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The program 'netcat' is an advanced form of the Telnet command when used in a hackers hands. 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 can also be used as a network debugging and exploration tool, since it can create almost any kind of connection you would need and has several interesting built-in capabilities. Standard input is normally sent to the host, and anything that comes back across the connection is sent to standard output. This continues indefinitely, until the network side of the connection shuts down. Netcat can also function as a server, by listening for inbound connections on arbitrary ports and then doing the same reading and writing. 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.
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