1 .\" $OpenBSD: nc.1,v 1.50 2009/06/05 06:47:12 jmc Exp $
3 .\" Copyright (c) 1996 David Sacerdote
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35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DdEhklnorStUuvz
40 .Op Fl e Ar IPsec_policy
45 .Op Fl P Ar proxy_username
46 .Op Fl p Ar source_port
47 .Op Fl s Ar source_ip_address
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
66 It can open TCP connections, send UDP packets, listen on arbitrary
67 TCP and UDP ports, do port scanning, and deal with both IPv4 and
72 scripts nicely, and separates error messages onto standard error instead
73 of sending them to standard output, as
79 .Bl -bullet -offset indent -compact
83 shell-script based HTTP clients and servers
85 network daemon testing
87 a SOCKS or HTTP ProxyCommand for
93 The options are as follows:
98 to use IPv4 addresses only.
102 to use IPv6 addresses only.
104 Enable debugging on the socket.
106 Do not attempt to read from stdin.
110 .Li "-e 'in ipsec esp/transport//require'"
111 .Li "-e 'out ipsec esp/transport//require'"
113 which enables IPsec ESP transport mode in both
116 If IPsec support is available, then one can specify the IPsec policies
117 to be used using the syntax described in
118 .Xr ipsec_set_policy 3 .
119 This flag can be specified up to two times, as typically one policy for
120 each direction is needed.
126 Specifies the size of the TCP receive buffer.
128 Specifies a delay time interval between lines of text sent and received.
129 Also causes a delay time between connections to multiple ports.
133 to stay listening for another connection after its current connection
135 It is an error to use this option without the
141 should listen for an incoming connection rather than initiate a
142 connection to a remote host.
143 It is an error to use this option in conjunction with the
149 Additionally, any timeouts specified with the
153 Do not do any DNS or service lookups on any specified addresses,
156 Disables the use of TCP options on the socket, by setting the boolean
160 Specifies the size of the TCP send buffer.
166 does not terminate on EOF condition on input,
167 but continues until the network side has been closed down.
170 will make it terminate on EOF as well.
171 .It Fl P Ar proxy_username
172 Specifies a username to present to a proxy server that requires authentication.
173 If no username is specified then authentication will not be attempted.
174 Proxy authentication is only supported for HTTP CONNECT proxies at present.
175 .It Fl p Ar source_port
176 Specifies the source port
178 should use, subject to privilege restrictions and availability.
179 It is an error to use this option in conjunction with the
183 Specifies that source and/or destination ports should be chosen randomly
184 instead of sequentially within a range or in the order that the system
187 Enables the RFC 2385 TCP MD5 signature option.
188 .It Fl s Ar source_ip_address
189 Specifies the IP of the interface which is used to send the packets.
190 It is an error to use this option in conjunction with the
194 Specifies IP Type of Service (ToS) for the connection.
195 Valid values are the tokens
199 or an 8-bit hexadecimal value preceded by
204 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
205 This makes it possible to use
207 to script telnet sessions.
209 Specifies to use Unix Domain Sockets.
211 Use UDP instead of the default option of TCP.
213 Set the routing table (FIB).
218 give more verbose output.
220 If a connection and stdin are idle for more than
222 seconds, then the connection is silently closed.
225 flag has no effect on the
229 will listen forever for a connection, with or without the
232 The default is no timeout.
233 .It Fl X Ar proxy_protocol
236 should use the specified protocol when talking to the proxy server.
237 Supported protocols are
245 If the protocol is not specified, SOCKS version 5 is used.
247 .Fl x Ar proxy_address Ns Oo : Ns
260 is not specified, the well-known port for the proxy protocol is used (1080
261 for SOCKS, 3128 for HTTPS).
265 should just scan for listening daemons, without sending any data to them.
266 It is an error to use this option in conjunction with the
272 can be a numerical IP address or a symbolic hostname
276 In general, a hostname must be specified,
280 (in which case the local host is used).
283 can be a single integer or a range of ports.
