Bash/nc: netcat as a simple port scanner

gistfile1.txt

PORT SCANNING
     It may be useful to know which ports are open and running services on a target machine.  The -z flag can be used to tell nc to report open ports, rather than initiate a connection. Usually it's useful to turn on verbose output to stderr by use this
     option in conjunction with -v option.

     For example:

           $ nc -zv host.example.com 20-30
           Connection to host.example.com 22 port [tcp/ssh] succeeded!
           Connection to host.example.com 25 port [tcp/smtp] succeeded!

     The port range was specified to limit the search to ports 20 - 30, and is scanned by increasing order.

     You can also specify a list of ports to scan, for example:

           $ nc -zv host.example.com 80 20 22
           nc: connect to host.example.com 80 (tcp) failed: Connection refused
           nc: connect to host.example.com 20 (tcp) failed: Connection refused
           Connection to host.example.com port [tcp/ssh] succeeded!

     The ports are scanned by the order you given.

     Alternatively, it might be useful to know which server software is running, and which versions.  This information is often contained within the greeting banners.  In order to retrieve these, it is necessary to first make a connection, and then break the
     connection when the banner has been retrieved.  This can be accomplished by specifying a small timeout with the -w flag, or perhaps by issuing a "QUIT" command to the server:

           $ echo "QUIT" | nc host.example.com 20-30
           SSH-1.99-OpenSSH_3.6.1p2
           Protocol mismatch.
           220 host.example.com IMS SMTP Receiver Version 0.84 Ready

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