Annotation of src/usr.bin/nc/nc.1, Revision 1.86
1.86 ! jsing 1: .\" $OpenBSD: nc.1,v 1.85 2017/05/10 21:56:53 bluhm Exp $
1.1 deraadt 2: .\"
3: .\" Copyright (c) 1996 David Sacerdote
4: .\" All rights reserved.
5: .\"
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7: .\" modification, are permitted provided that the following conditions
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1.86 ! jsing 28: .Dd $Mdocdate: May 10 2017 $
1.3 aaron 29: .Dt NC 1
1.4 deraadt 30: .Os
1.1 deraadt 31: .Sh NAME
32: .Nm nc
1.28 jmc 33: .Nd arbitrary TCP and UDP connections and listens
1.2 deraadt 34: .Sh SYNOPSIS
1.1 deraadt 35: .Nm nc
1.70 jmc 36: .Op Fl 46cDdFhklNnrStUuvz
37: .Op Fl C Ar certfile
38: .Op Fl e Ar name
39: .Op Fl H Ar hash
1.47 jmc 40: .Op Fl I Ar length
1.1 deraadt 41: .Op Fl i Ar interval
1.70 jmc 42: .Op Fl K Ar keyfile
1.73 jca 43: .Op Fl M Ar ttl
44: .Op Fl m Ar minttl
1.47 jmc 45: .Op Fl O Ar length
1.77 beck 46: .Op Fl o Ar staplefile
1.42 djm 47: .Op Fl P Ar proxy_username
1.28 jmc 48: .Op Fl p Ar source_port
1.70 jmc 49: .Op Fl R Ar CAfile
1.57 jeremy 50: .Op Fl s Ar source
1.69 beck 51: .Op Fl T Ar keyword
1.54 guenther 52: .Op Fl V Ar rtable
1.85 bluhm 53: .Op Fl W Ar recvlimit
1.6 aaron 54: .Op Fl w Ar timeout
1.33 djm 55: .Op Fl X Ar proxy_protocol
1.71 schwarze 56: .Op Fl x Ar proxy_address Ns Op : Ns Ar port
1.83 beck 57: .Op Fl Z Ar peercertfile
1.57 jeremy 58: .Op Ar destination
1.48 sobrado 59: .Op Ar port
1.1 deraadt 60: .Sh DESCRIPTION
61: The
1.6 aaron 62: .Nm
1.1 deraadt 63: (or
64: .Nm netcat )
1.57 jeremy 65: utility is used for just about anything under the sun involving TCP,
66: UDP, or
67: .Ux Ns -domain
68: sockets.
1.13 ericj 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
71: IPv6.
1.7 aaron 72: Unlike
1.1 deraadt 73: .Xr telnet 1 ,
1.6 aaron 74: .Nm
1.1 deraadt 75: scripts nicely, and separates error messages onto standard error instead
1.6 aaron 76: of sending them to standard output, as
1.24 pvalchev 77: .Xr telnet 1
1.6 aaron 78: does with some.
1.1 deraadt 79: .Pp
80: Common uses include:
1.7 aaron 81: .Pp
82: .Bl -bullet -offset indent -compact
1.1 deraadt 83: .It
1.3 aaron 84: simple TCP proxies
1.1 deraadt 85: .It
1.28 jmc 86: shell-script based HTTP clients and servers
1.1 deraadt 87: .It
1.13 ericj 88: network daemon testing
1.1 deraadt 89: .It
1.33 djm 90: a SOCKS or HTTP ProxyCommand for
91: .Xr ssh 1
92: .It
1.1 deraadt 93: and much, much more
94: .El
95: .Pp
96: The options are as follows:
97: .Bl -tag -width Ds
1.13 ericj 98: .It Fl 4
99: Forces
100: .Nm
101: to use IPv4 addresses only.
102: .It Fl 6
103: Forces
104: .Nm
105: to use IPv6 addresses only.
1.70 jmc 106: .It Fl C Ar certfile
1.69 beck 107: Specifies the filename from which the public key part of the TLS
1.70 jmc 108: certificate is loaded, in PEM format.
109: May only be used with TLS.
1.69 beck 110: .It Fl c
1.70 jmc 111: If using a TCP socket to connect or listen, use TLS.
112: Illegal if not using TCP sockets.
1.32 markus 113: .It Fl D
114: Enable debugging on the socket.
1.29 tedu 115: .It Fl d
116: Do not attempt to read from stdin.
