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1 .\" $OpenBSD: nc.1,v 1.45 2007/05/31 19:20:13 jmc Exp $
2 .\"
3 .\" Copyright (c) 1996 David Sacerdote
4 .\" All rights reserved.
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17 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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28 .\" $FreeBSD$
29 .\"
30 .Dd $Mdocdate$
31 .Dt NC 1
32 .Os
33 .Sh NAME
34 .Nm nc
35 .Nd arbitrary TCP and UDP connections and listens
36 .Sh SYNOPSIS
37 .Nm nc
38 .Bk -words
39 .Op Fl 46DdEhklnOorStUuvz
40 .Op Fl e Ar IPsec_policy
41 .Op Fl i Ar interval
42 .Op Fl P Ar proxy_username
43 .Op Fl p Ar source_port
44 .Op Fl s Ar source_ip_address
45 .Op Fl T Ar ToS
46 .Op Fl w Ar timeout
47 .Op Fl X Ar proxy_protocol
48 .Oo Xo
49 .Fl x Ar proxy_address Ns Oo : Ns
50 .Ar port Oc Oc
51 .Xc
52 .Op Ar hostname
53 .Op Ar port Ns Bq Ar s
54 .Ek
55 .Sh DESCRIPTION
56 The
57 .Nm
58 (or
59 .Nm netcat )
60 utility is used for just about anything under the sun involving TCP
61 or UDP.
62 It can open TCP connections, send UDP packets, listen on arbitrary
63 TCP and UDP ports, do port scanning, and deal with both IPv4 and
64 IPv6.
65 Unlike
66 .Xr telnet 1 ,
67 .Nm
68 scripts nicely, and separates error messages onto standard error instead
69 of sending them to standard output, as
70 .Xr telnet 1
71 does with some.
72 .Pp
73 Common uses include:
74 .Pp
75 .Bl -bullet -offset indent -compact
76 .It
77 simple TCP proxies
78 .It
79 shell-script based HTTP clients and servers
80 .It
81 network daemon testing
82 .It
83 a SOCKS or HTTP ProxyCommand for
84 .Xr ssh 1
85 .It
86 and much, much more
87 .El
88 .Pp
89 The options are as follows:
90 .Bl -tag -width Ds
91 .It Fl 4
92 Forces
93 .Nm
94 to use IPv4 addresses only.
95 .It Fl 6
96 Forces
97 .Nm
98 to use IPv6 addresses only.
99 .It Fl D
100 Enable debugging on the socket.
101 .It Fl d
102 Do not attempt to read from stdin.
103 .It Fl E
104 Shortcut for
105 .Qo
106 .Li "-e 'in ipsec esp/transport//require'"
107 .Li "-e 'out ipsec esp/transport//require'"
108 .Qc ,
109 which enables IPsec ESP transport mode in both
110 directions.
111 .It Fl e
112 If IPsec support is available, then one can specify the IPsec policies
113 to be used using the syntax described in
114 .Xr ipsec_set_policy 3 .
115 This flag can be specified up to two times, as typically one policy for
116 each direction is needed.
117 .It Fl h
118 Prints out
119 .Nm
120 help.
121 .It Fl i Ar interval
122 Specifies a delay time interval between lines of text sent and received.
123 Also causes a delay time between connections to multiple ports.
124 .It Fl k
125 Forces
126 .Nm
127 to stay listening for another connection after its current connection
128 is completed.
129 It is an error to use this option without the
130 .Fl l
131 option.
132 .It Fl l
133 Used to specify that
134 .Nm
135 should listen for an incoming connection rather than initiate a
136 connection to a remote host.
137 It is an error to use this option in conjunction with the
138 .Fl p ,
139 .Fl s ,
140 or
141 .Fl z
142 options.
143 Additionally, any timeouts specified with the
144 .Fl w
145 option are ignored.
146 .It Fl n
147 Do not do any DNS or service lookups on any specified addresses,
148 hostnames or ports.
149 .It Fl O
150 Disables the use of TCP options on the socket, by setting the boolean
151 TCP_NOOPT
152 socket option.
153 .It Fl o
154 .Dq Once-only mode .
155 By default,
156 .Nm
157 does not terminate on EOF condition on input,
158 but continues until the network side has been closed down.
159 Specifying
160 .Fl o
161 will make it terminate on EOF as well.
