Annotation of src/usr.bin/nc/nc.1, Revision 1.95
1.95 ! schwarze 1: .\" $OpenBSD: nc.1,v 1.94 2019/10/24 17:27:08 kn Exp $
1.1 deraadt 2: .\"
3: .\" Copyright (c) 1996 David Sacerdote
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1.95 ! schwarze 28: .Dd $Mdocdate: October 24 2019 $
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
1.95 ! schwarze 50: .Op Fl s Ar sourceaddr
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
1.89 schwarze 99: Use IPv4 addresses only.
1.13 ericj 100: .It Fl 6
1.89 schwarze 101: Use IPv6 addresses only.
1.70 jmc 102: .It Fl C Ar certfile
1.89 schwarze 103: Load the public key part of the TLS peer certificate from
104: .Ar certfile ,
105: in PEM format.
106: Requires
107: .Fl c .
1.69 beck 108: .It Fl c
1.89 schwarze 109: Use TLS to connect or listen.
110: Cannot be used together with any of the options
111: .Fl FuU .
1.32 markus 112: .It Fl D
113: Enable debugging on the socket.
1.29 tedu 114: .It Fl d
115: Do not attempt to read from stdin.
1.69 beck 116: .It Fl e Ar name
1.89 schwarze 117: Only accept the TLS peer certificate if it contains the
118: .Ar name .
119: Requires
120: .Fl c .
121: If not specified,
122: .Ar destination
123: is used.
1.64 djm 124: .It Fl F
125: Pass the first connected socket using
126: .Xr sendmsg 2
127: to stdout and exit.
128: This is useful in conjunction with
129: .Fl X
130: to have
131: .Nm
132: perform connection setup with a proxy but then leave the rest of the
1.65 jmc 133: connection to another program (e.g.\&
1.64 djm 134: .Xr ssh 1
135: using the
136: .Xr ssh_config 5
1.68 tobias 137: .Cm ProxyUseFdpass
1.64 djm 138: option).
1.89 schwarze 139: Cannot be used with
140: .Fl c
141: or
142: .Fl U .
1.70 jmc 143: .It Fl H Ar hash
1.89 schwarze 144: Only accept the TLS peer certificate if its hash returned from
145: .Xr tls_peer_cert_hash 3
146: matches
147: .Ar hash .
148: Requires
149: .Fl c
150: and cannot be used with
151: .Fl T Cm noverify .
1.13 ericj 152: .It Fl h
1.89 schwarze 153: Print out the
1.13 ericj 154: .Nm
1.89 schwarze 155: help text and exit.
1.47 jmc 156: .It Fl I Ar length
1.89 schwarze 157: Specify the size of the TCP receive buffer.
1.13 ericj 158: .It Fl i Ar interval
1.89 schwarze 159: Sleep for
160: .Ar interval
161: seconds between lines of text sent and received.
1.1 deraadt 162: Also causes a delay time between connections to multiple ports.
1.70 jmc 163: .It Fl K Ar keyfile
1.89 schwarze 164: Load the TLS private key from
165: .Ar keyfile ,
166: in PEM format.
167: Requires
168: .Fl c .
1.13 ericj 169: .It Fl k
1.89 schwarze 170: When a connection is completed, listen for another one.
171: Requires
172: .Fl l .
1.61 haesbaer 173: When used together with the
174: .Fl u
175: option, the server socket is not connected and it can receive UDP datagrams from
176: multiple hosts.
1.1 deraadt 177: .It Fl l
1.89 schwarze 178: Listen for an incoming connection rather than initiating a
1.7 aaron 179: connection to a remote host.
1.89 schwarze 180: Cannot be used together with any of the options
181: .Fl psxz .
1.36 jmc 182: Additionally, any timeouts specified with the
1.35 jmc 183: .Fl w
1.36 jmc 184: option are ignored.
1.73 jca 185: .It Fl M Ar ttl
1.74 jmc 186: Set the TTL / hop limit of outgoing packets.
1.73 jca 187: .It Fl m Ar minttl
1.74 jmc 188: Ask the kernel to drop incoming packets whose TTL / hop limit is under
1.73 jca 189: .Ar minttl .
