Setup postfix DKIM with dkim-milter

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

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

# rpm 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

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.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

The contents of 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

Also add this to your zone file.

_ssp._domainkey IN TXT "t=y; dkim=unknown"

• 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"

• 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

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

Send a message to autorespond+dkim@dk.elandsys.com the system will return a response to let you know if DKIM is working. Examine the headers of mail from domains like gmail to see if your system is checking the DKIM signatures of inbound mail.