1.\" $OpenBSD: nc.1,v 1.57 2011/01/09 22:16:46 jeremy Exp $ 2.\" 3.\" Copyright (c) 1996 David Sacerdote 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. The name of the author may not be used to endorse or promote products 15.\" derived from this software without specific prior written permission 16.\" 17.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 18.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 19.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 20.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 21.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 22.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 26.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27.\" 28.\" $FreeBSD$ 29.\" 30.Dd January 8, 2011 31.Dt NC 1 32.Os 33.Sh NAME 34.Nm nc 35.Nd arbitrary TCP and UDP connections and listens 36.Sh SYNOPSIS 37.Nm nc 38.Bk -words 39.Op Fl 46DdEhklnrStUuvz 40.Op Fl e Ar IPsec_policy 41.Op Fl I Ar length 42.Op Fl i Ar interval 43.Op Fl -no-tcpopt 44.Op Fl O Ar length 45.Op Fl P Ar proxy_username 46.Op Fl p Ar source_port 47.Op Fl s Ar source 48.Op Fl T Ar ToS 49.Op Fl V Ar rtable 50.Op Fl w Ar timeout 51.Op Fl X Ar proxy_protocol 52.Oo Xo 53.Fl x Ar proxy_address Ns Oo : Ns 54.Ar port Oc 55.Xc Oc 56.Op Ar destination 57.Op Ar port 58.Ek 59.Sh DESCRIPTION 60The 61.Nm 62(or 63.Nm netcat ) 64utility is used for just about anything under the sun involving TCP, 65UDP, or 66.Ux Ns -domain 67sockets. 68It can open TCP connections, send UDP packets, listen on arbitrary 69TCP and UDP ports, do port scanning, and deal with both IPv4 and 70IPv6. 71Unlike 72.Xr telnet 1 , 73.Nm 74scripts nicely, and separates error messages onto standard error instead 75of sending them to standard output, as 76.Xr telnet 1 77does with some. 78.Pp 79Common uses include: 80.Pp 81.Bl -bullet -offset indent -compact 82.It 83simple TCP proxies 84.It 85shell-script based HTTP clients and servers 86.It 87network daemon testing 88.It 89a SOCKS or HTTP ProxyCommand for 90.Xr ssh 1 91.It 92and much, much more 93.El 94.Pp 95The options are as follows: 96.Bl -tag -width Ds 97.It Fl 4 98Forces 99.Nm 100to use IPv4 addresses only. 101.It Fl 6 102Forces 103.Nm 104to use IPv6 addresses only. 105.It Fl D 106Enable debugging on the socket. 107.It Fl d 108Do not attempt to read from stdin. 109.It Fl E 110Shortcut for 111.Qo 112.Li "-e 'in ipsec esp/transport//require'" 113.Li "-e 'out ipsec esp/transport//require'" 114.Qc , 115which enables IPsec ESP transport mode in both 116directions. 117.It Fl e 118If IPsec support is available, then one can specify the IPsec policies 119to be used using the syntax described in 120.Xr ipsec_set_policy 3 . 121This flag can be specified up to two times, as typically one policy for 122each direction is needed. 123.It Fl h 124Prints out 125.Nm 126help. 127.It Fl I Ar length 128Specifies the size of the TCP receive buffer. 129.It Fl i Ar interval 130Specifies a delay time interval between lines of text sent and received. 131Also causes a delay time between connections to multiple ports. 132.It Fl k 133Forces 134.Nm 135to stay listening for another connection after its current connection 136is completed. 137It is an error to use this option without the 138.Fl l 139option. 140.It Fl l 141Used to specify that 142.Nm 143should listen for an incoming connection rather than initiate a 144connection to a remote host. 145It is an error to use this option in conjunction with the 146.Fl p , 147.Fl s , 148or 149.Fl z 150options. 151Additionally, any timeouts specified with the 152.Fl w 153option are ignored. 154.It Fl n 155Do not do any DNS or service lookups on any specified addresses, 156hostnames or ports. 157.It Fl -no-tcpopt 158Disables the use of TCP options on the socket, by setting the boolean 159TCP_NOOPT 160socket option. 161.It Fl O Ar length 162Specifies the size of the TCP send buffer. 163.It Fl P Ar proxy_username 164Specifies a username to present to a proxy server that requires authentication. 165If no username is specified then authentication will not be attempted. 166Proxy authentication is only supported for HTTP CONNECT proxies at present. 167.It Fl p Ar source_port 168Specifies the source port 169.Nm 170should use, subject to privilege restrictions and availability. 171It is an error to use this option in conjunction with the 172.Fl l 173option. 