1.69 beck 117: .It Fl e Ar name
1.70 jmc 118: Specify the name that must be present in the peer certificate when using TLS.
1.69 beck 119: Illegal if not using TLS.
1.64 djm 120: .It Fl F
121: Pass the first connected socket using
122: .Xr sendmsg 2
123: to stdout and exit.
124: This is useful in conjunction with
125: .Fl X
126: to have
127: .Nm
128: perform connection setup with a proxy but then leave the rest of the
1.65 jmc 129: connection to another program (e.g.\&
1.64 djm 130: .Xr ssh 1
131: using the
132: .Xr ssh_config 5
1.68 tobias 133: .Cm ProxyUseFdpass
1.64 djm 134: option).
1.70 jmc 135: .It Fl H Ar hash
1.69 beck 136: Specifies the required hash string of the peer certificate when using TLS.
137: The string format required is that used by
138: .Xr tls_peer_cert_hash 3 .
139: Illegal if not using TLS, and may not be used with -T noverify.
1.13 ericj 140: .It Fl h
141: Prints out
142: .Nm
143: help.
1.47 jmc 144: .It Fl I Ar length
1.46 djm 145: Specifies the size of the TCP receive buffer.
1.13 ericj 146: .It Fl i Ar interval
1.1 deraadt 147: Specifies a delay time interval between lines of text sent and received.
148: Also causes a delay time between connections to multiple ports.
1.70 jmc 149: .It Fl K Ar keyfile
1.69 beck 150: Specifies the filename from which the private key
1.70 jmc 151: is loaded in PEM format.
152: May only be used with TLS.
1.13 ericj 153: .It Fl k
154: Forces
155: .Nm
1.21 ericj 156: to stay listening for another connection after its current connection
1.13 ericj 157: is completed.
1.28 jmc 158: It is an error to use this option without the
159: .Fl l
160: option.
1.61 haesbaer 161: When used together with the
162: .Fl u
163: option, the server socket is not connected and it can receive UDP datagrams from
164: multiple hosts.
1.1 deraadt 165: .It Fl l
1.13 ericj 166: Used to specify that
1.6 aaron 167: .Nm
1.13 ericj 168: should listen for an incoming connection rather than initiate a
1.7 aaron 169: connection to a remote host.
1.28 jmc 170: It is an error to use this option in conjunction with the
171: .Fl p ,
172: .Fl s ,
173: or
174: .Fl z
175: options.
1.36 jmc 176: Additionally, any timeouts specified with the
1.35 jmc 177: .Fl w
1.36 jmc 178: option are ignored.
1.73 jca 179: .It Fl M Ar ttl
1.74 jmc 180: Set the TTL / hop limit of outgoing packets.
1.73 jca 181: .It Fl m Ar minttl
1.74 jmc 182: Ask the kernel to drop incoming packets whose TTL / hop limit is under
1.73 jca 183: .Ar minttl .
1.62 sthen 184: .It Fl N
185: .Xr shutdown 2
186: the network socket after EOF on the input.
187: Some servers require this to finish their work.
1.1 deraadt 188: .It Fl n
1.21 ericj 189: Do not do any DNS or service lookups on any specified addresses,
190: hostnames or ports.
1.47 jmc 191: .It Fl O Ar length
192: Specifies the size of the TCP send buffer.
1.77 beck 193: .It Fl o Ar staplefile
194: Specifies the filename from which to load data to be stapled
195: during the TLS handshake.
1.81 jmc 196: The file is expected to contain an OCSP response from an OCSP server in
1.78 jmc 197: DER format.
1.77 beck 198: May only be used with TLS and when a certificate is being used.
1.42 djm 199: .It Fl P Ar proxy_username
200: Specifies a username to present to a proxy server that requires authentication.
201: If no username is specified then authentication will not be attempted.
202: Proxy authentication is only supported for HTTP CONNECT proxies at present.
1.28 jmc 203: .It Fl p Ar source_port
1.1 deraadt 204: Specifies the source port
1.6 aaron 205: .Nm
1.1 deraadt 206: should use, subject to privilege restrictions and availability.
1.28 jmc 207: It is an error to use this option in conjunction with the
208: .Fl l
209: option.
1.70 jmc 210: .It Fl R Ar CAfile
211: Specifies the filename from which the root CA bundle for certificate
212: verification is loaded, in PEM format.
213: Illegal if not using TLS.
214: The default is
1.69 beck 215: .Pa /etc/ssl/cert.pem .
