The Debian opendkim package is a full-featured DKIM implementation suitable for use with MTAs (mail servers) such as Postfix.
DomainKeys Identified Mail (DKIM) combines several existing antiphishing and antispam methods to improve the quality of the classification and identification of legitimate e-mail. Instead of the traditional IP-address, to determine the message sender DKIM adds a digital signature associated with the domain name of the organization.
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This section has everything to set up a fully functional, minimal installation of opendkim for signing and verifying integrated with Postfix.
This is the quick, five-minute version of opendkim setup suitable only for a single domain. For a fuller discussion of the different setup options available please refer to the subsequent sections.
Let’s go! First, install opendkim.
sudo apt install opendkim opendkim-tools
Next, generate the private key for your domain and selector:
sudo opendkim-genkey -D /etc/dkimkeys -d yourdomain.com -s 2020 -S
Now, edit /etc/opendkim.conf. At a minimum, four parameters need to be adapted: the domain/selector/key file triple, and the socket. For the socket, the easiest option is to use a TCP socket listening on a local port.
Domain yourdomain.com Selector 2020 KeyFile /etc/dkimkeys/2020.private Socket inet:8891@localhost
That’s it for opendkim. Restart the service with sudo systemctl restart opendkim.
The final step is integrating the new opendkim service with Postfix. Edit /etc/postfix/main.cf to connect the two:
smtpd_milters = inet:localhost:8891 non_smtpd_milters = $smtpd_milters
And finally reload the Postfix configuration with sudo systemctl reload postfix.
That’s it! Your mail is now being signed and verified. Don’t forget to publish your public key as a TXT record in DNS at 2020._domainkey.yourdomain.com. The generated file /etc/dkimkeys/2020.txt contains that record for your convenience.
The following sections discuss opendkim configuration options in more detail.
These sections assume that you have installed the opendkim and opendkim-tools packages.
For key generation, the opendkim-tools package provides the opendkim-genkey program.
sudo opendkim-genkey \ --directory=/etc/dkimkeys \ --domain=yourdomain.com \ --selector=2020 \ --nosubdomains
You will want to tweak some of these options. For example, you might not want to forbid subdomain signing with --nosubdomains, or you might want to restrict usage to email with --restrict. The cryptography has reasonably strong default settings, so it is usually not necessary to specify --bits (default: 2048) and --hash-algorithms (default: SHA-256).
The directory /etc/dkimkeys is created by the opendkim package as the Debian-specific canonical key storage location. It is owned by user opendkim. You could give it to root instead, but see the next section for a discussion of key ownership and permissions.
Notice how we execute opendkim-genkey as root. That way, opendkim-genkey produces key files with the correct, restricted permissions, owned by root.
In the next section, an alternative scheme is described, where opendkim runs as unprivileged user opendkim even when it reads the key file. In that case, it is important that the key file be owned by user opendkim. So, if you plan to use that scheme, be sure to execute opendkim-genkey as user opendkim instead:
sudo -u opendkim opendkim-genkey ...
User and privileges
By default, the opendkim service runs as user opendkim. This is because the default configuration contains a setting for parameter UserID:
This setting instructs opendkim to become user opendkim.
So, more accurately, the opendkim service starts life as root, does everything it needs to do as root – such as reading in keys –, and then, before beginning normal operation, it drops the root privileges and becomes user opendkim.
An alternative setup is possible where the opendkim service runs as an unprivileged user from the very start, and this is described in the following section.
Running as an unprivileged user
Using a local UNIX socket
Postfix and opendkim
Install the package:
apt-get install opendkim opendkim-tools
Add to the Postfix signature opendkim. In Debian stretch, the recommended place for the private key is /etc/dkimkeys (it already exists with owner opendkim, group opendkim, and rwx permissions for user only).
