Netcat -- reading from and writing to network connections on either TCP or UDP

Our sample setup
-----------------------
HostA // 192.168.1.1
------------------------
           sda
        NETWORK
           sdb
------------------------
HostB // 192.168.1.2
-------------------------
Your task is copy HostA /dev/sda to HostB's /dev/sdb using netcat command. First login as root user

Command to type on hostB (receiving end ~ write image mode)

You need to open port on hostB using netcat, enter :
# netcat -p 2222 -l |bzip2 -d | dd of=/dev/sdb
Where,

-p 2222 : Specifies the source port nc should use, subject to privilege restrictions and availability. Make sure port 2222 is not used by another process.

-l : Used to specify that nc should listen for an incoming connection rather than initiate a connection to a remote host.

bzip2 -d : Compresses image using the Burrows-Wheeler block sorting text compression algorithm, and Huffman coding. This will speed up network transfer ( -d : force decompression mode)

dd of=/dev/sda : /dev/sda is your hard disk. You can also specify partition such as /dev/sda1

Command to type on hostA (send data over a network ~ read image mode)

Now all you have to do is start copying image. Again login as root and enter:
# bzip2 -c /dev/sda | netcat hostA 2222
OR use IP address:
# bzip2 -c /dev/sda | netcat 192.168.1.1 2222

This process takes its own time.

A note about latest netcat version 1.84-10 and above

If you are using latest nc / netcat version above syntax will generate an error. It is an error to use -l option in conjunction with the -p, -s, or -z options. Additionally, any timeouts specified with the -w option are ignored. So use nc command as follows.

On hostA, enter:
# nc -l 2222 > /dev/sdb
On hostB, enter:
# nc hostA 2222< /dev/sda
OR
# nc 192.168.1.1 2222< /dev/sda

Using a second machine (hostB), connect to the listening nc process at 2222 (hostA), feeding it the file (/dev/sda)which is to be transferred. You can use bzip2 as follows.
On hostA, enter:
# nc -l 2222 | bzip2 -d > /dev/sdb
On hostB, enter:
# bzip2 -c /dev/sda | nc 192.168.1.1 2222

Further readings

netcat command other usage - Linux port scanning with netcat (nc) command (you can also use nmap command for port scanning)

nc and bzip2 command man page

A good collection of netcat command usage

Another good netcat disk image howto

How do I improve performance?

As suggested by anonymous user:

You should definitely use bs=16M or something like that. Otherwise, the copy will take forever. Copying a 300 GB hard drive over a 1 Gbps cross-over cable took about 1 1/2 hours or so using bs=16M Without this option, the same thing would have taken about 7 hours.

In short use command as follows:
# netcat -p 2222 -l |bzip2 -d | dd of=/dev/sdb

News	See Also	Recommended Links	Pipes	Sockets	Nmap
ntop	ngrep	nettee	Netcat	Humor	Etc

Option	Description
-d	Allows netcat to detach from the console on Windows NT.
-e	Executes a program if netcat is compiled with the –DGAPING_SECURITY_HOLE.
-i	Sets the interval time. Netcat uses large 8K reads and writes. This basically sends data one line at a time. This is normally used when data is read from files or pipes.
-g	Used to construct a loose-source-routed path for your connection. This is modeled after "traceroute".
-G	Positions the "hop pointer" within the list.
-l	Forces netcat to listen for an inbound connection. An example "nc –l –p 1234 <filename" tells netcat to listen for a connection on port 1234 and once a connection is made to send the file named filename. The file is sent whether the connecting system wants it or not. If you specify a target host netcat will only accept an bound connection only from that host and if you specify one, only from the specified foreign source port.
-L	Restarts Netcat with the same command line that was used when the connection was started.. This way you can connect over and over to the same Netcat process.
-n	Forces netcat to only accept numeric IP addresses and to not do any DNS lookups for anything
-o	Used to obtain a hex dump file of the data sent either way, use "-o logfile". The dump lines begin with "<" or ">" to respectively indicate "from the net" or "to the net", and contain the total count per direction, and hex or ascii representations of the traffic.
-p	Required for outbound connections. The parameter can be numeric or a name as listed in the services file. If –p is not used netcat will bind to whatever unused port the systems gives it, unless the –r option is used.
-r	Causes port scanning to be done randomly. Normally it is done highest to lowest.
-s	Used to specifiy local network source address. Usage "-s ip-addr" or "-s name".
-t	Enables netcat to respond to telnet option negotiation if netcat is compiled with –DTELNET parameter. Telnet daemons will get no useful answers, as they would from a telnet program.
-u	Tells netcat to use UDP instead of TCP.
-v	Controls the level of verbosity. (without –n) netcat will do a full forward and reverse name and address lookup for the host, and warn you about the all-to-common problem of mismatched names in the DNS. Usually want to use the –w 3, which limits the time spent trying to make a connection. If multiple ports are given –v must be specified twice.
-w	Limits the time spent trying to make a connection.
-z	Prevents sending any data to a TCP connection and very limited probe data to a UDP connection. Use –i to insert a delay between each port probe. This is useful as a fast scanning mode just to see what ports the target is listening on.

Netcat -- reading from and writing to network connections on either TCP or UDP

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