Category: Security

openDKIM and Postfix on Ubuntu Server 12.04LTS

You probably found this post because you already know what DKIM is and want to implement it, but for those of you who need more information Wikipedia has a good entry on the subject;

This little walkthrough has been based upon a Ubuntu 12.04LTS server with Postfix; your mileage may very depending on your setup.  At a minimum you will need:

1. Root access to your mail server
2. Access to update the dns records for your domain

Firstly, install opendkim from the repositories.  There is some good information available there too.

sudo apt-get install opendkim

You must decide on what “selector” you wish to use.  The selector is essentially a word to describe the key you wish to use.  Here I am going to use the selector 201204 as the key became valid in April 2012 (cunning eh?).  Obviously, my domain will be brabeum.com.

Setup

So onward.  Generate the keys in your home directory, using your chosen selector and domain.

opendkim-genkey -s 201204 -d brabeum.com

This will create two files, 201204.txt and 201204.private containing the public and private keys respectively.  You must publish the content of the public key to your authoritative DNS server.  Three things to note;

1. openDKIM 2.5.2 on Ubuntu omits the k= after v=DKIM1;
2. you should add the t=y flag to indicate to receiving servers that you are testing DKIM but not actively using it yet.
3. You should use a short TTL (time to live) so that you can change the key without waiting ages for it to propagate though DNS.

DNS implementations vary, but you should end up with an entry something like this (I’ve shortened the key because of line breaks);

201204._domainkey IN TXT v=DKIM1; k=rsa; t=y; p=MIGfM..[snip]..QIDAQAB

Remember to reload your DNS configuration after addition.

Now, test the key…

opendkim-testkey -d brabeum.com -s 201204 -k 201204.private -vvv

And you should get output similar to below.

opendkim-testkey: key loaded from 201204.private
opendkim-testkey: checking key '201204._domainkey.brabeum.com'
opendkim-testkey: key not secure
opendkim-testkey: key OK

Note that opendkim is reporting that the key is not secure.  This relates to the fact that DNSSEC is not implemented on my DNS server and theoretically somebody could intercept the DNS lookup and replace it with their own key.

Once we are happy with the key test, move the private key to /etc/mail

cd /etc/mail
sudo mv /home/basil/201204.private .

Setup openDKIM by editing /etc/default/opendkim with your favourite editor and add the following line to the end.

SOCKET="inet:8891@localhost"

Then edit /etc/opendkim.conf and add the following lines

Domain                  brabeum.com
KeyFile                 /etc/mail/201204.private
Selector                201204

And restart opendkim

sudo service opendkim restart

Lastly we configure postfix.  Edit /etc/postfix/main.cf and add the lines to the end

smtpd_milters=inet:localhost:8891
non_smtpd_milters=inet:localhost:8891

And restart

sudo service postfix restart

Testing

So now you need to send some test messages through your newly configured postfix server.  At the receiving end you should see some new headers, one of which will show similar to below (note that Google recognize it as test mode as we still have the t=y flag applied).

Once you are happy that all is well, you can remove the testing flag and increase TTL.

Done!