1 .\" $OpenBSD: nc.1,v 1.43 2006/01/31 09:34:12 jmc Exp $
3 .\" Copyright (c) 1996 David Sacerdote
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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 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,
153 does not terminate on EOF condition on input,
154 but continues until the network side has been closed down.
157 will make it terminate on EOF as well.
158 .It Fl P Ar proxy_username
159 Specifies a username to present to a proxy server that requires authentication.
160 If no username is specified then authentication will not be attempted.
161 Proxy authentication is only supported for HTTP CONNECT proxies at present.
162 .It Fl p Ar source_port
163 Specifies the source port
165 should use, subject to privilege restrictions and availability.
166 It is an error to use this option in conjunction with the
170 Specifies that source and/or destination ports should be chosen randomly
171 instead of sequentially within a range or in the order that the system
174 Enables the RFC 2385 TCP MD5 signature option.
175 .It Fl s Ar source_ip_address
176 Specifies the IP of the interface which is used to send the packets.
177 It is an error to use this option in conjunction with the
181 Specifies IP Type of Service (ToS) for the connection.
182 Valid values are the tokens
186 or an 8-bit hexadecimal value preceded by
191 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
192 This makes it possible to use
194 to script telnet sessions.
196 Specifies to use Unix Domain Sockets.
198 Use UDP instead of the default option of TCP.
202 give more verbose output.
204 If a connection and stdin are idle for more than
206 seconds, then the connection is silently closed.
209 flag has no effect on the
213 will listen forever for a connection, with or without the
216 The default is no timeout.
217 .It Fl X Ar proxy_protocol
220 should use the specified protocol when talking to the proxy server.
221 Supported protocols are
229 If the protocol is not specified, SOCKS version 5 is used.
231 .Fl x Ar proxy_address Ns Oo : Ns
244 is not specified, the well-known port for the proxy protocol is used (1080
245 for SOCKS, 3128 for HTTPS).
249 should just scan for listening daemons, without sending any data to them.
250 It is an error to use this option in conjunction with the
256 can be a numerical IP address or a symbolic hostname
260 In general, a hostname must be specified,
264 (in which case the local host is used).
267 can be single integers or ranges.
268 Ranges are in the form nn-mm.
270 a destination port must be specified,
274 (in which case a socket must be specified).
275 .Sh CLIENT/SERVER MODEL
276 It is quite simple to build a very basic client/server model using
278 On one console, start
280 listening on a specific port for a connection.
286 is now listening on port 1234 for a connection.
288 .Pq or a second machine ,
289 connect to the machine and port being listened on:
291 .Dl $ nc 127.0.0.1 1234
293 There should now be a connection between the ports.
294 Anything typed at the second console will be concatenated to the first,
296 After the connection has been set up,
298 does not really care which side is being used as a
300 and which side is being used as a
302 The connection may be terminated using an
306 The example in the previous section can be expanded to build a
307 basic data transfer model.
308 Any information input into one end of the connection will be output
309 to the other end, and input and output can be easily captured in order to
310 emulate file transfer.
314 to listen on a specific port, with output captured into a file:
316 .Dl $ nc -l 1234 \*(Gt filename.out
318 Using a second machine, connect to the listening
320 process, feeding it the file which is to be transferred:
322 .Dl $ nc host.example.com 1234 \*(Lt filename.in
324 After the file has been transferred, the connection will close automatically.
325 .Sh TALKING TO SERVERS
326 It is sometimes useful to talk to servers
328 rather than through a user interface.
329 It can aid in troubleshooting,
330 when it might be necessary to verify what data a server is sending
331 in response to commands issued by the client.
332 For example, to retrieve the home page of a web site:
333 .Bd -literal -offset indent
334 $ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
337 Note that this also displays the headers sent by the web server.
338 They can be filtered, using a tool such as
342 More complicated examples can be built up when the user knows the format
343 of requests required by the server.
344 As another example, an email may be submitted to an SMTP server using:
345 .Bd -literal -offset indent
346 $ nc localhost 25 \*(Lt\*(Lt EOF
347 HELO host.example.com
348 MAIL FROM: \*(Ltuser@host.example.com\*(Gt
349 RCPT TO: \*(Ltuser2@host.example.com\*(Gt
357 It may be useful to know which ports are open and running services on
361 flag can be used to tell
363 to report open ports,
364 rather than initiate a connection.
366 .Bd -literal -offset indent
367 $ nc -z host.example.com 20-30
368 Connection to host.example.com 22 port [tcp/ssh] succeeded!
369 Connection to host.example.com 25 port [tcp/smtp] succeeded!
372 The port range was specified to limit the search to ports 20 \- 30.
374 Alternatively, it might be useful to know which server software
375 is running, and which versions.
376 This information is often contained within the greeting banners.
377 In order to retrieve these, it is necessary to first make a connection,
378 and then break the connection when the banner has been retrieved.
379 This can be accomplished by specifying a small timeout with the
381 flag, or perhaps by issuing a
383 command to the server:
384 .Bd -literal -offset indent
385 $ echo "QUIT" | nc host.example.com 20-30
386 SSH-1.99-OpenSSH_3.6.1p2
388 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
391 Open a TCP connection to port 42 of host.example.com, using port 31337 as
392 the source port, with a timeout of 5 seconds:
394 .Dl $ nc -p 31337 -w 5 host.example.com 42
396 Open a UDP connection to port 53 of host.example.com:
398 .Dl $ nc -u host.example.com 53
400 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
401 IP for the local end of the connection:
403 .Dl $ nc -s 10.1.2.3 host.example.com 42
405 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
406 incoming and outgoing traffic.
408 .Dl $ nc -E host.example.com 42
410 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
411 outgoing traffic only.
413 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
415 Create and listen on a Unix Domain Socket:
417 .Dl $ nc -lU /var/tmp/dsocket
419 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
421 This example could also be used by
427 for more information.
429 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
431 The same example again, this time enabling proxy authentication with username
433 if the proxy requires it:
435 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
440 Original implementation by *Hobbit*
441 .Aq hobbit@avian.org .
443 Rewritten with IPv6 support by
444 .An Eric Jackson Aq ericj@monkey.org .
446 UDP port scans will always succeed
447 (i.e. report the port as open),
450 combination of flags relatively useless.