Setup your own email server (MTA) on the cheap - part 5

Our PostFix based MTA has been running well now, and all these last changes are just securing the server from SPAM and Viruses. Traditionally SPF was a great protection from SPAM which works by comparing the server sending the message to a DNS record to confirm the validity of the sending server. A great system but it's far from perfect, and most implementations could be generously described as half arsed. To improve on SPF DKIM has been developed which uses keys to sign sent messages, then the receiving server can pull a DNS record to confirm the validity of the mail it's receiving. To do this we're going to install and use opendkim.

    1. Install opendkim, of course:

    sudo apt-get install opendkim opendkim-tools

    2. Open /etc/opendkim.conf in your favourite editor and set these four lines:

    Domain          mydomain.com
    KeyFile         /etc/postfix/dkim.key
    Selector        mail
    SOCKET          inet:8891@localhost

    2a. I've complicated things in my setup to use different keys for each of the domains I use. To support this, I've also added these lines to my opendkim.conf

    KeyTable                       /etc/opendkim/KeyTable
    SigningTable              /etc/opendkim/SigningTable
    ExternalIgnoreList /etc/opendkim/TrustedHosts
    InternalHosts           /etc/opendkim/TrustedHosts

    2b. For my multi-domain setup to work I need to populate these referenced files to map the domains to the keys used (which we'll create soon).
    /etc/opendkim/KeyTable - this maps the domain and selector to a key file:

    mail._domainkey.domain1.com domain1.com:mail:/etc/opendkim/keys/domain1.com.dkim.key
    mail._domainkey.domain2.com domain2.com:mail:/etc/opendkim/keys/domain2.com.dkim.key
    /etc/opendkim/SigningTable - this maps a mail domain to a domain and selector:

    domain1.com mail._domainkey.domain1.com
    domain2.com mail._domainkey.domain2.com

    /etc/opendkim/TrustedHosts - this just ignores local subnets:

    127.0.0.1
    localhost
    172.31.0.0/16

    3. Now we need to create our signing keys. For a simple one domian configuration, run these:

    sudo opendkim-genkey -t -s mail -d domain.com 
    sudo cp mail.private /etc/postfix/dkim.key
    sudo chown opendkim:opendkim /etc/postfix/dkim.key

    3a. Of course, I complicate things so for each domain I need to run similar commands:

    sudo opendkim-genkey -t -s mail -d domain1.com
    sudo cp mail.private /etc/opendkim/keys/domain1.com.dkim.key
    sudo cp mail.txt ~/domain1.com.mail.txt

    sudo chown opendkim:opendkim /etc/opendkim/keys/domain1.com.dkim.key

    sudo opendkim-genkey -t -s mail -d domain2.com
    sudo cp mail.private /etc/opendkim/keys/domain2.com.dkim.key
    sudo cp mail.txt ~/domain2.mail.txt

    sudo chown opendkim:opendkim /etc/opendkim/keys/domain2.com.dkim.key

    These commands are: using opendkim-genkey to generate a key with the selector mail and the domain provided. We then copy the key into /etc/opendkim/keys/ then backup the DNS details it creates in mail.txt, then update the ownership of the key file so the opendkim daemon can access it.

    4. Now you'll need to create TXT dns records for your domain(s) according to the contents of either mail.txt or domainx.mail.txt. These should look something like this:

    Hostname: mail._domainkey
    Type: TXT
    Value: k=rsa;p=somehugerandomstring

    While we're editing DNS, lets also setup DMARC. This DNS record tells mail servers what they should do if DKIM or SPF fails. I'd recommend looking into a service to help report mail from your domains, like: https://dmarc.postmarkapp.com/. When you go through the setup on this site, it will give you details of the DNS records required - although I'd suggest change the p=none; part to p=quarantine; and once you've had a few reports and all looks well, change it to p=reject;

    5. Finally, reload everything and you should be on your way

    sudo service opendkim restart

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