This tutorial will focus on installing and configuring DomainKeys Identified Mail (DKIM) an open source implementation of the DKIM sender authentication system. OpenDKIM is really useful for signing your mail messages by generated pair of private key which is afterwards validated by public key stored as TXT DNS record for lookup by other servers.
I assume you have properly installed and configured sendmail. Check if it’s running:
$ service sendmail status
$ aptitude install opendkim opendkim-tools
Create new directory for your keys
$ mkdir -p /etc/opendkim/keys/your-domain.com
Generate Signing Keys
You need to generate a privateand a public key for each of the domains for which you wish to sign mail. The private key is stored away from prying eyes on your server, while the public key gets published in your domain’s DNS records so that receiving mail servers can verify your DKIM-signed mail.
If you’re really hard-core, you can always build the keys manually. Or, you can use the easy script included with OpenDKIM to do it for you.
Before running this script, decide now what the name of yourselector is going to be. A selector is a unique keyword that is associated with both keys (public and private), included in all the signatures, and published in your DNS records. For simplicity, I use the word default as my default selector.
$ opendkim-genkey -D /etc/opendkim/keys/your-domain.com -d your-domain.com -s default
The above command will create two files under the newly created directory – default.private and default.txt You can do a man opendkim-genkey if you’re interested in what additional options are available when creating your keys. In this example, I used the -D (directory) option, the -d (domain) option, and the -s (selector) options.
Change the ownership to opendkim
$ chown -R opendkim:opendkim /etc/opendkim/keys/your-domain.com $ chmod 640 /etc/opendkim/keys/your-domain.com/default.private $ chmod 644 /etc/opendkim/keys/your-domain.com/default.txt
A couple of files must be created and edited in order to configure OpenDKIM. Open the openDKIM configuration file /etc/opendkim.conf , and set following options:
AutoRestart Yes AutoRestartRate 10/1h UMask 002 Syslog yes SyslogSuccess Yes LogWhy Yes Canonicalization relaxed/simple ExternalIgnoreList refile:/etc/opendkim/TrustedHosts InternalHosts refile:/etc/opendkim/TrustedHosts KeyTable refile:/etc/opendkim/KeyTable SigningTable refile:/etc/opendkim/SigningTable Mode sv PidFile /var/run/opendkim/opendkim.pid SignatureAlgorithm rsa-sha256 UserID opendkim:opendkim Socket inet:firstname.lastname@example.org Domain your-domain.com Selector default
To learn about options do
man opendkim.conf or go here.
Next, you’ll need to create the three files you just declared on opendkim.conf. Create and Open /etc/opendkim/KeyTable and add the following line:
The KeyTable file tells OpenDKIM where to find your keys. Each entry in the KeyTable file is a single line for each key location (for example, all of the text in the above example should be on a single line in your file). If you’re going to use multiple keys (to sign mail for virtual domains with different keys, for example), you’ll need to create a separate line in the KeyTable file for each domain, like this:
default._domainkey.your-domain.com your-domain.com:default:/etc/opendkim/keys/your-domain.com/default.private default._domainkey.your-domain2.com your-domain2.com:default:/etc/opendkim/keys/your-domain2.com/default.private
Create and Open /etc/opendkim/SigningTable and add the following lines:
*@your-domain.com default._domainkey.your-domain.com # email@example.com default._domainkey.your-domain2.com # firstname.lastname@example.org default._domainkey.your-domain2.com
This SigningTable file is used for declaring the domains/email addresses and their selectors. I’m saying that everyone (*) sending mail from the server “your-domain.com” should use the selector named “default”. But, if you uncomment last two lines, only contact and no-reply can sign mail for “your-domain2.com” (also using a selector named default). It’s important to note that the * wildcard symbol will only work if the SigningTable option uses the refile: prefix before the filename (see the opendkim.conf documentation for more details).
Create and Open /etc/opendkim/TrustedHosts and add the following lines:
127.0.0.1 localhost your-domain.com
The TrustedHosts file tells OpenDKIM who to let use your keys. Because it’s referenced by the ExternalIgnoreList directive in your conf file, OpenDKIM will ignore this list of hosts when verifyingincoming mail. And, because it’s also referenced by theInternalHosts directive, this same list of hosts will be considered “internal,” and OpenDKIM will sign their outgoing mail.
Tell sendmail about OpenDKIM
Edit the .mc configuration file (used to be on /etc/mail/sendmail.mc) that was used to build your current sendmail.cf file. Add the following line:
Then build and install a new sendmail.cf by restarting sendmail with following command:
Start OpenDKIM and restart sendmail
It’s time to fire things up! Assuming you’re using bash, do
hash -r to rehash your shell so you can find the init script.
Now start OpenDKIM with:
$ service opendkim start
Check opendkim is listening right port
$ netstat -nlp | grep 8891 tcp 0 0 127.0.0.1:8891 0.0.0.0:* LISTEN 5115/opendkim
and Sendmail users should do:
$ service sendmail restart
If everything looks good, I recommend running chkconfig (system tool to enable or disable system services, if not installed run aptitude install chkconfig) on OpenDKIM to make sure it starts when you boot your server:
$ chkconfig opendkim on
Adding DNS Records
Now that your mail server is signing outgoing mail and verifying incoming mail, you’ll need to put some information in your DNS records to tell other mail servers how your keys are set up, and provide the public key for them to check that your mail is properly signed. Do:
$ cat /etc/opendkim/keys/your-domain.com/default.txt default._domainkey IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDHp6PyqC13hdWHKeD4lrX8okexumtWnZp2T80GYrYfHNAsCbD5r7CCdwYK850GcCawALpQpM2977HlktA379Rt6kYZxsSCAA+D27PekdRe6NGD4WgQF2S1S7dJvgCB9Zus1x/rfsE22DVgigBim5qfc8X+4g4V7eBK7e2Xn+TDvQIDAQAB" ; ----- DKIM key default for your-domain.com
If you’re using a web-based DNS interface (like GoDaddy or CloudFlare), the Name of the TXT record would default._domainkey and the Value of the TXT record would beeverything from the first quote to the last quote (starting with “v=).
Testing Things Out
The best way to see that everything is working on the server side is to keep an eye on your/var/log/mail.log file.
When you send a mail that gets successfully signed, you should see:
opendkim: t8IFlgM8005195: DKIM-Signature header added (s=default, d=your-domain.com)
To check that your signed mail is being authenticated and that your DNS records are properly set up send a signed email to free testing service email@example.com
$ sendmail firstname.lastname@example.org From: email@example.com To: firstname.lastname@example.org Hi .
Et voila !!