1 .\" $OpenBSD: nc.1,v 1.68 2015/03/26 10:35:04 tobias 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 46DdEFhklMNnrStUuvz
40 .Op Fl e Ar IPsec_policy
46 .Op Fl P Ar proxy_username
47 .Op Fl p Ar source_port
49 .Op Fl T Ar toskeyword
52 .Op Fl X Ar proxy_protocol
54 .Fl x Ar proxy_address Ns Oo : Ns
65 utility is used for just about anything under the sun involving TCP,
69 It can open TCP connections, send UDP packets, listen on arbitrary
70 TCP and UDP ports, do port scanning, and deal with both IPv4 and
75 scripts nicely, and separates error messages onto standard error instead
76 of sending them to standard output, as
82 .Bl -bullet -offset indent -compact
86 shell-script based HTTP clients and servers
88 network daemon testing
90 a SOCKS or HTTP ProxyCommand for
96 The options are as follows:
101 to use IPv4 addresses only.
105 to use IPv6 addresses only.
107 Enable debugging on the socket.
109 Do not attempt to read from stdin.
113 .Li "-e 'in ipsec esp/transport//require'"
114 .Li "-e 'out ipsec esp/transport//require'"
116 which enables IPsec ESP transport mode in both
119 If IPsec support is available, then one can specify the IPsec policies
120 to be used using the syntax described in
121 .Xr ipsec_set_policy 3 .
122 This flag can be specified up to two times, as typically one policy for
123 each direction is needed.
125 Pass the first connected socket using
128 This is useful in conjunction with
132 perform connection setup with a proxy but then leave the rest of the
133 connection to another program (e.g.\&
144 Specifies the size of the TCP receive buffer.
146 Specifies a delay time interval between lines of text sent and received.
147 Also causes a delay time between connections to multiple ports.
151 to stay listening for another connection after its current connection
153 It is an error to use this option without the
156 When used together with the
158 option, the server socket is not connected and it can receive UDP datagrams from
163 should listen for an incoming connection rather than initiate a
164 connection to a remote host.
165 It is an error to use this option in conjunction with the
171 Additionally, any timeouts specified with the
175 Collect per-connection TCP statistics using the
177 framework and print them in JSON format to
179 after the connection is closed.
182 the network socket after EOF on the input.
183 Some servers require this to finish their work.
185 Do not do any DNS or service lookups on any specified addresses,
188 Disables the use of TCP options on the socket, by setting the boolean
192 Use SCTP instead of the default option of TCP.
194 Specifies the size of the TCP send buffer.
195 .It Fl P Ar proxy_username
196 Specifies a username to present to a proxy server that requires authentication.
197 If no username is specified then authentication will not be attempted.
198 Proxy authentication is only supported for HTTP CONNECT proxies at present.
199 .It Fl p Ar source_port
200 Specifies the source port
202 should use, subject to privilege restrictions and availability.
203 It is an error to use this option in conjunction with the
207 Specifies that source and/or destination ports should be chosen randomly
208 instead of sequentially within a range or in the order that the system
211 Enables the RFC 2385 TCP MD5 signature option.
213 Specifies the IP of the interface which is used to send the packets.
216 datagram sockets, specifies the local temporary socket file
217 to create and use so that datagrams can be received.
218 It is an error to use this option in conjunction with the
221 .It Fl T Ar toskeyword
222 Change IPv4 TOS value.
231 or one of the DiffServ Code Points:
235 or a number in either hex or decimal.
239 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
240 This makes it possible to use
242 to script telnet sessions.
248 Use UDP instead of the default option of TCP.
251 sockets, use a datagram socket instead of a stream socket.
254 socket is used, a temporary receiving socket is created in
260 Set the routing table
266 give more verbose output.
268 Connections which cannot be established or are idle timeout after
273 flag has no effect on the
277 will listen forever for a connection, with or without the
280 The default is no timeout.
281 .It Fl X Ar proxy_protocol
284 should use the specified protocol when talking to the proxy server.
285 Supported protocols are
293 If the protocol is not specified, SOCKS version 5 is used.
295 .Fl x Ar proxy_address Ns Oo : Ns
308 is not specified, the well-known port for the proxy protocol is used (1080
309 for SOCKS, 3128 for HTTPS).
313 should just scan for listening daemons, without sending any data to them.
