1.\" $OpenBSD: nc.1,v 1.47 2008/05/06 16:21:03 jmc 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 $Mdocdate: May 6 2008 $ 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 46DdEhklnorStUuvz 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_ip_address 48.Op Fl T Ar ToS 49.Op Fl w Ar timeout 50.Op Fl X Ar proxy_protocol 51.Oo Xo 52.Fl x Ar proxy_address Ns Oo : Ns 53.Ar port Oc Oc 54.Xc 55.Op Ar hostname 56.Op Ar port Ns Bq Ar s 57.Ek 58.Sh DESCRIPTION 59The 60.Nm 61(or 62.Nm netcat ) 63utility is used for just about anything under the sun involving TCP 64or UDP. 65It can open TCP connections, send UDP packets, listen on arbitrary 66TCP and UDP ports, do port scanning, and deal with both IPv4 and 67IPv6. 68Unlike 69.Xr telnet 1 , 70.Nm 71scripts nicely, and separates error messages onto standard error instead 72of sending them to standard output, as 73.Xr telnet 1 74does with some. 75.Pp 76Common uses include: 77.Pp 78.Bl -bullet -offset indent -compact 79.It 80simple TCP proxies 81.It 82shell-script based HTTP clients and servers 83.It 84network daemon testing 85.It 86a SOCKS or HTTP ProxyCommand for 87.Xr ssh 1 88.It 89and much, much more 90.El 91.Pp 92The options are as follows: 93.Bl -tag -width Ds 94.It Fl 4 95Forces 96.Nm 97to use IPv4 addresses only. 98.It Fl 6 99Forces 100.Nm 101to use IPv6 addresses only. 102.It Fl D 103Enable debugging on the socket. 104.It Fl d 105Do not attempt to read from stdin. 106.It Fl E 107Shortcut for 108.Qo 109.Li "-e 'in ipsec esp/transport//require'" 110.Li "-e 'out ipsec esp/transport//require'" 111.Qc , 112which enables IPsec ESP transport mode in both 113directions. 114.It Fl e 115If IPsec support is available, then one can specify the IPsec policies 116to be used using the syntax described in 117.Xr ipsec_set_policy 3 . 118This flag can be specified up to two times, as typically one policy for 119each direction is needed. 120.It Fl h 121Prints out 122.Nm 123help. 124.It Fl I Ar length 125Specifies the size of the TCP receive buffer. 126.It Fl i Ar interval 127Specifies a delay time interval between lines of text sent and received. 128Also causes a delay time between connections to multiple ports. 129.It Fl k 130Forces 131.Nm 132to stay listening for another connection after its current connection 133is completed. 134It is an error to use this option without the 135.Fl l 136option. 137.It Fl l 138Used to specify that 139.Nm 140should listen for an incoming connection rather than initiate a 141connection to a remote host. 142It is an error to use this option in conjunction with the 143.Fl p , 144.Fl s , 145or 146.Fl z 147options. 148Additionally, any timeouts specified with the 149.Fl w 150option are ignored. 151.It Fl n 152Do not do any DNS or service lookups on any specified addresses, 153hostnames or ports. 154.It Fl -no-tcpopt 155Disables the use of TCP options on the socket, by setting the boolean 156TCP_NOOPT 157socket option. 158.It Fl O Ar length 159Specifies the size of the TCP send buffer. 160When 161.It Fl o 162.Dq Once-only mode . 163By default, 164.Nm 165does not terminate on EOF condition on input, 166but continues until the network side has been closed down. 167Specifying 168.Fl o 169will make it terminate on EOF as well. 170.It Fl P Ar proxy_username 171Specifies a username to present to a proxy server that requires authentication. 172If no username is specified then authentication will not be attempted. 173Proxy authentication is only supported for HTTP CONNECT proxies at present. 174.It Fl p Ar source_port 175Specifies the source port 176.Nm 177should use, subject to privilege restrictions and availability. 178It is an error to use this option in conjunction with the 179.Fl l 180option. 181.It Fl r 182Specifies that source and/or destination ports should be chosen randomly 183instead of sequentially within a range or in the order that the system 184assigns them. 185.It Fl S 186Enables the RFC 2385 TCP MD5 signature option. 