162 .It Fl P Ar proxy_username
163 Specifies a username to present to a proxy server that requires authentication.
164 If no username is specified then authentication will not be attempted.
165 Proxy authentication is only supported for HTTP CONNECT proxies at present.
166 .It Fl p Ar source_port
167 Specifies the source port
168 .Nm
169 should use, subject to privilege restrictions and availability.
170 It is an error to use this option in conjunction with the
171 .Fl l
172 option.
173 .It Fl r
174 Specifies that source and/or destination ports should be chosen randomly
175 instead of sequentially within a range or in the order that the system
176 assigns them.
177 .It Fl S
178 Enables the RFC 2385 TCP MD5 signature option.
179 .It Fl s Ar source_ip_address
180 Specifies the IP of the interface which is used to send the packets.
181 It is an error to use this option in conjunction with the
182 .Fl l
183 option.
184 .It Fl T Ar ToS
185 Specifies IP Type of Service (ToS) for the connection.
186 Valid values are the tokens
187 .Dq lowdelay ,
188 .Dq throughput ,
189 .Dq reliability ,
190 or an 8-bit hexadecimal value preceded by
191 .Dq 0x .
192 .It Fl t
193 Causes
194 .Nm
195 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
196 This makes it possible to use
197 .Nm
198 to script telnet sessions.
199 .It Fl U
200 Specifies to use Unix Domain Sockets.
201 .It Fl u
202 Use UDP instead of the default option of TCP.
203 .It Fl v
204 Have
205 .Nm
206 give more verbose output.
207 .It Fl w Ar timeout
208 If a connection and stdin are idle for more than
209 .Ar timeout
210 seconds, then the connection is silently closed.
211 The
212 .Fl w
213 flag has no effect on the
214 .Fl l
215 option, i.e.\&
216 .Nm
217 will listen forever for a connection, with or without the
218 .Fl w
219 flag.
220 The default is no timeout.
221 .It Fl X Ar proxy_protocol
222 Requests that
223 .Nm
224 should use the specified protocol when talking to the proxy server.
225 Supported protocols are
226 .Dq 4
227 (SOCKS v.4),
228 .Dq 5
229 (SOCKS v.5)
230 and
231 .Dq connect
232 (HTTPS proxy).
233 If the protocol is not specified, SOCKS version 5 is used.
234 .It Xo
235 .Fl x Ar proxy_address Ns Oo : Ns
236 .Ar port Oc
237 .Xc
238 Requests that
239 .Nm
240 should connect to
241 .Ar hostname
242 using a proxy at
243 .Ar proxy_address
244 and
245 .Ar port .
246 If
247 .Ar port
248 is not specified, the well-known port for the proxy protocol is used (1080
249 for SOCKS, 3128 for HTTPS).
250 .It Fl z
251 Specifies that
252 .Nm
253 should just scan for listening daemons, without sending any data to them.
254 It is an error to use this option in conjunction with the
255 .Fl l
256 option.
257 .El
258 .Pp
259 .Ar hostname
260 can be a numerical IP address or a symbolic hostname
261 (unless the
262 .Fl n
263 option is given).
264 In general, a hostname must be specified,
265 unless the
266 .Fl l
267 option is given
268 (in which case the local host is used).
269 .Pp
270 .Ar port Ns Op Ar s
271 can be single integers or ranges.
272 Ranges are in the form nn-mm.
273 In general,
274 a destination port must be specified,
275 unless the
276 .Fl U
277 option is given
278 (in which case a socket must be specified).
279 .Sh CLIENT/SERVER MODEL
280 It is quite simple to build a very basic client/server model using
281 .Nm .
282 On one console, start
283 .Nm
284 listening on a specific port for a connection.
285 For example:
286 .Pp
287 .Dl $ nc -l 1234
288 .Pp
289 .Nm
290 is now listening on port 1234 for a connection.
291 On a second console
292 .Pq or a second machine ,
293 connect to the machine and port being listened on:
294 .Pp
295 .Dl $ nc 127.0.0.1 1234
296 .Pp
297 There should now be a connection between the ports.
298 Anything typed at the second console will be concatenated to the first,
299 and vice-versa.
300 After the connection has been set up,
301 .Nm
302 does not really care which side is being used as a
303 .Sq server
304 and which side is being used as a
305 .Sq client .
306 The connection may be terminated using an
307 .Dv EOF
308 .Pq Sq ^D .
309 .Sh DATA TRANSFER
310 The example in the previous section can be expanded to build a
311 basic data transfer model.
312 Any information input into one end of the connection will be output
313 to the other end, and input and output can be easily captured in order to
314 emulate file transfer.