1.62 sthen 190: .It Fl N
191: .Xr shutdown 2
192: the network socket after EOF on the input.
193: Some servers require this to finish their work.
1.1 deraadt 194: .It Fl n
1.94 kn 195: Do not perform domain name resolution.
196: If a name cannot be resolved without DNS, an error will be reported.
1.47 jmc 197: .It Fl O Ar length
1.89 schwarze 198: Specify the size of the TCP send buffer.
1.77 beck 199: .It Fl o Ar staplefile
1.89 schwarze 200: During the TLS handshake, load data to be stapled from
201: .Ar staplefile ,
202: which is expected to contain an OCSP response from an OCSP server in
1.78 jmc 203: DER format.
1.89 schwarze 204: Requires
205: .Fl c
206: and
207: .Fl C .
1.42 djm 208: .It Fl P Ar proxy_username
209: Specifies a username to present to a proxy server that requires authentication.
210: If no username is specified then authentication will not be attempted.
211: Proxy authentication is only supported for HTTP CONNECT proxies at present.
1.28 jmc 212: .It Fl p Ar source_port
1.89 schwarze 213: Specify the source port
1.6 aaron 214: .Nm
1.1 deraadt 215: should use, subject to privilege restrictions and availability.
1.89 schwarze 216: Cannot be used together with
217: .Fl l .
1.70 jmc 218: .It Fl R Ar CAfile
1.89 schwarze 219: Load the root CA bundle for TLS certificate verification from
220: .Ar CAfile ,
221: in PEM format, instead of
1.69 beck 222: .Pa /etc/ssl/cert.pem .
1.89 schwarze 223: Requires
224: .Fl c .
1.1 deraadt 225: .It Fl r
1.89 schwarze 226: Choose source and/or destination ports randomly
1.13 ericj 227: instead of sequentially within a range or in the order that the system
1.21 ericj 228: assigns them.
1.28 jmc 229: .It Fl S
1.89 schwarze 230: Enable the RFC 2385 TCP MD5 signature option.
1.95 ! schwarze 231: .It Fl s Ar sourceaddr
! 232: Set the source address to send packets from,
! 233: which is useful on machines with multiple interfaces.
1.56 jeremy 234: For
235: .Ux Ns -domain
236: datagram sockets, specifies the local temporary socket file
237: to create and use so that datagrams can be received.
1.89 schwarze 238: Cannot be used together with
1.28 jmc 239: .Fl l
1.89 schwarze 240: or
241: .Fl x .
1.69 beck 242: .It Fl T Ar keyword
1.91 jmc 243: Change the IPv4 TOS/IPv6 traffic class value or the TLS options.
1.89 schwarze 244: .Pp
245: For TLS options,
1.69 beck 246: .Ar keyword
1.87 jmc 247: may be one of:
1.89 schwarze 248: .Cm noverify ,
1.70 jmc 249: which disables certificate verification;
1.89 schwarze 250: .Cm noname ,
1.75 beck 251: which disables certificate name checking;
1.89 schwarze 252: .Cm clientcert ,
1.75 beck 253: which requires a client certificate on incoming connections; or
1.89 schwarze 254: .Cm muststaple ,
1.76 jmc 255: which requires the peer to provide a valid stapled OCSP response
256: with the handshake.
1.89 schwarze 257: The following TLS options specify a value in the form of a
258: .Ar key Ns = Ns Ar value
259: pair:
260: .Cm ciphers ,
1.88 jsing 261: which allows the supported TLS ciphers to be specified (see
262: .Xr tls_config_set_ciphers 3
263: for further details);
1.89 schwarze 264: .Cm protocols ,
1.88 jsing 265: which allows the supported TLS protocols to be specified (see
266: .Xr tls_config_parse_protocols 3
267: for further details).
1.90 jmc 268: Specifying TLS options requires
1.89 schwarze 269: .Fl c .
