Netcat

Netcat is a wonderfully versatile tool that has been dubbed the “hackers’ Swiss army knife.” The simplest definition of Netcat is “a tool that can read and write to TCP and UDP ports.” This dual functionality suggests that Netcat runs in two modes: client and server.

Connecting to a TCP/UDP port can be useful in several situations:

• To check if a port is open or closed
• To read a banner from the port
• To connect to a network service manually

Please take time to inspect Netcat’s command line options:
root@bt:~# nc -h
[v1.10-38]
connect to somewhere: nc [-options] hostname port[s] [ports] …
listen for inbound: nc -l -p port [-options] [hostname] [port]
options:
-c shell commands as `-e’; use /bin/sh to exec [dangerous!!]
-e filename program to exec after connect [dangerous!!]
-b allow broadcasts
-g gateway source-routing hop point[s], up to 8
-G num source-routing pointer: 4, 8, 12, …
-h this cruft
-i secs delay interval for lines sent, ports scanned
-k set keepalive option on socket
-l listen mode, for inbound connects
-n numeric-only IP addresses, no DNS
-o file hex dump of traffic
-p port local port number
-r randomize local and remote ports
-q secs quit after EOF on stdin and delay of secs
-s addr local source address
-T tos set Type Of Service
-t answer TELNET negotiation
-u UDP mode
-v verbose [use twice to be more verbose]
-w secs timeout for connects and final net reads
-z zero-I/O mode [used for scanning]
port numbers can be individual or ranges: lo-hi [inclusive];
hyphens in port names must be backslash escaped (e.g. ‘ftp\-data’).
root@bt:~#