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28 .Dd $Mdocdate: February 9 2017 $
33 .Nd arbitrary TCP and UDP connections and listens
36 .Op Fl 46cDdFhklNnrStUuvz
46 .Op Fl o Ar staplefile
47 .Op Fl P Ar proxy_username
48 .Op Fl p Ar source_port
54 .Op Fl X Ar proxy_protocol
55 .Op Fl x Ar proxy_address Ns Op : Ns Ar port
63 utility is used for just about anything under the sun involving TCP,
67 It can open TCP connections, send UDP packets, listen on arbitrary
68 TCP and UDP ports, do port scanning, and deal with both IPv4 and
73 scripts nicely, and separates error messages onto standard error instead
74 of sending them to standard output, as
80 .Bl -bullet -offset indent -compact
84 shell-script based HTTP clients and servers
86 network daemon testing
88 a SOCKS or HTTP ProxyCommand for
94 The options are as follows:
99 to use IPv4 addresses only.
103 to use IPv6 addresses only.
105 Specifies the filename from which the public key part of the TLS
106 certificate is loaded, in PEM format.
107 May only be used with TLS.
109 If using a TCP socket to connect or listen, use TLS.
110 Illegal if not using TCP sockets.
112 Enable debugging on the socket.
114 Do not attempt to read from stdin.
116 Specify the name that must be present in the peer certificate when using TLS.
117 Illegal if not using TLS.
119 Pass the first connected socket using
122 This is useful in conjunction with
126 perform connection setup with a proxy but then leave the rest of the
127 connection to another program (e.g.\&
134 Specifies the required hash string of the peer certificate when using TLS.
135 The string format required is that used by
136 .Xr tls_peer_cert_hash 3 .
137 Illegal if not using TLS, and may not be used with -T noverify.
143 Specifies the size of the TCP receive buffer.
145 Specifies a delay time interval between lines of text sent and received.
146 Also causes a delay time between connections to multiple ports.
148 Specifies the filename from which the private key
149 is loaded in PEM format.
150 May only be used with TLS.
154 to stay listening for another connection after its current connection
156 It is an error to use this option without the
159 When used together with the
161 option, the server socket is not connected and it can receive UDP datagrams from
166 should listen for an incoming connection rather than initiate a
167 connection to a remote host.
168 It is an error to use this option in conjunction with the
174 Additionally, any timeouts specified with the
178 Set the TTL / hop limit of outgoing packets.
180 Ask the kernel to drop incoming packets whose TTL / hop limit is under
184 the network socket after EOF on the input.
185 Some servers require this to finish their work.
187 Do not do any DNS or service lookups on any specified addresses,
190 Specifies the size of the TCP send buffer.
191 .It Fl o Ar staplefile
192 Specifies the filename from which to load data to be stapled
193 during the TLS handshake.
194 The file is expected to contain an OCSP response from an OCSP server in
196 May only be used with TLS and when a certificate is being used.
197 .It Fl P Ar proxy_username
198 Specifies a username to present to a proxy server that requires authentication.
199 If no username is specified then authentication will not be attempted.
200 Proxy authentication is only supported for HTTP CONNECT proxies at present.
201 .It Fl p Ar source_port
202 Specifies the source port
204 should use, subject to privilege restrictions and availability.
205 It is an error to use this option in conjunction with the
209 Specifies the filename from which the root CA bundle for certificate
210 verification is loaded, in PEM format.
211 Illegal if not using TLS.
213 .Pa /etc/ssl/cert.pem .
215 Specifies that source and/or destination ports should be chosen randomly
216 instead of sequentially within a range or in the order that the system
219 Enables the RFC 2385 TCP MD5 signature option.
221 Specifies the IP of the interface which is used to send the packets.
224 datagram sockets, specifies the local temporary socket file
225 to create and use so that datagrams can be received.
226 It is an error to use this option in conjunction with the
230 Change IPv4 TOS value or TLS options.
235 which allows the use of all supported TLS protocols and ciphers,
237 which disables certificate verification;
239 which disables certificate name checking;
241 which requires a client certificate on incoming connections; or
243 which requires the peer to provide a valid stapled OCSP response
245 It is illegal to specify TLS options if not using TLS.
256 or one of the DiffServ Code Points:
260 or a number in either hex or decimal.
264 to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
265 This makes it possible to use
267 to script telnet sessions.
273 Use UDP instead of the default option of TCP.
276 sockets, use a datagram socket instead of a stream socket.
279 socket is used, a temporary receiving socket is created in
285 Set the routing table to be used.
289 give more verbose output.
291 Connections which cannot be established or are idle timeout after
296 flag has no effect on the
300 will listen forever for a connection, with or without the
303 The default is no timeout.
304 .It Fl X Ar proxy_protocol
307 should use the specified protocol when talking to the proxy server.
