Annotation of src/usr.bin/nc/nc.1, Revision 1.75
1.75 ! beck 1: .\" $OpenBSD: nc.1,v 1.74 2016/07/02 05:58:00 jmc Exp $
1.1 deraadt 2: .\"
3: .\" Copyright (c) 1996 David Sacerdote
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1.75 ! beck 28: .Dd $Mdocdate: July 2 2016 $
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
1.42 djm 46: .Op Fl P Ar proxy_username
1.28 jmc 47: .Op Fl p Ar source_port
1.70 jmc 48: .Op Fl R Ar CAfile
1.57 jeremy 49: .Op Fl s Ar source
1.69 beck 50: .Op Fl T Ar keyword
1.54 guenther 51: .Op Fl V Ar rtable
1.6 aaron 52: .Op Fl w Ar timeout
1.33 djm 53: .Op Fl X Ar proxy_protocol
1.71 schwarze 54: .Op Fl x Ar proxy_address Ns Op : Ns Ar port
1.57 jeremy 55: .Op Ar destination
1.48 sobrado 56: .Op Ar port
1.1 deraadt 57: .Sh DESCRIPTION
58: The
1.6 aaron 59: .Nm
1.1 deraadt 60: (or
61: .Nm netcat )
1.57 jeremy 62: utility is used for just about anything under the sun involving TCP,
63: UDP, or
64: .Ux Ns -domain
65: sockets.
1.13 ericj 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
68: IPv6.
1.7 aaron 69: Unlike
1.1 deraadt 70: .Xr telnet 1 ,
1.6 aaron 71: .Nm
1.1 deraadt 72: scripts nicely, and separates error messages onto standard error instead
1.6 aaron 73: of sending them to standard output, as
1.24 pvalchev 74: .Xr telnet 1
1.6 aaron 75: does with some.
1.1 deraadt 76: .Pp
77: Common uses include:
1.7 aaron 78: .Pp
79: .Bl -bullet -offset indent -compact
1.1 deraadt 80: .It
1.3 aaron 81: simple TCP proxies
1.1 deraadt 82: .It
1.28 jmc 83: shell-script based HTTP clients and servers
1.1 deraadt 84: .It
1.13 ericj 85: network daemon testing
1.1 deraadt 86: .It
1.33 djm 87: a SOCKS or HTTP ProxyCommand for
88: .Xr ssh 1
89: .It
1.1 deraadt 90: and much, much more
91: .El
92: .Pp
93: The options are as follows:
94: .Bl -tag -width Ds
1.13 ericj 95: .It Fl 4
96: Forces
97: .Nm
98: to use IPv4 addresses only.
99: .It Fl 6
100: Forces
101: .Nm
102: to use IPv6 addresses only.
1.70 jmc 103: .It Fl C Ar certfile
1.69 beck 104: Specifies the filename from which the public key part of the TLS
1.70 jmc 105: certificate is loaded, in PEM format.
106: May only be used with TLS.
1.69 beck 107: .It Fl c
1.70 jmc 108: If using a TCP socket to connect or listen, use TLS.
109: Illegal if not using TCP sockets.
1.32 markus 110: .It Fl D
111: Enable debugging on the socket.
1.29 tedu 112: .It Fl d
113: Do not attempt to read from stdin.
1.69 beck 114: .It Fl e Ar name
1.70 jmc 115: Specify the name that must be present in the peer certificate when using TLS.
1.69 beck 116: Illegal if not using TLS.
1.64 djm 117: .It Fl F
118: Pass the first connected socket using
119: .Xr sendmsg 2
120: to stdout and exit.
121: This is useful in conjunction with
122: .Fl X
123: to have
124: .Nm
125: perform connection setup with a proxy but then leave the rest of the
1.65 jmc 126: connection to another program (e.g.\&
1.64 djm 127: .Xr ssh 1
128: using the
129: .Xr ssh_config 5
1.68 tobias 130: .Cm ProxyUseFdpass
1.64 djm 131: option).
