1 .\" $OpenBSD: nc.1,v 1.61 2012/07/07 15:33:02 haesbaert 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 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
140 When used together with the
142 option, the server socket is not connected and it can receive UDP datagrams from
147 should listen for an incoming connection rather than initiate a
148 connection to a remote host.
149 It is an error to use this option in conjunction with the
155 Additionally, any timeouts specified with the
159 Do not do any DNS or service lookups on any specified addresses,
162 Disables the use of TCP options on the socket, by setting the boolean
166 Specifies the size of the TCP send buffer.
167 .It Fl P Ar proxy_username
168 Specifies a username to present to a proxy server that requires authentication.
169 If no username is specified then authentication will not be attempted.
170 Proxy authentication is only supported for HTTP CONNECT proxies at present.
171 .It Fl p Ar source_port
172 Specifies the source port
174 should use, subject to privilege restrictions and availability.
175 It is an error to use this option in conjunction with the
179 Specifies that source and/or destination ports should be chosen randomly
180 instead of sequentially within a range or in the order that the system
183 Enables the RFC 2385 TCP MD5 signature option.
185 Specifies the IP of the interface which is used to send the packets.
188 datagram sockets, specifies the local temporary socket file
189 to create and use so that datagrams can be received.
190 It is an error to use this option in conjunction with the
193 .It Fl T Ar toskeyword
194 Change IPv4 TOS value.
203 or one of the DiffServ Code Points:
207 or a number in either hex or decimal.
211 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
212 This makes it possible to use
214 to script telnet sessions.
220 Use UDP instead of the default option of TCP.
223 sockets, use a datagram socket instead of a stream socket.
226 socket is used, a temporary receiving socket is created in
232 Set the routing table
239 give more verbose output.
241 Connections which cannot be established or are idle timeout after
246 flag has no effect on the
250 will listen forever for a connection, with or without the
253 The default is no timeout.
254 .It Fl X Ar proxy_protocol
257 should use the specified protocol when talking to the proxy server.
258 Supported protocols are
266 If the protocol is not specified, SOCKS version 5 is used.
268 .Fl x Ar proxy_address Ns Oo : Ns
281 is not specified, the well-known port for the proxy protocol is used (1080
282 for SOCKS, 3128 for HTTPS).
286 should just scan for listening daemons, without sending any data to them.
287 It is an error to use this option in conjunction with the
293 can be a numerical IP address or a symbolic hostname
297 In general, a destination must be specified,
301 (in which case the local host is used).
304 sockets, a destination is required and is the socket path to connect to
310 can be a single integer or a range of ports.
311 Ranges are in the form nn-mm.
313 a destination port must be specified,
317 .Sh CLIENT/SERVER MODEL
318 It is quite simple to build a very basic client/server model using
320 On one console, start
322 listening on a specific port for a connection.
328 is now listening on port 1234 for a connection.
330 .Pq or a second machine ,
331 connect to the machine and port being listened on:
333 .Dl $ nc 127.0.0.1 1234
335 There should now be a connection between the ports.
336 Anything typed at the second console will be concatenated to the first,
338 After the connection has been set up,
340 does not really care which side is being used as a
342 and which side is being used as a
344 The connection may be terminated using an
348 The example in the previous section can be expanded to build a
349 basic data transfer model.
350 Any information input into one end of the connection will be output
351 to the other end, and input and output can be easily captured in order to
352 emulate file transfer.
356 to listen on a specific port, with output captured into a file:
358 .Dl $ nc -l 1234 \*(Gt filename.out
360 Using a second machine, connect to the listening
362 process, feeding it the file which is to be transferred:
364 .Dl $ nc host.example.com 1234 \*(Lt filename.in
366 After the file has been transferred, the connection will close automatically.
367 .Sh TALKING TO SERVERS
368 It is sometimes useful to talk to servers
370 rather than through a user interface.
371 It can aid in troubleshooting,
372 when it might be necessary to verify what data a server is sending
373 in response to commands issued by the client.
374 For example, to retrieve the home page of a web site:
375 .Bd -literal -offset indent
376 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
379 Note that this also displays the headers sent by the web server.
380 They can be filtered, using a tool such as
384 More complicated examples can be built up when the user knows the format
385 of requests required by the server.
386 As another example, an email may be submitted to an SMTP server using:
387 .Bd -literal -offset indent
388 $ nc localhost 25 \*(Lt\*(Lt EOF
389 HELO host.example.com
390 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
391 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
399 It may be useful to know which ports are open and running services on
403 flag can be used to tell
405 to report open ports,
406 rather than initiate a connection.
408 .Bd -literal -offset indent
409 $ nc -z host.example.com 20-30
410 Connection to host.example.com 22 port [tcp/ssh] succeeded!
411 Connection to host.example.com 25 port [tcp/smtp] succeeded!
414 The port range was specified to limit the search to ports 20 \- 30.
416 Alternatively, it might be useful to know which server software
417 is running, and which versions.
418 This information is often contained within the greeting banners.
419 In order to retrieve these, it is necessary to first make a connection,
420 and then break the connection when the banner has been retrieved.
421 This can be accomplished by specifying a small timeout with the
423 flag, or perhaps by issuing a
425 command to the server:
426 .Bd -literal -offset indent
427 $ echo "QUIT" | nc host.example.com 20-30
428 SSH-1.99-OpenSSH_3.6.1p2
430 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
433 Open a TCP connection to port 42 of host.example.com, using port 31337 as
434 the source port, with a timeout of 5 seconds:
436 .Dl $ nc -p 31337 -w 5 host.example.com 42
438 Open a UDP connection to port 53 of host.example.com:
440 .Dl $ nc -u host.example.com 53
442 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
443 IP for the local end of the connection:
445 .Dl $ nc -s 10.1.2.3 host.example.com 42
447 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
448 incoming and outgoing traffic.
450 .Dl $ nc -E host.example.com 42
452 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
453 outgoing traffic only.
455 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
457 Create and listen on a
461 .Dl $ nc -lU /var/tmp/dsocket
463 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
465 This example could also be used by
471 for more information.
473 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
475 The same example again, this time enabling proxy authentication with username
477 if the proxy requires it:
479 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
488 Original implementation by *Hobbit*
489 .Aq hobbit@avian.org .
491 Rewritten with IPv6 support by
492 .An Eric Jackson Aq ericj@monkey.org .
494 UDP port scans using the
496 combination of flags will always report success irrespective of
497 the target machine's state.
499 in conjunction with a traffic sniffer either on the target machine
500 or an intermediary device,
503 combination could be useful for communications diagnostics.
504 Note that the amount of UDP traffic generated may be limited either
505 due to hardware resources and/or configuration settings.