DomainKeys Identified Mail is a digital email signing/verification technology, which is already supported by some common mail providers. (For example yahoo, google, etc).
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
You may add user for opendkim or use existing one (for example: postfix)
- Generate key:
openssl genrsa -out private.key 1024 openssl rsa -in private.key -pubout -out public.key
- Create /etc/opendkim/opendkim.conf (see example in the same directory)
Domain YOUR-DOMAIN1.com, YOUR-DOMAIN2.com KeyFile /path/to/private.key Selector server1 Socket inet:8891@localhost UserID postfix
- Add a DNS TXT record with your selector (see Selector in config, you may choose random name) and public key:
server1._domainkey IN TXT "k=rsa; p=MHwwDQYJK ... OprwIDAQAB; t=y"
p= is your public key, k= the algorithm (rsa by default). There are two possible flags for the t= value: y means testing (unsigned mail is not to be treated as spam/unauthorized), s means the key is not valid for subdomains.
- Run it with /etc/rc.d/opendkim start or add it to DAEMONS in /etc/rc.conf
into main.cf or smtpd options in master.cf
smtp inet n - n - - smtpd -o smtpd_client_connection_count_limit=10 -o smtpd_milters=inet:127.0.0.1:8891 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:127.0.0.1:8891
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.