squeeze), all dependencies are
available in either the main or backports repositories:
[sudo] apt install python3-milter python3-nacl python3-dnspython
[sudo] apt install -t stretch-backports python3-authres python3-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. Because pymilter and PyNaCl are compiled
Python extensions, the system will need appropriate development packages and
an C compiler. Alternately, install these dependencies from distribution/OS
packages and then pip install dkimpy_milter.
The milter will work with either py3dns (DNS) or dnspython (dns), preferring
dnspython if both are available. The dkimpy DKIM module also works with
either.
SETUP
====
SIGNING KEYS
============
In order to create DKIM signatures, a private key must be available. Signing
keys should be protected (owned by root:root with permissions 600 in a
directory that is not world readable). Different keys are required for RSA
and (if used) Ed25519.
RSA
===
Both public and private keys for RSA have standard formats and there are many
tools available to create them. Keys must (RFC 8302) have a minimum size of
1024 bits and should have a size of at least 2048 bits. The dknewkey script
that is provided with dkimpy is one such tool:
dknewkey exampleprivkey
will produce both the private key file (.key suffix) and a file with the DKIM
public key record to be published DNS (.dns suffix). RSA is the default key
type. 2048 bits is the default key size.
ED25519
=======
There is no standardized non-binary representation for Ed25519 private keys,
so in order to generate Ed25519 keys for dkimpy-milter, dkimpy specific tools
must be used to be compatible. The same dknewkey script support Ed25519:
dknewkey --ktype ed25519 anothernewkey
will provide both the private key file (.key suffix) and a file with the DKIM
public key record to be published DNS (.dns suffix). Ed25519 keys do not have
variable bit lengths.
MTA INTEGRATION
==============
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 uses start-stop-deamon
from Debian. It is not portable to systems without that available.
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 /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).
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:/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
ISSUES USING SENDMAIL TO SIGN AND VERIFY
========================================
When using the sendmail MTA in both signing and verifying mode, there are
a few issues of which to be aware that might cause operational problems
and deserve consideration.
(a) When the MTA will be used for relaying emails, e.g. delivering to other
hosts using the aliases mechanism, it is important not to break
signatures inserted by the original sender. This is particularly sensitive
particular when the sending domain has published a "reject" DMARC policy.
By default, sendmail quotes to address header fields when there are no
quotes and the display part of the address contains a period or an
apostrophe. However, opendkim only sees the raw, unmodified form of
the header field, and so the content that gets verified and what gets
signed will not be the same, guaranteeing the attached signature is not
valid.
To direct sendmail not to modify the headers, add this to your sendmail.mc:
conf(`confMUST_QUOTE_CHARS', `')
(b) As stated in sendmail's KNOWNBUGS file, sendmail truncates header field
values longer than 256 characters, which could mean truncating the domain
of a long From: header field value and invalidating the signature.
You may wish to consider increasing MAXNAME in sendmail/conf.h to mitigate
changing the messages and invalidating their signatures. This change
requires recompiling sendmail.
(c) Similar to (a) above, sendmail may wrap very long single-line recipient
fields for presentation purposes; for example:
To: very long name <a@example.org>,anotherloo...ong name b <b@example.org>
...might be rewritten as:
To: very long name <a@example.org>,
anotherloo...ong name b <b@example.org>
This rewrite is also done after opendkim has seen the message, meaning
the signature opendkim attaches to the message does not match the
content it signed. There is not a known configuration change to
mitigate this mutation.
The only known mechanism for dealing with this is to have distinct
instances of opendkim do the verifying (inbound) and signing (outbound)
so that the version that arrives at the signing instance is already
in the rewritten form, guaranteeing the input and output are the same
and thus the signature matches the payload.
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 that 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
...
NOTES
=====
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.
DKIM with Ed25519 signatures are described in RFC 8463. Version 1.0.0 and
later support Ed25519 signing and verification. RFC 8301 removed rsa-sha1
from DKIM. dkimpy-milter does not sign with rsa-sha1, but still considers
rsa-sha1 signatures as valid for verification because they are still in
common use and are not known to be cryptographically broken.