1.1 deraadt 216: .It Fl r
1.13 ericj 217: Specifies that source and/or destination ports should be chosen randomly
218: instead of sequentially within a range or in the order that the system
1.21 ericj 219: assigns them.
1.28 jmc 220: .It Fl S
221: Enables the RFC 2385 TCP MD5 signature option.
1.57 jeremy 222: .It Fl s Ar source
1.3 aaron 223: Specifies the IP of the interface which is used to send the packets.
1.56 jeremy 224: For
225: .Ux Ns -domain
226: datagram sockets, specifies the local temporary socket file
227: to create and use so that datagrams can be received.
1.28 jmc 228: It is an error to use this option in conjunction with the
229: .Fl l
230: option.
1.69 beck 231: .It Fl T Ar keyword
232: Change IPv4 TOS value or TLS options.
233: For TLS options
234: .Ar keyword
235: may be one of
1.80 jmc 236: .Ar tlsall ;
1.79 beck 237: which allows the use of all supported TLS protocols and ciphers,
1.86 ! jsing 238: .Ar tlscompat ;
! 239: which allows the use of all supported TLS protocols and "compat" ciphers,
1.80 jmc 240: .Ar noverify ;
1.70 jmc 241: which disables certificate verification;
1.69 beck 242: .Ar noname ,
1.75 beck 243: which disables certificate name checking;
1.70 jmc 244: .Ar clientcert ,
1.75 beck 245: which requires a client certificate on incoming connections; or
246: .Ar muststaple ,
1.76 jmc 247: which requires the peer to provide a valid stapled OCSP response
248: with the handshake.
249: It is illegal to specify TLS options if not using TLS.
1.70 jmc 250: .Pp
1.69 beck 251: For IPv4 TOS value
252: .Ar keyword
1.58 haesbaer 253: may be one of
254: .Ar critical ,
255: .Ar inetcontrol ,
256: .Ar lowdelay ,
257: .Ar netcontrol ,
258: .Ar throughput ,
259: .Ar reliability ,
260: or one of the DiffServ Code Points:
261: .Ar ef ,
262: .Ar af11 ... af43 ,
263: .Ar cs0 ... cs7 ;
264: or a number in either hex or decimal.
1.1 deraadt 265: .It Fl t
266: Causes
1.6 aaron 267: .Nm
1.25 jmc 268: to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
1.7 aaron 269: This makes it possible to use
1.6 aaron 270: .Nm
1.7 aaron 271: to script telnet sessions.
1.28 jmc 272: .It Fl U
1.51 sobrado 273: Specifies to use
1.52 sobrado 274: .Ux Ns -domain
275: sockets.
1.1 deraadt 276: .It Fl u
1.13 ericj 277: Use UDP instead of the default option of TCP.
1.56 jeremy 278: For
279: .Ux Ns -domain
280: sockets, use a datagram socket instead of a stream socket.
281: If a
282: .Ux Ns -domain
283: socket is used, a temporary receiving socket is created in
284: .Pa /tmp
285: unless the
286: .Fl s
287: flag is given.
1.54 guenther 288: .It Fl V Ar rtable
289: Set the routing table to be used.
1.1 deraadt 290: .It Fl v
1.13 ericj 291: Have
1.6 aaron 292: .Nm
1.13 ericj 293: give more verbose output.
1.85 bluhm 294: .It Fl W Ar recvlimit
295: Terminate after receiving
296: .Ar recvlimit
297: packets from the network.
1.26 jmc 298: .It Fl w Ar timeout
1.59 fgsch 299: Connections which cannot be established or are idle timeout after
1.26 jmc 300: .Ar timeout
1.59 fgsch 301: seconds.
1.26 jmc 302: The
303: .Fl w
304: flag has no effect on the
305: .Fl l
306: option, i.e.\&
307: .Nm
308: will listen forever for a connection, with or without the
309: .Fl w
310: flag.
311: The default is no timeout.
1.43 jmc 312: .It Fl X Ar proxy_protocol
1.28 jmc 313: Requests that
314: .Nm
1.33 djm 315: should use the specified protocol when talking to the proxy server.
316: Supported protocols are
317: .Dq 4
318: (SOCKS v.4),
319: .Dq 5
320: (SOCKS v.5)
321: and
322: .Dq connect
323: (HTTPS proxy).
324: If the protocol is not specified, SOCKS version 5 is used.
