Set Up Postfix DKIM With dkim-milter

This howto has been superseded by http://www.topdog.za.net/postfix_dkim_milter

Introduction

DKIM is an authentication framework which stores public-keys in DNS and digitally signs emails on a domain basis. It was created as a result of merging Yahoo's domainkeys and Cisco's Identified Internet mail specification. It is defined in RFC 4871.

We will be using the milter implementation of dkim http://dkim-milter.sf.net on CentOS 5.1

Installation

Install the rpm, ignore dependencies as csh is a dependency but it does not affect dkim-milter; it's only required for some sample scripts that are shipped with the rpm.

rpm -Uvh http://www.c-corp.net/linux/centos/5/general/RPMS/i386/dkim-milter-2.2.1-1.i386.rpm --nodeps
mkdir /etc/dkim-milter
chown dkim-milt.dkim-milt /etc/dkim-milter
chmod 700 /etc/dkim-milter
chgrp postfix /var/run/dkim-milter
chmod 770 /var/run/dkim-milter

Generate The Keys

Download this script that you can use to easily generate the keys for signing the mail:

./dkim-genkey.sh -d <domainname>

Replace <domainname> with the domain name you will be signing mail for. This will create two files default.txt and default.private, default.txt is the line you need to add to your zone file - a sample is below:

default._domainkey IN TXT "v=DKIM1; g=*; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDG81CNNVOlWwfhENOZEnJKNlikTB3Dnb5kUC8/zvht/S8SQnx+YgZ/KG7KOus0By8cIDDvwn3ElVRVQ6Jhz/HcvPU5DXCAC5owLBf/gX5tvAnjF1vSL8ZBetxquVHyJQpMFH3VW37m/mxPTGmDL+zJVW+CKpUcI8BJD03iW2l1CwIDAQAB"
; ----- DKIM default for topdog-software.com

default.private contains your private key. Move this file into /etc/dkim-milter and rename it <domainname>_default.key.pem:

mv default.private /etc/dkim-milter/<domainname>_default.key.pem

Edit the file /etc/sysconfig/dkim-milter and set the variables:

USER="dkim-milt"
PORT=local:/var/run/dkim-milter/dkim.sock
SIGNING_DOMAIN="<domainname>"
SELECTOR_NAME="default"
KEYFILE="/etc/dkim-milter/${SIGNING_DOMAIN}_${SELECTOR_NAME}.key.pem"
SIGNER=yes
VERIFIER=yes
CANON=simple
SIGALG=rsa-sha1
REJECTION="bad=r,dns=t,int=t,no=a,miss=r"
EXTRA_ARGS="-h -l -D"

Init Script Fix

Install my modified init script as the one that is supplied with the rpm has a bug.

wget http://www.topdog-software.com/files/dkim-milter -O /etc/init.d/dkim-milter
chkconfig --level 345 dkim-milter on 
service dkim-milter start

Configure Postfix

Add this to the postfix configuration file /etc/postfix/main.cf:

smtpd_milters = unix:/var/run/dkim-milter/dkim.sock
non_smtpd_milters = unix:/var/run/dkim-milter/dkim.sock

Append to the existing milters if you have other milters already configured.

Start dkim-milter and restart Postfix:

 service dkim-milter start

service postfix restart

Testing

Send a message to [email protected]; the system will return a response to let you know if DKIM is working. Examine the headers of mails from domains like gmail to see if your system is checking the DKIM signatures of inbound mail.