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Linux 2.6 - man page for netcat (linux section 1)

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

     nc -- arbitrary TCP and UDP connections and listens

     nc [-46DdhklnrStUuvzC] [-i interval] [-P proxy_username] [-p source_port]
	[-s source_ip_address] [-T ToS] [-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.

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

     -k      Forces nc to stay listening for another connection after its current 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 initi-
	     ate a connection to a remote host.  It is an error to use this option in conjunction
	     with the -p, -s, or -z options.  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.

     -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 authenti-
	     cation is only supported for HTTP CONNECT proxies at present.

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

     -q      after EOF on stdin, wait the specified number of seconds and then quit. If seconds
	     is negative, wait forever.

     -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 packets.  It is an error
	     to use this option in conjunction with the -l option.

     -T ToS  Specifies IP Type of Service (ToS) for the connection.  Valid values are the tokens
	     ``lowdelay'', ``throughput'', ``reliability'', or an 8-bit hexadecimal value pre-
	     ceded by ``0x''.

     -C      Send CRLF as line-ending

     -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 connec-
	     tion is silently closed.  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

     -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 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 HTTPS).

     -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 des-
     tination 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 captured 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 host.example.com 1234 < filename.in

     After the file has been transferred, the connection will close automatically.

     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 necessary to verify what data a server is
     sending in response to commands issued by the client.  For example, to retrieve the home
     page of a web site:

	   $ echo -n "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

	   $ nc [-C] localhost 25 << EOF
	   HELO host.example.com
	   MAIL FROM:<user@host.example.com>
	   RCPT TO:<user2@host.example.com>
	   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

	   $ 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 running, and which ver-
     sions.  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
	   Protocol mismatch.
	   220 host.example.com IMS SMTP Receiver Version 0.84 Ready

     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 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 as the IP for the local
     end of the connection:

	   $ nc -s host.example.com 42

     Create and listen on a Unix Domain Socket:

	   $ nc -lU /var/tmp/dsocket

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

	   $ 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

     cat(1), ssh(1)

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

     UDP port scans will always succeed (i.e. report the port as open), rendering the -uz combi-
     nation of flags relatively useless.

BSD					 August 15, 2018				      BSD

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