John Horne with the rkhunter project replied, and his suggestion does indeed solve the problem for me, so we can go ahead and close this.
For some reason, I thought that rkhunter's ALLOW_SSH_ROOT_USER directive wanted "PermitRootLogin", instead of PermitRootLogin's *value* in /etc/ssh/sshd_config! This was operator error on my part.
Rather, rkhunter's ALLOW_SSH_ROOT_USER directive wants the same *value* that is assigned to PermitRootLogin in /etc/ssh/sshd_config.
Changing rkhunter's config to ALLOW_SSH_ROOT_USER=yes fixes the issue. My bad!
Thank you, François, I really appreciate it!
John Horne with the rkhunter project replied, and his suggestion does indeed solve the problem for me, so we can go ahead and close this.
For some reason, I thought that rkhunter's ALLOW_SSH_ROOT_USER directive wanted "PermitRootLogin", instead of PermitRootLogin's *value* in /etc/ssh/ sshd_config! This was operator error on my part.
Rather, rkhunter's ALLOW_SSH_ROOT_USER directive wants the same *value* that is assigned to PermitRootLogin in /etc/ssh/ sshd_config.
Changing rkhunter's config to ALLOW_SSH_ ROOT_USER= yes fixes the issue. My bad!
Nothing to see here, folks!