Fixing OpenDKIM Signing After Upgrading Ubuntu 18.04 LTS → 24.04 LTS

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After a long-planned upgrade from Ubuntu 18 LTS straight to Ubuntu 24 LTS, I discovered that outgoing e-mail was no longer being signed with OpenDKIM. What followed was a small adventure involving masked services, silent failures, and a missing package. Here is the full story for anyone who runs into the same issue.

1. First symptoms: DKIM signing stopped working

Right after the upgrade, outgoing mail lost its DKIM signatures.
A look into /var/log/mail.log showed a clear clue:

postfix/smtpd[153304]: warning: connect to Milter service inet:localhost:8892: Connection refused

Port 8892, where OpenDKIM normally listens, was not responding:

# ss -tuln | grep 8892
(no output)

So the DKIM milter was simply not running.

2. Trying to start OpenDKIM — and finding it masked

My next step was obvious: check the service.

# systemctl start opendkim
Failed to start opendkim.service: Unit opendkim.service is masked.

Status confirmed it:

# systemctl status opendkim
○ opendkim.service
     Loaded: masked (Reason: Unit opendkim.service is masked.)
     Active: inactive (dead)

Why the upgrade masked the service is unclear, but unmasking is straightforward:

# systemctl unmask opendkim
# systemctl start opendkim

This time the service started—at least according to systemd.
But DKIM signatures were still missing.

3. Adding debug logging

To see what was going on, I enabled more verbose logging by adding to /etc/opendkim.conf:

SyslogSuccess           yes
LogWhy                  yes

After that:

# systemctl restart opendkim

The service looked healthy, no visible errors... but still no signs of DKIM signing.

4. The surprising discovery: OpenDKIM wasn’t installed (!?)

Running locate gave the final hint:

# locate opendkim
opendkim:

It essentially returned nothing.
At this point it became clear: the service files were present, but the actual OpenDKIM binary was not installed (a side effect of the distribution jump).

A quick explicit install solved everything:

# apt install opendkim

And — voilà — DKIM signing immediately started working again.

5. Conclusion

Upgrading directly from Ubuntu 18 → 24 can leave some services in a strange state. In my case:

• opendkim.service was masked after the upgrade

• The service files survived, but the binary package was missing

• Postfix failed to talk to the milter (Connection refused)

• Installing OpenDKIM manually restored full functionality