BackupPC gotchas for PCBSD.

I am getting BackupPC configured to back up a new PCBSD desktop. Every time I do this, I have to relearn what tricks are required, so here is my troubleshooting checklist. Probably applicable to most Linux and Unix-like Operating Systems as well. If you've tried getting this set up before, the useless BackupPC error "Unable to read 4 bytes" has probably become the bane of your existence.

1. Does the client sshd configuration allow root logins? Check /etc/ssh/sshd_config, specifically the the PermitRootLogin parameter. Unless you absolutely need password authentication for root, use the following configuration: PermitRootLogin without-password
2. Make sure that the client has the public ssh for the BackupPC user on the server included in /root/.ssh/authorized_keys. If you haven't generated an ssh key for BackupPC, you should do that.
3. The permissions for /root/.ssh/authorized_keys on the client should be 640. /root/.ssh should be 750. Ownership should be root:wheel.
4. The .ssh/known_hosts file for the server BackupPC user should contain the fingerprint for the client. If you have connected to the client from the server before as another user (root), the fingerprint may already be in that user's known_hosts file. You can copy it into the BackupPC user's file. If you have not already connected to this host, you can verify the client configuration by connecting with the BackupPC ssh keys like so: ssh -i /path/to/bpc/.ssh/id_rsa client.example.org. The hostname you use (and known_hosts contains) MUST contain the exact hostname BackupPC is configured to connect to. If you connect to 'client' (leveraging your domain search list), but BackupPC is configured to connect to 'client.example.org', the ssh connection will fail.