How to configure OpenDKIM with Postfix

In this HowTo, you will find out how we configured OpenDKIM and Postfix(3.1.0) to sign our @postfix.io emails with DKIM. I will not talk about what is DKIM and settings like hash algorithms, DKIM Identity, Selectors etc.

Install OpenDKIM and tools.

apt-get install opendkim opendkim-tools

Add the following settings to /etc/opendkim.conf after “UserID” directive.

Map AuthorDomains to RSA keys.
KeyTable /etc/dkimkeys/rsakeys.table
SigningTable refile:/etc/dkimkeys/signingdomains.table

# "simple" recommended by DKIMCore
Canonicalization simple

Mode sv
SubDomains no
AutoRestart yes
AutoRestartRate 10/1M
Background yes
DNSTimeout 5
SignatureAlgorithm rsa-sha256
OversignHeaders From

Generate RSA key for @postfix.io

cd /etc/dkimkeys/
opendkim-genkey --bits=1024 --selector=key1 --domain=postfix.io --append-domain

It will create two files. “key1.private” which is for server side and “key1.txt” which contain the following DNS record that needs to be created in @postfix.io zone. You can lookup my existing record with dig key1._domainkey.postfix.io TXT +short

key1._domainkey.postfix.io. IN TXT ( "v=DKIM1;k=rsa;p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDVgaYb2qaO92yF1DuoSIWybPgiwQ3dfjN1XhzstnEqfi/GroqtN87BrjEr9BGTTiisocbMZOtfErgfCSq+sCjHohEySdngfnxPUqLYqco+Xe3RlESYngKFU9YUUKXE9OcT3dt3v921h1pZ9BJwQ2RyJ+xANYR5DivfRT2gPCdIWwIDAQAB" )  ; ----- DKIM key key1 for postfix.io

mv key1.private key1.postfix.io.rsa

Add RSAkey reference to KeyTable file in /etc/dkimkeys/rsakeys.table

postfixdkim postfix.io:key1:/etc/dkimkeys/key1.postfix.io.rsa

Add AuthorDomain and RSAKey reference in /etc/dkimkeys/signingdomains.table

*@postfix.io postfixdkim

“*@postfix.io” says domain with any local-part should be signed with this key. “postfixdkim” is reference to RSA key in KeyTable.

Connect OpenDKIM to Postfix. Since Postfix runs in jail environment so its better to keep OpenDKIM also inside Postfix spool directory.

mkdir /var/spool/postfix/opendkim

Update opendkim.sock path to new location in /etc/default/opendkim

SOCKET="local:/var/spool/postfix/opendkim/opendkim.sock"

Configure opendkim in /etc/postfix/main.cf

# Connect OpenDKIM
milter_default_action = accept
milter_protocol = 6
smtpd_milters = local:/opendkim/opendkim.sock
non_smtpd_milters = local:/opendkim/opendkim.sock

Setup proper permissions.

chown -R opendkim:opendkim /etc/opendkim.conf /etc/dkimkeys
chown opendkim:postfix /var/spool/postfix/opendkim

Restart opendkim and postfix

systemctl restart opendkim.service
systemctl restart postfix.service

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