1 .\" $OpenBSD: nc.1,v 1.36 2005/01/07 10:11:31 jmc Exp $
3 .\" Copyright (c) 1996 David Sacerdote
4 .\" All rights reserved.
6 .\" Redistribution and use in source and binary forms, with or without
7 .\" modification, are permitted provided that the following conditions
9 .\" 1. Redistributions of source code must retain the above copyright
10 .\" notice, this list of conditions and the following disclaimer.
11 .\" 2. Redistributions in binary form must reproduce the above copyright
12 .\" notice, this list of conditions and the following disclaimer in the
13 .\" documentation and/or other materials provided with the distribution.
14 .\" 3. The name of the author may not be used to endorse or promote products
15 .\" derived from this software without specific prior written permission
17 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
18 .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
19 .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
20 .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
21 .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
22 .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
26 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DEdhklnorStUuvz
40 .Op Fl e Ar IPsec policy
42 .Op Fl p Ar source_port
43 .Op Fl s Ar source_ip_address
45 .Op Fl X Ar proxy_protocol
47 .Fl x Ar proxy_address Ns Oo : Ns
51 .Op Ar port Ns Bq Ar s
58 utility is used for just about anything under the sun involving TCP
60 It can open TCP connections, send UDP packets, listen on arbitrary
61 TCP and UDP ports, do port scanning, and deal with both IPv4 and
66 scripts nicely, and separates error messages onto standard error instead
67 of sending them to standard output, as
73 .Bl -bullet -offset indent -compact
77 shell-script based HTTP clients and servers
79 network daemon testing
81 a SOCKS or HTTP ProxyCommand for
87 The options are as follows:
92 to use IPv4 addresses only.
96 to use IPv6 addresses only.
98 Enable debugging on the socket.
100 Do not attempt to read from stdin.
106 Shortcut for "-e 'in ipsec esp/transport//require' -e 'out ipsec
107 esp/transport//require'", which enables IPsec ESP transport mode in both
110 If IPsec support is available, then one can specify the IPsec policies
111 to be used using the syntax described in
112 .Xr ipsec_set_policy 3 .
113 This flag can be specified up to two times, as typically one policy for
114 each direction is needed.
116 Specifies a delay time interval between lines of text sent and received.
117 Also causes a delay time between connections to multiple ports.
121 to stay listening for another connection after its current connection
123 It is an error to use this option without the
129 should listen for an incoming connection rather than initiate a
130 connection to a remote host.
131 It is an error to use this option in conjunction with the
137 Additionally, any timeouts specified with the
141 Do not do any DNS or service lookups on any specified addresses,
147 does not terminate on EOF condition on input,
148 but continues until the network side has been closed down.
151 will make it terminate on EOF as well.
152 .It Fl p Ar source_port
153 Specifies the source port
155 should use, subject to privilege restrictions and availability.
156 It is an error to use this option in conjunction with the
160 Specifies that source and/or destination ports should be chosen randomly
161 instead of sequentially within a range or in the order that the system
164 Enables the RFC 2385 TCP MD5 signature option.
165 .It Fl s Ar source_ip_address
166 Specifies the IP of the interface which is used to send the packets.
167 It is an error to use this option in conjunction with the
173 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
174 This makes it possible to use
176 to script telnet sessions.
178 Specifies to use Unix Domain Sockets.
180 Use UDP instead of the default option of TCP.
184 give more verbose output.
186 If a connection and stdin are idle for more than
188 seconds, then the connection is silently closed.
191 flag has no effect on the
195 will listen forever for a connection, with or without the
198 The default is no timeout.
199 .It Fl X Ar proxy_version
202 should use the specified protocol when talking to the proxy server.
203 Supported protocols are
211 If the protocol is not specified, SOCKS version 5 is used.
213 .Fl x Ar proxy_address Ns Oo : Ns
226 is not specified, the well-known port for the proxy protocol is used (1080
227 for SOCKS, 3128 for HTTPS).
231 should just scan for listening daemons, without sending any data to them.
232 It is an error to use this option in conjunction with the
238 can be a numerical IP address or a symbolic hostname
242 In general, a hostname must be specified,
246 (in which case the local host is used).
249 can be single integers or ranges.
250 Ranges are in the form nn-mm.
252 a destination port must be specified,
256 (in which case a socket must be specified).
