.\" $OpenBSD: nc.1,v 1.40 2005/07/17 19:18:47 jmc Exp $ .\" .\" Copyright (c) 1996 David Sacerdote .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd June 25, 2001 .Dt NC 1 .Os .Sh NAME .Nm nc .Nd arbitrary TCP and UDP connections and listens .Sh SYNOPSIS .Nm nc .Bk -words .Op Fl 46AcDCdhklnrtUuvz .Op Fl b Ar boundif .Op Fl i Ar interval .Op Fl p Ar source_port .Op Fl s Ar source_ip_address .Op Fl w Ar timeout .Op Fl X Ar proxy_protocol .Oo Xo .Fl x Ar proxy_address Ns Oo : Ns .Ar port Oc Oc .Xc .Op Ar hostname .Op Ar port Ns Bq Ar s .Ek .Sh DESCRIPTION The .Nm (or .Nm netcat ) utility is used for just about anything under the sun involving TCP or UDP. It can open TCP connections, send UDP packets, listen on arbitrary TCP and UDP ports, do port scanning, and deal with both IPv4 and IPv6. Unlike .Xr telnet 1 , .Nm scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as .Xr telnet 1 does with some. .Pp Common uses include: .Pp .Bl -bullet -offset indent -compact .It simple TCP proxies .It shell-script based HTTP clients and servers .It network daemon testing .It a SOCKS or HTTP ProxyCommand for .Xr ssh 1 .It and much, much more .El .Pp The options are as follows: .Bl -tag -width Ds .It Fl 4 Forces .Nm to use IPv4 addresses only. .It Fl 6 Forces .Nm to use IPv6 addresses only. .It Fl A Set SO_RECV_ANYIF on socket. .It Fl b Ar boundif Specifies the interface to bind the socket to. .It Fl c Send CRLF as line-ending .It Fl D Enable debugging on the socket. .It Fl C Forces .Nm not to use cellular data context. .It Fl d Do not attempt to read from stdin. .It Fl h Prints out .Nm help. .It Fl i Ar interval Specifies a delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports. .It Fl G Ar conntimeout TCP connection timeout in seconds. .It Fl H Ar keepidle Initial TCP keep alive timeout in seconds. .It Fl I Ar keepintvl Interval for repeating TCP keep alive timeouts in seconds. .It Fl J Ar keepcnt Number of times to repeat TCP keep alive packets. .It Fl k Forces .Nm to stay listening for another connection after its current connection is completed. It is an error to use this option without the .Fl l option. .It Fl l Used to specify that .Nm should listen for an incoming connection rather than initiate a connection to a remote host. It is an error to use this option in conjunction with the .Fl p , .Fl s , or .Fl z options. Additionally, any timeouts specified with the .Fl w option are ignored. .It Fl L Ar num_probes Number of probes to send to the peer before declaring that the peer is not reachable and generating an adpative timeout read/write event. .It Fl n Do not do any DNS or service lookups on any specified addresses, hostnames or ports. .It Fl p Ar source_port Specifies the source port .Nm should use, subject to privilege restrictions and availability. It is an error to use this option in conjunction with the .Fl l option. .It Fl r Specifies that source and/or destination ports should be chosen randomly instead of sequentially within a range or in the order that the system assigns them. .It Fl s Ar source_ip_address Specifies the IP of the interface which is used to send the packets. It is an error to use this option in conjunction with the .Fl l option. .It Fl t Causes .Nm to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use .Nm to script telnet sessions. .It Fl U Specifies to use Unix Domain Sockets. .It Fl u Use UDP instead of the default option of TCP. .It Fl v Have .Nm give more verbose output. .It Fl w Ar timeout If a connection and stdin are idle for more than .Ar timeout seconds, then the connection is silently closed. The .Fl w flag has no effect on the .Fl l option, i.e.\& .Nm will listen forever for a connection, with or without the .Fl w flag. The default is no timeout. .It Fl X Ar proxy_version Requests that .Nm should use the specified protocol when talking to the proxy server. Supported protocols are .Dq 4 (SOCKS v.4), .Dq 5 (SOCKS v.5) and .Dq connect (HTTPS proxy). If the protocol is not specified, SOCKS version 5 is used. .It Xo .Fl x Ar proxy_address Ns Oo : Ns .Ar port Oc .Xc Requests that .Nm should connect to .Ar hostname using a proxy at .Ar proxy_address and .Ar port . If .Ar port is not specified, the well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS). .It Fl z Specifies that .Nm should just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunction with the .Fl l option. .El .Pp .Ar hostname can be a numerical IP address or a symbolic hostname (unless the .Fl n option is given). In general, a hostname must be specified, unless the .Fl l option is given (in which case the local host is used). .Pp .Ar