Man1 - netcat.1

netcat - GNU Netcat Manual

netcat [options] hostname port [port] …

netcat -l -p port [options] [hostname] [port] …

netcat -L hostname:port -p port [options] …

Netcat is a simple Unix utility which reads and writes data across network connections, using TCP or UDP protocol. It is designed to be a reliable back-end tool that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities. Netcat, or nc as the original program was named, should have been supplied long ago as another one of those cryptic but standard Unix tools.

Netcat has three main modes of functionality. These are the connect mode, the listen mode, and the tunnel mode.

The most common mode is the connect mode, which for example allows the output of a locally called command to be redirected for example to a remote netcat listening or to any other kind of daemon waiting for a connection.

On the other hand, the listen mode can be used to obtain some kind of stream of data from a remote site.

The most new feature is the tunnel mode, which is a powerful and reliable mode that allows tunneling a remote site towards any other remote site, allowing to specify for example from which interface create the connection and from which port.

-V

–version

Display the version of netcat and exit.

-h

–help

Print a help message describing most common netcat’s command-line switches and a short description.

-v

–verbose

Prints status messages, usually needed for using netcat as user front-end. All messages are printed to stderr in order not to affect the data stream. Use this option double to get more messages.

-t

–tcp

Selects the TCP protocol, this is the default. It may be useful (see Tunnel Mode) to specify this option after for example the UDP option in order to allow a cross-protocol bridge between TCP and UDP.

-u

–udp

Selects the UDP protocol. See the –tcp option.

-p NUM

–local-port=NUM

Selects the local port. In listen and tunnel mode, it specifies which port to use for listening, while in connect mode it specifies the source port (the port from which originating the connection). If this option is not specified, the OS will assign a random available port.

-s ADDRESS

–source=ADDRESS

Specifies the source address used for creating sockets. In listen mode and tunnel mode this switch specifies the bound address, and it is generally a good idea not to specify this, which causes netcat to bind to a generic interface. In the connect mode, this switch is used to specify the source address for connecting to the outside world. Again, if it’s not specified a proper address for the destination route will be used.

-P NUM

–tunnel-port=NUM

Same as –port, but affects only the connect phase (thus this option has no effect in listen mode). This switch is useful in tunnel mode for specifying the source port for the connecting socket.

-S ADDRESS

–tunnel-source=ADDRESS

Same as –source, but affects only the connect phase (thus this has no effects in listen mode). This switch is useful in tunnel mode for specifying the source address for the connecting socket.

-i SECS

–interval SECS

sets the buffering output delay time. This affects all the current modes and makes the connection sock to buffer outgoing data. This means that in tunnel mode everything received from the listening socket is buffered for the connect socket.

-n

–dont-resolve

Don’t do DNS lookups on any of the specified addresses or hostnames, or names of port numbers from /etc/services.

-r

–randomize

Randomizes the target remote ports ranges. If more than one range is specified it will randomize the ports in the whole global range.

-w

–wait=SECS

Specifies the starting inactivity delay after which netcat will exit with an error status. In connect mode and in tunnel mode this specifies the timeout for the connecting socket, while in listen mode it specifies the time to wait for a VALID incoming connection (see listen mode).

-T

–telnet

Answers the telnet codes as described in RFC0854. This makes possible to use netcat to script telnet sessions. The incoming telnet codes are parsed inside the receiving queue and are stripped off before forwarding the data as they were never received, so the application doesn’t have to parse the codes itself (this behaviour can be disabled at compile time with –enable-oldtelnet or with –enable-compat).

-z

–zero

Sets the zero I/O flag for the selected mode. In connect mode it means that as soon as the port is open it is immediately shutdown and closed. This may be useful for probing or scanning (even if there are faster portscanners out there, but this may be useful for scripting purposes). In listen mode, it makes netcat refusing all the incoming connections thus running in timeout (if set), or waiting forever. In both cases, no data is transfered. This option is incompatible with the tunnel mode.

GNU Info entry for netcat.

Originally written by Giovanni Giacobbi <giovanni@giacobbi.net>.

Copyright (c) 2002 - 2004 Giovanni Giacobbi

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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being ``GNU General Public License’’ and ``GNU Free Documentation License’’, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License’’.