Debian – Backuppc won’t backup due to no ping success

You don't necessarily need to have the computers establish a VPN connection on a timed basis. BackupPC will to backups on laptops and such whenever they are reachable. Partially, completed backups will be finished next time it connects.

Initial backups require transferring all the files being backed up. These will be the longest backups. Once that is done all subsequent backups will only require the differences from the last backup to be transferred. About 1% of the files will be transferred to validate the contents on the server.

You can use the alias to provide an alternate address to contact the fixed PCs at remote locations. You can also override the commands used to determine if a PC is reachable on a client by client basis. You can also run commands before and after a backup. This should give you lots of flexibility to handle remote PCs with fixed.

Ideally, you want the laptops to be reachable from time to time. However, if you can script a post to the web interface, the laptops should be able to trigger their own backups whenever they connect.

It may be better to install VPN software on the laptops and let BackupPC pickup the laptops when they are available. BackupPC will email the user specified in the host's user column if the backup gets too old.

The --server --sender are options that used when you are running rsync over a pipe. The rsync server is setup on the remote side so that it can do all checksum and delta-encoding magic so that you are only transferring the changed bits over the wire. This are probably not documented in the man page since they are automatically added, and usually don't need to know or care about them.

From man rsync(1)

The options --server and --sender are used internally by rsync, and should never be typed by a user under normal circumstances. Some awareness of these options may be needed in certain scenarios, such as when setting up a login that can only run an rsync command. For instance, the support directory of the rsync distribution has an example script named rrsync (for restricted rsync) that can be used with a restricted ssh login.