OpenDKIM

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DomainKeys Identified Mail is a digital email signing/verification technology, which is already supported by some common mail providers (for example yahoo, google, etc).

The idea

Basically DKIM means digitally signing all messages on the server to verify the message actually was sent from the domain in question and is not spam or pishing (and has not been modified).

  • The sender's mail server signs outgoing email with the private key.
  • When the message arrives, the receiver (or his server) requests the public key from the domain's DNS and verifies the signature.

This ensures the message was sent from a server who's private key matches the domain's public key.

For more info see RFC 6376

Installation

Install the package opendkim from the Official repositories.

Basic configuration

  • To generate a secret signing key, you need to specify the domain used to send mails and a selector which is used to refer to the key. You may choose anything you like, see the RFC for details, but alpha-numeric strings should be OK:
opendkim-genkey -r -s myselector -d example.com
  • Copy/move the sample configuration file /etc/opendkim/opendkim.conf.sample to /etc/opendkim/opendkim.conf and change the following options:
/etc/opendkim/opendkim.conf
Domain                  example.com
KeyFile                 /path/to/keys/server1.private
Selector                myselector
Socket                  inet:8891@localhost
UserID                  opendkim
  • Add a DNS TXT record with your selector and public key. The correct record is generated with the private key and can be found in myselector.txt in the same location as the private key. Example:
myselector._domainkey   IN	 TXT	"v=DKIM1; k=rsa; s=email; p=...................."

Check that your DNS record has been correctly updated:

host -t TXT myselector._domainkey.example.com

You may also check that your DKIM DNS record is properly formated using one of the DKIM Key checker available on the web.

  • Enable and start the opendkim.service. Read Daemons for more information.

Postfix integration

Either add the following lines to main.cf:

 non_smtpd_milters=inet:127.0.0.1:8891
 smtpd_milters=inet:127.0.0.1:8891

Or change smtpd options in master.cf:

smtp      inet  n       -       n       -       -       smtpd
  -o smtpd_client_connection_count_limit=10
  -o smtpd_milters=inet:127.0.0.1:8891

submission inet n       -       n       -       -       smtpd
  -o smtpd_enforce_tls=no
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o smtpd_sasl_path=smtpd
  -o cyrus_sasl_config_path=/etc/sasl2
  -o smtpd_milters=inet:127.0.0.1:8891

Security

The default configuration for the OpenDKIM daemon is less than ideal from a security point of view (all those are minor security issues):

  • The opendkim user is created with an incorrect default shell. You may want to change it to /bin/false or /sbin/login (bug report).
  • The OpenDKIM daemon does not need to run as root at all (the configuration suggested earlier will have OpenDKIM drop root privileges by himself but systemd can do this too and much earlier).
  • If your mail daemon is on the same host as the OpenDKIM daemon, there is no need for localhost tcp sockets and unix sockets may be used instead, allowing classic user/group access controls.
  • OpenDKIM is using the /tmp folder by default whereas it could use its own folder with additional access restrictions.

The following configurations files will fix most of those issues (assuming you're using Postfix) and drop some unnecessary options in the systemd service unit:

/etc/tmpfiles.d/opendkim.conf
D /run/opendkim 0750 opendkim postfix
/etc/opendkim/opendkim.conf
BaseDirectory           /var/lib/opendkim
Domain                  example.com
KeyFile                 /etc/opendkim/myselector.private
Selector                myselector
Socket                  local:/run/opendkim/socket
Syslog                  Yes
TemporaryDirectory      /run/opendkim
UMask                   002
/etc/systemd/system/opendkim.service
[Unit]
Description=OpenDKIM daemon
After=network.target remote-fs.target nss-lookup.target

[Service]
Type=forking
User=opendkim
Group=postfix
ExecStart=/usr/bin/opendkim -x /etc/opendkim/opendkim.conf

[Install]
WantedBy=multi-user.target

Notes

While you're about to fight spam and increase people's trust in your server, you might want to take a look at Sender Policy Framework, which basically means adding a DNS Record stating which servers are authorized to send email for your domain.