1.70 jmc 132: .It Fl H Ar hash
1.69 beck 133: Specifies the required hash string of the peer certificate when using TLS.
134: The string format required is that used by
135: .Xr tls_peer_cert_hash 3 .
136: Illegal if not using TLS, and may not be used with -T noverify.
1.13 ericj 137: .It Fl h
138: Prints out
139: .Nm
140: help.
1.47 jmc 141: .It Fl I Ar length
1.46 djm 142: Specifies the size of the TCP receive buffer.
1.13 ericj 143: .It Fl i Ar interval
1.1 deraadt 144: Specifies a delay time interval between lines of text sent and received.
145: Also causes a delay time between connections to multiple ports.
1.70 jmc 146: .It Fl K Ar keyfile
1.69 beck 147: Specifies the filename from which the private key
1.70 jmc 148: is loaded in PEM format.
149: May only be used with TLS.
1.13 ericj 150: .It Fl k
151: Forces
152: .Nm
1.21 ericj 153: to stay listening for another connection after its current connection
1.13 ericj 154: is completed.
1.28 jmc 155: It is an error to use this option without the
156: .Fl l
157: option.
1.61 haesbaer 158: When used together with the
159: .Fl u
160: option, the server socket is not connected and it can receive UDP datagrams from
161: multiple hosts.
1.1 deraadt 162: .It Fl l
1.13 ericj 163: Used to specify that
1.6 aaron 164: .Nm
1.13 ericj 165: should listen for an incoming connection rather than initiate a
1.7 aaron 166: connection to a remote host.
1.28 jmc 167: It is an error to use this option in conjunction with the
168: .Fl p ,
169: .Fl s ,
170: or
171: .Fl z
172: options.
1.36 jmc 173: Additionally, any timeouts specified with the
1.35 jmc 174: .Fl w
1.36 jmc 175: option are ignored.
1.73 jca 176: .It Fl M Ar ttl
1.74 jmc 177: Set the TTL / hop limit of outgoing packets.
1.73 jca 178: .It Fl m Ar minttl
1.74 jmc 179: Ask the kernel to drop incoming packets whose TTL / hop limit is under
1.73 jca 180: .Ar minttl .
1.62 sthen 181: .It Fl N
182: .Xr shutdown 2
183: the network socket after EOF on the input.
184: Some servers require this to finish their work.
1.1 deraadt 185: .It Fl n
1.21 ericj 186: Do not do any DNS or service lookups on any specified addresses,
187: hostnames or ports.
1.47 jmc 188: .It Fl O Ar length
189: Specifies the size of the TCP send buffer.
1.42 djm 190: .It Fl P Ar proxy_username
191: Specifies a username to present to a proxy server that requires authentication.
192: If no username is specified then authentication will not be attempted.
193: Proxy authentication is only supported for HTTP CONNECT proxies at present.
1.28 jmc 194: .It Fl p Ar source_port
1.1 deraadt 195: Specifies the source port
1.6 aaron 196: .Nm
1.1 deraadt 197: should use, subject to privilege restrictions and availability.
1.28 jmc 198: It is an error to use this option in conjunction with the
199: .Fl l
200: option.
1.70 jmc 201: .It Fl R Ar CAfile
202: Specifies the filename from which the root CA bundle for certificate
203: verification is loaded, in PEM format.
204: Illegal if not using TLS.
205: The default is
1.69 beck 206: .Pa /etc/ssl/cert.pem .
1.1 deraadt 207: .It Fl r
1.13 ericj 208: Specifies that source and/or destination ports should be chosen randomly
209: instead of sequentially within a range or in the order that the system
1.21 ericj 210: assigns them.
1.28 jmc 211: .It Fl S
212: Enables the RFC 2385 TCP MD5 signature option.
1.57 jeremy 213: .It Fl s Ar source
1.3 aaron 214: Specifies the IP of the interface which is used to send the packets.
1.56 jeremy 215: For
216: .Ux Ns -domain
217: datagram sockets, specifies the local temporary socket file
218: to create and use so that datagrams can be received.
