[rdiff-backup-users] Backing up the backup server

[Brian C]

Hi,

I have a backup server running OpenSUSE, call it 'backup-server' and a 
dns server running Debian, call it 'dns-server'. They are on the same 
subnet. Last night I got rdiff-backup 1.0.4 installed on both so that 
backup-server backs up dns-server via a cron job, basically following 
the instructions at http://www.howtoforge.com/linux_rdiff_backup.

However, I cannot similarly get backup-server to backup itself via the 
backup user's cron job. Instead I have to let root run the cron job. 
Every time the backup user runs rdiff-backup it requests a password 
rather than running without requiring interaction.

Question 1: For a purely local backup such as this, is there any reason 
(security concerns, for instance) to prefer that it not run as root, and 
instead to have it run as the backup user?

If the answer to question 1 is yes, then:

Question 2: Why does rdiff-backup keep asking me for a password, instead 
of working without interaction as it does when it backs up dns-server? 
What are the things I should check?

I already looked at /home/backup/.ssh/config and 
/root/.ssh/authorized_keys and /etc/ssh/sshd_config and everything looks 
just as good as it did for the server that works.

I also tried the sudoers method described here:
http://www.arctic.org/~dean/rdiff-backup/unattended.html

and I also get asked for a password by sudo! Aack!?!

Thanks for any help.

Brian