— 3 min read


This is part 3 of my guide to getting a mail server configured with all the sexy bits to improve deliverability, spam and virus protection.

You can view part 1 here and part 2 here.

The key pair

We need to create a key pair to sign emails with:

.. code-block:: bash
openssl genrsa -out private.key 1024 openssl rsa -in private.key -out public.key -pubout -outform PEM sudo mkdir /etc/dk/ sudo cp private.key /etc/dk/dk.key

Now we can move on to DK and DKIM signing, make sure you keep the public key for later.


First we’ll need to install an application to sign our emails.

sudo apt-get install dkim-filter

Once installed we need to configure it, open up /etc/default/dkim-filter, modify the file to look like below replacing <DOMAIN> with the domain you want to sign email from.

DAEMON_OPTS="-l -o X-DomainKeys,DomainKey-Signature"
DAEMON_OPTS="$DAEMON_OPTS -d <DOMAIN> -k /etc/dk/dk.key -s mail"
SOCKET="inet:12345@localhost" # listen on loopback on port 12345

We use the -o flag to tell the filter to ignore any DomainKeys headers when signing.

DK (DomainKeys)

The dk-filter package is not installable from APT on Debian 6 for me, try to install it using APT

sudo apt-get install dk-filter

If it doesn’t work then install it by hand, go to https://ftp.us.debian.org/debian/pool/main/d/dk-milter/ and download the .deb file for your architecture. To install it run the command below replacing <FILE> with the file you downloaded.

sudo dpkg -i <FILE>

Now that it’s installed we can configure it, open up /etc/default/dk-filter, modify the file to look like below replacing <DOMAIN> with the domain you want to sign email from.

DAEMON_OPTS="-l -o DKIM-Signature,X-DKIM"
DAEMON_OPTS="$DAEMON_OPTS -d <DOMAIN> -s /etc/dk/dk.key -S mail"
SOCKET="inet:12346@localhost" # listen on loopback on port 12346

We use the -o flag to tell the filter to ignore any DKIM headers when signing.


Install with

sudo apt-get install postfix-policyd-spf-python


Open up /etc/postfix/main.cf and add the following lines to it

milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:localhost:12345 inet:localhost:12346
non_smtpd_milters = inet:localhost:12345 inet:localhost:12346
spf-policyd_time_limit = 3600s

This tells Postfix to pass incoming and outgoing email through the DK and DKIM filters, as well as mail that arrives from the queue, local commands like sendmail and cleanup. It also sets a time limit on SPF checks.

Add the following to smtpd_recipient_restrictions = it should look like this

smtpd_recipient_restrictions = permit_mynetworks,

check_policy_service unix:private/policy-spf

Now open up /etc/postfix/master.cf and add the following

policy-spf unix - n n - - spawn
    user=nobody argv=/usr/bin/policyd-spf


We need to modify your DNS entries so that DK and DKIM actually work and we also need to add SPF records.

We need to create 3 TXT records, 1 for SPF and 2 for DK/DKIM.

Creating the SPF record is easy, create a new TXT record called <DOMAIN>. e.g. syslog.tv. with the following content. replacing <IP> with the IP of your mail server.

v=spf1 a mx ip4:<IP>

The DK and DKIM records are a little trickier, first create a TXT record with the following name _domainkey.<DOMAIN>. e.g. _domainkey.syslog.tv with the following content

t=y; o=-

With t set to y it puts your DK and DKIM in test mode, just in-case.

Now we need to create a second record called mail._domainkey.<DOMAIN>. e.g. mail._domainkey.syslog.tv, you’ll need to copy the contents of the public key file we created earlier. Open it up and copy everything between ——-BEGIN PUBLIC KEY——- and ——-END PUBLIC KEY——- in to one long line. Once done put it in the DNS record like below

k=rsa; p=<KEY_CONTENT>

like this

k=rsa; p=MIGfMA0GCSqGSIb3DQE ... snip ... 03hFbY5y2QbQIDAQAB


sudo /etc/init.d/dk-filter restart
sudo /etc/init.d/dkim-filter restart
sudo /etc/init.d/postfix restart

Try send an email to yourself, you should see both DK and DKIM signatures in the source.

« Part 2 - Postfix + SpamAssassin + ClamAV + Procmail

Part 4 - SpamAssassin + Razor + Pyzor »


Anarchist. Pessimist. Bipolar. Hacker. Hyperpolyglot. Musician. Ex-(semi-)pro gamer. They/Them.

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