1.70 jmc 270: .Pp
1.91 jmc 271: For the IPv4 TOS/IPv6 traffic class value,
1.69 beck 272: .Ar keyword
1.58 haesbaer 273: may be one of
1.89 schwarze 274: .Cm critical ,
275: .Cm inetcontrol ,
276: .Cm lowdelay ,
277: .Cm netcontrol ,
278: .Cm throughput ,
279: .Cm reliability ,
1.58 haesbaer 280: or one of the DiffServ Code Points:
1.89 schwarze 281: .Cm ef ,
282: .Cm af11 No ... Cm af43 ,
283: .Cm cs0 No ... Cm cs7 ;
1.58 haesbaer 284: or a number in either hex or decimal.
1.1 deraadt 285: .It Fl t
1.89 schwarze 286: Send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
1.7 aaron 287: This makes it possible to use
1.6 aaron 288: .Nm
1.7 aaron 289: to script telnet sessions.
1.28 jmc 290: .It Fl U
1.89 schwarze 291: Use
1.52 sobrado 292: .Ux Ns -domain
293: sockets.
1.89 schwarze 294: Cannot be used together with any of the options
295: .Fl cFx .
1.1 deraadt 296: .It Fl u
1.89 schwarze 297: Use UDP instead of TCP.
298: Cannot be used together with
299: .Fl c
300: or
301: .Fl x .
1.56 jeremy 302: For
303: .Ux Ns -domain
304: sockets, use a datagram socket instead of a stream socket.
305: If a
306: .Ux Ns -domain
307: socket is used, a temporary receiving socket is created in
308: .Pa /tmp
309: unless the
310: .Fl s
311: flag is given.
1.54 guenther 312: .It Fl V Ar rtable
313: Set the routing table to be used.
1.1 deraadt 314: .It Fl v
1.89 schwarze 315: Produce more verbose output.
1.85 bluhm 316: .It Fl W Ar recvlimit
317: Terminate after receiving
318: .Ar recvlimit
319: packets from the network.
1.26 jmc 320: .It Fl w Ar timeout
1.59 fgsch 321: Connections which cannot be established or are idle timeout after
1.26 jmc 322: .Ar timeout
1.59 fgsch 323: seconds.
1.26 jmc 324: The
325: .Fl w
326: flag has no effect on the
327: .Fl l
328: option, i.e.\&
329: .Nm
330: will listen forever for a connection, with or without the
331: .Fl w
332: flag.
333: The default is no timeout.
1.43 jmc 334: .It Fl X Ar proxy_protocol
1.89 schwarze 335: Use
336: .Ar proxy_protocol
337: when talking to the proxy server.
1.33 djm 338: Supported protocols are
1.89 schwarze 339: .Cm 4
1.33 djm 340: (SOCKS v.4),
1.89 schwarze 341: .Cm 5
1.33 djm 342: (SOCKS v.5)
343: and
1.89 schwarze 344: .Cm connect
1.33 djm 345: (HTTPS proxy).
346: If the protocol is not specified, SOCKS version 5 is used.
1.71 schwarze 347: .It Fl x Ar proxy_address Ns Op : Ns Ar port
1.89 schwarze 348: Connect to
1.57 jeremy 349: .Ar destination
1.33 djm 350: using a proxy at
1.28 jmc 351: .Ar proxy_address
352: and
353: .Ar port .
354: If
355: .Ar port
1.33 djm 356: is not specified, the well-known port for the proxy protocol is used (1080
357: for SOCKS, 3128 for HTTPS).
1.82 jca 358: An IPv6 address can be specified unambiguously by enclosing
359: .Ar proxy_address
360: in square brackets.
1.89 schwarze 361: A proxy cannot be used with any of the options
362: .Fl lsuU .
1.84 jmc 363: .It Fl Z Ar peercertfile
1.89 schwarze 364: Save the peer certificates to
365: .Ar peercertfile ,
1.84 jmc 366: in PEM format.
1.89 schwarze 367: Requires
368: .Fl c .
1.1 deraadt 369: .It Fl z
1.89 schwarze 370: Only scan for listening daemons, without sending any data to them.
371: Cannot be used together with
372: .Fl l .
1.28 jmc 373: .El
1.35 jmc 374: .Pp
1.57 jeremy 375: .Ar destination
1.35 jmc 376: can be a numerical IP address or a symbolic hostname
377: (unless the
378: .Fl n
379: option is given).
