1 .\" $OpenBSD: nc.1,v 1.60 2012/02/07 12:11:43 lum Exp $
3 .\" Copyright (c) 1996 David Sacerdote
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35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DdEhklnrStUuvz
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.
128 Specifies the size of the TCP receive buffer.
130 Specifies a delay time interval between lines of text sent and received.
131 Also causes a delay time between connections to multiple ports.
135 to stay listening for another connection after its current connection
137 It is an error to use this option without the
143 should listen for an incoming connection rather than initiate a
144 connection to a remote host.
145 It is an error to use this option in conjunction with the
151 Additionally, any timeouts specified with the
155 Do not do any DNS or service lookups on any specified addresses,
158 Disables the use of TCP options on the socket, by setting the boolean
162 Specifies the size of the TCP send buffer.
163 .It Fl P Ar proxy_username
164 Specifies a username to present to a proxy server that requires authentication.
165 If no username is specified then authentication will not be attempted.
166 Proxy authentication is only supported for HTTP CONNECT proxies at present.
167 .It Fl p Ar source_port
168 Specifies the source port
170 should use, subject to privilege restrictions and availability.
171 It is an error to use this option in conjunction with the
175 Specifies that source and/or destination ports should be chosen randomly
176 instead of sequentially within a range or in the order that the system
179 Enables the RFC 2385 TCP MD5 signature option.
181 Specifies the IP of the interface which is used to send the packets.
184 datagram sockets, specifies the local temporary socket file
185 to create and use so that datagrams can be received.
186 It is an error to use this option in conjunction with the
189 .It Fl T Ar toskeyword
190 Change IPv4 TOS value.
199 or one of the DiffServ Code Points:
203 or a number in either hex or decimal.
207 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
208 This makes it possible to use
210 to script telnet sessions.
216 Use UDP instead of the default option of TCP.
219 sockets, use a datagram socket instead of a stream socket.
222 socket is used, a temporary receiving socket is created in
228 Set the routing table
235 give more verbose output.
237 Connections which cannot be established or are idle timeout after
242 flag has no effect on the
246 will listen forever for a connection, with or without the
249 The default is no timeout.
250 .It Fl X Ar proxy_protocol
253 should use the specified protocol when talking to the proxy server.
254 Supported protocols are
262 If the protocol is not specified, SOCKS version 5 is used.
264 .Fl x Ar proxy_address Ns Oo : Ns
277 is not specified, the well-known port for the proxy protocol is used (1080
278 for SOCKS, 3128 for HTTPS).
282 should just scan for listening daemons, without sending any data to them.
283 It is an error to use this option in conjunction with the
289 can be a numerical IP address or a symbolic hostname
293 In general, a destination must be specified,
297 (in which case the local host is used).
300 sockets, a destination is required and is the socket path to connect to
306 can be a single integer or a range of ports.
307 Ranges are in the form nn-mm.
309 a destination port must be specified,
313 .Sh CLIENT/SERVER MODEL
314 It is quite simple to build a very basic client/server model using
316 On one console, start
318 listening on a specific port for a connection.
324 is now listening on port 1234 for a connection.
326 .Pq or a second machine ,
327 connect to the machine and port being listened on:
329 .Dl $ nc 127.0.0.1 1234
331 There should now be a connection between the ports.
332 Anything typed at the second console will be concatenated to the first,
334 After the connection has been set up,
336 does not really care which side is being used as a
338 and which side is being used as a
340 The connection may be terminated using an
344 The example in the previous section can be expanded to build a
345 basic data transfer model.
346 Any information input into one end of the connection will be output
347 to the other end, and input and output can be easily captured in order to
348 emulate file transfer.
352 to listen on a specific port, with output captured into a file:
354 .Dl $ nc -l 1234 \*(Gt filename.out
356 Using a second machine, connect to the listening
358 process, feeding it the file which is to be transferred:
360 .Dl $ nc host.example.com 1234 \*(Lt filename.in
362 After the file has been transferred, the connection will close automatically.
363 .Sh TALKING TO SERVERS
364 It is sometimes useful to talk to servers
366 rather than through a user interface.
367 It can aid in troubleshooting,
368 when it might be necessary to verify what data a server is sending
369 in response to commands issued by the client.
370 For example, to retrieve the home page of a web site:
371 .Bd -literal -offset indent
372 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
375 Note that this also displays the headers sent by the web server.
376 They can be filtered, using a tool such as
380 More complicated examples can be built up when the user knows the format
381 of requests required by the server.
382 As another example, an email may be submitted to an SMTP server using:
383 .Bd -literal -offset indent
384 $ nc localhost 25 \*(Lt\*(Lt EOF
385 HELO host.example.com
386 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
387 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
395 It may be useful to know which ports are open and running services on
399 flag can be used to tell
401 to report open ports,
402 rather than initiate a connection.
404 .Bd -literal -offset indent
405 $ nc -z host.example.com 20-30
406 Connection to host.example.com 22 port [tcp/ssh] succeeded!
407 Connection to host.example.com 25 port [tcp/smtp] succeeded!
410 The port range was specified to limit the search to ports 20 \- 30.
412 Alternatively, it might be useful to know which server software
413 is running, and which versions.
414 This information is often contained within the greeting banners.
415 In order to retrieve these, it is necessary to first make a connection,
416 and then break the connection when the banner has been retrieved.
417 This can be accomplished by specifying a small timeout with the
419 flag, or perhaps by issuing a
421 command to the server:
422 .Bd -literal -offset indent
423 $ echo "QUIT" | nc host.example.com 20-30
424 SSH-1.99-OpenSSH_3.6.1p2
426 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
429 Open a TCP connection to port 42 of host.example.com, using port 31337 as
430 the source port, with a timeout of 5 seconds:
432 .Dl $ nc -p 31337 -w 5 host.example.com 42
434 Open a UDP connection to port 53 of host.example.com:
436 .Dl $ nc -u host.example.com 53
438 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
439 IP for the local end of the connection:
441 .Dl $ nc -s 10.1.2.3 host.example.com 42
443 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
444 incoming and outgoing traffic.
446 .Dl $ nc -E host.example.com 42
448 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
449 outgoing traffic only.
451 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
453 Create and listen on a
457 .Dl $ nc -lU /var/tmp/dsocket
459 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
461 This example could also be used by
467 for more information.
469 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
471 The same example again, this time enabling proxy authentication with username
473 if the proxy requires it:
475 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
484 Original implementation by *Hobbit*
485 .Aq hobbit@avian.org .
487 Rewritten with IPv6 support by
488 .An Eric Jackson Aq ericj@monkey.org .
490 UDP port scans using the
492 combination of flags will always report success irrespective of
493 the target machine's state.
495 in conjunction with a traffic sniffer either on the target machine
496 or an intermediary device,
499 combination could be useful for communications diagnostics.
500 Note that the amount of UDP traffic generated may be limited either
501 due to hardware resources and/or configuration settings.