174.It Fl r 175Specifies that source and/or destination ports should be chosen randomly 176instead of sequentially within a range or in the order that the system 177assigns them. 178.It Fl S 179Enables the RFC 2385 TCP MD5 signature option. 180.It Fl s Ar source 181Specifies the IP of the interface which is used to send the packets. 182For 183.Ux Ns -domain 184datagram sockets, specifies the local temporary socket file 185to create and use so that datagrams can be received. 186It is an error to use this option in conjunction with the 187.Fl l 188option. 189.It Fl T Ar ToS 190Specifies IP Type of Service (ToS) for the connection. 191Valid values are the tokens 192.Dq lowdelay , 193.Dq throughput , 194.Dq reliability , 195or an 8-bit hexadecimal value preceded by 196.Dq 0x . 197.It Fl t 198Causes 199.Nm 200to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. 201This makes it possible to use 202.Nm 203to script telnet sessions. 204.It Fl U 205Specifies to use 206.Ux Ns -domain 207sockets. 208.It Fl u 209Use UDP instead of the default option of TCP. 210For 211.Ux Ns -domain 212sockets, use a datagram socket instead of a stream socket. 213If a 214.Ux Ns -domain 215socket is used, a temporary receiving socket is created in 216.Pa /tmp 217unless the 218.Fl s 219flag is given. 220.It Fl V Ar rtable 221Set the routing table 222.Pq Dq FIB 223to be used. 224The default is 0. 225.It Fl v 226Have 227.Nm 228give more verbose output. 229.It Fl w Ar timeout 230If a connection and stdin are idle for more than 231.Ar timeout 232seconds, then the connection is silently closed. 233The 234.Fl w 235flag has no effect on the 236.Fl l 237option, i.e.\& 238.Nm 239will listen forever for a connection, with or without the 240.Fl w 241flag. 242The default is no timeout. 243.It Fl X Ar proxy_protocol 244Requests that 245.Nm 246should use the specified protocol when talking to the proxy server. 247Supported protocols are 248.Dq 4 249(SOCKS v.4), 250.Dq 5 251(SOCKS v.5) 252and 253.Dq connect 254(HTTPS proxy). 255If the protocol is not specified, SOCKS version 5 is used. 256.It Xo 257.Fl x Ar proxy_address Ns Oo : Ns 258.Ar port Oc 259.Xc 260Requests that 261.Nm 262should connect to 263.Ar destination 264using a proxy at 265.Ar proxy_address 266and 267.Ar port . 268If 269.Ar port 270is not specified, the well-known port for the proxy protocol is used (1080 271for SOCKS, 3128 for HTTPS). 272.It Fl z 273Specifies that 274.Nm 275should just scan for listening daemons, without sending any data to them. 276It is an error to use this option in conjunction with the 277.Fl l 278option. 279.El 280.Pp 281.Ar destination 282can be a numerical IP address or a symbolic hostname 283(unless the 284.Fl n 285option is given). 286In general, a destination must be specified, 287unless the 288.Fl l 289option is given 290(in which case the local host is used). 291For 292.Ux Ns -domain 293sockets, a destination is required and is the socket path to connect to 294(or listen on if the 295.Fl l 296option is given). 297.Pp 298.Ar port 299can be a single integer or a range of ports. 300Ranges are in the form nn-mm. 301In general, 302a destination port must be specified, 303unless the 304.Fl U 305option is given. 306.Sh CLIENT/SERVER MODEL 307It is quite simple to build a very basic client/server model using 308.Nm . 309On one console, start 310.Nm 311listening on a specific port for a connection. 312For example: 313.Pp 314.Dl $ nc -l 1234 315.Pp 316.Nm 317is now listening on port 1234 for a connection. 318On a second console 319.Pq or a second machine , 320connect to the machine and port being listened on: 321.Pp 322.Dl $ nc 127.0.0.1 1234 323.Pp 324There should now be a connection between the ports. 325Anything typed at the second console will be concatenated to the first, 326and vice-versa. 327After the connection has been set up, 328.Nm 329does not really care which side is being used as a 330.Sq server 331and which side is being used as a 332.Sq client . 333The connection may be terminated using an 334.Dv EOF 335.Pq Sq ^D . 336.Sh DATA TRANSFER 337The example in the previous section can be expanded to build a 338basic data transfer model. 339Any information input into one end of the connection will be output 340to the other end, and input and output can be easily captured in order to 341emulate file transfer. 