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PROGRAM:

NAME

nc - TCP/IP swiss army knife

SYNOPSIS

nc [-options] hostname port[s] [ports] ...
nc -l -p port [-options] [hostname] [port]

DESCRIPTION

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 actual program is named, should have been supplied long ago as another one
of those cryptic but standard Unix tools.

In the simplest usage, "nc host port" creates a TCP connection to the given port on the
given target host. Your standard input is then sent to the host, and anything that comes
back across the connection is sent to your standard output. This continues indefinitely,
until the network side of the connection shuts down. Note that this behavior is different
from most other applications which shut everything down and exit after an end-of-file on
the standard input.

Netcat can also function as a server, by listening for inbound connections on arbitrary
ports and then doing the same reading and writing. With minor limitations, netcat doesn't
really care if it runs in "client" or "server" mode -- it still shovels data back and
forth until there isn't any more left. In either mode, shutdown can be forced after a
configurable time of inactivity on the network side.

And it can do this via UDP too, so netcat is possibly the "udp telnet-like" application
you always wanted for testing your UDP-mode servers. UDP, as the "U" implies, gives less
reliable data transmission than TCP connections and some systems may have trouble sending
large amounts of data that way, but it's still a useful capability to have.

You may be asking "why not just use telnet to connect to arbitrary ports?" Valid question,
and here are some reasons. Telnet has the "standard input EOF" problem, so one must
introduce calculated delays in driving scripts to allow network output to finish. This is
the main reason netcat stays running until the *network* side closes. Telnet also will
not transfer arbitrary binary data, because certain characters are interpreted as telnet
options and are thus removed from the data stream. Telnet also emits some of its
diagnostic messages to standard output, where netcat keeps such things religiously
separated from its *output* and will never modify any of the real data in transit unless
you *really* want it to. And of course telnet is incapable of listening for inbound
connections, or using UDP instead. Netcat doesn't have any of these limitations, is much
smaller and faster than telnet, and has many other advantages.

OPTIONS

-c string specify shell commands to exec after connect (use with caution). The string
is passed to /bin/sh -c for execution. See the -e option if you don't have a
working /bin/sh (Note that POSIX-conformant system must have one).

-e filename specify filename to exec after connect (use with caution). See the -c option
for enhanced functionality.

-g gateway source-routing hop point[s], up to 8

-G num source-routing pointer: 4, 8, 12, ...

-h display help

-i secs delay interval for lines sent, ports scanned

-l listen mode, for inbound connects

-n numeric-only IP addresses, no DNS

-o file hex dump of traffic

-p port local port number (port numbers can be individual or ranges: lo-hi
[inclusive])

-q seconds after EOF on stdin, wait the specified number of seconds and then quit. If
seconds is negative, wait forever.

-b allow UDP broadcasts

-r randomize local and remote ports

-s addr local source address

-t enable telnet negotiation

-u UDP mode

-v verbose [use twice to be more verbose]

-w secs timeout for connects and final net reads

-C Send CRLF as line-ending

-z zero-I/O mode [used for scanning]

-T type set TOS flag (type may be one of "Minimize-Delay", "Maximize-Throughput",
"Maximize-Reliability", or "Minimize-Cost".)

COPYRIGHT

Netcat is entirely my own creation, although plenty of other code was used as examples.
It is freely given away to the Internet community in the hope that it will be useful, with
no restrictions except giving credit where it is due. No GPLs, Berkeley copyrights or any
of that nonsense. The author assumes NO responsibility for how anyone uses it. If netcat
makes you rich somehow and you're feeling generous, mail me a check. If you are
affiliated in any way with Microsoft Network, get a life. Always ski in control.
Comments, questions, and patches to [email protected].

NOTES

Some port names in /etc/services contain hyphens -- netcat currently will not correctly
parse those unless you escape the hyphens with backslashes (e.g. "netcat localhost
'ftp\-data'").

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