Generate a key for mail.example.com server
opendkim-genkey -D /etc/dkimkeys/ -d example.com -s mail
resulting in the directory /etc/dkimkeys/ 2 files : mail.private and mail.txt (private and public key, respectively). Ensure that only the opendkim user can read them as that is enough:
chgrp opendkim /etc/dkimkeys/* chmod go-rwx /etc/dkimkeys/*
Postfix and opendkim will communicate through a unix socket (faster and more secure than a tcp socket). The default configuration of postfix in debian runs under a chroot (/var/spool/postfix) so the socket must be created there :
sockdir=/var/spool/postfix/var/run/opendkim mkdir -p $sockdir chown opendkim. $sockdir chmod go-rwx $sockdir chmod g+x $sockdir
Setup the /etc/opendkim.conf:
All the available options can be found on the page: http://www.opendkim.org/opendkim.conf.5.html
Syslog yes # Required to use local socket with MTAs that access the socket as a non- # privileged user (e.g. Postfix) UMask 007 # Signature mode and signature verification Mode sv # not needed as this is the default # DO NOT BELIEVE that /etc/default/opendkim overrides the following : # (In stretch, it does not on 2019-08-13) Socket local:/var/spool/postfix/var/run/opendkim/opendkim.sock # Specify the list of keys KeyTable file:/etc/dkimkeys/keytable # Match keys and domains. To use regular expressions in the file, use refile: instead of file: SigningTable refile:/etc/dkimkeys/signingtable # Match a list of hosts whose messages will be signed. By default, only localhost is considered as internal host. InternalHosts refile:/etc/dkimkeys/trustedhosts
Now in the file /etc/dkimkeys/keytable, put information about the private key:
In the file /etc/dkimkeys/signingtable, specify which key will sign a domain:
# Domain example.com *@example.com mail._domainkey.example.com # You can specify multiple domains # Example.net www._domainkey.example.net
In the file /etc/dkimkeys/trustedhosts, specify which hosts will have messages signed. If needed, include localhost as it is not implicit:
127.0.0.1 10.1.0.0/16 188.8.131.52/24
You do not need to modify /etc/default/opendkim.
Now restart opendkim and check that everything is ok:
# service opendkim restart # service opendkim status # ls -ld /var/run/opendkim drwxr-x--- 2 opendkim opendkim 60 Aug 13 13:57 /var/run/opendkim # ls -l /var/run/opendkim total 4 -rw-rw---- 1 root root 6 Aug 13 13:57 opendkim.pid # ls -ld /var/spool/postfix/var/run/opendkim drwx--x--- 2 opendkim opendkim 4096 Aug 13 13:57 /var/spool/postfix/var/run/opendkim # ls -l /var/spool/postfix/var/run/opendkim total 0 srwxrwx--- 1 opendkim opendkim 0 Aug 13 13:57 opendkim.sock root@serveur:~/config#
Now, add postfix to the group opendkim so that it can communicate with opendkim:
adduser postfix opendkim
And add the following lines to /etc/postfix/main.cf:
milter_default_action = accept milter_protocol = 6 # from inside the chroot, the socket will be in /var/run/opendkim smtpd_milters = unix:/var/run/opendkim/opendkim.sock non_smtpd_milters = unix:/var/run/opendkim/opendkim.sock
The setup of opendkim and postfix is complete.
service postfix restart service postfix status
Try to send a mail. If you see in /var/log/mail.log something like
Aug 13 13:18:00 yourhostname opendkim: OpenDKIM Filter: Unable to bind to port local:/var/spool/postfix/var/run/opendkim/opendkim.sock: No such file or directory Aug 13 13:18:00 yourhostname opendkim: OpenDKIM Filter: Unable to create listening socket on conn local:/var/spool/postfix/var/run/opendkim/opendkim.sock
then that probably means that you did not create the directory for the socket (see above) or you gave it the wrong permissions. Double-check!
If you see
Aug 13 13:46:19 yourhostname postfix/cleanup: warning: connect to Milter service unix:/var/run/opendkim/opendkim.sock: No such file or directory
then that means postfix could not read the socket. Did you put postfix in group opendkim? Are the permissions on /var/spool/postfix/var/run/opendkim/opendkim.sock correct?
If everything is correct, that does not mean your configuration of DKIM is complete: you must configure the DNS.
Add a TXT record for your example.com domain
v=DKIM1; k=rsa; p=MI.. (take it from /etc/dkimkeys/mail.txt file; remove the >"< and connect the lines after p= to one key.)
You can test your installation with opendkim-testkey:
# opendkim-testkey -d example.com -s mail -vvv opendkim-testkey: using default configfile /etc/opendkim.conf opendkim-testkey: checking key 'mail._domainkey.example.com' opendkim-testkey: key not secure opendkim-testkey: key OK
http://www.opendkim.org/ : Official website