1.28 jmc 219: It is an error to use this option in conjunction with the
220: .Fl l
221: option.
1.69 beck 222: .It Fl T Ar keyword
223: Change IPv4 TOS value or TLS options.
224: For TLS options
225: .Ar keyword
226: may be one of
227: .Ar tlslegacy ,
1.70 jmc 228: which allows legacy TLS protocols;
1.69 beck 229: .Ar noverify ,
1.70 jmc 230: which disables certificate verification;
1.69 beck 231: .Ar noname ,
1.75 ! beck 232: which disables certificate name checking;
1.70 jmc 233: .Ar clientcert ,
1.75 ! beck 234: which requires a client certificate on incoming connections; or
! 235: .Ar muststaple ,
! 236: which requires the peer to provide a valid stapled OCSP response
! 237: with the handshake. It is illegal to specify TLS options if not using TLS.
1.70 jmc 238: .Pp
1.69 beck 239: For IPv4 TOS value
240: .Ar keyword
1.58 haesbaer 241: may be one of
242: .Ar critical ,
243: .Ar inetcontrol ,
244: .Ar lowdelay ,
245: .Ar netcontrol ,
246: .Ar throughput ,
247: .Ar reliability ,
248: or one of the DiffServ Code Points:
249: .Ar ef ,
250: .Ar af11 ... af43 ,
251: .Ar cs0 ... cs7 ;
252: or a number in either hex or decimal.
1.1 deraadt 253: .It Fl t
254: Causes
1.6 aaron 255: .Nm
1.25 jmc 256: to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
1.7 aaron 257: This makes it possible to use
1.6 aaron 258: .Nm
1.7 aaron 259: to script telnet sessions.
1.28 jmc 260: .It Fl U
1.51 sobrado 261: Specifies to use
1.52 sobrado 262: .Ux Ns -domain
263: sockets.
1.1 deraadt 264: .It Fl u
1.13 ericj 265: Use UDP instead of the default option of TCP.
1.56 jeremy 266: For
267: .Ux Ns -domain
268: sockets, use a datagram socket instead of a stream socket.
269: If a
270: .Ux Ns -domain
271: socket is used, a temporary receiving socket is created in
272: .Pa /tmp
273: unless the
274: .Fl s
275: flag is given.
1.54 guenther 276: .It Fl V Ar rtable
277: Set the routing table to be used.
1.1 deraadt 278: .It Fl v
1.13 ericj 279: Have
1.6 aaron 280: .Nm
1.13 ericj 281: give more verbose output.
1.26 jmc 282: .It Fl w Ar timeout
1.59 fgsch 283: Connections which cannot be established or are idle timeout after
1.26 jmc 284: .Ar timeout
1.59 fgsch 285: seconds.
1.26 jmc 286: The
287: .Fl w
288: flag has no effect on the
289: .Fl l
290: option, i.e.\&
291: .Nm
292: will listen forever for a connection, with or without the
293: .Fl w
294: flag.
295: The default is no timeout.
1.43 jmc 296: .It Fl X Ar proxy_protocol
1.28 jmc 297: Requests that
298: .Nm
1.33 djm 299: should use the specified protocol when talking to the proxy server.
300: Supported protocols are
301: .Dq 4
302: (SOCKS v.4),
303: .Dq 5
304: (SOCKS v.5)
305: and
306: .Dq connect
307: (HTTPS proxy).
308: If the protocol is not specified, SOCKS version 5 is used.
1.71 schwarze 309: .It Fl x Ar proxy_address Ns Op : Ns Ar port
1.19 jakob 310: Requests that
311: .Nm
312: should connect to
1.57 jeremy 313: .Ar destination
1.33 djm 314: using a proxy at
1.28 jmc 315: .Ar proxy_address
316: and
317: .Ar port .
318: If
319: .Ar port
1.33 djm 320: is not specified, the well-known port for the proxy protocol is used (1080
321: for SOCKS, 3128 for HTTPS).
1.1 deraadt 322: .It Fl z
323: Specifies that
1.6 aaron 324: .Nm
1.13 ericj 325: should just scan for listening daemons, without sending any data to them.
