Beware rdiff-backup. It certainly does turn rsync (not a backup program) into a backup program.

However, I used rdiff-backup in the past and it can be a bit problematic. If I remember correctly, every “snapshot” you keep in rdiff-backup uses as many inodes as the thing you are backing up. (Because every “file” in the snapshot is either a file or a hard link to an identical version of that file in another snapshot.) So this can be a problem if you store many snapshots of many files.

But it does make rsync a backup solution; a snapshot or a redundant copy is very useful, but it’s not a backup.

(OTOH, rsync is still wonderful for large transfers.)