257 .Sh CLIENT/SERVER MODEL
258 It is quite simple to build a very basic client/server model using
260 On one console, start
262 listening on a specific port for a connection.
268 is now listening on port 1234 for a connection.
270 .Pq or a second machine ,
271 connect to the machine and port being listened on:
273 .Dl $ nc 127.0.0.1 1234
275 There should now be a connection between the ports.
276 Anything typed at the second console will be concatenated to the first,
278 After the connection has been set up,
280 does not really care which side is being used as a
282 and which side is being used as a
284 The connection may be terminated using an
288 The example in the previous section can be expanded to build a
289 basic data transfer model.
290 Any information input into one end of the connection will be output
291 to the other end, and input and output can be easily captured in order to
292 emulate file transfer.
296 to listen on a specific port, with output captured into a file:
298 .Dl $ nc -l 1234 \*(Gt filename.out
300 Using a second machine, connect to the listening
302 process, feeding it the file which is to be transferred:
304 .Dl $ nc host.example.com 1234 \*(Lt filename.in
306 After the file has been transferred, the connection will close automatically.
307 .Sh TALKING TO SERVERS
308 It is sometimes useful to talk to servers
310 rather than through a user interface.
311 It can aid in troubleshooting,
312 when it might be necessary to verify what data a server is sending
313 in response to commands issued by the client.
314 For example, to retrieve the home page of a web site:
316 .Dl $ echo \&"GET\&" | nc host.example.com 80
318 Note that this also displays the headers sent by the web server.
319 They can be filtered, using a tool such as
323 More complicated examples can be built up when the user knows the format
324 of requests required by the server.
325 As another example, an email may be submitted to an SMTP server using:
326 .Bd -literal -offset indent
327 $ nc localhost 25 \*(Lt\*(Lt EOF
328 HELO host.example.com
329 MAIL FROM: \*(Ltuser@host.example.com\*(Gt
330 RCPT TO: \*(Ltuser2@host.example.com\*(Gt
338 It may be useful to know which ports are open and running services on
342 flag can be used to tell
344 not to initiate a connection,
349 to report open ports.
351 .Bd -literal -offset indent
352 $ nc -vz host.example.com 20-30
353 Connection to host.example.com 22 port [tcp/ssh] succeeded!
354 Connection to host.example.com 25 port [tcp/smtp] succeeded!
357 The port range was specified to limit the search to ports 20 \- 30.
359 Alternatively, it might be useful to know which server software
360 is running, and which versions.
361 This information is often contained within the greeting banners.
362 In order to retrieve these, it is necessary to first make a connection,
363 and then break the connection when the banner has been retrieved.
364 This can be accomplished by specifying a small timeout with the
366 flag, or perhaps by issuing a
368 command to the server:
369 .Bd -literal -offset indent
370 $ echo "QUIT" | nc host.example.com 20-30
371 SSH-1.99-OpenSSH_3.6.1p2
373 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
376 Open a TCP connection to port 42 of hostname, using port 31337 as
377 the source port, with a timeout of 5 seconds:
379 .Dl $ nc -p 31337 -w 5 hostname 42
381 Open a UDP connection to port 53 of hostname:
383 .Dl $ nc -u hostname 53
385 Open a TCP connection to port 42 of example.host using 10.1.2.3 as the
386 IP for the local end of the connection:
387 .It Li "nc -E example.host 42"
388 Open a TCP connection to port 42 of example.host using IPsec ESP for
389 incoming and outgoing traffic.
390 .It Li "nc -e 'out ipsec esp/transport//require' example.host 42"
391 Open a TCP connection to port 42 of example.host using IPsec ESP for
392 outgoing traffic only.
394 .Dl $ nc -s 10.1.2.3 example.host 42
396 Send UDP packets to ports 20-30 of example.host, and report which ones
397 responded with an ICMP packet after three seconds:
399 .Dl $ nc -uvz -w 3 hostname 20-30
401 Create and listen on a Unix Domain Socket:
403 .Dl $ nc -lU /var/tmp/dsocket
405 Connect to port 42 of hostname via an HTTP proxy at 10.2.3.4, port 8080:
407 .Dl $ nc -x10.2.3.4:8080 -Xconnect hostname 42
411 Original implementation by *Hobbit*
412 .Aq hobbit@avian.org .
414 Rewritten with IPv6 support by
415 .An Eric Jackson Aq ericj@monkey.org .