1 .\" $OpenBSD: nc.1,v 1.44 2006/12/02 01:08:30 jmc Exp $
3 .\" Copyright (c) 1996 David Sacerdote
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35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DEdhklnoOrStUuvz
40 .Op Fl e Ar IPsec_policy
42 .Op Fl P Ar proxy_username
43 .Op Fl p Ar source_port
44 .Op Fl s Ar source_ip_address
47 .Op Fl X Ar proxy_protocol
49 .Fl x Ar proxy_address Ns Oo : Ns
53 .Op Ar port Ns Bq Ar s
60 utility is used for just about anything under the sun involving TCP
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
68 scripts nicely, and separates error messages onto standard error instead
69 of sending them to standard output, as
75 .Bl -bullet -offset indent -compact
79 shell-script based HTTP clients and servers
81 network daemon testing
83 a SOCKS or HTTP ProxyCommand for
89 The options are as follows:
94 to use IPv4 addresses only.
98 to use IPv6 addresses only.
100 Enable debugging on the socket.
102 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.
122 Specifies a delay time interval between lines of text sent and received.
123 Also causes a delay time between connections to multiple ports.
127 to stay listening for another connection after its current connection
129 It is an error to use this option without the
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
143 Additionally, any timeouts specified with the
147 Do not do any DNS or service lookups on any specified addresses,
154 does not terminate on EOF condition on input,
155 but continues until the network side has been closed down.
158 will make it terminate on EOF as well.
160 Disables the use of TCP options on the socket, by setting the boolean
163 .It Fl P Ar proxy_username
164 Specifies a username to present to a proxy server that requires authentication.
165 If no username is specified then authentication will not be attempted.
166 Proxy authentication is only supported for HTTP CONNECT proxies at present.
167 .It Fl p Ar source_port
168 Specifies the source port
170 should use, subject to privilege restrictions and availability.
171 It is an error to use this option in conjunction with the
175 Specifies that source and/or destination ports should be chosen randomly
176 instead of sequentially within a range or in the order that the system
179 Enables the RFC 2385 TCP MD5 signature option.
180 .It Fl s Ar source_ip_address
181 Specifies the IP of the interface which is used to send the packets.
182 It is an error to use this option in conjunction with the
186 Specifies IP Type of Service (ToS) for the connection.
187 Valid values are the tokens
191 or an 8-bit hexadecimal value preceded by
196 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
197 This makes it possible to use
199 to script telnet sessions.
201 Specifies to use Unix Domain Sockets.
203 Use UDP instead of the default option of TCP.
207 give more verbose output.
209 If a connection and stdin are idle for more than
211 seconds, then the connection is silently closed.
214 flag has no effect on the
218 will listen forever for a connection, with or without the
221 The default is no timeout.
222 .It Fl X Ar proxy_protocol
225 should use the specified protocol when talking to the proxy server.
226 Supported protocols are
234 If the protocol is not specified, SOCKS version 5 is used.
236 .Fl x Ar proxy_address Ns Oo : Ns
249 is not specified, the well-known port for the proxy protocol is used (1080
250 for SOCKS, 3128 for HTTPS).
254 should just scan for listening daemons, without sending any data to them.
255 It is an error to use this option in conjunction with the
261 can be a numerical IP address or a symbolic hostname
265 In general, a hostname must be specified,
269 (in which case the local host is used).
272 can be single integers or ranges.
273 Ranges are in the form nn-mm.
275 a destination port must be specified,
279 (in which case a socket must be specified).
280 .Sh CLIENT/SERVER MODEL
281 It is quite simple to build a very basic client/server model using
283 On one console, start
285 listening on a specific port for a connection.
291 is now listening on port 1234 for a connection.
293 .Pq or a second machine ,
294 connect to the machine and port being listened on:
296 .Dl $ nc 127.0.0.1 1234
298 There should now be a connection between the ports.
299 Anything typed at the second console will be concatenated to the first,
301 After the connection has been set up,
303 does not really care which side is being used as a
305 and which side is being used as a
307 The connection may be terminated using an
311 The example in the previous section can be expanded to build a
312 basic data transfer model.
313 Any information input into one end of the connection will be output
314 to the other end, and input and output can be easily captured in order to
315 emulate file transfer.
319 to listen on a specific port, with output captured into a file:
321 .Dl $ nc -l 1234 \*(Gt filename.out
323 Using a second machine, connect to the listening
325 process, feeding it the file which is to be transferred:
327 .Dl $ nc host.example.com 1234 \*(Lt filename.in
329 After the file has been transferred, the connection will close automatically.
330 .Sh TALKING TO SERVERS
331 It is sometimes useful to talk to servers
333 rather than through a user interface.
334 It can aid in troubleshooting,
335 when it might be necessary to verify what data a server is sending
336 in response to commands issued by the client.
337 For example, to retrieve the home page of a web site:
338 .Bd -literal -offset indent
339 $ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
342 Note that this also displays the headers sent by the web server.
343 They can be filtered, using a tool such as
347 More complicated examples can be built up when the user knows the format
348 of requests required by the server.
349 As another example, an email may be submitted to an SMTP server using:
350 .Bd -literal -offset indent
351 $ nc localhost 25 \*(Lt\*(Lt EOF
352 HELO host.example.com
353 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
354 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
362 It may be useful to know which ports are open and running services on
366 flag can be used to tell
368 to report open ports,
369 rather than initiate a connection.
371 .Bd -literal -offset indent
372 $ nc -z host.example.com 20-30
373 Connection to host.example.com 22 port [tcp/ssh] succeeded!
374 Connection to host.example.com 25 port [tcp/smtp] succeeded!
377 The port range was specified to limit the search to ports 20 \- 30.
379 Alternatively, it might be useful to know which server software
380 is running, and which versions.
381 This information is often contained within the greeting banners.
382 In order to retrieve these, it is necessary to first make a connection,
383 and then break the connection when the banner has been retrieved.
384 This can be accomplished by specifying a small timeout with the
386 flag, or perhaps by issuing a
388 command to the server:
389 .Bd -literal -offset indent
390 $ echo "QUIT" | nc host.example.com 20-30
391 SSH-1.99-OpenSSH_3.6.1p2
393 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
396 Open a TCP connection to port 42 of host.example.com, using port 31337 as
397 the source port, with a timeout of 5 seconds:
399 .Dl $ nc -p 31337 -w 5 host.example.com 42
401 Open a UDP connection to port 53 of host.example.com:
403 .Dl $ nc -u host.example.com 53
405 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
406 IP for the local end of the connection:
408 .Dl $ nc -s 10.1.2.3 host.example.com 42
410 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
411 incoming and outgoing traffic.
413 .Dl $ nc -E host.example.com 42
415 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
416 outgoing traffic only.
418 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
420 Create and listen on a Unix Domain Socket:
422 .Dl $ nc -lU /var/tmp/dsocket
424 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
426 This example could also be used by
432 for more information.
434 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
436 The same example again, this time enabling proxy authentication with username
438 if the proxy requires it:
440 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
446 Original implementation by *Hobbit*
447 .Aq hobbit@avian.org .
449 Rewritten with IPv6 support by
450 .An Eric Jackson Aq ericj@monkey.org .
452 UDP port scans will always succeed
453 (i.e. report the port as open),
456 combination of flags relatively useless.