Hi,

Paolo Basenghi wrote on 2014-07-01 16:28:45 +0200 [Re: [BackupPC-users] How 
does backuppc really work?]:
> 
> Il 01/07/2014 15:37, HAL9000 ha scritto:
> > [...] My specific question regarding this is how after the backuppc
> > server logs into a remote machine using ssh then backup the data to
> > the backuppc server.

BackupPC doesn't really "log into a remote machine" (it doesn't start an
interactive ssh session). It executes something like
"ssh -l someuser somehost rsync --server --sender ..." (or tar instead of
rsync), i.e. it "executes a command on a remote machine" and writes/reads the
standard in-/output of that command (via ssh). This pretty much answers the
following question.

> > How, once logged in to the remote machine, does the remote client
> > machine send the information to be backed up the backuppc server?
> > Does it go back through the same ssh tunnel that was created when
> > logging into the client,

Yes.

> > or does it make a new tunnel altogether?

No.

The requirements of BackupPC are that the BackupPC server can create a
network connection to the client machine(s), not the other way around.
This is also true for SMB and rsyncd backups. In many cases it would
even be undesirable to have connections from the client to the server
machine (there might be firewalls in between, and the server might even
belong to a different company that provides backup services to many
different clients).

In many simple cases server and clients are within one administrative
domain, and you might be able to freely ssh between them. That is perfectly
ok, but there is absolutely no point in setting up *passwordless* ssh
from client to server. I'm just pointing that out, because it has come
up on the list in the past.

> [...] This tunnel becomes the transport layer for the rsync perl
> library on backuppc side, that instantiate the rsync program on the
> other side and get the data to backup.

In other words, it works exactly like 'rsync -e ssh ...', except that
the local rsync instance is replaced by BackupPC.

Regards,
Holger

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