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NC(1)			FreeBSD	General	Commands Manual			 NC(1)

NAME
     nc	-- arbitrary TCP and UDP connections and listens

SYNOPSIS
     nc	[-46cDdFhklNnrStUuvz] [-C certfile] [-e	name] [-H hash]	[-I length]
	[-i interval] [-K keyfile] [-M ttl] [-m	minttl]	[-O length]
	[-o staplefile]	[-P proxy_username] [-p	source_port] [-R CAfile]
	[-s sourceaddr]	[-T keyword] [-V rtable] [-W recvlimit]	[-w timeout]
	[-X proxy_protocol] [-x	proxy_address[:port]] [-Z peercertfile]
	[destination] [port]

DESCRIPTION
     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	     Use IPv4 addresses	only.

     -6	     Use IPv6 addresses	only.

     -C	certfile
	     Load the public key part of the TLS peer certificate from
	     certfile, in PEM format.  Requires	-c.

     -c	     Use TLS to	connect	or listen.  Cannot be used together with any
	     of	the options -FuU.

     -D	     Enable debugging on the socket.

     -d	     Do	not attempt to read from stdin.

     -e	name
	     Only accept the TLS peer certificate if it	contains the name.
	     Requires -c.  If not specified, destination is used.

     -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).  Cannot be	used with -c or	-U.

     -H	hash
	     Only accept the TLS peer certificate if its hash returned from
	     tls_peer_cert_hash(3) matches hash.  Requires -c and cannot be
	     used with -T noverify.

     -h	     Print out the nc help text	and exit.

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

     -i	interval
	     Sleep for interval	seconds	between	lines of text sent and re-
	     ceived.  Also causes a delay time between connections to multiple
	     ports.

     -K	keyfile
	     Load the TLS private key from keyfile, in PEM format.  Requires
	     -c.

     -k	     When a connection is completed, listen for	another	one.  Requires
	     -l.  When used together with the -u option, the server socket is
	     not connected and it can receive UDP datagrams from multiple
	     hosts.

     -l	     Listen for	an incoming connection rather than initiating a	con-
	     nection to	a remote host.	Cannot be used together	with any of
	     the options -psxz.	 Additionally, any timeouts specified with the
	     -w	option are ignored.

     -M	ttl  Set the TTL / hop limit of	outgoing packets.

     -m	minttl
	     Ask the kernel to drop incoming packets whose TTL / hop limit is
	     under minttl.

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

     -n	     Do	not perform domain name	resolution.  If	a name cannot be re-
	     solved without DNS, an error will be reported.

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

     -o	staplefile
	     During the	TLS handshake, load data to be stapled from
	     staplefile, which is expected to contain an OCSP response from an
	     OCSP server in DER	format.	 Requires -c and -C.

     -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
	     Specify the source	port nc	should use, subject to privilege re-
	     strictions	and availability.  Cannot be used together with	-l.

     -R	CAfile
	     Load the root CA bundle for TLS certificate verification from
	     CAfile, in	PEM format, instead of /etc/ssl/cert.pem.  Requires
	     -c.

     -r	     Choose source and/or destination ports randomly instead of	se-
	     quentially	within a range or in the order that the	system assigns
	     them.

     -S	     Enable the	RFC 2385 TCP MD5 signature option.

     -s	sourceaddr
	     Set the source address to send packets from, which	is useful on
	     machines with multiple interfaces.	 For UNIX-domain datagram
	     sockets, specifies	the local temporary socket file	to create and
	     use so that datagrams can be received.  Cannot be used together
	     with -l or	-x.

     -T	keyword
	     Change the	IPv4 TOS/IPv6 traffic class value or the TLS options.

	     For TLS options, keyword may be one of: noverify, which disables
	     certificate verification; noname, which disables certificate name
	     checking; clientcert, which requires a client certificate on in-
	     coming connections; or muststaple,	which requires the peer	to
	     provide a valid stapled OCSP response with	the handshake.	The
	     following TLS options specify a value in the form of a key=value
	     pair: ciphers, which allows the supported TLS ciphers to be spec-
	     ified (see	tls_config_set_ciphers(3) for further details);
	     protocols,	which allows the supported TLS protocols to be speci-
	     fied (see tls_config_parse_protocols(3) for further details).
	     Specifying	TLS options requires -c.