1.57 jeremy 380: In general, a destination must be specified,
1.35 jmc 381: unless the
382: .Fl l
383: option is given
384: (in which case the local host is used).
1.57 jeremy 385: For
386: .Ux Ns -domain
387: sockets, a destination is required and is the socket path to connect to
388: (or listen on if the
389: .Fl l
390: option is given).
1.35 jmc 391: .Pp
1.48 sobrado 392: .Ar port
1.93 jmc 393: can be specified as a numeric port number or as a service name.
1.92 tedu 394: Port ranges may be specified as numeric port numbers of the form
1.89 schwarze 395: .Ar nn Ns - Ns Ar mm .
1.35 jmc 396: In general,
397: a destination port must be specified,
398: unless the
399: .Fl U
1.57 jeremy 400: option is given.
1.28 jmc 401: .Sh CLIENT/SERVER MODEL
402: It is quite simple to build a very basic client/server model using
403: .Nm .
404: On one console, start
405: .Nm
406: listening on a specific port for a connection.
407: For example:
408: .Pp
409: .Dl $ nc -l 1234
410: .Pp
411: .Nm
412: is now listening on port 1234 for a connection.
413: On a second console
414: .Pq or a second machine ,
415: connect to the machine and port being listened on:
416: .Pp
417: .Dl $ nc 127.0.0.1 1234
418: .Pp
419: There should now be a connection between the ports.
420: Anything typed at the second console will be concatenated to the first,
421: and vice-versa.
422: After the connection has been set up,
423: .Nm
424: does not really care which side is being used as a
425: .Sq server
426: and which side is being used as a
427: .Sq client .
428: The connection may be terminated using an
429: .Dv EOF
430: .Pq Sq ^D .
431: .Sh DATA TRANSFER
432: The example in the previous section can be expanded to build a
433: basic data transfer model.
434: Any information input into one end of the connection will be output
435: to the other end, and input and output can be easily captured in order to
436: emulate file transfer.
437: .Pp
438: Start by using
439: .Nm
440: to listen on a specific port, with output captured into a file:
441: .Pp
442: .Dl $ nc -l 1234 \*(Gt filename.out
443: .Pp
444: Using a second machine, connect to the listening
445: .Nm
446: process, feeding it the file which is to be transferred:
447: .Pp
1.66 jmc 448: .Dl $ nc -N host.example.com 1234 \*(Lt filename.in
1.28 jmc 449: .Pp
450: After the file has been transferred, the connection will close automatically.
451: .Sh TALKING TO SERVERS
452: It is sometimes useful to talk to servers
453: .Dq by hand
454: rather than through a user interface.
455: It can aid in troubleshooting,
456: when it might be necessary to verify what data a server is sending
457: in response to commands issued by the client.
458: For example, to retrieve the home page of a web site:
1.40 jmc 459: .Bd -literal -offset indent
1.55 guenther 460: $ printf "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
1.40 jmc 461: .Ed
1.28 jmc 462: .Pp
463: Note that this also displays the headers sent by the web server.
464: They can be filtered, using a tool such as
465: .Xr sed 1 ,
466: if necessary.
467: .Pp
468: More complicated examples can be built up when the user knows the format
469: of requests required by the server.
470: As another example, an email may be submitted to an SMTP server using:
471: .Bd -literal -offset indent
472: $ nc localhost 25 \*(Lt\*(Lt EOF
473: HELO host.example.com
1.44 jmc 474: MAIL FROM:\*(Ltuser@host.example.com\*(Gt
475: RCPT TO:\*(Ltuser2@host.example.com\*(Gt
1.28 jmc 476: DATA
477: Body of email.
478: \&.
479: QUIT
480: EOF
481: .Ed
482: .Sh PORT SCANNING
483: It may be useful to know which ports are open and running services on
484: a target machine.
485: The
486: .Fl z
487: flag can be used to tell
1.22 markus 488: .Nm
1.39 jmc 489: to report open ports,
490: rather than initiate a connection.
1.28 jmc 491: For example:
492: .Bd -literal -offset indent
1.39 jmc 493: $ nc -z host.example.com 20-30
1.28 jmc 494: Connection to host.example.com 22 port [tcp/ssh] succeeded!