port Ns Op Ar s can be single integers or ranges. Ranges are in the form nn-mm. In general, a destination port must be specified, unless the .Fl U option is given (in which case a socket must be specified). .Sh CLIENT/SERVER MODEL It is quite simple to build a very basic client/server model using .Nm . On one console, start .Nm listening on a specific port for a connection. For example: .Pp .Dl $ nc -l 1234 .Pp .Nm is now listening on port 1234 for a connection. On a second console .Pq or a second machine , connect to the machine and port being listened on: .Pp .Dl $ nc 127.0.0.1 1234 .Pp There should now be a connection between the ports. Anything typed at the second console will be concatenated to the first, and vice-versa. After the connection has been set up, .Nm does not really care which side is being used as a .Sq server and which side is being used as a .Sq client . The connection may be terminated using an .Dv EOF .Pq Sq ^D . .Sh DATA TRANSFER The example in the previous section can be expanded to build a basic data transfer model. Any information input into one end of the connection will be output to the other end, and input and output can be easily captured in order to emulate file transfer. .Pp Start by using .Nm to listen on a specific port, with output captured into a file: .Pp .Dl $ nc -l 1234 \*(Gt filename.out .Pp Using a second machine, connect to the listening .Nm process, feeding it the file which is to be transferred: .Pp .Dl $ nc host.example.com 1234 \*(Lt filename.in .Pp After the file has been transferred, the connection will close automatically. .Sh TALKING TO SERVERS It is sometimes useful to talk to servers .Dq by hand rather than through a user interface. It can aid in troubleshooting, when it might be necessary to verify what data a server is sending in response to commands issued by the client. For example, to retrieve the home page of a web site: .Bd -literal -offset indent $ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80 .Ed .Pp Note that this also displays the headers sent by the web server. They can be filtered, using a tool such as .Xr sed 1 , if necessary. .Pp More complicated examples can be built up when the user knows the format of requests required by the server. As another example, an email may be submitted to an SMTP server using: .Bd -literal -offset indent $ nc localhost 25 \*(Lt\*(Lt EOF HELO host.example.com MAIL FROM: \*(Ltuser@host.example.com\*(Gt RCPT TO: \*(Ltuser2@host.example.com\*(Gt DATA Body of email. \&. QUIT EOF .Ed .Sh PORT SCANNING It may be useful to know which ports are open and running services on a target machine. The .Fl z flag can be used to tell .Nm to report open ports, rather than initiate a connection. For example: .Bd -literal -offset indent $ nc -z host.example.com 20-30 Connection to host.example.com 22 port [tcp/ssh] succeeded! Connection to host.example.com 25 port [tcp/smtp] succeeded! .Ed .Pp The port range was specified to limit the search to ports 20 \- 30. .Pp Alternatively, it might be useful to know which server software is running, and which versions. This information is often contained within the greeting banners. In order to retrieve these, it is necessary to first make a connection, and then break the connection when the banner has been retrieved. This can be accomplished by specifying a small timeout with the .Fl w flag, or perhaps by issuing a .Qq Dv QUIT command to the server: .Bd -literal -offset indent $ echo "QUIT" | nc host.example.com 20-30 SSH-1.99-OpenSSH_3.6.1p2 Protocol mismatch. 220 host.example.com IMS SMTP Receiver Version 0.84 Ready .Ed .Sh EXAMPLES Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds: .Pp .Dl $ nc -p 31337 -w 5 host.example.com 42 .Pp Open a UDP connection to port 53 of host.example.com: .Pp .Dl $ nc -u host.example.com 53 .Pp Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the IP for the local end of the connection: .Pp .Dl $ nc -s 10.1.2.3 host.example.com 42 .Pp Create and listen on a Unix Domain Socket: .Pp .Dl $ nc -lU /var/tmp/dsocket .Pp Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080. This example could also be used by .Xr ssh 1 ; see the .Cm ProxyCommand directive in .Xr ssh_config 5 for more information. .Pp .Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42 .Sh SEE ALSO .Xr cat 1 , .Xr ssh 1 .Sh AUTHORS Original implementation by *Hobbit* .Aq email@example.com . .br Rewritten with IPv6 support by .An Eric Jackson Aq firstname.lastname@example.org . .Sh CAVEATS UDP port scans will always succeed (i.e. report the port as open), rendering the .Fl uz combination of flags relatively useless.