Transfer files between two host with nc

Problem: you have two hosts that you can access but there is no mechanism
to transfer files between them - no ssh(scp/sftp), no ftp etc.
How to do it ?!

Solution: use nc and tar/dd/echo ...

# transfer by tar of a directory
|destination_host|                     |source_host|
nc -l 9000 | tar xvf -                  tar cvf - /my_dir  | nc destination_host 9000

# we listen on all interfaces          # we tar my_dir to STDOUT(-) and all is piped to 
# on port 9000, all that comes in      # nc that will connect on destination_host on port 9000
# is piped to tar xvf (will extract)   # and will transfer what ever is given
# - means take the STDIN

# transfer by dd of a partition /dev/sda3
|destination_host|                     |source_host|
nc -l 9000 | dd of=/backup_device      dd if=/dev/sda3  | nc destination_host 9000

# we listen on all interfaces          # we dd in /dev/sda3 (reading) all is piped to 
# on port 9000, all that comes in      # nc that will connect on destination_host on port 9000
# is piped to dd and dd will write it  # and will transfer what ever is given
# all to /backup_device

As you can see this becomes very useful because you can open the destination port as you need it
and even transfer from a block device as /dev/sda3 in the example. Once the transfer is done
on the destination host nc stops to listen for the port you asked(there is a switch to make it
to continue listening -k, this won't work -l however).