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NC(1)			  BSD General Commands Manual			 NC(1)

     nc	-- arbitrary TCP and UDP connections and listens

     nc	[-46DEdhklnorStUuvz] [-e IPsec_policy] [-i interval] [-p source_port]
	[-s source_ip_address] [-w timeout] [-X	proxy_protocol]	[-x
	proxy_address[:port]] [hostname] [port[s]]

     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:

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

     The options are as	follows:

     -4	     Forces nc to use IPv4 addresses only.

     -6	     Forces nc to use IPv6 addresses only.

     -D	     Enable debugging on the socket.

     -d	     Do	not attempt to read from stdin.

     -h	     Prints out	nc help.

     -E	     Shortcut for "-e 'in ipsec	esp/transport//require'	-e 'out	ipsec
	     esp/transport//require'", which enables IPsec ESP transport mode
	     in	both directions.

     -e	     If	IPsec support is available, then one can specify the IPsec
	     policies to be used using the syntax described in
	     ipsec_set_policy(3).  This	flag can be specified up to two	times,
	     as	typically one policy for each direction	is needed.

     -i	interval
	     Specifies a delay time interval between lines of text sent	and
	     received.	Also causes a delay time between connections to	multi-
	     ple ports.

     -k	     Forces nc to stay listening for another connection	after its cur-
	     rent connection is	completed.  It is an error to use this option
	     without the -l option.

     -l	     Used to specify that nc should listen for an incoming connection
	     rather than initiate a connection to a remote host.  It is	an er-
	     ror to use	this option in conjunction with	the -p,	-s, or -z op-
	     tions.  Additionally, any timeouts	specified with the -w option
	     are ignored.

     -n	     Do	not do any DNS or service lookups on any specified addresses,
	     hostnames or ports.

     -o	     "Once-only	mode".	By default, nc does not	terminate on EOF con-
	     dition on input, but continues until the network side has been
	     closed down.  Specifying -o will make it terminate	on EOF as

     -p	source_port
	     Specifies the source port nc should use, subject to privilege re-
	     strictions	and availability.  It is an error to use this option
	     in	conjunction with the -l	option.

     -r	     Specifies that source and/or destination ports should be chosen
	     randomly instead of sequentially within a range or	in the order
	     that the system assigns them.

     -S	     Enables the RFC 2385 TCP MD5 signature option.

     -s	source_ip_address
	     Specifies the IP of the interface which is	used to	send the pack-
	     ets.  It is an error to use this option in	conjunction with the
	     -l	option.

     -t	     Causes nc to send RFC 854 DON'T and WON'T responses to RFC	854 DO
	     and WILL requests.	 This makes it possible	to use nc to script
	     telnet sessions.

     -U	     Specifies to use Unix Domain Sockets.

     -u	     Use UDP instead of	the default option of TCP.

     -v	     Have nc give more verbose output.

     -w	timeout
	     If	a connection and stdin are idle	for more than timeout seconds,
	     then the connection is silently closed.  The -w flag has no ef-
	     fect on the -l option, i.e. nc will listen	forever	for a connec-
	     tion, with	or without the -w flag.	 The default is	no timeout.

     -X	proxy_version
	     Requests that nc should use the specified protocol	when talking
	     to	the proxy server.  Supported protocols are "4" (SOCKS v.4),
	     "5" (SOCKS	v.5) and "connect" (HTTPS proxy).  If the protocol is
	     not specified, SOCKS version 5 is used.

     -x	proxy_address[:port]
	     Requests that nc should connect to	hostname using a proxy at
	     proxy_address and port.  If port is not specified,	the well-known
	     port for the proxy	protocol is used (1080 for SOCKS, 3128 for

     -z	     Specifies that nc should just scan	for listening daemons, without
	     sending any data to them.	It is an error to use this option in
	     conjunction with the -l option.

     hostname can be a numerical IP address or a symbolic hostname (unless the
     -n	option is given).  In general, a hostname must be specified, unless
     the -l option is given (in	which case the local host is used).

     port[s] can be single integers or ranges.	Ranges are in the form nn-mm.
     In	general, a destination port must be specified, unless the -U option is
     given (in which case a socket must	be specified).

     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 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').

     The example in the	previous section can be	expanded to build a basic data
     transfer model.  Any information input into one end of the	connection
     will be output to the other end, and input	and output can be easily cap-
     tured in order to emulate file transfer.

     Start by using nc to listen on a specific port, with output captured into
     a file:

	   $ nc	-l 1234	> filename.out

     Using a second machine, connect to	the listening nc process, feeding it
     the file which is to be transferred:

	   $ nc 1234 <

     After the file has	been transferred, the connection will close automati-

     It	is sometimes useful to talk to servers "by hand" rather	than through a
     user interface.  It can aid in troubleshooting, when it might be neces-
     sary to verify what data a	server is sending in response to commands is-
     sued by the client.  For example, to retrieve the home page of a web

	   $ echo "GET"	| nc 80

     Note that this also displays the headers sent by the web server.  They
     can be filtered, using a tool such	as sed(1), if necessary.

     More complicated examples can be built up when the	user knows the format
     of	requests required by the server.  As another example, an email may be
     submitted to an SMTP server using:

	   $ nc	localhost 25 <<	EOF
	   MAIL	FROM: <>
	   RCPT	TO: <>
	   Body	of email.

     It	may be useful to know which ports are open and running services	on a
     target machine.  The -z flag can be used to tell nc not to	initiate a
     connection, together with the -v (verbose)	flag, to report	open ports.
     For example:

	   $ nc	-vz 20-30
	   Connection to 22 port [tcp/ssh] succeeded!
	   Connection to 25 port [tcp/smtp] succeeded!

     The port range was	specified to limit the search to ports 20 - 30.

     Alternatively, it might be	useful to know which server software is	run-
     ning, and which versions.	This information is often contained within the
     greeting banners.	In order to retrieve these, it is necessary to first
     make a connection,	and then break the connection when the banner has been
     retrieved.	 This can be accomplished by specifying	a small	timeout	with
     the -w flag, or perhaps by	issuing	a "QUIT" command to the	server:

	   $ echo "QUIT" | nc 20-30
	   Protocol mismatch.
	   220	IMS SMTP Receiver Version 0.84 Ready

     Open a TCP	connection to port 42 of hostname, using port 31337 as the
     source port, with a timeout of 5 seconds:

	   $ nc	-p 31337 -w 5 hostname 42

     Open a UDP	connection to port 53 of hostname:

	   $ nc	-u hostname 53

     Open a TCP	connection to port 42 of using as	the IP
     for the local end of the connection:

	   nc -E 42

     Open a TCP	connection to port 42 of using IPsec ESP for in-
     coming and	outgoing traffic.

	   nc -e 'out ipsec esp/transport//require' 42

     Open a TCP	connection to port 42 of using IPsec ESP for out-
     going traffic only.

	   $ nc	-s 42

     Send UDP packets to ports 20-30 of, and report which ones
     responded with an ICMP packet after three seconds:

	   $ nc	-uvz -w	3 hostname 20-30

     Create and	listen on a Unix Domain	Socket:

	   $ nc	-lU /var/tmp/dsocket

     Connect to	port 42	of hostname via	an HTTP	proxy at, port	8080:

	   $ nc	-x10.2.3.4:8080	-Xconnect hostname 42


     Original implementation by	*Hobbit* <>.
     Rewritten with IPv6 support by Eric Jackson <>.

BSD			       January 30, 2005				   BSD


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