1 .\" $OpenBSD: nc.1,v 1.57 2011/01/09 22:16:46 jeremy Exp $
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
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,
68 It can open TCP connections, send UDP packets, listen on arbitrary
69 TCP and UDP ports, do port scanning, and deal with both IPv4 and
74 scripts nicely, and separates error messages onto standard error instead
75 of sending them to standard output, as
81 .Bl -bullet -offset indent -compact
85 shell-script based HTTP clients and servers
87 network daemon testing
89 a SOCKS or HTTP ProxyCommand for
95 The options are as follows:
100 to use IPv4 addresses only.
104 to use IPv6 addresses only.
106 Enable debugging on the socket.
108 Do not attempt to read from stdin.
112 .Li "-e 'in ipsec esp/transport//require'"
113 .Li "-e 'out ipsec esp/transport//require'"
115 which enables IPsec ESP transport mode in both
118 If IPsec support is available, then one can specify the IPsec policies
119 to be used using the syntax described in
120 .Xr ipsec_set_policy 3 .
121 This flag can be specified up to two times, as typically one policy for
122 each direction is needed.
128 Specifies the size of the TCP receive buffer.
130 Specifies a delay time interval between lines of text sent and received.
131 Also causes a delay time between connections to multiple ports.
135 to stay listening for another connection after its current connection
137 It is an error to use this option without the
143 should listen for an incoming connection rather than initiate a
144 connection to a remote host.
145 It is an error to use this option in conjunction with the
151 Additionally, any timeouts specified with the
155 Do not do any DNS or service lookups on any specified addresses,
158 Disables the use of TCP options on the socket, by setting the boolean
162 Specifies the size of the TCP send buffer.
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.
181 Specifies the IP of the interface which is used to send the packets.
184 datagram sockets, specifies the local temporary socket file
185 to create and use so that datagrams can be received.
186 It is an error to use this option in conjunction with the
190 Specifies IP Type of Service (ToS) for the connection.
191 Valid values are the tokens
195 or an 8-bit hexadecimal value preceded by
200 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
201 This makes it possible to use
203 to script telnet sessions.
209 Use UDP instead of the default option of TCP.
212 sockets, use a datagram socket instead of a stream socket.
215 socket is used, a temporary receiving socket is created in
221 Set the routing table
228 give more verbose output.
230 If a connection and stdin are idle for more than
232 seconds, then the connection is silently closed.
235 flag has no effect on the
239 will listen forever for a connection, with or without the
242 The default is no timeout.
243 .It Fl X Ar proxy_protocol
246 should use the specified protocol when talking to the proxy server.
247 Supported protocols are
255 If the protocol is not specified, SOCKS version 5 is used.
257 .Fl x Ar proxy_address Ns Oo : Ns
270 is not specified, the well-known port for the proxy protocol is used (1080
271 for SOCKS, 3128 for HTTPS).
275 should just scan for listening daemons, without sending any data to them.
276 It is an error to use this option in conjunction with the
282 can be a numerical IP address or a symbolic hostname
286 In general, a destination must be specified,
290 (in which case the local host is used).
293 sockets, a destination is required and is the socket path to connect to
299 can be a single integer or a range of ports.
300 Ranges are in the form nn-mm.
302 a destination port must be specified,
306 .Sh CLIENT/SERVER MODEL
307 It is quite simple to build a very basic client/server model using
309 On one console, start
311 listening on a specific port for a connection.
317 is now listening on port 1234 for a connection.
319 .Pq or a second machine ,
320 connect to the machine and port being listened on:
322 .Dl $ nc 127.0.0.1 1234
324 There should now be a connection between the ports.
325 Anything typed at the second console will be concatenated to the first,
327 After the connection has been set up,
329 does not really care which side is being used as a
331 and which side is being used as a
333 The connection may be terminated using an
337 The example in the previous section can be expanded to build a
338 basic data transfer model.
339 Any information input into one end of the connection will be output
340 to the other end, and input and output can be easily captured in order to
341 emulate file transfer.
345 to listen on a specific port, with output captured into a file:
347 .Dl $ nc -l 1234 \*(Gt filename.out
349 Using a second machine, connect to the listening
351 process, feeding it the file which is to be transferred:
353 .Dl $ nc host.example.com 1234 \*(Lt filename.in
355 After the file has been transferred, the connection will close automatically.
356 .Sh TALKING TO SERVERS
357 It is sometimes useful to talk to servers
359 rather than through a user interface.
360 It can aid in troubleshooting,
361 when it might be necessary to verify what data a server is sending
362 in response to commands issued by the client.
363 For example, to retrieve the home page of a web site:
364 .Bd -literal -offset indent
365 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
368 Note that this also displays the headers sent by the web server.
369 They can be filtered, using a tool such as
373 More complicated examples can be built up when the user knows the format
374 of requests required by the server.
375 As another example, an email may be submitted to an SMTP server using:
376 .Bd -literal -offset indent
377 $ nc localhost 25 \*(Lt\*(Lt EOF
378 HELO host.example.com
379 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
380 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
388 It may be useful to know which ports are open and running services on
392 flag can be used to tell
394 to report open ports,
395 rather than initiate a connection.
397 .Bd -literal -offset indent
398 $ nc -z host.example.com 20-30
399 Connection to host.example.com 22 port [tcp/ssh] succeeded!
400 Connection to host.example.com 25 port [tcp/smtp] succeeded!
403 The port range was specified to limit the search to ports 20 \- 30.
405 Alternatively, it might be useful to know which server software
406 is running, and which versions.
407 This information is often contained within the greeting banners.
408 In order to retrieve these, it is necessary to first make a connection,
409 and then break the connection when the banner has been retrieved.
410 This can be accomplished by specifying a small timeout with the
412 flag, or perhaps by issuing a
414 command to the server:
415 .Bd -literal -offset indent
416 $ echo "QUIT" | nc host.example.com 20-30
417 SSH-1.99-OpenSSH_3.6.1p2
419 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
422 Open a TCP connection to port 42 of host.example.com, using port 31337 as
423 the source port, with a timeout of 5 seconds:
425 .Dl $ nc -p 31337 -w 5 host.example.com 42
427 Open a UDP connection to port 53 of host.example.com:
429 .Dl $ nc -u host.example.com 53
431 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
432 IP for the local end of the connection:
434 .Dl $ nc -s 10.1.2.3 host.example.com 42
436 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
437 incoming and outgoing traffic.
439 .Dl $ nc -E host.example.com 42
441 Open a TCP connection to port 42 of host.example.com using IPsec ESP for
442 outgoing traffic only.
444 .Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
446 Create and listen on a
450 .Dl $ nc -lU /var/tmp/dsocket
452 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
454 This example could also be used by
460 for more information.
462 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
464 The same example again, this time enabling proxy authentication with username
466 if the proxy requires it:
468 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
477 Original implementation by *Hobbit*
478 .Aq hobbit@avian.org .
480 Rewritten with IPv6 support by
481 .An Eric Jackson Aq ericj@monkey.org .
483 UDP port scans will always succeed
484 (i.e. report the port as open),
487 combination of flags relatively useless.