1.28 jmc 326: It is an error to use this option in conjunction with the
327: .Fl l
328: option.
329: .El
1.35 jmc 330: .Pp
1.57 jeremy 331: .Ar destination
1.35 jmc 332: can be a numerical IP address or a symbolic hostname
333: (unless the
334: .Fl n
335: option is given).
1.57 jeremy 336: In general, a destination must be specified,
1.35 jmc 337: unless the
338: .Fl l
339: option is given
340: (in which case the local host is used).
1.57 jeremy 341: For
342: .Ux Ns -domain
343: sockets, a destination is required and is the socket path to connect to
344: (or listen on if the
345: .Fl l
346: option is given).
1.35 jmc 347: .Pp
1.48 sobrado 348: .Ar port
1.72 beck 349: can be a specified as a numeric port number, or as a service name.
350: Ports may be specified in a range of the form nn-mm.
1.35 jmc 351: In general,
352: a destination port must be specified,
353: unless the
354: .Fl U
1.57 jeremy 355: option is given.
1.28 jmc 356: .Sh CLIENT/SERVER MODEL
357: It is quite simple to build a very basic client/server model using
358: .Nm .
359: On one console, start
360: .Nm
361: listening on a specific port for a connection.
362: For example:
363: .Pp
364: .Dl $ nc -l 1234
365: .Pp
366: .Nm
367: is now listening on port 1234 for a connection.
368: On a second console
369: .Pq or a second machine ,
370: connect to the machine and port being listened on:
371: .Pp
372: .Dl $ nc 127.0.0.1 1234
373: .Pp
374: There should now be a connection between the ports.
375: Anything typed at the second console will be concatenated to the first,
376: and vice-versa.
377: After the connection has been set up,
378: .Nm
379: does not really care which side is being used as a
380: .Sq server
381: and which side is being used as a
382: .Sq client .
383: The connection may be terminated using an
384: .Dv EOF
385: .Pq Sq ^D .
386: .Sh DATA TRANSFER
387: The example in the previous section can be expanded to build a
388: basic data transfer model.
389: Any information input into one end of the connection will be output
390: to the other end, and input and output can be easily captured in order to
391: emulate file transfer.
392: .Pp
393: Start by using
394: .Nm
395: to listen on a specific port, with output captured into a file:
396: .Pp
397: .Dl $ nc -l 1234 \*(Gt filename.out
398: .Pp
399: Using a second machine, connect to the listening
400: .Nm
401: process, feeding it the file which is to be transferred:
402: .Pp
1.66 jmc 403: .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
1.28 jmc 404: .Pp
405: After the file has been transferred, the connection will close automatically.
406: .Sh TALKING TO SERVERS
407: It is sometimes useful to talk to servers
408: .Dq by hand
409: rather than through a user interface.
410: It can aid in troubleshooting,
411: when it might be necessary to verify what data a server is sending
412: in response to commands issued by the client.
413: For example, to retrieve the home page of a web site:
1.40 jmc 414: .Bd -literal -offset indent
1.55 guenther 415: $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
1.40 jmc 416: .Ed
1.28 jmc 417: .Pp
418: Note that this also displays the headers sent by the web server.
419: They can be filtered, using a tool such as
420: .Xr sed 1 ,
421: if necessary.
422: .Pp
423: More complicated examples can be built up when the user knows the format
424: of requests required by the server.
425: As another example, an email may be submitted to an SMTP server using:
426: .Bd -literal -offset indent
427: $ nc localhost 25 \*(Lt\*(Lt EOF
428: HELO host.example.com
1.44 jmc 429: MAIL FROM:\*(Ltuser@host.example.com\*(Gt
430: RCPT TO:\*(Ltuser2@host.example.com\*(Gt
1.28 jmc 431: DATA
432: Body of email.
433: \&.
434: QUIT
435: EOF
436: .Ed
437: .Sh PORT SCANNING
438: It may be useful to know which ports are open and running services on
439: a target machine.
