Linux and Open Source | Main / NetcatUtility

nc - arbitrary TCP and UDP connections and listens

INSTALL

[centos@centos ~]$ sudo yum install nc
Loaded plugins: fastestmirror, refresh-packagekit
Loading mirror speeds from cached hostfile
 * base: ftp.iitm.ac.in
 * extras: ftp.iitm.ac.in
 * rpmforge: fr2.rpmfind.net
 * updates: ftp.iitm.ac.in
Setting up Install Process
Resolving Dependencies
--> Running transaction check
---> Package nc.x86_64 0:1.84-22.el6 set to be updated
--> Finished Dependency Resolution

nc.x86_64 : Reads and writes data across network connections using TCP or UDP

The nc (or netcat) utility is used for just about anything under the sun involving TCP or UDP. It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6. Unlike telnet(1), nc scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as telnet(1) does with some.

     Common uses include:

           ·   simple TCP proxies
           ·   shell-script based HTTP clients and servers
           ·   network daemon testing
           ·   a SOCKS or HTTP ProxyCommand for ssh(1)
           ·   and much, much more

EXAMPLE:

[centos@centos ~]$ sudo nc -v 127.0.0.1 80

Connection to 127.0.0.1 80 port [tcp/http] succeeded!
[centos@centos ~]$

HELP

[centos@centos ~]$ nc -h
usage: nc [-46DdhklnrStUuvzC] [-i interval] [-p source_port]
	  [-s source_ip_address] [-T ToS] [-w timeout] [-X proxy_version]
	  [-x proxy_address[:port]] [hostname] [port[s]]
	Command Summary:
		-4		Use IPv4
		-6		Use IPv6
		-D		Enable the debug socket option
		-d		Detach from stdin
		-h		This help text
		-i secs		Delay interval for lines sent, ports scanned
		-k		Keep inbound sockets open for multiple connects
		-l		Listen mode, for inbound connects
		-n		Suppress name/port resolutions
		-p port		Specify local port for remote connects
		-r		Randomize remote ports
 		-S		Enable the TCP MD5 signature option
		-s addr		Local source address
		-T ToS		Set IP Type of Service
		-C		Send CRLF as line-ending
		-t		Answer TELNET negotiation
		-U		Use UNIX domain socket
		-u		UDP mode
		-v		Verbose
		-w secs		Timeout for connects and final net reads
		-X proto	Proxy protocol: "4", "5" (SOCKS) or "connect"
		-x addr[:port]	Specify proxy address and port
		-z		Zero-I/O mode [used for scanning]
	Port numbers can be individual or ranges: lo-hi [inclusive]
[centos@centos ~]$

CLIENT/SERVER MODEL

It is quite simple to build a very basic client/server model using nc. On one console, start nc listening on a specific port for a connection. For example:

$ nc -l 1234

nc is now listening on port 1234 for a connection. On a second console (or a second machine), connect to the machine and port being listened on:

$ nc 127.0.0.1 1234

There should now be a connection between the ports. Anything typed at the second console will be concatenated to the first, and vice-versa. After the connection has been set up, nc does not really care which side is being used as a ‘server’ and which side is being used as a ‘client’. The connection may be terminated using an EOF (‘^D’).