187.It Fl s Ar source_ip_address 188Specifies the IP of the interface which is used to send the packets. 189It is an error to use this option in conjunction with the 190.Fl l 191option. 192.It Fl T Ar ToS 193Specifies IP Type of Service (ToS) for the connection. 194Valid values are the tokens 195.Dq lowdelay , 196.Dq throughput , 197.Dq reliability , 198or an 8-bit hexadecimal value preceded by 199.Dq 0x . 200.It Fl t 201Causes 202.Nm 203to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. 204This makes it possible to use 205.Nm 206to script telnet sessions. 207.It Fl U 208Specifies to use Unix Domain Sockets. 209.It Fl u 210Use UDP instead of the default option of TCP. 211.It Fl v 212Have 213.Nm 214give more verbose output. 215.It Fl w Ar timeout 216If a connection and stdin are idle for more than 217.Ar timeout 218seconds, then the connection is silently closed. 219The 220.Fl w 221flag has no effect on the 222.Fl l 223option, i.e.\& 224.Nm 225will listen forever for a connection, with or without the 226.Fl w 227flag. 228The default is no timeout. 229.It Fl X Ar proxy_protocol 230Requests that 231.Nm 232should use the specified protocol when talking to the proxy server. 233Supported protocols are 234.Dq 4 235(SOCKS v.4), 236.Dq 5 237(SOCKS v.5) 238and 239.Dq connect 240(HTTPS proxy). 241If the protocol is not specified, SOCKS version 5 is used. 242.It Xo 243.Fl x Ar proxy_address Ns Oo : Ns 244.Ar port Oc 245.Xc 246Requests that 247.Nm 248should connect to 249.Ar hostname 250using a proxy at 251.Ar proxy_address 252and 253.Ar port . 254If 255.Ar port 256is not specified, the well-known port for the proxy protocol is used (1080 257for SOCKS, 3128 for HTTPS). 258.It Fl z 259Specifies that 260.Nm 261should just scan for listening daemons, without sending any data to them. 262It is an error to use this option in conjunction with the 263.Fl l 264option. 265.El 266.Pp 267.Ar hostname 268can be a numerical IP address or a symbolic hostname 269(unless the 270.Fl n 271option is given). 272In general, a hostname must be specified, 273unless the 274.Fl l 275option is given 276(in which case the local host is used). 277.Pp 278.Ar port Ns Op Ar s 279can be single integers or ranges. 280Ranges are in the form nn-mm. 281In general, 282a destination port must be specified, 283unless the 284.Fl U 285option is given 286(in which case a socket must be specified). 287.Sh CLIENT/SERVER MODEL 288It is quite simple to build a very basic client/server model using 289.Nm . 290On one console, start 291.Nm 292listening on a specific port for a connection. 293For example: 294.Pp 295.Dl $ nc -l 1234 296.Pp 297.Nm 298is now listening on port 1234 for a connection. 299On a second console 300.Pq or a second machine , 301connect to the machine and port being listened on: 302.Pp 303.Dl $ nc 127.0.0.1 1234 304.Pp 305There should now be a connection between the ports. 306Anything typed at the second console will be concatenated to the first, 307and vice-versa. 308After the connection has been set up, 309.Nm 310does not really care which side is being used as a 311.Sq server 312and which side is being used as a 313.Sq client . 314The connection may be terminated using an 315.Dv EOF 316.Pq Sq ^D . 317.Sh DATA TRANSFER 318The example in the previous section can be expanded to build a 319basic data transfer model. 320Any information input into one end of the connection will be output 321to the other end, and input and output can be easily captured in order to 322emulate file transfer. 323.Pp 324Start by using 325.Nm 326to listen on a specific port, with output captured into a file: 327.Pp 328.Dl $ nc -l 1234 \*(Gt filename.out 329.Pp 330Using a second machine, connect to the listening 331.Nm 332process, feeding it the file which is to be transferred: 333.Pp 334.Dl $ nc host.example.com 1234 \*(Lt filename.in 335.Pp 336After the file has been transferred, the connection will close automatically. 