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3 .\" Copyright (c) 1996 David Sacerdote
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30 .Dd September 26, 2015
35 .Nd arbitrary TCP and UDP connections and listens
39 .Op Fl 46DdEFhklNnrStUuvz
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
176 the network socket after EOF on the input.
177 Some servers require this to finish their work.
179 Do not do any DNS or service lookups on any specified addresses,
182 Disables the use of TCP options on the socket, by setting the boolean
186 Use SCTP instead of the default option of TCP.
188 Specifies the size of the TCP send buffer.
189 .It Fl P Ar proxy_username
190 Specifies a username to present to a proxy server that requires authentication.
191 If no username is specified then authentication will not be attempted.
192 Proxy authentication is only supported for HTTP CONNECT proxies at present.
193 .It Fl p Ar source_port
194 Specifies the source port
196 should use, subject to privilege restrictions and availability.
197 It is an error to use this option in conjunction with the
201 Specifies that source and/or destination ports should be chosen randomly
202 instead of sequentially within a range or in the order that the system
205 Enables the RFC 2385 TCP MD5 signature option.
207 Specifies the IP of the interface which is used to send the packets.
210 datagram sockets, specifies the local temporary socket file
211 to create and use so that datagrams can be received.
212 It is an error to use this option in conjunction with the
215 .It Fl T Ar toskeyword
216 Change IPv4 TOS value.
225 or one of the DiffServ Code Points:
229 or a number in either hex or decimal.
233 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
234 This makes it possible to use
236 to script telnet sessions.
242 Use UDP instead of the default option of TCP.
245 sockets, use a datagram socket instead of a stream socket.
248 socket is used, a temporary receiving socket is created in
254 Set the routing table
260 give more verbose output.
262 Connections which cannot be established or are idle timeout after
267 flag has no effect on the
271 will listen forever for a connection, with or without the
274 The default is no timeout.
275 .It Fl X Ar proxy_protocol
278 should use the specified protocol when talking to the proxy server.
279 Supported protocols are
287 If the protocol is not specified, SOCKS version 5 is used.
289 .Fl x Ar proxy_address Ns Oo : Ns
302 is not specified, the well-known port for the proxy protocol is used (1080
303 for SOCKS, 3128 for HTTPS).
307 should just scan for listening daemons, without sending any data to them.
308 It is an error to use this option in conjunction with the
314 can be a numerical IP address or a symbolic hostname
318 In general, a destination must be specified,
322 (in which case the local host is used).
325 sockets, a destination is required and is the socket path to connect to
331 can be a single integer or a range of ports.
332 Ranges are in the form nn-mm.
334 a destination port must be specified,
338 .Sh CLIENT/SERVER MODEL
339 It is quite simple to build a very basic client/server model using
341 On one console, start
343 listening on a specific port for a connection.
349 is now listening on port 1234 for a connection.
351 .Pq or a second machine ,
352 connect to the machine and port being listened on:
354 .Dl $ nc 127.0.0.1 1234
356 There should now be a connection between the ports.
357 Anything typed at the second console will be concatenated to the first,
359 After the connection has been set up,
361 does not really care which side is being used as a
363 and which side is being used as a
365 The connection may be terminated using an
369 The example in the previous section can be expanded to build a
370 basic data transfer model.
371 Any information input into one end of the connection will be output
372 to the other end, and input and output can be easily captured in order to
373 emulate file transfer.
377 to listen on a specific port, with output captured into a file:
379 .Dl $ nc -l 1234 \*(Gt filename.out
381 Using a second machine, connect to the listening
383 process, feeding it the file which is to be transferred:
385 .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
387 After the file has been transferred, the connection will close automatically.
388 .Sh TALKING TO SERVERS
389 It is sometimes useful to talk to servers
391 rather than through a user interface.
392 It can aid in troubleshooting,
393 when it might be necessary to verify what data a server is sending
394 in response to commands issued by the client.
395 For example, to retrieve the home page of a web site:
396 .Bd -literal -offset indent
397 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
400 Note that this also displays the headers sent by the web server.
401 They can be filtered, using a tool such as
405 More complicated examples can be built up when the user knows the format
406 of requests required by the server.
407 As another example, an email may be submitted to an SMTP server using:
408 .Bd -literal -offset indent
409 $ nc localhost 25 \*(Lt\*(Lt EOF
410 HELO host.example.com
411 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
412 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
420 It may be useful to know which ports are open and running services on
424 flag can be used to tell
426 to report open ports,
427 rather than initiate a connection.
429 .Bd -literal -offset indent
430 $ nc -z host.example.com 20-30
431 Connection to host.example.com 22 port [tcp/ssh] succeeded!
432 Connection to host.example.com 25 port [tcp/smtp] succeeded!
435 The port range was specified to limit the search to ports 20 \- 30.
437 Alternatively, it might be useful to know which server software
438 is running, and which versions.
439 This information is often contained within the greeting banners.
440 In order to retrieve these, it is necessary to first make a connection,
441 and then break the connection when the banner has been retrieved.
442 This can be accomplished by specifying a small timeout with the
444 flag, or perhaps by issuing a
446 command to the server:
447 .Bd -literal -offset indent
448 $ echo "QUIT" | nc host.example.com 20-30
449 SSH-1.99-OpenSSH_3.6.1p2
451 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
454 Open a TCP connection to port 42 of host.example.com, using port 31337 as
455 the source port, with a timeout of 5 seconds:
457 .Dl $ nc -p 31337 -w 5 host.example.com 42
459 Open a UDP connection to port 53 of host.example.com:
461 .Dl $ nc -u host.example.com 53
463 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
464 IP for the local end of the connection:
466 .Dl $ nc -s 10.1.2.3 host.example.com 42
468 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
469 incoming and outgoing traffic.
471 .Dl $ nc -E host.example.com 42
473 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
474 outgoing traffic only.
476 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
478 Create and listen on a
482 .Dl $ nc -lU /var/tmp/dsocket
484 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
486 This example could also be used by
492 for more information.
494 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
496 The same example again, this time enabling proxy authentication with username
498 if the proxy requires it:
500 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
509 Original implementation by *Hobbit*
510 .Aq Mt hobbit@avian.org .
512 Rewritten with IPv6 support by
513 .An Eric Jackson Aq Mt ericj@monkey.org .
515 UDP port scans using the
517 combination of flags will always report success irrespective of
518 the target machine's state.
520 in conjunction with a traffic sniffer either on the target machine
521 or an intermediary device,
524 combination could be useful for communications diagnostics.
525 Note that the amount of UDP traffic generated may be limited either
526 due to hardware resources and/or configuration settings.