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The opendkim package contains a full-featured DKIM milter 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.

Jump to #Quickstart for an overview and minimal setup, and refer to #Configuration for the complete picture.

Quickstart

The quickstart instructions in this section describe setting up a minimal, but functional installation of opendkim for signing and verifying, integrated with Postfix. This is the five-minute version of opendkim configuration for the impatient. For a fuller discussion of the 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 pair:

sudo -u opendkim opendkim-genkey -D /etc/dkimkeys -d yourdomain.com -s 2020

Now, edit /etc/opendkim.conf. 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 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.

Done! 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.

Configuration

The following sections discuss opendkim configuration options in more detail. See the manual page opendkim.conf(5) for reference.

These sections assume that you have installed the opendkim and opendkim-tools packages.

The opendkim configuration file can be found at /etc/opendkim.conf. All configuration parameters should be set in this file.

This needs to be stated, because there is a lot of older, now misleading information on this online. Previously, one would edit the default settings at /etc/default/opendkim, and then execute /lib/opendkim/opendkim.service.generate to generate systemd override files at /etc/systemd/system/opendkim.service.d/override.conf and /etc/tmpfiles.d/opendkim.conf. While this is still possible, it is now recommended to adjust the settings directly in /etc/opendkim.conf.

Generating keys

For key generation, the opendkim-tools package provides the opendkim-genkey program. This program generates a private key named <selector>.private in the specified directory, as well as a public key ready for installation in a DNS zone file. An example invocation:

sudo --user opendkim 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. In Debian, the cryptographic options use reasonably strong defaults, so it is usually not necessary to specify --bits (default: 2048) and --hash-algorithms (default: SHA-256).

In Debian, the directory /etc/dkimkeys serves as the canonical key storage location, and is created by the opendkim package on installation. The owner is set to opendkim by default.

Notice how we execute opendkim-genkey as user opendkim. That way, opendkim-genkey produces key files with the correct, restricted permissions, owned by opendkim.

It is also possible to restrict key ownership further to user root by invoking opendkim-genkey directly as root:

sudo opendkim-genkey ...

However, while this does work, opendkim will not be able to reload keys during operation: during startup, the key file may be read into memory as root, but subsequently, after root privileges are dropped, the keys will be accessed as user opendkim. This is discussed in the following section.

User and privileges

By default, the opendkim service runs as user opendkim. This is because the default configuration contains a setting for parameter UserID in /etc/opendkim.conf:

UserID opendkim

This setting instructs opendkim to become user opendkim. So, more accurately, what happens is that the opendkim service is started as root, does everything it needs to do as root – such as reading private keys and writing the pid file –, and then, before beginning normal operation, it drops the root privileges and becomes user opendkim. This is a standard, secure procedure that should be appropriate for most users.

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

For setups that have additional security requirements, it is possible to run the opendkim service as user opendkim from the very beginning, with no root privileges involved at any stage. Please note that most users do not need this.

Create a systemd override file at /etc/systemd/system/opendkim.service.d/override.conf (you may need to create the directory too), with the following contents:

[Service]
User=opendkim
Group=opendkim

The UserID opendkim setting in /etc/opendkim.conf can now be removed, as no privilege dropping is necessary. Reload the systemd configuration with sudo systemctl daemon-reload, and restart the opendkim service. It now runs as an unprivileged user.

Make sure that the unprivileged user can actually read the keys in /etc/dkimkeys (see above), and write the pid file. (Since both /etc/dkimkeys and the runtime directory /run/opendkim come owned by user/group opendkim, this should work without further adjustment.)

Key selection

For a single-domain DKIM setup with only a single key, the configuration shown in #Quickstart, using the three parameters Domain, Selector, KeyFile is enough. However, opendkim configuration supports multiple domains and keys, read from a variety of sources (files, SQL databases, Lua scripts, …). KeyTable and SigningTable are the configuration parameters that enable this.

Setup the /etc/opendkim.conf:

# 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:

mail._domainkey.example.com example.com:mail:/etc/dkimkeys/mail.private 

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
1.2.3.4/24

Socket

The opendkim service has to provide a communication channel for the MTA (Postfix). A TCP socket listening on a port only accessible locally is a reasonable choice that is also easy to set up.

Socket inet:8891@localhost

Sockets can be of IPv4 or IPv6 type, and can listen on all interfaces or on a specific interface only.

Some prefer setting up a UNIX domain socket instead, as a faster and more secure channel (though opinion on this point varies). This requires a little more configuration work, and is described in the following section.

