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I have several installs of Ubuntu 18.04.2 inside LXC containers in several nodes running Proxmox 4.4-24 (Debian 8.11).

On any of these, When I try to login as root on the console, I enter "root", push enter, and after a few seconds I get "Login incorrect".

Logging in as a non-root user works fine.

Logging in as root works fine in other installs of Ubuntu 18.04.2 inside LXC containers in several nodes running Proxmox 5.3-11 (Debian 9.8).

How do I even begin to debug this problem?

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I debugged this by looking in /var/log/auth.log.

The unsuccessful logins under Proxmox 4.4 looked like

Feb 28 10:19:56 host1 login[19577]: pam_securetty(login:auth): access denied: tty '/dev/pts/0' is not secure !
Feb 28 10:19:59 host1 login[19577]: FAILED LOGIN (1) on '/dev/pts/0' FOR 'root', Authentication failure

while the successful logins under Proxmox 5.3 looked like (ignore that middle line)

Feb 28 10:37:37 host2 login[3519]: pam_unix(login:session): session opened for user root by LOGIN(uid=0)
Feb 28 10:37:37 host2 login[3519]: pam_systemd(login:session): Failed to create session: Seat has no VTs but VT number not 0
Feb 28 10:37:37 host2 login[3557]: ROOT LOGIN  on '/dev/tty1'

Something changed in Proxmox 5.3 so that the virtual console is now on /dev/tty1 rather than /dev/pts/0 under Proxmox 4.4

I have solved this by adding

pts/0

to /etc/securetty. I'm not sure if this is introducing a security hole though. I hope we can upgrade our Proxmox nodes soon.

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