Bharathi Subramanian

nc -- TCP/IP Tool to read and write data across network connection

Summary:
netcat(nc) is a simple utility which reads and writes data across network connections, using IP, TCP or UDP protocol. It is designed to be a reliable "back-end" tool that can be used directly or easily driven by other programs and scripts. It is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities.

We can use netcat with-in local system. But just for better understanding, I use two systems A and B. IP of A is 192.168.1.1 and B is 192.168.1.2.

Examples:

 
(A) $ nc B 21 -- Connect to port 21 on B 
 
(B) $ nc -l -p 5000 -- Listen on port 5000 in B. 
 
(A) $ nc B 5000 -- Connect to B on port 5000. Whatever typed
in A will goto B and vice-versa. 
 
(A) $ nc -o dump.txt B 5000 -- Same as above. But all traffic
will be dumped in the file in hex format. 
 
(B) cat fileB.txt | gzip -9 | nc -l 5000 -- Send the file through
port 5000 and on-the-fly compress it. 
 
(A) nc B 5000 > fileA.gz -- Receive a file from B and save it in A.
 
(A) $ echo "Hello" | nc -w 1 B 5000 -- Connect to B on port 5000
and pass "Hello" message to it. 
 
(B) nc -l -p 5000 -e /bin/bash -- Listen on port 5000. If anyone
connected this, provide the bash shell. (Simple way to open a 
backdoor). 
 
(A) $ nc -zv B 10-100 -- Simple TCP port scan on B. Option -z
makes the netcat not to wait for any response from B. 
 
(A) $ nc -zvu B 10-100 -- Simple UDP port scan on B.