1 .\" $OpenBSD: nc.1,v 1.68 2015/03/26 10:35:04 tobias Exp $
3 .\" Copyright (c) 1996 David Sacerdote
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35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DdEFhklMNnrStUuvz
40 .Op Fl e Ar IPsec_policy
45 .Op Fl P Ar proxy_username
46 .Op Fl p Ar source_port
48 .Op Fl T Ar toskeyword
51 .Op Fl X Ar proxy_protocol
53 .Fl x Ar proxy_address Ns Oo : Ns
64 utility is used for just about anything under the sun involving TCP,
68 It can open TCP connections, send UDP packets, listen on arbitrary
69 TCP and UDP ports, do port scanning, and deal with both IPv4 and
74 scripts nicely, and separates error messages onto standard error instead
75 of sending them to standard output, as
81 .Bl -bullet -offset indent -compact
85 shell-script based HTTP clients and servers
87 network daemon testing
89 a SOCKS or HTTP ProxyCommand for
95 The options are as follows:
100 to use IPv4 addresses only.
104 to use IPv6 addresses only.
106 Enable debugging on the socket.
108 Do not attempt to read from stdin.
112 .Li "-e 'in ipsec esp/transport//require'"
113 .Li "-e 'out ipsec esp/transport//require'"
115 which enables IPsec ESP transport mode in both
118 If IPsec support is available, then one can specify the IPsec policies
119 to be used using the syntax described in
120 .Xr ipsec_set_policy 3 .
121 This flag can be specified up to two times, as typically one policy for
122 each direction is needed.
124 Pass the first connected socket using
127 This is useful in conjunction with
131 perform connection setup with a proxy but then leave the rest of the
132 connection to another program (e.g.\&
143 Specifies the size of the TCP receive buffer.
145 Specifies a delay time interval between lines of text sent and received.
146 Also causes a delay time between connections to multiple ports.
150 to stay listening for another connection after its current connection
152 It is an error to use this option without the
155 When used together with the
157 option, the server socket is not connected and it can receive UDP datagrams from
162 should listen for an incoming connection rather than initiate a
163 connection to a remote host.
164 It is an error to use this option in conjunction with the
170 Additionally, any timeouts specified with the
174 Collect per-connection TCP statistics using the
176 framework and print them in JSON format to
178 after the connection is closed.
181 the network socket after EOF on the input.
182 Some servers require this to finish their work.
184 Do not do any DNS or service lookups on any specified addresses,
187 Disables the use of TCP options on the socket, by setting the boolean
191 Specifies the size of the TCP send buffer.
192 .It Fl P Ar proxy_username
193 Specifies a username to present to a proxy server that requires authentication.
194 If no username is specified then authentication will not be attempted.
195 Proxy authentication is only supported for HTTP CONNECT proxies at present.
196 .It Fl p Ar source_port
197 Specifies the source port
199 should use, subject to privilege restrictions and availability.
200 It is an error to use this option in conjunction with the
204 Specifies that source and/or destination ports should be chosen randomly
205 instead of sequentially within a range or in the order that the system
208 Enables the RFC 2385 TCP MD5 signature option.
210 Specifies the IP of the interface which is used to send the packets.
213 datagram sockets, specifies the local temporary socket file
214 to create and use so that datagrams can be received.
215 It is an error to use this option in conjunction with the
218 .It Fl T Ar toskeyword
219 Change IPv4 TOS value.
228 or one of the DiffServ Code Points:
232 or a number in either hex or decimal.
236 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
237 This makes it possible to use
239 to script telnet sessions.
245 Use UDP instead of the default option of TCP.
248 sockets, use a datagram socket instead of a stream socket.
251 socket is used, a temporary receiving socket is created in
257 Set the routing table
263 give more verbose output.
265 Connections which cannot be established or are idle timeout after
270 flag has no effect on the
274 will listen forever for a connection, with or without the
277 The default is no timeout.
278 .It Fl X Ar proxy_protocol
281 should use the specified protocol when talking to the proxy server.
282 Supported protocols are
290 If the protocol is not specified, SOCKS version 5 is used.
292 .Fl x Ar proxy_address Ns Oo : Ns
305 is not specified, the well-known port for the proxy protocol is used (1080
306 for SOCKS, 3128 for HTTPS).
310 should just scan for listening daemons, without sending any data to them.