337.Sh TALKING TO SERVERS 338It is sometimes useful to talk to servers 339.Dq by hand 340rather than through a user interface. 341It can aid in troubleshooting, 342when it might be necessary to verify what data a server is sending 343in response to commands issued by the client. 344For example, to retrieve the home page of a web site: 345.Bd -literal -offset indent 346$ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80 347.Ed 348.Pp 349Note that this also displays the headers sent by the web server. 350They can be filtered, using a tool such as 351.Xr sed 1 , 352if necessary. 353.Pp 354More complicated examples can be built up when the user knows the format 355of requests required by the server. 356As another example, an email may be submitted to an SMTP server using: 357.Bd -literal -offset indent 358$ nc localhost 25 \*(Lt\*(Lt EOF 359HELO host.example.com 360MAIL FROM:\*(Ltuser@host.example.com\*(Gt 361RCPT TO:\*(Ltuser2@host.example.com\*(Gt 362DATA 363Body of email. 364\&. 365QUIT 366EOF 367.Ed 368.Sh PORT SCANNING 369It may be useful to know which ports are open and running services on 370a target machine. 371The 372.Fl z 373flag can be used to tell 374.Nm 375to report open ports, 376rather than initiate a connection. 377For example: 378.Bd -literal -offset indent 379$ nc -z host.example.com 20-30 380Connection to host.example.com 22 port [tcp/ssh] succeeded! 381Connection to host.example.com 25 port [tcp/smtp] succeeded! 382.Ed 383.Pp 384The port range was specified to limit the search to ports 20 \- 30. 385.Pp 386Alternatively, it might be useful to know which server software 387is running, and which versions. 388This information is often contained within the greeting banners. 389In order to retrieve these, it is necessary to first make a connection, 390and then break the connection when the banner has been retrieved. 391This can be accomplished by specifying a small timeout with the 392.Fl w 393flag, or perhaps by issuing a 394.Qq Dv QUIT 395command to the server: 396.Bd -literal -offset indent 397$ echo "QUIT" | nc host.example.com 20-30 398SSH-1.99-OpenSSH_3.6.1p2 399Protocol mismatch. 400220 host.example.com IMS SMTP Receiver Version 0.84 Ready 401.Ed 402.Sh EXAMPLES 403Open a TCP connection to port 42 of host.example.com, using port 31337 as 404the source port, with a timeout of 5 seconds: 405.Pp 406.Dl $ nc -p 31337 -w 5 host.example.com 42 407.Pp 408Open a UDP connection to port 53 of host.example.com: 409.Pp 410.Dl $ nc -u host.example.com 53 411.Pp 412Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the 413IP for the local end of the connection: 414.Pp 415.Dl $ nc -s 10.1.2.3 host.example.com 42 416.Pp 417Open a TCP connection to port 42 of host.example.com using IPsec ESP for 418incoming and outgoing traffic. 419.Pp 420.Dl $ nc -E host.example.com 42 421.Pp 422Open a TCP connection to port 42 of host.example.com using IPsec ESP for 423outgoing traffic only. 424.Pp 425.Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42 426.Pp 427Create and listen on a Unix Domain Socket: 428.Pp 429.Dl $ nc -lU /var/tmp/dsocket 430.Pp 431Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, 432port 8080. 433This example could also be used by 434.Xr ssh 1 ; 435see the 436.Cm ProxyCommand 437directive in 438.Xr ssh_config 5 439for more information. 440.Pp 441.Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42 442.Pp 443The same example again, this time enabling proxy authentication with username 444.Dq ruser 445if the proxy requires it: 446.Pp 447.Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42 448.Sh SEE ALSO 449.Xr cat 1 , 450.Xr ssh 1 , 451.Xr tcp 4 452.Sh AUTHORS 453Original implementation by *Hobbit* 454.Aq hobbit@avian.org . 455.br 456Rewritten with IPv6 support by 457.An Eric Jackson Aq ericj@monkey.org . 458.Sh CAVEATS 459UDP port scans will always succeed 460(i.e. report the port as open), 461rendering the 462.Fl uz 463combination of flags relatively useless. 464