Re: [gentoo-user] {OT} Are "push" backups flawed?

[Grant]

>> A little while ago I set up an automated backup system to back up the
>> data from 3 machines to a backup server.  I decided to use a
>> push-style layout where the 3 machines push their data to the backup
>> server.  Public SSH keys for the 3 machines are stored on the backup
>> server and restricted to the rdiff-backup command.  Each of the 3
>> machines pushes their data to the backup server as a different user
>> and the top directory of each backup is chmod 700 to prevent any of
>> the 3 machines from reading or writing a backup from another machine.
>>
>> I've run into a problem with this layout that I can't seem to solve,
>> and I'm wondering if I should switch to a pull-style layout where the
>> backup server pulls data from each of the 3 machines.
>>
>> The problem with my current push-style layout is that if one of the 3
>> machines is compromised, the attacker can delete or alter the backup
>> of the compromised machine on the backup server.  I can rsync the
>> backups from the backup server to another machine, but if the backups
>> are deleted or altered on the backup server, the rsync'ed copy on the
>> next machine will also be deleted or altered.
>>
>> If I run a pull-style layout and the backup server is compromised, the
>> attacker would have root read access to each of the 3 machines, but
>> the attacker would already have access to backups from each of the 3
>> machines stored on the backup server itself so that's not really an
>> issue.  I would also have the added inconvenience of using openvpn or
>> ssh -R for my laptop so the backup server can pull from it through any
>> router.
>>
>> What do you think guys?  Are push-style backups flawed and unacceptable?
>>
>
> No, it's not flawed, as long as the implementation is right: versioning and
> deduplication.
>
> With versioning, an attacker (or infiltrator, in this matter) might try to
> taint the backup, but all she can do is just push a new version to the
> server. You can recover your data by reverting to a prior version.

Is that true?  Wouldn't the infiltrator be able to craft some sort of
rdiff-backup command that deletes the entire backup?  I can't come up
with such a command myself, but I thought I was essentially giving
full read/write access of a system's backup to an infiltrator by
putting that system's public key on the backup server.  I do restrict
the key like command="rdiff-backup --server" but I didn't expect that
to completely prevent the backup from being wiped out.  Does it?

- Grant


> The deduplication part is only to save storage space. It's less necessary if
> you have a robust versioning system that can categorize each push as either
> canonical/perpetual/permanent or ephemeral/temporary. The system can just
> discard old ephemeral pushes when storage becomes critical.