284 Ranges are in the form nn-mm.
286 a destination port must be specified,
290 (in which case a socket must be specified).
291 .Sh CLIENT/SERVER MODEL
292 It is quite simple to build a very basic client/server model using
294 On one console, start
296 listening on a specific port for a connection.
302 is now listening on port 1234 for a connection.
304 .Pq or a second machine ,
305 connect to the machine and port being listened on:
307 .Dl $ nc 127.0.0.1 1234
309 There should now be a connection between the ports.
310 Anything typed at the second console will be concatenated to the first,
312 After the connection has been set up,
314 does not really care which side is being used as a
316 and which side is being used as a
318 The connection may be terminated using an
322 The example in the previous section can be expanded to build a
323 basic data transfer model.
324 Any information input into one end of the connection will be output
325 to the other end, and input and output can be easily captured in order to
326 emulate file transfer.
330 to listen on a specific port, with output captured into a file:
332 .Dl $ nc -l 1234 \*(Gt filename.out
334 Using a second machine, connect to the listening
336 process, feeding it the file which is to be transferred:
338 .Dl $ nc host.example.com 1234 \*(Lt filename.in
340 After the file has been transferred, the connection will close automatically.
341 .Sh TALKING TO SERVERS
342 It is sometimes useful to talk to servers
344 rather than through a user interface.
345 It can aid in troubleshooting,
346 when it might be necessary to verify what data a server is sending
347 in response to commands issued by the client.
348 For example, to retrieve the home page of a web site:
349 .Bd -literal -offset indent
350 $ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
353 Note that this also displays the headers sent by the web server.
354 They can be filtered, using a tool such as
358 More complicated examples can be built up when the user knows the format
359 of requests required by the server.
360 As another example, an email may be submitted to an SMTP server using:
361 .Bd -literal -offset indent
362 $ nc localhost 25 \*(Lt\*(Lt EOF
363 HELO host.example.com
364 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
365 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
373 It may be useful to know which ports are open and running services on
377 flag can be used to tell
379 to report open ports,
380 rather than initiate a connection.
382 .Bd -literal -offset indent
383 $ nc -z host.example.com 20-30
384 Connection to host.example.com 22 port [tcp/ssh] succeeded!
385 Connection to host.example.com 25 port [tcp/smtp] succeeded!
388 The port range was specified to limit the search to ports 20 \- 30.
390 Alternatively, it might be useful to know which server software
391 is running, and which versions.
392 This information is often contained within the greeting banners.
393 In order to retrieve these, it is necessary to first make a connection,
394 and then break the connection when the banner has been retrieved.
395 This can be accomplished by specifying a small timeout with the
397 flag, or perhaps by issuing a
399 command to the server:
400 .Bd -literal -offset indent
401 $ echo "QUIT" | nc host.example.com 20-30
402 SSH-1.99-OpenSSH_3.6.1p2
404 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
407 Open a TCP connection to port 42 of host.example.com, using port 31337 as
408 the source port, with a timeout of 5 seconds:
410 .Dl $ nc -p 31337 -w 5 host.example.com 42
412 Open a UDP connection to port 53 of host.example.com:
414 .Dl $ nc -u host.example.com 53
416 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
417 IP for the local end of the connection:
419 .Dl $ nc -s 10.1.2.3 host.example.com 42
421 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
422 incoming and outgoing traffic.
424 .Dl $ nc -E host.example.com 42
426 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
427 outgoing traffic only.
429 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
431 Create and listen on a Unix Domain Socket:
433 .Dl $ nc -lU /var/tmp/dsocket
435 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
437 This example could also be used by
443 for more information.
445 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
447 The same example again, this time enabling proxy authentication with username
449 if the proxy requires it:
451 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
460 Original implementation by *Hobbit*
461 .Aq hobbit@avian.org .
463 Rewritten with IPv6 support by
464 .An Eric Jackson Aq ericj@monkey.org .
466 UDP port scans will always succeed
467 (i.e. report the port as open),
470 combination of flags relatively useless.