440: The
441: .Fl z
442: flag can be used to tell
1.22 markus 443: .Nm
1.39 jmc 444: to report open ports,
445: rather than initiate a connection.
1.28 jmc 446: For example:
447: .Bd -literal -offset indent
1.39 jmc 448: $ nc -z host.example.com 20-30
1.28 jmc 449: Connection to host.example.com 22 port [tcp/ssh] succeeded!
450: Connection to host.example.com 25 port [tcp/smtp] succeeded!
451: .Ed
452: .Pp
453: The port range was specified to limit the search to ports 20 \- 30.
454: .Pp
455: Alternatively, it might be useful to know which server software
456: is running, and which versions.
457: This information is often contained within the greeting banners.
458: In order to retrieve these, it is necessary to first make a connection,
459: and then break the connection when the banner has been retrieved.
460: This can be accomplished by specifying a small timeout with the
461: .Fl w
462: flag, or perhaps by issuing a
463: .Qq Dv QUIT
464: command to the server:
465: .Bd -literal -offset indent
466: $ echo "QUIT" | nc host.example.com 20-30
467: SSH-1.99-OpenSSH_3.6.1p2
468: Protocol mismatch.
469: 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
470: .Ed
1.1 deraadt 471: .Sh EXAMPLES
1.37 jmc 472: Open a TCP connection to port 42 of host.example.com, using port 31337 as
1.28 jmc 473: the source port, with a timeout of 5 seconds:
474: .Pp
1.37 jmc 475: .Dl $ nc -p 31337 -w 5 host.example.com 42
1.69 beck 476: .Pp
1.70 jmc 477: Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
478: Check for a different name in the certificate for validation.
1.69 beck 479: .Pp
480: .Dl $ nc -v -c -e adsf.au.doubleclick.net www.google.ca 443
1.28 jmc 481: .Pp
1.37 jmc 482: Open a UDP connection to port 53 of host.example.com:
1.28 jmc 483: .Pp
1.37 jmc 484: .Dl $ nc -u host.example.com 53
1.28 jmc 485: .Pp
1.37 jmc 486: Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
1.28 jmc 487: IP for the local end of the connection:
488: .Pp
1.37 jmc 489: .Dl $ nc -s 10.1.2.3 host.example.com 42
1.28 jmc 490: .Pp
1.51 sobrado 491: Create and listen on a
1.52 sobrado 492: .Ux Ns -domain
1.57 jeremy 493: stream socket:
1.28 jmc 494: .Pp
495: .Dl $ nc -lU /var/tmp/dsocket
1.33 djm 496: .Pp
1.37 jmc 497: Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
1.38 jmc 498: port 8080.
499: This example could also be used by
500: .Xr ssh 1 ;
501: see the
502: .Cm ProxyCommand
503: directive in
504: .Xr ssh_config 5
505: for more information.
1.33 djm 506: .Pp
1.37 jmc 507: .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
1.42 djm 508: .Pp
509: The same example again, this time enabling proxy authentication with username
510: .Dq ruser
511: if the proxy requires it:
512: .Pp
513: .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
1.1 deraadt 514: .Sh SEE ALSO
1.38 jmc 515: .Xr cat 1 ,
516: .Xr ssh 1
1.15 smart 517: .Sh AUTHORS
518: Original implementation by *Hobbit*
1.63 schwarze 519: .Aq Mt hobbit@avian.org .
1.28 jmc 520: .br
521: Rewritten with IPv6 support by
1.63 schwarze 522: .An Eric Jackson Aq Mt ericj@monkey.org .
1.39 jmc 523: .Sh CAVEATS
1.60 lum 524: UDP port scans using the
1.39 jmc 525: .Fl uz
1.60 lum 526: combination of flags will always report success irrespective of
527: the target machine's state.
528: However,
529: in conjunction with a traffic sniffer either on the target machine
530: or an intermediary device,
531: the
532: .Fl uz
533: combination could be useful for communications diagnostics.
534: Note that the amount of UDP traffic generated may be limited either
535: due to hardware resources and/or configuration settings.
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