308 Supported protocols are
316 If the protocol is not specified, SOCKS version 5 is used.
317 .It Fl x Ar proxy_address Ns Op : Ns Ar port
328 is not specified, the well-known port for the proxy protocol is used (1080
329 for SOCKS, 3128 for HTTPS).
330 An IPv6 address can be specified unambiguously by enclosing
336 should just scan for listening daemons, without sending any data to them.
337 It is an error to use this option in conjunction with the
343 can be a numerical IP address or a symbolic hostname
347 In general, a destination must be specified,
351 (in which case the local host is used).
354 sockets, a destination is required and is the socket path to connect to
360 can be a specified as a numeric port number, or as a service name.
361 Ports may be specified in a range of the form nn-mm.
363 a destination port must be specified,
367 .Sh CLIENT/SERVER MODEL
368 It is quite simple to build a very basic client/server model using
370 On one console, start
372 listening on a specific port for a connection.
378 is now listening on port 1234 for a connection.
380 .Pq or a second machine ,
381 connect to the machine and port being listened on:
383 .Dl $ nc 127.0.0.1 1234
385 There should now be a connection between the ports.
386 Anything typed at the second console will be concatenated to the first,
388 After the connection has been set up,
390 does not really care which side is being used as a
392 and which side is being used as a
394 The connection may be terminated using an
398 The example in the previous section can be expanded to build a
399 basic data transfer model.
400 Any information input into one end of the connection will be output
401 to the other end, and input and output can be easily captured in order to
402 emulate file transfer.
406 to listen on a specific port, with output captured into a file:
408 .Dl $ nc -l 1234 \*(Gt filename.out
410 Using a second machine, connect to the listening
412 process, feeding it the file which is to be transferred:
414 .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
416 After the file has been transferred, the connection will close automatically.
417 .Sh TALKING TO SERVERS
418 It is sometimes useful to talk to servers
420 rather than through a user interface.
421 It can aid in troubleshooting,
422 when it might be necessary to verify what data a server is sending
423 in response to commands issued by the client.
424 For example, to retrieve the home page of a web site:
425 .Bd -literal -offset indent
426 $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
429 Note that this also displays the headers sent by the web server.
430 They can be filtered, using a tool such as
434 More complicated examples can be built up when the user knows the format
435 of requests required by the server.
436 As another example, an email may be submitted to an SMTP server using:
437 .Bd -literal -offset indent
438 $ nc localhost 25 \*(Lt\*(Lt EOF
439 HELO host.example.com
440 MAIL FROM:\*(Ltuser@host.example.com\*(Gt
441 RCPT TO:\*(Ltuser2@host.example.com\*(Gt
449 It may be useful to know which ports are open and running services on
453 flag can be used to tell
455 to report open ports,
456 rather than initiate a connection.
458 .Bd -literal -offset indent
459 $ nc -z host.example.com 20-30
460 Connection to host.example.com 22 port [tcp/ssh] succeeded!
461 Connection to host.example.com 25 port [tcp/smtp] succeeded!
464 The port range was specified to limit the search to ports 20 \- 30.
466 Alternatively, it might be useful to know which server software
467 is running, and which versions.
468 This information is often contained within the greeting banners.
469 In order to retrieve these, it is necessary to first make a connection,
470 and then break the connection when the banner has been retrieved.
471 This can be accomplished by specifying a small timeout with the
473 flag, or perhaps by issuing a
475 command to the server:
476 .Bd -literal -offset indent
477 $ echo "QUIT" | nc host.example.com 20-30
478 SSH-1.99-OpenSSH_3.6.1p2
480 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
483 Open a TCP connection to port 42 of host.example.com, using port 31337 as
484 the source port, with a timeout of 5 seconds:
486 .Dl $ nc -p 31337 -w 5 host.example.com 42
488 Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
489 Check for a different name in the certificate for validation.
491 .Dl $ nc -v -c -e adsf.au.doubleclick.net www.google.ca 443
493 Open a UDP connection to port 53 of host.example.com:
495 .Dl $ nc -u host.example.com 53
497 Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
498 IP for the local end of the connection:
500 .Dl $ nc -s 10.1.2.3 host.example.com 42
502 Create and listen on a
506 .Dl $ nc -lU /var/tmp/dsocket
508 Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
510 This example could also be used by
516 for more information.
518 .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
520 The same example again, this time enabling proxy authentication with username
522 if the proxy requires it:
524 .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
529 Original implementation by *Hobbit*
530 .Aq Mt hobbit@avian.org .
532 Rewritten with IPv6 support by
533 .An Eric Jackson Aq Mt ericj@monkey.org .
535 UDP port scans using the
537 combination of flags will always report success irrespective of
538 the target machine's state.
540 in conjunction with a traffic sniffer either on the target machine
541 or an intermediary device,
544 combination could be useful for communications diagnostics.
545 Note that the amount of UDP traffic generated may be limited either
546 due to hardware resources and/or configuration settings.