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3 .\" Copyright (c) 1996 David Sacerdote
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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
47 .Op Fl s Ar source_ip_address
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
66 It can open TCP connections, send UDP packets, listen on arbitrary
67 TCP and UDP ports, do port scanning, and deal with both IPv4 and
72 scripts nicely, and separates error messages onto standard error instead
73 of sending them to standard output, as
79 .Bl -bullet -offset indent -compact
83 shell-script based HTTP clients and servers
85 network daemon testing
87 a SOCKS or HTTP ProxyCommand for
93 The options are as follows:
98 to use IPv4 addresses only.
102 to use IPv6 addresses only.
104 Enable debugging on the socket.
106 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.
126 Specifies the size of the TCP receive buffer.
128 Specifies a delay time interval between lines of text sent and received.
129 Also causes a delay time between connections to multiple ports.
133 to stay listening for another connection after its current connection
135 It is an error to use this option without the
141 should listen for an incoming connection rather than initiate a
142 connection to a remote host.
143 It is an error to use this option in conjunction with the
149 Additionally, any timeouts specified with the
153 Do not do any DNS or service lookups on any specified addresses,
156 Disables the use of TCP options on the socket, by setting the boolean
160 Specifies the size of the TCP send buffer.
161 .It Fl P Ar proxy_username
162 Specifies a username to present to a proxy server that requires authentication.
163 If no username is specified then authentication will not be attempted.
164 Proxy authentication is only supported for HTTP CONNECT proxies at present.
165 .It Fl p Ar source_port
166 Specifies the source port
168 should use, subject to privilege restrictions and availability.
169 It is an error to use this option in conjunction with the
173 Specifies that source and/or destination ports should be chosen randomly
174 instead of sequentially within a range or in the order that the system
177 Enables the RFC 2385 TCP MD5 signature option.
178 .It Fl s Ar source_ip_address
179 Specifies the IP of the interface which is used to send the packets.
180 It is an error to use this option in conjunction with the
184 Specifies IP Type of Service (ToS) for the connection.
185 Valid values are the tokens
189 or an 8-bit hexadecimal value preceded by
194 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
195 This makes it possible to use
197 to script telnet sessions.
203 Use UDP instead of the default option of TCP.
205 Set the routing table (FIB).
210 give more verbose output.
212 If a connection and stdin are idle for more than
214 seconds, then the connection is silently closed.
217 flag has no effect on the
221 will listen forever for a connection, with or without the
224 The default is no timeout.
225 .It Fl X Ar proxy_protocol
228 should use the specified protocol when talking to the proxy server.
229 Supported protocols are
237 If the protocol is not specified, SOCKS version 5 is used.
239 .Fl x Ar proxy_address Ns Oo : Ns
252 is not specified, the well-known port for the proxy protocol is used (1080
253 for SOCKS, 3128 for HTTPS).
257 should just scan for listening daemons, without sending any data to them.
258 It is an error to use this option in conjunction with the
264 can be a numerical IP address or a symbolic hostname
268 In general, a hostname must be specified,
272 (in which case the local host is used).
275 can be a single integer or a range of ports.
276 Ranges are in the form nn-mm.
278 a destination port must be specified,
282 (in which case a socket must be specified).
283 .Sh CLIENT/SERVER MODEL
284 It is quite simple to build a very basic client/server model using
286 On one console, start
288 listening on a specific port for a connection.
294 is now listening on port 1234 for a connection.
296 .Pq or a second machine ,
297 connect to the machine and port being listened on:
299 .Dl $ nc 127.0.0.1 1234
301 There should now be a connection between the ports.
302 Anything typed at the second console will be concatenated to the first,
304 After the connection has been set up,
306 does not really care which side is being used as a
308 and which side is being used as a
310 The connection may be terminated using an
314 The example in the previous section can be expanded to build a
315 basic data transfer model.
316 Any information input into one end of the connection will be output
317 to the other end, and input and output can be easily captured in order to
318 emulate file transfer.
322 to listen on a specific port, with output captured into a file:
324 .Dl $ nc -l 1234 \*(Gt filename.out
326 Using a second machine, connect to the listening
328 process, feeding it the file which is to be transferred:
330 .Dl $ nc host.example.com 1234 \*(Lt filename.in
332 After the file has been transferred, the connection will close automatically.
333 .Sh TALKING TO SERVERS
334 It is sometimes useful to talk to servers
336 rather than through a user interface.
337 It can aid in troubleshooting,
338 when it might be necessary to verify what data a server is sending
339 in response to commands issued by the client.
340 For example, to retrieve the home page of a web site:
341 .Bd -literal -offset indent
342 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
345 Note that this also displays the headers sent by the web server.
346 They can be filtered, using a tool such as
350 More complicated examples can be built up when the user knows the format
351 of requests required by the server.
352 As another example, an email may be submitted to an SMTP server using:
353 .Bd -literal -offset indent
354 $ nc localhost 25 \*(Lt\*(Lt EOF
355 HELO host.example.com
356 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
357 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
365 It may be useful to know which ports are open and running services on
369 flag can be used to tell
371 to report open ports,
372 rather than initiate a connection.
374 .Bd -literal -offset indent
375 $ nc -z host.example.com 20-30
376 Connection to host.example.com 22 port [tcp/ssh] succeeded!
377 Connection to host.example.com 25 port [tcp/smtp] succeeded!
380 The port range was specified to limit the search to ports 20 \- 30.
382 Alternatively, it might be useful to know which server software
383 is running, and which versions.
384 This information is often contained within the greeting banners.
385 In order to retrieve these, it is necessary to first make a connection,
386 and then break the connection when the banner has been retrieved.
387 This can be accomplished by specifying a small timeout with the
389 flag, or perhaps by issuing a
391 command to the server:
392 .Bd -literal -offset indent
393 $ echo "QUIT" | nc host.example.com 20-30
394 SSH-1.99-OpenSSH_3.6.1p2
396 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
399 Open a TCP connection to port 42 of host.example.com, using port 31337 as
400 the source port, with a timeout of 5 seconds:
402 .Dl $ nc -p 31337 -w 5 host.example.com 42
404 Open a UDP connection to port 53 of host.example.com:
406 .Dl $ nc -u host.example.com 53
408 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
409 IP for the local end of the connection:
411 .Dl $ nc -s 10.1.2.3 host.example.com 42
413 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
414 incoming and outgoing traffic.
416 .Dl $ nc -E host.example.com 42
418 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
419 outgoing traffic only.
421 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
423 Create and listen on a
427 .Dl $ nc -lU /var/tmp/dsocket
429 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
431 This example could also be used by
437 for more information.
439 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
441 The same example again, this time enabling proxy authentication with username
443 if the proxy requires it:
445 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
454 Original implementation by *Hobbit*
455 .Aq hobbit@avian.org .
457 Rewritten with IPv6 support by
458 .An Eric Jackson Aq ericj@monkey.org .
460 UDP port scans will always succeed
461 (i.e. report the port as open),
464 combination of flags relatively useless.