311 It is an error to use this option in conjunction with the
317 can be a numerical IP address or a symbolic hostname
321 In general, a destination must be specified,
325 (in which case the local host is used).
328 sockets, a destination is required and is the socket path to connect to
334 can be a single integer or a range of ports.
335 Ranges are in the form nn-mm.
337 a destination port must be specified,
341 .Sh CLIENT/SERVER MODEL
342 It is quite simple to build a very basic client/server model using
344 On one console, start
346 listening on a specific port for a connection.
352 is now listening on port 1234 for a connection.
354 .Pq or a second machine ,
355 connect to the machine and port being listened on:
357 .Dl $ nc 127.0.0.1 1234
359 There should now be a connection between the ports.
360 Anything typed at the second console will be concatenated to the first,
362 After the connection has been set up,
364 does not really care which side is being used as a
366 and which side is being used as a
368 The connection may be terminated using an
372 The example in the previous section can be expanded to build a
373 basic data transfer model.
374 Any information input into one end of the connection will be output
375 to the other end, and input and output can be easily captured in order to
376 emulate file transfer.
380 to listen on a specific port, with output captured into a file:
382 .Dl $ nc -l 1234 \*(Gt filename.out
384 Using a second machine, connect to the listening
386 process, feeding it the file which is to be transferred:
388 .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
390 After the file has been transferred, the connection will close automatically.
391 .Sh TALKING TO SERVERS
392 It is sometimes useful to talk to servers
394 rather than through a user interface.
395 It can aid in troubleshooting,
396 when it might be necessary to verify what data a server is sending
397 in response to commands issued by the client.
398 For example, to retrieve the home page of a web site:
399 .Bd -literal -offset indent
400 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
403 Note that this also displays the headers sent by the web server.
404 They can be filtered, using a tool such as
408 More complicated examples can be built up when the user knows the format
409 of requests required by the server.
410 As another example, an email may be submitted to an SMTP server using:
411 .Bd -literal -offset indent
412 $ nc localhost 25 \*(Lt\*(Lt EOF
413 HELO host.example.com
414 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
415 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
423 It may be useful to know which ports are open and running services on
427 flag can be used to tell
429 to report open ports,
430 rather than initiate a connection.
432 .Bd -literal -offset indent
433 $ nc -z host.example.com 20-30
434 Connection to host.example.com 22 port [tcp/ssh] succeeded!
435 Connection to host.example.com 25 port [tcp/smtp] succeeded!
438 The port range was specified to limit the search to ports 20 \- 30.
440 Alternatively, it might be useful to know which server software
441 is running, and which versions.
442 This information is often contained within the greeting banners.
443 In order to retrieve these, it is necessary to first make a connection,
444 and then break the connection when the banner has been retrieved.
445 This can be accomplished by specifying a small timeout with the
447 flag, or perhaps by issuing a
449 command to the server:
450 .Bd -literal -offset indent
451 $ echo "QUIT" | nc host.example.com 20-30
452 SSH-1.99-OpenSSH_3.6.1p2
454 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
457 Open a TCP connection to port 42 of host.example.com, using port 31337 as
458 the source port, with a timeout of 5 seconds:
460 .Dl $ nc -p 31337 -w 5 host.example.com 42
462 Open a UDP connection to port 53 of host.example.com:
464 .Dl $ nc -u host.example.com 53
466 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
467 IP for the local end of the connection:
469 .Dl $ nc -s 10.1.2.3 host.example.com 42
471 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
472 incoming and outgoing traffic.
474 .Dl $ nc -E host.example.com 42
476 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
477 outgoing traffic only.
479 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
481 Create and listen on a
485 .Dl $ nc -lU /var/tmp/dsocket
487 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
489 This example could also be used by
495 for more information.
497 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
499 The same example again, this time enabling proxy authentication with username
501 if the proxy requires it:
503 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
512 Original implementation by *Hobbit*
513 .Aq Mt hobbit@avian.org .
515 Rewritten with IPv6 support by
516 .An Eric Jackson Aq Mt ericj@monkey.org .
518 UDP port scans using the
520 combination of flags will always report success irrespective of
521 the target machine's state.
523 in conjunction with a traffic sniffer either on the target machine
524 or an intermediary device,
527 combination could be useful for communications diagnostics.
528 Note that the amount of UDP traffic generated may be limited either
529 due to hardware resources and/or configuration settings.