	     For the IPv4 TOS/IPv6 traffic class value,	keyword	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	     Send RFC 854 DON'T	and WON'T responses to RFC 854 DO and WILL re-
	     quests.  This makes it possible to	use nc to script telnet	ses-
	     sions.

     -U	     Use UNIX-domain sockets.  Cannot be used together with any	of the
	     options -cFx.

     -u	     Use UDP instead of	TCP.  Cannot be	used together with -c or -x.
	     For UNIX-domain sockets, use a datagram socket instead of a
	     stream socket.  If	a UNIX-domain socket is	used, a	temporary re-
	     ceiving socket is created in /tmp unless the -s flag is given.

     -V	rtable
	     Set the routing table to be used.

     -v	     Produce more verbose output.

     -W	recvlimit
	     Terminate after receiving recvlimit packets from the network.

     -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
	     Use proxy_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]
	     Connect to	destination using a proxy at proxy_address and port.
	     If	port is	not specified, the well-known port for the proxy pro-
	     tocol is used (1080 for SOCKS, 3128 for HTTPS).  An IPv6 address
	     can be specified unambiguously by enclosing proxy_address in
	     square brackets.  A proxy cannot be used with any of the options
	     -lsuU.

     -Z	peercertfile
	     Save the peer certificates	to peercertfile, in PEM	format.	 Re-
	     quires -c.

     -z	     Only scan for listening daemons, without sending any data to
	     them.  Cannot be used together with -l.

     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 specified as a	numeric	port number or as a service name.
     Port ranges may be	specified as numeric port numbers of the form nn-mm.
     In	general, a destination port must be specified, unless the -U option is
     given.

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').

DATA TRANSFER
     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 host.example.com 1234 < filename.in

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

TALKING	TO SERVERS
     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
     site:

	   $ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 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
	   HELO	host.example.com
	   MAIL	FROM:<user@host.example.com>
	   RCPT	TO:<user2@host.example.com>
	   DATA
	   Body	of email.
	   .
	   QUIT
	   EOF

PORT SCANNING
     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 host.example.com 20-30
	   Connection to host.example.com 22 port [tcp/ssh] succeeded!
	   Connection to host.example.com 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 host.example.com 20-30
	   SSH-1.99-OpenSSH_3.6.1p2
	   Protocol mismatch.
	   220 host.example.com	IMS SMTP Receiver Version 0.84 Ready

EXAMPLES
     Open a TCP	connection to port 42 of host.example.com, using port 31337 as
     the source	port, with a timeout of	5 seconds:

	   $ nc	-p 31337 -w 5 host.example.com 42

     Open a TCP	connection to port 443 of www.example.com, and negotiate TLS
     with any supported	TLS protocol version and "compat" ciphers:

	   $ nc	-cv -T protocols=all -T	ciphers=compat www.example.com 443

     Open a TCP	connection to port 443 of www.google.ca, and negotiate TLS.
     Check for a different name	in the certificate for validation:

	   $ nc	-cv -e adsf.au.doubleclick.net www.google.ca 443

     Open a UDP	connection to port 53 of host.example.com:

	   $ nc	-u host.example.com 53

     Open a TCP	connection to port 42 of host.example.com using	10.1.2.3 as
     the IP for	the local end of the connection:

	   $ nc	-s 10.1.2.3 host.example.com 42

     Create and	listen on a UNIX-domain	stream socket:

	   $ nc	-lU /var/tmp/dsocket

     Connect to	port 42	of host.example.com via	an HTTP	proxy at 10.2.3.4,
     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 host.example.com 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 host.example.com 42

SEE ALSO
     cat(1), ssh(1)

AUTHORS
     Original implementation by	*Hobbit* <hobbit@avian.org>.
     Rewritten with IPv6 support by
     Eric Jackson <ericj@monkey.org>.

CAVEATS
     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.

FreeBSD	13.0		       February	12, 2020		  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | CLIENT/SERVER MODEL | DATA TRANSFER | TALKING TO SERVERS | PORT SCANNING | EXAMPLES | SEE ALSO | AUTHORS | CAVEATS

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