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=== Running as an unprivileged user ==
Domain Keys 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.
Page is being revised.
This section has everything to set up to a fully functional, minimal installation of opendkim for signing and verifying integrated with Postfix.
This is the quick, five-minute version of opendkim setup. 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:
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. However, to let others verify your signatures don’t forget to publish the public key as a TXT record 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 tool.
TODO Discuss defaults, permissions
User and privileges
=== 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 10.1.0.0/16 126.96.36.199/24
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