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Revision as of 23:11, 30 August 2012 by Kwirkie (talk | contribs) (Updated with key generation using opendkim built in program "opendkim-genkey".)
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DomainKeys Identified Mail is a digital email signing/verification technology, which is already supported by some common mail providers. (For example yahoo, google, etc).

The idea

Basically DKIM means digitally singing all messages on the server to verify the message actually was sent from the domain in question and is not spam or pishing (and has not been modified).

  • The sender's mail server signs outgoing email with the private key.
  • When the message arrives, the receiver (or his server) requests the public key from the domain's DNS and verifies the signature.

This ensures the message was sent from a server who's private key matches the domain's public key.

For more info see RFC 6376


Install opendkim

You may add user for opendkim or use existing one (for example: postfix)

Basic configuration

  • Generate key:
opendkim-genkey -r -s server1 -d
  • Create /etc/opendkim/opendkim.conf (see example in the same directory)

Minimal config:

 KeyFile                 /path/to/keys/server1.private
 Selector                server1
 Socket                  inet:8891@localhost
 UserID                  opendkim
  • Add a DNS TXT record with your selector and public key. The correct record is generated with the private key and can be found in "server1.txt" in the same location as the private key.
  • Run it with /etc/rc.d/opendkim start or add it to DAEMONS in /etc/rc.conf

Postfix integration

Just add




into or smtpd options in example:

smtp      inet  n       -       n       -       -       smtpd
    -o smtpd_client_connection_count_limit=10
    -o smtpd_milters=inet:

submission inet n       -       n       -       -       smtpd
  -o smtpd_enforce_tls=no
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o smtpd_sasl_path=smtpd
  -o cyrus_sasl_config_path=/etc/sasl2
  -o smtpd_milters=inet:


While you're about to fight spam and increase people's trust in your server, you might want to take a look at Sender Policy Framework, which basically means adding a DNS Record stating which servers are authorized to send email for your domain.