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DomainKeys Identified Mail (DKIM)

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DomainKeys Identified Mail (DKIM) lets an organization take responsibility for a message while it is in transit. The organization is a handler of the message, either as its originator or as an intermediary. Their reputation is the basis for evaluating whether to trust the message for delivery. Technically DKIM provides a method for validating a domain name identity that is associated with a message through cryptographic authentication.

Contents

Installing DKIM-Milter with Postfix

FedoraLogo infinity.png Note: This Page was written with Fedora 10 in mind, and may not have been tested on any other versions.

Building Sendmail's libmailter From Source

First we need to download Sendmail so we can build against it's libraries. 

cd /var/src
wget ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.14.3.tar.gz
tar -xzf sendmail.8.14.3.tar.gz
cd sendmail-8.14.3/libmilter/

Now lets compile the libmailter 

make
echo $?

And install it 

make install
echo $?

Building DKIM-Milter From Source 

ln -s /usr/share/man /usr/man
cd /var/src
wget http://superb-east.dl.sourceforge.net/sourceforge/dkim-milter/dkim-milter-2.8.1.tar.gz
tar -xzf dkim-milter-2.8.1.tar.gz
cd dkim-milter-2.8.1

We can now compile 

make
echo $?

Installing DKIM-Milter 

make install
echo $?

cd dkim-filter/
cp -f dkim-genkey.sh /usr/local/bin/dkim-genkey.sh

If this is the first time you are installing DKIM-Milter run the following: 

mkdir /etc/postfix/dkim-milter
chown postfix:postfix /etc/postfix/dkim-milter

You also need to install an init script, mine is explained below. 

wget http://wiki.mattrude.com/images/4/43/Dkim-milter.sh
mv Dkim-milter.sh /etc/init.d/dkim-milter
chmod 755 /etc/init.d/dkim-milter
chkconfig dkim-milter on
service dkim-milter start

Building the DKIM Keys

To build a key, one for each domain. From the source directory from above, go into the dkim-filter folder: 

cd dkim-filter/

And Run dkim-genkey.sh to create the key as below. 

dkim-genkey.sh -d example1.com
mv default.private /etc/dkim-milter/example1.com_default.key.pem
mv default.txt /etc/dkim-milter/example1.com.txt

Now add the domain and the key file to the /etc/init.d/dkim-milter. To have multiple domains setup your init file as below: 

SIGNING_DOMAIN="example1.com,example2.com,example3.com"
KEYFILE="/etc/dkim-milter/example1.com_default.key.pem"
KEYFILE="/etc/dkim-milter/example2.com_default.key.pem"
KEYFILE="/etc/dkim-milter/example3.com_default.key.pem"

DNS Zone Setup

Configuring Postfix for use with DKIM-Milter

In your /etc/postfix/master.cf file, add the following (BOLD) entry's. 

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
smtp      inet  n       -       -       -       -       smtpd                
         -o content_filter=spamassassin
         -o smtpd_milters=unix:/var/run/dkim-milter/dkim.sock

DKIM & DomainKey Testers

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