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Set Up DKIM On Postfix With dkim-milter (CentOS 5.2)


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 on CentOS 5.2.



I provide Centos rpms for Dkim-milter at so we will install the latest version.

  • Install the rpm, ( 32bit and 64bit intel supported )

# wget
# rpm –import andrew_topdog-software.com_key.txt
#$(uname -i).rpm


Generate the Keys

# sh /usr/share/doc/dkim-milter-2.8.2/ -r -d <domain_name>

Replace <domain_name> with the domain name you will be signing the mail for. The command will create two files.

  • default.txt – contains the public key you publish via DNS
  • default.private – the private key you use for signing your email

Move the private key to the dkim-milter directory and secure it.

# mv default.private /etc/mail/dkim/default.key.pem
# chmod 600 /etc/mail/dkim/default.key.pem
# chown dkim-milt.dkim-milt /etc/mail/dkim/default.key.pem


DNS Setup

You need to publish your public key via DNS, client servers use this key to verify your signed email. 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

Also add this to your zone file.

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



  • Create the file /etc/sysconfig/dkim-milter with the contents below overwriting the existing sample file that was installed by the rpm, Make sure you set the SIGNING_DOMAIN variable to the domain or domains you will be signing mail for.

EXTRA_ARGS="-h -l -D"

Configure Postfix

You need to add the following options to the postfix file to enable it to use the milter.

smtpd_milters = inet:localhost:20209
non_smtpd_milters = inet:localhost:20209

Append the dkim-milter options 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 an email to or, you will receive a response stating if your setup is working correctly. If you have a Gmail account you can send an email to that account and look at the message details similar to the picture below, you should see signed-by “your domain” if your setup was done correctly.


Previous versions




Updated rpms are always provided at