1.71 schwarze 325: .It Fl x Ar proxy_address Ns Op : Ns Ar port
1.19 jakob 326: Requests that
327: .Nm
328: should connect to
1.57 jeremy 329: .Ar destination
1.33 djm 330: using a proxy at
1.28 jmc 331: .Ar proxy_address
332: and
333: .Ar port .
334: If
335: .Ar port
1.33 djm 336: is not specified, the well-known port for the proxy protocol is used (1080
337: for SOCKS, 3128 for HTTPS).
1.82 jca 338: An IPv6 address can be specified unambiguously by enclosing
339: .Ar proxy_address
340: in square brackets.
1.84 jmc 341: .It Fl Z Ar peercertfile
342: Specifies the filename in which the peer supplied certificates will be saved
343: in PEM format.
344: May only be used with TLS.
1.1 deraadt 345: .It Fl z
346: Specifies that
1.6 aaron 347: .Nm
1.13 ericj 348: should just scan for listening daemons, without sending any data to them.
1.28 jmc 349: It is an error to use this option in conjunction with the
350: .Fl l
351: option.
352: .El
1.35 jmc 353: .Pp
1.57 jeremy 354: .Ar destination
1.35 jmc 355: can be a numerical IP address or a symbolic hostname
356: (unless the
357: .Fl n
358: option is given).
1.57 jeremy 359: In general, a destination must be specified,
1.35 jmc 360: unless the
361: .Fl l
362: option is given
363: (in which case the local host is used).
1.57 jeremy 364: For
365: .Ux Ns -domain
366: sockets, a destination is required and is the socket path to connect to
367: (or listen on if the
368: .Fl l
369: option is given).
1.35 jmc 370: .Pp
1.48 sobrado 371: .Ar port
1.72 beck 372: can be a specified as a numeric port number, or as a service name.
373: Ports may be specified in a range of the form nn-mm.
1.35 jmc 374: In general,
375: a destination port must be specified,
376: unless the
377: .Fl U
1.57 jeremy 378: option is given.
1.28 jmc 379: .Sh CLIENT/SERVER MODEL
380: It is quite simple to build a very basic client/server model using
381: .Nm .
382: On one console, start
383: .Nm
384: listening on a specific port for a connection.
385: For example:
386: .Pp
387: .Dl $ nc -l 1234
388: .Pp
389: .Nm
390: is now listening on port 1234 for a connection.
391: On a second console
392: .Pq or a second machine ,
393: connect to the machine and port being listened on:
394: .Pp
395: .Dl $ nc 127.0.0.1 1234
396: .Pp
397: There should now be a connection between the ports.
398: Anything typed at the second console will be concatenated to the first,
399: and vice-versa.
400: After the connection has been set up,
401: .Nm
402: does not really care which side is being used as a
403: .Sq server
404: and which side is being used as a
405: .Sq client .
406: The connection may be terminated using an
407: .Dv EOF
408: .Pq Sq ^D .
409: .Sh DATA TRANSFER
410: The example in the previous section can be expanded to build a
411: basic data transfer model.
412: Any information input into one end of the connection will be output
413: to the other end, and input and output can be easily captured in order to
414: emulate file transfer.
415: .Pp
416: Start by using
417: .Nm
418: to listen on a specific port, with output captured into a file:
419: .Pp
420: .Dl $ nc -l 1234 \*(Gt filename.out
421: .Pp
422: Using a second machine, connect to the listening
423: .Nm
424: process, feeding it the file which is to be transferred:
425: .Pp
1.66 jmc 426: .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
1.28 jmc 427: .Pp
428: After the file has been transferred, the connection will close automatically.
429: .Sh TALKING TO SERVERS
430: It is sometimes useful to talk to servers
431: .Dq by hand
432: rather than through a user interface.
433: It can aid in troubleshooting,
434: when it might be necessary to verify what data a server is sending
435: in response to commands issued by the client.
436: For example, to retrieve the home page of a web site:
1.40 jmc 437: .Bd -literal -offset indent
1.55 guenther 438: $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
1.40 jmc 439: .Ed
1.28 jmc 440: .Pp
441: Note that this also displays the headers sent by the web server.
442: They can be filtered, using a tool such as
443: .Xr sed 1 ,
444: if necessary.
445: .Pp
446: More complicated examples can be built up when the user knows the format
447: of requests required by the server.
