Here is an article which already covers the group 0 part of your question.
Well I'm not sure if this is the best solution, but this is what I did, and I welcome comments/improvements/suggestions.
I recently set up a remote backup for a clients drupal website, my centralised backup server was accessing a remote server and pulling. I followed this principle.
- Created a user (on remote) which I wanted to use as admin, this user was permitted to login via ssh, but not using a password, only via a public/private key. (which you do need to keep safe and backed up at your admin location).
- Disabled root login via ssh, on the remote.
- Permitted my admin user to become root via sudo.
Note the user who would use this is me, and I don't sit on the backup server. So I used this to set up the backup below.
Now I can admin remote, root can't login only my special admin user and only via key certs.
Created a backup user (the user who will perform the backups) on both remote and local, it wasn't the same username, I called her something a bit unusual (on remote), so even if the key was obtained, they would need to guess the remote user name too. The user is not priviledged and could not access the directories (or dump the database without password) on the remote.
I permitted this User on the remote to run one command in /etc/sudoers
myobscurebackupuser ALL=NOPASSWD: /usr/bin/rsync
On my central backup server, under the user (who again is un-priviledged) I store the public/private key and the backup script which I want to run. Only this user can gain access to script/keys for reading (well apart from root).
my backup was two steps
'ssh -i /path/to/key/file obscureuser@remote mysqldump [options] | gzip' | gunzip > local_dbdump.sql
_Note The first half is excuted on remote (up to gzip), and second half runs locally.
Step 2 rsync remote directory structure
The password and the database dump are not stored on the remote, they are passed in over the ssh connection. The db_dump is piped directly into gzip which sends it back over stdout. I pipe this through gunzip and direct stdout into local db dump file.