315 .Pp
316 Start by using
317 .Nm
318 to listen on a specific port, with output captured into a file:
319 .Pp
320 .Dl $ nc -l 1234 \*(Gt filename.out
321 .Pp
322 Using a second machine, connect to the listening
323 .Nm
324 process, feeding it the file which is to be transferred:
325 .Pp
326 .Dl $ nc host.example.com 1234 \*(Lt filename.in
327 .Pp
328 After the file has been transferred, the connection will close automatically.
329 .Sh TALKING TO SERVERS
330 It is sometimes useful to talk to servers
331 .Dq by hand
332 rather than through a user interface.
333 It can aid in troubleshooting,
334 when it might be necessary to verify what data a server is sending
335 in response to commands issued by the client.
336 For example, to retrieve the home page of a web site:
337 .Bd -literal -offset indent
338 $ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
339 .Ed
340 .Pp
341 Note that this also displays the headers sent by the web server.
342 They can be filtered, using a tool such as
343 .Xr sed 1 ,
344 if necessary.
345 .Pp
346 More complicated examples can be built up when the user knows the format
347 of requests required by the server.
348 As another example, an email may be submitted to an SMTP server using:
349 .Bd -literal -offset indent
350 $ nc localhost 25 \*(Lt\*(Lt EOF
351 HELO host.example.com
352 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
353 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
354 DATA
355 Body of email.
356 \&.
357 QUIT
358 EOF
359 .Ed
360 .Sh PORT SCANNING
361 It may be useful to know which ports are open and running services on
362 a target machine.
363 The
364 .Fl z
365 flag can be used to tell
366 .Nm
367 to report open ports,
368 rather than initiate a connection.
369 For example:
370 .Bd -literal -offset indent
371 $ nc -z host.example.com 20-30
372 Connection to host.example.com 22 port [tcp/ssh] succeeded!
373 Connection to host.example.com 25 port [tcp/smtp] succeeded!
374 .Ed
375 .Pp
376 The port range was specified to limit the search to ports 20 \- 30.
377 .Pp
378 Alternatively, it might be useful to know which server software
379 is running, and which versions.
380 This information is often contained within the greeting banners.
381 In order to retrieve these, it is necessary to first make a connection,
382 and then break the connection when the banner has been retrieved.
383 This can be accomplished by specifying a small timeout with the
384 .Fl w
385 flag, or perhaps by issuing a
386 .Qq Dv QUIT
387 command to the server:
388 .Bd -literal -offset indent
389 $ echo "QUIT" | nc host.example.com 20-30
390 SSH-1.99-OpenSSH_3.6.1p2
391 Protocol mismatch.
392 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
393 .Ed
394 .Sh EXAMPLES
395 Open a TCP connection to port 42 of host.example.com, using port 31337 as
396 the source port, with a timeout of 5 seconds:
397 .Pp
398 .Dl $ nc -p 31337 -w 5 host.example.com 42
399 .Pp
400 Open a UDP connection to port 53 of host.example.com:
401 .Pp
402 .Dl $ nc -u host.example.com 53
403 .Pp
404 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
405 IP for the local end of the connection:
406 .Pp
407 .Dl $ nc -s 10.1.2.3 host.example.com 42
408 .Pp
409 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
410 incoming and outgoing traffic.
411 .Pp
412 .Dl $ nc -E host.example.com 42
413 .Pp
414 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
415 outgoing traffic only.
416 .Pp
417 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
418 .Pp
419 Create and listen on a Unix Domain Socket:
420 .Pp
421 .Dl $ nc -lU /var/tmp/dsocket
422 .Pp
423 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
424 port 8080.
425 This example could also be used by
426 .Xr ssh 1 ;
427 see the
428 .Cm ProxyCommand
429 directive in
430 .Xr ssh_config 5
431 for more information.
432 .Pp
433 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
434 .Pp
435 The same example again, this time enabling proxy authentication with username
436 .Dq ruser
437 if the proxy requires it:
438 .Pp
439 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
440 .Sh SEE ALSO
441 .Xr cat 1 ,
442 .Xr ssh 1 ,
443 .Xr tcp 4
444 .Sh AUTHORS
445 Original implementation by *Hobbit*
446 .Aq hobbit@avian.org .
447 .br
448 Rewritten with IPv6 support by
449 .An Eric Jackson Aq ericj@monkey.org .
450 .Sh CAVEATS
451 UDP port scans will always succeed
452 (i.e. report the port as open),
453 rendering the
454 .Fl uz
455 combination of flags relatively useless.