Scripts for automatically performing incremental backups with rsync.

Setup:
Setup takes place at n + 1 locations: the "server" setup (where backups will
be stored) and the n "client" setups (where backups will originate).

Security Settings:
----------------
SERVER: 
I recommend a dedicated "backup-operator" account with sudo
permissions as such:

  backup-operator ALL=NOPASSWD: /usr/bin/rsync, /usr/bin/find

With root privileges, excute visudo and add the above line to the
/etc/sudoers file. 

Besides this, that account should not have a whole lot of access. It
would probably be a good idea to disable password login for this
account (e.g. as root: passwd -d backup-operator), or to have a very good password for it.

CLIENT: 
I recommend a dedicated "backup-operator" account with sudo
permissions as such:

  backup-operator ALL=NOPASSWD: /usr/bin/rsync

Create an ssh public/private key pair password for your
backup user (create it with NO PASSWORD if you plan on running backups
automatically):

  ssh-keygen

Copy the public key into the SERVER backup-operator's
.ssh/authorized_keys section. I recommend temporarily allowing the
server backup operator to connect via ssh with a password, then using 

  ssh-copy-id backup-operator@server

to move it over. After this, disable password authentication for
backup-operator@server.

TODO's:
* would be nice to restrict permissions further (specific rsync
  commands, or something like that)
* create a security-setup script to automate this process: disallow
  commands to run unless checks pass (e.g. user is not root, etc)

Script Settings:
---------------
SERVER:
* Clone/copy the server directory to your server, under the
  backup-operator account's home directory. 
* create some directory owned by the backup-operator account to store
  the backups. it's recommeded that this be created under the
  backup-operator account's home directory.
* OPTIONAL: If you wish to do automated checks of backups:
  * Edit the settings file to specify how much time can pass between
    backups before a warning is issued.
  * Update the BACKUP_ROOT var in settings to reflect the location of
    your backup directory.

TODO's: 
* Make settingsFile a parameter here.

CLIENT:
* clone/copy the client directory to the machine which will be backed
  up.
* Fill in the variables in the settings file above the comment which
  says "computed variables below."
* NB: the SITE value should be unique across all of your backed-up
  machines. If more than one machine claims to be the same SITE, all
  hell will almost certainly break loose.

TODO's:
* better validation/safety-checks
* add dpkg --get-selections to backup script


Scheduling Settings:
--------------------
SERVER:
If you want to automatically scan backups to make sure they are taking
place, add an entry to the backup-operator's crontab to run the
check_backups.sh script periodically. an example of this that will
mail the results out at 8 AM every morning is:

  0 8 * * * ~/check_backups.sh | mail -s "My Backup Server: Status $(date)" myemail@mydomain.com


CLIENT:
If you want to automatically perform backups, add an entry to the
backup-operator's crontab to run the backup.sh script periodically.
For example:

  10 0 * * * /home/backup-operator/backup.sh >> rsync.log 2>&1

where the redirection is optional.

TODO's
* should create a directory for /etc/cron.d on each, or script the
   addition of cron jobs