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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.
162 .It Fl P Ar proxy_username
163 Specifies a username to present to a proxy server that requires authentication.
164 If no username is specified then authentication will not be attempted.
165 Proxy authentication is only supported for HTTP CONNECT proxies at present.
166 .It Fl p Ar source_port
167 Specifies the source port
169 should use, subject to privilege restrictions and availability.
170 It is an error to use this option in conjunction with the
174 Specifies that source and/or destination ports should be chosen randomly
175 instead of sequentially within a range or in the order that the system
178 Enables the RFC 2385 TCP MD5 signature option.
179 .It Fl s Ar source_ip_address
180 Specifies the IP of the interface which is used to send the packets.
181 It is an error to use this option in conjunction with the
185 Specifies IP Type of Service (ToS) for the connection.
186 Valid values are the tokens
190 or an 8-bit hexadecimal value preceded by
195 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
196 This makes it possible to use
198 to script telnet sessions.
204 Use UDP instead of the default option of TCP.
206 Set the routing table (FIB).
211 give more verbose output.
213 If a connection and stdin are idle for more than
215 seconds, then the connection is silently closed.
218 flag has no effect on the
222 will listen forever for a connection, with or without the
225 The default is no timeout.
226 .It Fl X Ar proxy_protocol
229 should use the specified protocol when talking to the proxy server.
230 Supported protocols are
238 If the protocol is not specified, SOCKS version 5 is used.
240 .Fl x Ar proxy_address Ns Oo : Ns
253 is not specified, the well-known port for the proxy protocol is used (1080
254 for SOCKS, 3128 for HTTPS).
258 should just scan for listening daemons, without sending any data to them.
259 It is an error to use this option in conjunction with the
265 can be a numerical IP address or a symbolic hostname
269 In general, a hostname must be specified,
273 (in which case the local host is used).
276 can be a single integer or a range of ports.
277 Ranges are in the form nn-mm.
279 a destination port must be specified,
283 (in which case a socket must be specified).
284 .Sh CLIENT/SERVER MODEL
285 It is quite simple to build a very basic client/server model using
287 On one console, start
289 listening on a specific port for a connection.
295 is now listening on port 1234 for a connection.
297 .Pq or a second machine ,
298 connect to the machine and port being listened on:
300 .Dl $ nc 127.0.0.1 1234
302 There should now be a connection between the ports.
303 Anything typed at the second console will be concatenated to the first,
305 After the connection has been set up,
307 does not really care which side is being used as a
309 and which side is being used as a
311 The connection may be terminated using an
315 The example in the previous section can be expanded to build a
316 basic data transfer model.
317 Any information input into one end of the connection will be output
318 to the other end, and input and output can be easily captured in order to
319 emulate file transfer.
323 to listen on a specific port, with output captured into a file:
325 .Dl $ nc -l 1234 \*(Gt filename.out
327 Using a second machine, connect to the listening
329 process, feeding it the file which is to be transferred:
331 .Dl $ nc host.example.com 1234 \*(Lt filename.in
333 After the file has been transferred, the connection will close automatically.
334 .Sh TALKING TO SERVERS
335 It is sometimes useful to talk to servers
337 rather than through a user interface.
338 It can aid in troubleshooting,
339 when it might be necessary to verify what data a server is sending
340 in response to commands issued by the client.
341 For example, to retrieve the home page of a web site:
342 .Bd -literal -offset indent
343 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
346 Note that this also displays the headers sent by the web server.
347 They can be filtered, using a tool such as
351 More complicated examples can be built up when the user knows the format
352 of requests required by the server.
353 As another example, an email may be submitted to an SMTP server using:
354 .Bd -literal -offset indent
355 $ nc localhost 25 \*(Lt\*(Lt EOF
356 HELO host.example.com
357 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
358 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
366 It may be useful to know which ports are open and running services on
370 flag can be used to tell
372 to report open ports,
373 rather than initiate a connection.
375 .Bd -literal -offset indent
376 $ nc -z host.example.com 20-30
377 Connection to host.example.com 22 port [tcp/ssh] succeeded!
378 Connection to host.example.com 25 port [tcp/smtp] succeeded!
381 The port range was specified to limit the search to ports 20 \- 30.
383 Alternatively, it might be useful to know which server software
384 is running, and which versions.
385 This information is often contained within the greeting banners.
386 In order to retrieve these, it is necessary to first make a connection,
387 and then break the connection when the banner has been retrieved.
388 This can be accomplished by specifying a small timeout with the
390 flag, or perhaps by issuing a
392 command to the server:
393 .Bd -literal -offset indent
394 $ echo "QUIT" | nc host.example.com 20-30
395 SSH-1.99-OpenSSH_3.6.1p2
397 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
400 Open a TCP connection to port 42 of host.example.com, using port 31337 as
401 the source port, with a timeout of 5 seconds:
403 .Dl $ nc -p 31337 -w 5 host.example.com 42
405 Open a UDP connection to port 53 of host.example.com:
407 .Dl $ nc -u host.example.com 53
409 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
410 IP for the local end of the connection:
412 .Dl $ nc -s 10.1.2.3 host.example.com 42
414 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
415 incoming and outgoing traffic.
417 .Dl $ nc -E host.example.com 42
419 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
420 outgoing traffic only.
422 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
424 Create and listen on a
428 .Dl $ nc -lU /var/tmp/dsocket
430 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
432 This example could also be used by
438 for more information.
440 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
442 The same example again, this time enabling proxy authentication with username
444 if the proxy requires it:
446 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
455 Original implementation by *Hobbit*
456 .Aq hobbit@avian.org .
458 Rewritten with IPv6 support by
459 .An Eric Jackson Aq ericj@monkey.org .
461 UDP port scans will always succeed
462 (i.e. report the port as open),
465 combination of flags relatively useless.