NETCAT(1) Netcat NETCAT(1)NAMEnetcat - GNU Netcat Manual
SYNOPSISnetcat [options] hostname port [port] ...
netcat-l -p port [options] [hostname] [port] ...
netcat-L hostname:port -p port [options] ...
DESCRIPTION
Netcat is a simple Unix utility which reads and writes data across net‐
work 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 reli‐
able 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.
OPTIONS
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.
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 avail‐
able 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 net‐
cat 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.
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 host‐
names, 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 pos‐
sible 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 wait‐
ing forever. In both cases, no data is transfered.
This option is incompatible with the tunnel mode.
SEE ALSO
GNU Info entry for netcat.
AUTHOR
Originally written by Giovanni Giacobbi <giovanni@giacobbi.net>.
COPYRIGHT
Copyright (c) 2002 - 2004 Giovanni Giacobbi
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
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''.
GNU Netcat 0.7.1 2004-01-11 NETCAT(1)