495: Connection to host.example.com 25 port [tcp/smtp] succeeded!
496: .Ed
497: .Pp
498: The port range was specified to limit the search to ports 20 \- 30.
499: .Pp
500: Alternatively, it might be useful to know which server software
501: is running, and which versions.
502: This information is often contained within the greeting banners.
503: In order to retrieve these, it is necessary to first make a connection,
504: and then break the connection when the banner has been retrieved.
505: This can be accomplished by specifying a small timeout with the
506: .Fl w
507: flag, or perhaps by issuing a
508: .Qq Dv QUIT
509: command to the server:
510: .Bd -literal -offset indent
511: $ echo "QUIT" | nc host.example.com 20-30
512: SSH-1.99-OpenSSH_3.6.1p2
513: Protocol mismatch.
514: 220 host.example.com IMS SMTP Receiver Version 0.84 Ready
515: .Ed
1.1 deraadt 516: .Sh EXAMPLES
1.37 jmc 517: Open a TCP connection to port 42 of host.example.com, using port 31337 as
1.28 jmc 518: the source port, with a timeout of 5 seconds:
519: .Pp
1.37 jmc 520: .Dl $ nc -p 31337 -w 5 host.example.com 42
1.69 beck 521: .Pp
1.88 jsing 522: Open a TCP connection to port 443 of www.example.com, and negotiate TLS with
523: any supported TLS protocol version and "compat" ciphers:
524: .Pp
525: .Dl $ nc -cv -T protocols=all -T ciphers=compat www.example.com 443
526: .Pp
1.70 jmc 527: Open a TCP connection to port 443 of www.google.ca, and negotiate TLS.
1.88 jsing 528: Check for a different name in the certificate for validation:
1.69 beck 529: .Pp
1.88 jsing 530: .Dl $ nc -cv -e adsf.au.doubleclick.net www.google.ca 443
1.28 jmc 531: .Pp
1.37 jmc 532: Open a UDP connection to port 53 of host.example.com:
1.28 jmc 533: .Pp
1.37 jmc 534: .Dl $ nc -u host.example.com 53
1.28 jmc 535: .Pp
1.37 jmc 536: Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
1.28 jmc 537: IP for the local end of the connection:
538: .Pp
1.37 jmc 539: .Dl $ nc -s 10.1.2.3 host.example.com 42
1.28 jmc 540: .Pp
1.51 sobrado 541: Create and listen on a
1.52 sobrado 542: .Ux Ns -domain
1.57 jeremy 543: stream socket:
1.28 jmc 544: .Pp
545: .Dl $ nc -lU /var/tmp/dsocket
1.33 djm 546: .Pp
1.37 jmc 547: Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
1.38 jmc 548: port 8080.
549: This example could also be used by
550: .Xr ssh 1 ;
551: see the
552: .Cm ProxyCommand
553: directive in
554: .Xr ssh_config 5
555: for more information.
1.33 djm 556: .Pp
1.37 jmc 557: .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
1.42 djm 558: .Pp
559: The same example again, this time enabling proxy authentication with username
560: .Dq ruser
561: if the proxy requires it:
562: .Pp
563: .Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
1.1 deraadt 564: .Sh SEE ALSO
1.38 jmc 565: .Xr cat 1 ,
566: .Xr ssh 1
1.15 smart 567: .Sh AUTHORS
1.89 schwarze 568: Original implementation by
569: .An *Hobbit* Aq Mt hobbit@avian.org .
1.28 jmc 570: .br
571: Rewritten with IPv6 support by
1.63 schwarze 572: .An Eric Jackson Aq Mt ericj@monkey.org .
1.39 jmc 573: .Sh CAVEATS
1.60 lum 574: UDP port scans using the
1.39 jmc 575: .Fl uz
1.60 lum 576: combination of flags will always report success irrespective of
577: the target machine's state.
578: However,
579: in conjunction with a traffic sniffer either on the target machine
580: or an intermediary device,
581: the
582: .Fl uz
583: combination could be useful for communications diagnostics.
584: Note that the amount of UDP traffic generated may be limited either
585: due to hardware resources and/or configuration settings.
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