Set Up Disk Cloning with dd, Netcat, and SSH Tunnel

dddisk-imagenetcatssh-tunnel

I would like to copy stuff in bulk (reimage disk using dd) with netcat from host A to B via ssh encrypted channel on Linux.

What commands should I type on both ends?

Best Answer

Copying from source to target where target has sshd running:

  • dd if=/dev/sda | gzip | ssh root@target 'gzip -d | dd of=/dev/sda'

Copying from source to target via sshd_host when target is not running sshd.

  • Target: nc -l -p 62222 | dd of=/dev/sda bs=$((16 * 1024 * 1024))
  • Source: ssh -L 62222:target:62222 sshd_host &

  • Source: dd if=/dev/sda | nc -w 3 localhost 62222

    dd - if= is the source, of= is the destination, bs= is the block size. Different block sizes may improve performance. 16 is usuually a fairly reasonable starting point. You can also use count= to indicate how many blocks to copy.

    nc - -p indicates the port to use for services. -l is used to start a service. -w sets up the time to wait for data in the pipline before quiting.

    ssh - -L sets up the tunnel on the remote host. The format of the argument is, local_port:target_host:target_port. Your local program (nc) connects to the local_port, this connection is tunneled and connected to target_port on the target_host.

The options defined are just the ones used for this. Look at the man pages for more details.

A few notes:

  1. If you are doing this over anything but a LAN, I'd suggest compressing the datastream with gzip or compress. Bzip2 would work too but it takes a bit more CPU time. The first one has an example of that usage.
  2. Its better if the source partition is not mounted or is mounted read-only. If not you will need to fsck the destination image.
  3. Unless one of the machines has netcat but not ssh, netcat isn't really needed here. That case would look like:

source machine dd -> nc -> ssh -> ssh tunnel -> sshd server -> nc on target -> dd

  1. dd works best if the source and targets are the same size. If not the target must be the bigger of the 2.
  2. If you are using ext2/3 or xfs, dump (or xfsdump) and restore may be a better option. It wont handle the boot sector but it works when the target and source are different sizes.
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