Using a UNIX domain socket

The UNIX domain socket file must be accessible to the MTA. In Debian, Postfix runs in a chroot jail in /var/spool/postfix by default, so the socket must be below that path.

Postfix does not prescribe a standard location for UNIX sockets in its chroot. You can mimic the /run directory hierarchy, and place the socket below /var/spool/postfix/run/opendkim, or you can simply claim a top-level directory like /var/spool/postfix/opendkim. Here we go with the latter.

First, create the directory, owned by opendkim and world-inaccessible:

sudo mkdir -m o-rwx /var/spool/postfix/opendkim
sudo chown opendkim: /var/spool/postfix/opendkim

Then, configure the socket in /etc/opendkim.conf:

Socket local:/var/spool/postfix/opendkim/opendkim.sock

Next, add user postfix to group opendkim. Postfix then relies on the group permissions to actually access the socket:

sudo adduser postfix opendkim

Finally, adjust the Postfix configuration in /etc/postfix/main.cf to use the desired socket path:

smtpd_milters = unix:/opendkim/opendkim.sock
non_smtpd_milters = $smtpd_milters

Don’t forget to restart opendkim and postfix to apply the settings.

DNS resolution

In Debian, opendkim is compiled with libunbound, a DNSSEC-capable asynchronous resolver library. It is important to be aware of this, because it means opendkim does DNS queries for DKIM keys independently, that is, it does not go through any local resolver and does not take into account configuration at /etc/resolv.conf.

The default opendkim configuration ships with a valid trust anchor setting, TrustAnchorFile /usr/share/dns/root.key, thus letting opendkim do DNSSEC queries out-of-the-box.

Advanced users should be aware of two additional configuration parameters.

The Nameservers parameter can be used to override the name servers that libunbound uses. For example, you may already have an Unbound resolver running locally (a relatively typical setup in a mail server). In that case, a setting like the following instructs opendkim to send DNS queries through that resolver:

Nameservers 127.0.0.1

The ResolverConfiguration parameter can be used to pass an Unbound configuration file (unbound.conf(5)) to libunbound. Using this, more sophisticated customization regarding DNS resolution in opendkim is possible.

ResolverConfiguration /etc/opendkim/unbound.conf

The Debian unbound package installs a default configuration file at /etc/unbound/unbound.conf. Do not attempt to use this file unchanged with ResolverConfiguration! opendkim will just quietly shut down.

The reason for the incompatibility is that the shipped unbound.conf includes an auto-trust-anchor-file setting, for which opendkim does not have the necessary permissions. Unfortunately, libunbound is rather fragile in this area. Prepare your own unbound.conf for opendkim and test carefully.

Postfix integration

The opendkim service functions as a milter, that is, a plugin software hooked into the SMTP processing of the Postfix MTA. To enable a milter, it is enough to tell Postfix on which socket the milter application is listening. Example /etc/postfix/main.cf:

smtpd_milters = inet:localhost:8891
non_smtpd_milters = $smtpd_milters

With opendkim, two additional milter configuration parameters in /etc/postfix/main.cf are useful.

The milter_default_action parameter determines what to do when a milter fails, for example, when it does not respond after a crash. In order to avoid losing mail, it is best to set this to accept:

milter_default_action = accept

Postfix does not pass internally-generated messages such as bounce messages to opendkim, so by default bounces are not DKIM-signed. This can be a problem if you also use a strict DMARC policy, because it may cause your unsigned bounce messages themselves to get rejected. The internal_mail_filter_classes parameter can be used to pass bounces through the milters as well:

internal_mail_filter_classes = bounce

Further details of milter usage in Postfix can be found in its MILTER_README.

Troubleshooting

Try to send a mail. If you see in /var/log/mail.log something like

Aug 13 13:18:00 yourhostname opendkim[15765]: OpenDKIM Filter: Unable to bind to port local:/var/spool/postfix/opendkim/opendkim.sock: No such file or directory
Aug 13 13:18:00 yourhostname opendkim[15765]: OpenDKIM Filter: Unable to create listening socket on conn local:/var/spool/postfix/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[17588]: warning: connect to Milter service unix:/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/opendkim/opendkim.sock correct?

If everything is correct, that does not mean your configuration of DKIM is complete: you must configure the DNS.

DNS Configuration

Add a TXT record for your example.com domain

Record Name

Record Type

Text

mail._domainkey

TXT

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.)

Testing

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

See also


CategoryNetwork