Introduction

DomainKeys Identified Mail (DKIM) is a method for E-mail authentication, allowing a person who receives email to verify that the message actually comes from the domain that it claims to have come from. The need for this type of authentication arises because spam often has forged headers.

DKIM uses public-key cryptography to allow the sender to electronically sign legitimate emails in a way that can be verified by recipients.

DKIM also guards against tampering with mail, offering almost end-to-end integrity from a signing to a verifying Mail transfer agent (MTA).

Read more on Wikipedia

dkim-milter is a milter-based application (dkim-filter) which plugs in to Postfix to provide DomainKeys Identified Mail service for your. NOTE: Starting with Ubuntu 10.04, dkim-filter is deprecated. Use the opendkim package instead. Configuration is generally similar to dkim-filter, but it is still actively maintained.

Installation

We assume you already successfully installed Postfix MTA, if not, please read the Postfix dedicated page.

To install dkim-milter, you need Universe repositories added, if so, use your favorite package manager and install the package. For example:

sudo aptitude install dkim-milter

Simply accept the defaults when the installation process asks questions. The configuration will be done in greater detail in the next stage.

Configuration

dkim-milter configuration consists of two files:

/etc/dkim-filter.conf
/etc/default/dkim-filter

Use your favorite editor to edit those files. Here's an example of /etc/dkim-filter.conf file already edited to suit my needs:

# Log to syslog
Syslog                  yes
# Required to use local socket with MTAs that access the socket as a non-
# privileged user (e.g. Postfix)
UMask                   105 # 'id postfix' in your sheel

# Sign for example.com with key in /etc/mail/dkim.key using
# selector '2007' (e.g. 2007._domainkey.example.com)
Domain                  ubuntu.ro
KeyFile                 /etc/mail/dkim.key # See bellow how to generate and set up the key
Selector                mail

# Common settings. See dkim-filter.conf(5) for more information.
AutoRestart             yes
Background              yes
Canonicalization        simple
DNSTimeout              5
Mode                    sv
SignatureAlgorithm      rsa-sha256
SubDomains              no
#UseASPDiscard          no
#Version                rfc4871
X-Header                no

###############################################
# Other (less-standard) configuration options #
###############################################
# 
# If enabled, log verification stats here
Statistics              /var/log/dkim-filter/dkim-stats
#
# KeyList is a file containing tuples of key information. Requires
# KeyFile to be unset. Each line of the file should be of the format:
#    sender glob:signing domain:signing key file
# Blank lines and lines beginning with # are ignored. Selector will be
# derived from the key's filename.
#KeyList                /etc/dkim-keys.conf
#
# If enabled, will generate verification failure reports for any messages
# that fail signature verification. These will be sent to the r= address
# in the policy record, if any.
#ReportInfo             yes
#
# If enabled, will issue a Sendmail QUARANTINE for any messages that fail
# signature verification, allowing them to be inspected later.
#Quarantine             yes
#
# If enabled, will check for required headers when processing messages.
# At a minimum, that means From: and Date: will be required. Messages not
# containing the required headers will not be signed or verified, but will
# be passed through
#RequiredHeaders        yes

Actually /etc/dkim-filter.conf is the most important file. It provides our milter with required information about selector (used for DNS requests and email verifications) and used signing key (the key is used for signing the outgoing emails).

Here's an example of /etc/defaults/dkim-filter. This file is used to literally connect the milter to MTA:

# Command-line options specified here will override the contents of
# /etc/dkim-filter.conf. See dkim-filter(8) for a complete list of options.
#DAEMON_OPTS=""
#
# Uncomment to specify an alternate socket
# Note that setting this will override any Socket value in dkim-filter.conf
#SOCKET="local:/var/run/dkim-filter/dkim-filter.sock" # Debian default
#SOCKET="inet:54321" # listen on all interfaces on port 54321
SOCKET="inet:8891@localhost" # Ubuntu default - listen on loopback on port 8891
#SOCKET="inet:12345@192.0.2.1" # listen on 192.0.2.1 on port 12345

In my case, this file needs no additional editing.

Now, to tell the Postfix about the existing milter, and where to connect with it, edit your Postfix main.cf file /etc/postfix/main.cf, and append to its content the following data:

# DKIM
milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891
non_smtpd_milters = inet:localhost:8891

If you are using already some milter (for example Postfix/DomainKeys), you can add the new one like this:

milter_default_action = accept
milter_protocol = 2
smtpd_milters = inet:localhost:8891,inet:localhost:8892
non_smtpd_milters = inet:localhost:8891,inet:localhost:8892

Key generation for dkim-milter and its setup with DNS

Actually it's not big deal. Generate an OpenSSL RSA key as you do it always, then move it private part to the location you indicated in your /etc/dkim-filter.conf:

openssl genrsa -out private.key 1024
openssl rsa -in private.key -out public.key -pubout -outform PEM
cp private.key /etc/mail/dkim.key

The DNS record should look like this:

mail._domainkey.ubuntu.ro. IN TXT "k=rsa; t=y; p=PpYHdE2tevfEpvL1Tk2dDYv0pF28/f 5MxU83x/0bsn4R4p7waPaz1IbOGs/6bm5QIDAQAB"

Where everything after p= is actually the content of the public key we generated above, public.key. To use it, strip out the comments inside it, this:

-----BEGIN PUBLIC KEY-----

and this:

-----END PUBLIC KEY-------

Startup and testing

Once configuration above was done, the daemon can be started with:

sudo /etc/init.d/dkim-filter start

If it doesn't start, search the logs for problems and see what it requires more:

grep -i dkim /var/log/mail.log

Also you can test your config file:

dkim-filter -x /etc/dkim-filter.conf

If you get the error like: dkim-filter: milter socket must be specified Then try manually specifing the socket, in our case it is local:

dkim-filter -x /etc/dkim-filter.conf -p local

Now restart the Postfix MTA, and check for email signing:

/etc/init.d/postfix restart

For testing purposes, I recommend you tools like:

Testing results should look like this in Gmail: http://stas.nerd.ro/blog/data/dkim-filter.png

Postfix/dkim-milter (last edited 2010-04-22 09:17:57 by spacelem)