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man : netcat
File: netcat.info, Node: Top, Next: Overview, Prev: (dir), Up: (dir)
Netcat 0.7.1
************
This manual documents version 0.7.1 of GNU Netcat, the freely
available utility for network connections.
Copyright (C) 1996, 1997, 1998, 2000, 2001 Free Software Foundation,
Inc.
* Menu:
* Overview:: Features of netcat.
* Invoking:: Netcat command-line arguments.
* The Connect Mode:: Using netcat in the connect mode.
* The Listen Mode:: Using netcat in the listen mode.
* The Tunnel Mode:: Using netcat in the tunnel mode.
* Examples:: Examples of usage.
File: netcat.info, Node: Overview, Next: Invoking, Prev: Top, Up: Top
Overview
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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.
File: netcat.info, Node: Invoking, Next: The Connect Mode, Prev: Overview, Up: Top
Invoking
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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.
* Menu:
* Basic Startup Options::
* Protocol and Interface Options::
* Advanced Options::
File: netcat.info, Node: Basic Startup Options, Next: Protocol and Interface Options, Prev: Invoking, Up: Invoking
Basic Startup Options
=====================
`-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.
File: netcat.info, Node: Protocol and Interface Options, Next: Advanced Options, Prev: Basic Startup Options, Up: Invoking
Protocol and Interface Options
==============================
`-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.
File: netcat.info, Node: Advanced Options, Prev: Protocol and Interface Options, Up: Invoking
Advanced Options
================
`-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.
File: netcat.info, Node: The Connect Mode, Next: The Listen Mode, Prev: Invoking, Up: Top
The Connect Mode
****************
In the connect mode, netcat connects to a remote host and simply
links his stdin and stdout to the remote data stream.
Basic usage is:
netcat [options] hostname ports ...
The specified ports indicates actually all ports to connect to. With
normal operations netcat will sequentially (or randomly, if you
specified the `-r' option) connect to all the specified ports and links
stdin/stdout. No more than one port is used at once.
Particular options: In this mode, the `-p' option specifies the
source port for connection, but it should never be specified unless you
know what you are doing. The `-w' (timeout) option specifies how long
to wait for the connection to succeed (if the remote host connects but
doesn't send any data, the timeout DOESN'T apply).
File: netcat.info, Node: The Listen Mode, Next: The Tunnel Mode, Prev: The Connect Mode, Up: Top
The Listen Mode
***************
In listen mode, netcat stays idle listening on a port, specified by
the `-p' switch, until some remote host connects. At this point, the
basic behaviour is the same of the connect mode.
Basic usage is:
netcat -l -p port [remote_hostname] [remote_ports] ...
The remote hostname specifies which host is allowed to connect and
from which ports. Usually these parameters are not specified, but if
you want to sort out a special connection.
File: netcat.info, Node: The Tunnel Mode, Next: Examples, Prev: The Listen Mode, Up: Top
The Tunnel Mode
***************
The tunnel mode ...
File: netcat.info, Node: Examples, Prev: The Tunnel Mode, Up: Top
Examples
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under writing ...
netcat [options] hostname port [port] ...
netcat -l -p port [options] [hostname] [port] ...
netcat -L hostname:port -p port [options] ...
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