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Sign DKIM signature on outgoing emails for new mail domain

Don't know what DKIM is? Check our tutorial here: What is a DKIM DNS record.

Don't know where Amavisd config file is? check this tutorial: Locations of configuration and log files of major components.

iRedMail configures Amavisd to sign outgoing emails for the first mail domain you added during iRedMail installation. If you added new mail domain, you should update Amavisd config file to sign DKIM signature for it.

Let's say your first mail domain added during iRedMail installation is mydomain.com, and new mail domain is new_domain.com, please follow below steps to enable DKIM signing for outgoing emails of this domain.

Use existing DKIM key for new mail domain

if you already have a working DKIM and valid DKIM DNS record, it's ok to use this existing DKIM key to sign emails sent by other hosted mail domains. This way, you don't need to ask your customer who owns this new domain to add DKIM DNS record.

• Find below setting in Amavisd config file amavisd.conf:

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Add one line in @dkim_signature_options_bysender_maps, after "mydomain.com" line like below:

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    "new_domain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

• Restart Amavisd service.

Generate new DKIM key for new mail domain

If you or your customer prefer to use their own DKIM key, you can generate a new DKIM key and ask your customer to add DKIM DNS record. Refer to our tutorial to add DKIM DNS record.

• Generate new DKIM key (key length 1024) for new domain, and set correct file owner and permission

  • on RHEL/CentOS, the command is amavisd, user/group is amavis:amavis.
  • on Debian/Ubuntu, the command is amavisd-new, user/group is amavis:amavis.
  • on FreeBSD, the command is amavisd, user/group is vscan:vscan.
  • on OpenBSD, the command is amavisd, user/group is _vscan:_vscan.

amavisd-new genrsa /var/lib/dkim/new_domain.com.pem 1024
chown amavis:amavis /var/lib/dkim/new_domain.com.pem
chmod 0400 /var/lib/dkim/new_domain.com.pem

• Find below setting in Amavisd config file amavisd.conf:

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");

Add one line after above line like below:

dkim_key('new_domain.com', "dkim", "/var/lib/dkim/new_domain.com.pem");

• Find below setting in Amavisd config file amavisd.conf:

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

Add one line after "mydomain.com" line like below:

@dkim_signature_options_bysender_maps = ( {
    ...
    "mydomain.com"  => { d => "mydomain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    "new_domain.com"  => { d => "new_domain.com", a => 'rsa-sha256', ttl => 10*24*3600 },
    ...
});

• Restart Amavisd service.

Again, don't forget to add DKIM DNS record for this new domain. The value of DKIM record can be checked with command below:

# amavisd-new showkeys

After added DKIM DNS record, please verify it with command:

# amavisd-new testkeys

Note: DNS vendor usually cache DNS records for 2 hours, so if above command shows "invalid" instead of "pass", you should try again later.

Use one DKIM key for all mail domains

For compatibility with dkim_milter the signing domain can include a '*' as a wildcard - this is not recommended as this way amavisd could produce signatures which have no corresponding public key published in DNS. The proper way is to have one dkim_key entry for each mail domain.

If you still want to try this, please follow below steps:

• Find below setting in Amavisd config file amavisd.conf:

dkim_key('mydomain.com', "dkim", "/var/lib/dkim/mydomain.com.pem");

• Replace it by below line:

dkim_key('*', "dkim", "/var/lib/dkim/mydomain.com.pem");

• Restart Amavisd serivce.

With above setting, all outbound emails with be signed with this dkim key. And Amavisd will show a warning message when start amavisd service:

dkim: wildcard in signing domain (key#1, *), may produce unverifiable signatures with no published public key, avoid!

References

• Amavisd official document: Setting up DKIM mail signing and verification