Install DKIM for Postfix on CentOS 6.3 using dkim-milter.

postfix_dkim.sh

#!/bin/bash

# Prerequisite: (EPEL repository)[http://www.rackspace.com/knowledge_center/article/installing-rhel-epel-repo-on-centos-5x-or-6x]
#
# Usage: postfix_dkim.sh domain selector
#
#
# After the script runs, you will need to create TXT DNS records for the domain key: 
#   <selector>._domainkey.<domain> IN TXT "copy and paste the content of /etc/mail/dkim-milter/keys/<domain>/<selector>.txt here"
#   _domainkey.<domain> IN TXT "o=~"

domain="$1"
selector="$2"

# install dkim-milter
yum --nogpgcheck -y install dkim-milter

# generate a key for your domain
cd /etc/mail/dkim-milter/keys
mkdir $domain
cd $domain
dkim-genkey -r -d $domain -s $selector
cd ..
chown -R dkim-milter:dkim-milter $domain
chmod -R 600 ${domain}/*

# configure dkim-milter's keylist
echo "*:${domain}:/etc/mail/dkim-milter/keys/${domain}/${selector}" >> /etc/mail/dkim-milter/keys/keylist

# configure dkim-milter's logging and network settings
# and disable original socket rule
sed -i "s/Socket/#Socket/g" /etc/mail/dkim-milter/dkim-filter.conf
echo "Syslog yes" >> /etc/mail/dkim-milter/dkim-filter.conf
echo "SyslogFacility mail" >> /etc/mail/dkim-milter/dkim-filter.conf
echo "Socket inet:8891@localhost" >> /etc/mail/dkim-milter/dkim-filter.conf

# configure dkim-milter InternalHosts
echo “127.0.0.1” > /etc/mail/dkim-milter/hosts
echo “InternalHosts /etc/mail/dkim-milter/hosts” >> /etc/mail/dkim-milter/dkim-filter.conf

# configure postfix to use dkim-milter
echo "milter_default_action = accept" >> /etc/postfix/main.cf
echo "milter_protocol = 2" >> /etc/postfix/main.cf
echo "smtpd_milters = inet:localhost:8891" >> /etc/postfix/main.cf
echo "non_smtpd_milters = inet:localhost:8891" >> /etc/postfix/main.cf

# restart services
service dkim-milter restart
service postfix restart

# print out DNS info
echo "Use the following information to your DNS records:"
cat /etc/mail/dkim-milter/keys/${domain}/${selector}.txt

Line 24: if ${domain} is clean changes system's permission. It must be changes to: chmod -R 600 ${domain} [EOL]