448: As another example, an email may be submitted to an SMTP server using:
449: .Bd -literal -offset indent
450: $ nc localhost 25 \*(Lt\*(Lt EOF
451: HELO host.example.com
1.44 jmc 452: MAIL FROM:\*(Ltuser@host.example.com\*(Gt
453: RCPT TO:\*(Ltuser2@host.example.com\*(Gt
1.28 jmc 454: DATA
455: Body of email.
456: \&.
457: QUIT
458: EOF
459: .Ed
460: .Sh PORT SCANNING
461: It may be useful to know which ports are open and running services on
462: a target machine.
463: The
464: .Fl z
465: flag can be used to tell
1.22 markus 466: .Nm
1.39 jmc 467: to report open ports,
468: rather than initiate a connection.
1.28 jmc 469: For example:
470: .Bd -literal -offset indent
1.39 jmc 471: $ nc -z host.example.com 20-30
1.28 jmc 472: Connection to host.example.com 22 port [tcp/ssh] succeeded!
473: Connection to host.example.com 25 port [tcp/smtp] succeeded!
474: .Ed
475: .Pp
476: The port range was specified to limit the search to ports 20 \- 30.
477: .Pp
478: Alternatively, it might be useful to know which server software
479: is running, and which versions.
480: This information is often contained within the greeting banners.
481: In order to retrieve these, it is necessary to first make a connection,
482: and then break the connection when the banner has been retrieved.
483: This can be accomplished by specifying a small timeout with the
484: .Fl w
485: flag, or perhaps by issuing a
486: .Qq Dv QUIT
487: command to the server:
488: .Bd -literal -offset indent
489: $ echo "QUIT" | nc host.example.com 20-30
490: SSH-1.99-OpenSSH_3.6.1p2
491: Protocol mismatch.
492: 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
493: .Ed
1.1 deraadt 494: .Sh EXAMPLES
1.37 jmc 495: Open a TCP connection to port 42 of host.example.com, using port 31337 as
1.28 jmc 496: the source port, with a timeout of 5 seconds:
497: .Pp
1.37 jmc 498: .Dl $ nc -p 31337 -w 5 host.example.com 42
1.69 beck 499: .Pp
1.70 jmc 500: Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
501: Check for a different name in the certificate for validation.
1.69 beck 502: .Pp
503: .Dl $ nc -v -c -e adsf.au.doubleclick.net www.google.ca 443
1.28 jmc 504: .Pp
1.37 jmc 505: Open a UDP connection to port 53 of host.example.com:
1.28 jmc 506: .Pp
1.37 jmc 507: .Dl $ nc -u host.example.com 53
1.28 jmc 508: .Pp
1.37 jmc 509: Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
1.28 jmc 510: IP for the local end of the connection:
511: .Pp
1.37 jmc 512: .Dl $ nc -s 10.1.2.3 host.example.com 42
1.28 jmc 513: .Pp
1.51 sobrado 514: Create and listen on a
1.52 sobrado 515: .Ux Ns -domain
1.57 jeremy 516: stream socket:
1.28 jmc 517: .Pp
518: .Dl $ nc -lU /var/tmp/dsocket
1.33 djm 519: .Pp
1.37 jmc 520: Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
1.38 jmc 521: port 8080.
522: This example could also be used by
523: .Xr ssh 1 ;
524: see the
525: .Cm ProxyCommand
526: directive in
527: .Xr ssh_config 5
528: for more information.
1.33 djm 529: .Pp
1.37 jmc 530: .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
1.42 djm 531: .Pp
532: The same example again, this time enabling proxy authentication with username
533: .Dq ruser
534: if the proxy requires it:
535: .Pp
536: .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
1.1 deraadt 537: .Sh SEE ALSO
1.38 jmc 538: .Xr cat 1 ,
539: .Xr ssh 1
1.15 smart 540: .Sh AUTHORS
541: Original implementation by *Hobbit*
1.63 schwarze 542: .Aq Mt hobbit@avian.org .
1.28 jmc 543: .br
544: Rewritten with IPv6 support by
1.63 schwarze 545: .An Eric Jackson Aq Mt ericj@monkey.org .
1.39 jmc 546: .Sh CAVEATS
1.60 lum 547: UDP port scans using the
1.39 jmc 548: .Fl uz
1.60 lum 549: combination of flags will always report success irrespective of
550: the target machine's state.
551: However,
552: in conjunction with a traffic sniffer either on the target machine
553: or an intermediary device,
554: the
555: .Fl uz
556: combination could be useful for communications diagnostics.
557: Note that the amount of UDP traffic generated may be limited either
558: due to hardware resources and/or configuration settings.
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