314 It is an error to use this option in conjunction with the
320 can be a numerical IP address or a symbolic hostname
324 In general, a destination must be specified,
328 (in which case the local host is used).
331 sockets, a destination is required and is the socket path to connect to
337 can be a single integer or a range of ports.
338 Ranges are in the form nn-mm.
340 a destination port must be specified,
344 .Sh CLIENT/SERVER MODEL
345 It is quite simple to build a very basic client/server model using
347 On one console, start
349 listening on a specific port for a connection.
355 is now listening on port 1234 for a connection.
357 .Pq or a second machine ,
358 connect to the machine and port being listened on:
360 .Dl $ nc 127.0.0.1 1234
362 There should now be a connection between the ports.
363 Anything typed at the second console will be concatenated to the first,
365 After the connection has been set up,
367 does not really care which side is being used as a
369 and which side is being used as a
371 The connection may be terminated using an
375 The example in the previous section can be expanded to build a
376 basic data transfer model.
377 Any information input into one end of the connection will be output
378 to the other end, and input and output can be easily captured in order to
379 emulate file transfer.
383 to listen on a specific port, with output captured into a file:
385 .Dl $ nc -l 1234 \*(Gt filename.out
387 Using a second machine, connect to the listening
389 process, feeding it the file which is to be transferred:
391 .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
393 After the file has been transferred, the connection will close automatically.
394 .Sh TALKING TO SERVERS
395 It is sometimes useful to talk to servers
397 rather than through a user interface.
398 It can aid in troubleshooting,
399 when it might be necessary to verify what data a server is sending
400 in response to commands issued by the client.
401 For example, to retrieve the home page of a web site:
402 .Bd -literal -offset indent
403 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
406 Note that this also displays the headers sent by the web server.
407 They can be filtered, using a tool such as
411 More complicated examples can be built up when the user knows the format
412 of requests required by the server.
413 As another example, an email may be submitted to an SMTP server using:
414 .Bd -literal -offset indent
415 $ nc localhost 25 \*(Lt\*(Lt EOF
416 HELO host.example.com
417 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
418 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
426 It may be useful to know which ports are open and running services on
430 flag can be used to tell
432 to report open ports,
433 rather than initiate a connection.
435 .Bd -literal -offset indent
436 $ nc -z host.example.com 20-30
437 Connection to host.example.com 22 port [tcp/ssh] succeeded!
438 Connection to host.example.com 25 port [tcp/smtp] succeeded!
441 The port range was specified to limit the search to ports 20 \- 30.
443 Alternatively, it might be useful to know which server software
444 is running, and which versions.
445 This information is often contained within the greeting banners.
446 In order to retrieve these, it is necessary to first make a connection,
447 and then break the connection when the banner has been retrieved.
448 This can be accomplished by specifying a small timeout with the
450 flag, or perhaps by issuing a
452 command to the server:
453 .Bd -literal -offset indent
454 $ echo "QUIT" | nc host.example.com 20-30
455 SSH-1.99-OpenSSH_3.6.1p2
457 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
460 Open a TCP connection to port 42 of host.example.com, using port 31337 as
461 the source port, with a timeout of 5 seconds:
463 .Dl $ nc -p 31337 -w 5 host.example.com 42
465 Open a UDP connection to port 53 of host.example.com:
467 .Dl $ nc -u host.example.com 53
469 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
470 IP for the local end of the connection:
472 .Dl $ nc -s 10.1.2.3 host.example.com 42
474 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
475 incoming and outgoing traffic.
477 .Dl $ nc -E host.example.com 42
479 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
480 outgoing traffic only.
482 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
484 Create and listen on a
488 .Dl $ nc -lU /var/tmp/dsocket
490 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
492 This example could also be used by
498 for more information.
500 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
502 The same example again, this time enabling proxy authentication with username
504 if the proxy requires it:
506 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
515 Original implementation by *Hobbit*
516 .Aq Mt hobbit@avian.org .
518 Rewritten with IPv6 support by
519 .An Eric Jackson Aq Mt ericj@monkey.org .
521 UDP port scans using the
523 combination of flags will always report success irrespective of
524 the target machine's state.
526 in conjunction with a traffic sniffer either on the target machine
527 or an intermediary device,
530 combination could be useful for communications diagnostics.
531 Note that the amount of UDP traffic generated may be limited either
532 due to hardware resources and/or configuration settings.