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

     nc	-- arbitrary TCP and UDP connections and listens

     nc	[-46DdEFhklMNnrStUuvz] [-e IPsec_policy] [-I length] [-i interval]
	[--no-tcpopt] [--sctp] [-O length] [-P proxy_username]
	[-p source_port] [-s source] [-T toskeyword] [-V rtable] [-w timeout]
	[-X proxy_protocol] [-x	proxy_address[:port]] [destination] [port]

     The nc (or	netcat)	utility	is used	for just about anything	under the sun
     involving TCP, UDP, or UNIX-domain	sockets.  It can open TCP connections,
     send UDP packets, listen on arbitrary TCP and UDP ports, do port scan-
     ning, and deal with both IPv4 and IPv6.  Unlike telnet(1),	nc scripts
     nicely, and separates error messages onto standard	error instead of send-
     ing 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.

     -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.

     -F	     Pass the first connected socket using sendmsg(2) to stdout	and
	     exit.  This is useful in conjunction with -X to have nc perform
	     connection	setup with a proxy but then leave the rest of the con-
	     nection to	another	program	(e.g. ssh(1) using the ssh_config(5)
	     ProxyUseFdpass option).

     -h	     Prints out	nc help.

     -I	length
	     Specifies the size	of the TCP receive buffer.

     -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.  When used together	with the -u option,
	     the server	socket is not connected	and it can receive UDP data-
	     grams from	multiple hosts.

     -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.

     -M	     Collect per-connection TCP	statistics using the stats(3) frame-
	     work and print them in JSON format	to stderr(4) after the connec-
	     tion is closed.

     -N	     shutdown(2) the network socket after EOF on the input.  Some
	     servers require this to finish their work.

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

	     Disables the use of TCP options on	the socket, by setting the
	     boolean TCP_NOOPT socket option.

     --sctp  Use SCTP instead of the default option of TCP.

     -O	length
	     Specifies the size	of the TCP send	buffer.

     -P	proxy_username
	     Specifies a username to present to	a proxy	server that requires
	     authentication.  If no username is	specified then authentication
	     will not be attempted.  Proxy authentication is only supported
	     for HTTP CONNECT proxies at present.

     -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
	     Specifies the IP of the interface which is	used to	send the pack-
	     ets.  For UNIX-domain datagram sockets, specifies the local tem-
	     porary socket file	to create and use so that datagrams can	be re-
	     ceived.  It is an error to	use this option	in conjunction with
	     the -l option.

     -T	toskeyword
	     Change IPv4 TOS value.  toskeyword	may be one of critical,
	     inetcontrol, lowdelay, netcontrol,	throughput, reliability, or
	     one of the	DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7;
	     or	a number in either hex or decimal.

     -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.  For	UNIX-domain
	     sockets, use a datagram socket instead of a stream	socket.	 If a
	     UNIX-domain socket	is used, a temporary receiving socket is cre-
	     ated in /tmp unless the -s	flag is	given.

     -V	rtable
	     Set the routing table ("FIB") to be used.

     -v	     Have nc give more verbose output.

     -w	timeout
	     Connections which cannot be established or	are idle timeout after
	     timeout seconds.  The -w flag has no effect on the	-l option,
	     i.e. nc will listen forever for a connection, with	or without the
	     -w	flag.  The default is no timeout.

     -X	proxy_protocol
	     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	destination 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.

     destination can be	a numerical IP address or a symbolic hostname (unless
     the -n option is given).  In general, a destination must be specified,
     unless the	-l option is given (in which case the local host is used).
     For UNIX-domain sockets, a	destination is required	and is the socket path
     to	connect	to (or listen on if the	-l option is given).

     port can be a single integer or a range of	ports.	Ranges are in the form
     nn-mm.  In	general, a destination port must be specified, unless the -U
     option is given.

     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	-N 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

	   $ printf "GET / HTTP/1.0\r\n\r\n" | 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 to report open	ports,
     rather than initiate a connection.	 For example:

	   $ nc	-z 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, using port 31337 as
     the source	port, with a timeout of	5 seconds:

	   $ nc	-p 31337 -w 5 42

     Open a UDP	connection to port 53 of

	   $ nc	-u 53

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

	   $ nc	-s 42

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

	   $ nc	-E 42

     Open a TCP	connection to port 42 of using	IPsec ESP for
     outgoing traffic only.

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

     Create and	listen on a UNIX-domain	stream socket:

	   $ nc	-lU /var/tmp/dsocket

     Connect to	port 42	of via	an HTTP	proxy at,
     port 8080.	 This example could also be used by ssh(1); see	the
     ProxyCommand directive in ssh_config(5) for more information.

	   $ nc	-x10.2.3.4:8080	-Xconnect 42

     The same example again, this time enabling	proxy authentication with
     username "ruser" if the proxy requires it:

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

     The nc utility exits 0 on success,	and >0 if an error occurs.

     cat(1), setfib(1),	ssh(1),	tcp(4)

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

     UDP port scans using the -uz combination of flags will always report suc-
     cess irrespective of the target machine's state.  However,	in conjunction
     with a traffic sniffer either on the target machine or an intermediary
     device, the -uz combination could be useful for communications diagnos-
     tics.  Note that the amount of UDP	traffic	generated may be limited ei-
     ther due to hardware resources and/or configuration settings.

BSD				 July 10, 2020				   BSD


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