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checking port open with a single `nc` (netcat) command: a comprehensive tutorial

this tutorial focuses on using the `nc` (netcat) command in linux (and similar unix-like systems like macos) to check if a port is open and listening on a remote server. we'll dive deep into various options and techniques, all achievable with a single command, making it a powerful and versatile tool for network diagnostics.

*what is `nc` (netcat)?*

netcat is a versatile networking utility often dubbed the "tcp/ip swiss army knife." it can:

*create tcp or udp connections:* connect to a specified host and port.
*listen on specified ports:* act as a simple server, waiting for incoming connections.
*transfer data:* send and receive data over the established connection.
*port scanning:* quickly check which ports are open on a target host.

our primary focus here is its port scanning capability, specifically the ability to determine if a port is open.

*the basic `nc` command structure for port checking*

the foundation of our port check command is:



let's break down the options:

*`nc`:* invokes the netcat program.
*`-v` (verbose):* provides more detailed output. without this, `nc` might not give any output unless a connection is established.
*`-z` (zero-i/o mode):* tells `nc` to simply scan for listening daemons without sending any data. this is crucial for port checking; we don't want to initiate a full connection, just determine if something is listening.
*`hostname or ip address`:* the domain name or ip address of the server you want to check. for example, `google.com` or `192.168.1.100`.
*`port number`:* the port number you want to test. for example, `80` (http), `443` (https), or `22` (ssh).

*example: checking if port 80 is open on google.com*



*interpreting the output*

the output from `nc` is straightforward:

*port is open:* you'll typically see a message like:

this confirms that a ...

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