Excalibur. After installation of fail2ban fails, dpkg fails too: 251204 09:31 ~ root@basil# dpkg --configure -a Setting up fail2ban (1.1.0-8) ... Starting Authentication failure monitor: fail2ban failed! invoke-rc.d: initscript fail2ban, action "start" failed. dpkg: error processing package fail2ban (--configure): installed fail2ban package post-installation script subprocess returned error exit status 1 Errors were encountered while processing: fail2ban 251204 10:37 ~ root@basil# Despite the above, fail2ban seems to be running anyway, but it complains: 251204 10:42 ~ root@basil# service fail2ban start Starting Authentication failure monitor: fail2ban2025-12-04 10:42:47,748 fail2ban [31706]: ERROR NOK: (38, 'Function not implemented') failed! 251204 10:42 ~ root@basil# After editing /etc/fail2ban/jail.d/defaults.debian so that... [sshd] backend = systemd ...becomes... [sshd] backend = auto then fail2ban can be started without complaint. (I didn't test its functionality.) However, the installation failure (via dpkg, again the same result as shown above) continues to obstruct further progress in my migration from Debian to Devuan. I seem to have little choice but to do without fail2ban, for now.