Fail2ban was working fine on Daedulus but failed when upgraded to Excalibur, with the same unhelpful ERROR message mentioned elsewhere.

I don't think I needed to change my logging target - I'm logging to /var/log/fail2ban.log - but for others perhaps worth checking the default which is set by

logtarget = /var/log/fail2ban.log

in /etc/fail2ban/fail2ban.conf .

There are two issues I found.

1) pyinotify. The package python3-pyinotify was not installed.

apt-get install python3-pyinotify

2) The folder /etc/fai2ban/jail.d contains the file defaults.debian.conf with the code:

[DEFAULT]
banaction = nftables
banaction_allports = nftables[type=allports]

[sshd]
backend = systemd
journalmatch = _SYSTEMD_UNIT=ssh.service + _COMM=sshd
enabled = true

I ban using nftables so those lines worked for me. If you use iptables or another firewall then you would need to change this.

However the only jail enabled [sshd] has backend = systemd, this is what breaks fail2ban on a Devuan install.
Ironically I don't strictly need this jail as my mail server in on my local network and I've closed port 22 to the internet at the router.

Rather than amend this package file, which, of course could be overwritten at an update, I found the solution that worked for me was to add backend=pyinotify to each jail I enabled in /etc/fai2ban/jail.local/ .

So:

[DEFAULT]
# use nftables
banaction = nftables
banaction_allports = nftables[type=allports]

# Whitelist specific IPs or ranges
ignoreip = 127.0.0.1/8 192.168.1.0/24

[sshd]
backend = pyinotify
enabled = true

[apache-auth]
backend = pyinotify
enabled = true

[dovecot]
backend = pyinotify
enabled = true
port    = pop3,pop3s,imap2,imaps,submission,465,sieve
maxretry = 3
findtime = 12h
bantime = 3d

[postfix]
backend = pyinotify
enabled = true
mode = auth
logpath = /var/log/mail.log
maxretry = 3
findtime = 12h
bantime = 3d

and so on for a couple more, non standard jails/filters I enable.

I see that python3-systemd is still marked as a dependency and python3-inotify simply as a recommend.
On my upgrade python3-systemd wasn't installed either, maybe it would have been on a systemd box.

This is exactly the problem Steve_V identified last year, not up to me whether it's worth making Devuan specific changes to make upgrading or installing fail2ban work out of the box.

TC

Cheers!

I didn't get any joy with the above examples or nixers script.

I eventuall tried to roll-back to fail2ban_1.0.2-2_all.deb.
python3-pyinotify was already installed.

When rollingback I got an error saying asynchat was not installed, so I installed python3-standard-asynchat_3.13.0-5_all.deb
and the fail2ban script started with out error.

I've only done one reboot since, so YMMV, but I hope it helps some of you here.
I'll see what happends when I eventually upgrade fail2ban later.
Cheers!

Nice one nuking the image BTW, did that land a little close to home?

(see above post)

maybe you are getting paid for your efforts.

I have posted the information required to solve the OPs problem, because nobody else was doing that. As a result, this thread is now [SOLVED].
What did you contribute here?

It looks like there you have a task to volunteer for.

You mean giving the forum autoresponder gramophone a whack, because it's stuck again?

There is no "licencing problem". Some APIs were deprecated or moved to external packages, that is all.

But apparently not reading, because g4sra already explained that.

As usual, a bunch of whinging and finger pointing about "them", "forced" changes, "clusterfuck" upstreams, and a bit of misdirection regarding "awful licence changes"...
While the real problem is right in our back yard, and has nothing to do with any of the favourite punching-bags. Nothing ever changes around here, does it?

But which package has the licensing problem? And if its python itself, say so.

If so, does this mean python has a problematic license like rust or java?

Asking...

@OP, make sure all your jails are using the 'polling', 'pyinotify' (if you have that installed) or 'auto' backends.

I had my backends set to "auto", and I even tried "polling".  I have not tried "pyinotify" yet, but if it works, I will report back here.

EDIT:  I installed

and it worked! 

In my jail.local file I have the backend set at:

Maybe the package does not need to be rebuilt, yet, but when this issue comes up again with someone else, pass the word about a missing dependency in devuan "python3-pyinotify"