342.Pp 343Start by using 344.Nm 345to listen on a specific port, with output captured into a file: 346.Pp 347.Dl $ nc -l 1234 \*(Gt filename.out 348.Pp 349Using a second machine, connect to the listening 350.Nm 351process, feeding it the file which is to be transferred: 352.Pp 353.Dl $ nc host.example.com 1234 \*(Lt filename.in 354.Pp 355After the file has been transferred, the connection will close automatically. 356.Sh TALKING TO SERVERS 357It is sometimes useful to talk to servers 358.Dq by hand 359rather than through a user interface. 360It can aid in troubleshooting, 361when it might be necessary to verify what data a server is sending 362in response to commands issued by the client. 363For 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 80 366.Ed 367.Pp 368Note that this also displays the headers sent by the web server. 369They can be filtered, using a tool such as 370.Xr sed 1 , 371if necessary. 372.Pp 373More complicated examples can be built up when the user knows the format 374of requests required by the server. 375As another example, an email may be submitted to an SMTP server using: 376.Bd -literal -offset indent 377$ nc localhost 25 \*(Lt\*(Lt EOF 378HELO host.example.com 379MAIL FROM:\*(Ltuser@host.example.com\*(Gt 380RCPT TO:\*(Ltuser2@host.example.com\*(Gt 381DATA 382Body of email. 383\&. 384QUIT 385EOF 386.Ed 387.Sh PORT SCANNING 388It may be useful to know which ports are open and running services on 389a target machine. 390The 391.Fl z 392flag can be used to tell 393.Nm 394to report open ports, 395rather than initiate a connection. 396For example: 397.Bd -literal -offset indent 398$ nc -z host.example.com 20-30 399Connection to host.example.com 22 port [tcp/ssh] succeeded! 400Connection to host.example.com 25 port [tcp/smtp] succeeded! 401.Ed 402.Pp 403The port range was specified to limit the search to ports 20 \- 30. 404.Pp 405Alternatively, it might be useful to know which server software 406is running, and which versions. 407This information is often contained within the greeting banners. 408In order to retrieve these, it is necessary to first make a connection, 409and then break the connection when the banner has been retrieved. 410This can be accomplished by specifying a small timeout with the 411.Fl w 412flag, or perhaps by issuing a 413.Qq Dv QUIT 414command to the server: 415.Bd -literal -offset indent 416$ echo "QUIT" | nc host.example.com 20-30 417SSH-1.99-OpenSSH_3.6.1p2 418Protocol mismatch. 419220 host.example.com IMS SMTP Receiver Version 0.84 Ready 420.Ed 421.Sh EXAMPLES 422Open a TCP connection to port 42 of host.example.com, using port 31337 as 423the source port, with a timeout of 5 seconds: 424.Pp 425.Dl $ nc -p 31337 -w 5 host.example.com 42 426.Pp 427Open a UDP connection to port 53 of host.example.com: 428.Pp 429.Dl $ nc -u host.example.com 53 430.Pp 431Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the 432IP for the local end of the connection: 433.Pp 434.Dl $ nc -s 10.1.2.3 host.example.com 42 435.Pp 436Open a TCP connection to port 42 of host.example.com using IPsec ESP for 437incoming and outgoing traffic. 438.Pp 439.Dl $ nc -E host.example.com 42 440.Pp 441Open a TCP connection to port 42 of host.example.com using IPsec ESP for 442outgoing traffic only. 443.Pp 444.Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42 445.Pp 446Create and listen on a 447.Ux Ns -domain 448stream socket: 449.Pp 450.Dl $ nc -lU /var/tmp/dsocket 451.Pp 452Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, 453port 8080. 454This example could also be used by 455.Xr ssh 1 ; 456see the 457.Cm ProxyCommand 458directive in 459.Xr ssh_config 5 460for more information. 461.Pp 462.Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42 463.Pp 464The same example again, this time enabling proxy authentication with username 465.Dq ruser 466if the proxy requires it: 467.Pp 468.Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42 469.Sh EXIT STATUS 470.Ex -std 471.Sh SEE ALSO 472.Xr cat 1 , 473.Xr setfib 1 , 474.Xr ssh 1 , 475.Xr tcp 4 476.Sh AUTHORS 477Original implementation by *Hobbit* 478.Aq hobbit@avian.org . 479.br 480Rewritten with IPv6 support by 481.An Eric Jackson Aq ericj@monkey.org . 482.Sh CAVEATS 483UDP port scans will always succeed 484(i.e. report the port as open), 485rendering the 486.Fl uz 487combination of flags relatively useless. 488