You need to do several things to make sure sshd works:
- ssh-keygen -A, in case a key is missing (happens when openssh updates add new key algorithms)
- useradd sshd, this user needs to exist for the SSH server to run
- enable permitRootLogin in sshd settings and allow the root user to log in with a password
- and of course, set the password using passwd
You can alternatively create other users for remote login, in which case you don’t need permitRootLogin. But such users will be restricted. They can’t connect to app_server, so they can’t launch any BApplication. This includes some command line tools such as vim and unzip.