UGH, this problem was ultimately due to a malformed iptables rule ...
The server in question that was failing to be backed up was our LDAP
server ... I guess due to the PAM hooks that fire off when BackupPC
runs QueueAllPCs() (nightly backup), it was causing a storm of LDAP
traffic as the BackupPC_dump commands were forking off ... This caused
our rule on the LDAP server that rate limits SSH traffic to
misinterpret the LDAP traffic as an SSH attack ...


On Mon, Jan 4, 2010 at 6:13 PM, David Esposito
<backuppc-users AT esposito.newnetco DOT com> wrote:
> I have a server that keeps failing its automated scheduled backup with
> the dreaded 'Unable to read 4 bytes'. However, if I initiate either a
> full or incremental backup via the web UI, it completes perfectly.
> This rules out any SSH keypair problems, in my opinion. I have a dozen
> successful backups, all initiated manually .. yet every night, I get
> the error email about not being able to back up the server.
>
> Here's my XferLOG.bad.z:
>
> incr backup started back to 2009-12-30 18:14:15 (backup #4) for directory /
> Running: /usr/bin/ssh -q -x -l root www.whatever.com /usr/bin/rsync
> --server --sender --numeric-ids --perms --owner --group -D --links
> --hard-links --times --block-size=2048 --recursive
> --checksum-seed=32761 . /
> Xfer PIDs are now 1952
> Read EOF: Connection reset by peer
> Tried again: got 0 bytes
> Done: 0 files, 0 bytes
> Got fatal error during xfer (Unable to read 4 bytes)
> Backup aborted (Unable to read 4 bytes)
>
> I have tried running the SSH command line above .. no errors .. it
> just sits there and waits ... But based on my reading and
> understanding of the --server and --sender options, that seems to be
> the expected behavior since it's waiting for the other end of the
> connection to connect and negotiate the rsync.
>
> I've spent a zillion hours trying to diagnose this and am at the end
> of my rope. I have 3 different BackupPC installations in our 3
> different data centers and have never seen this. Also, for this
> BackupPC server instance, I have a dozen other client machines that
> are successfully being backed up. It's just this one single box.
> Lastly, all of the client machines are 'in the cloud' with the same
> provider so there is no transient reachability issue. All of the
> client machines are also running the same CentOS (5.4) image so the
> kernel versions and rsync versions are identical.
>
> This is a mystery!
>
> Thanks in advance,
> Dave
>

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