Checking if a Port is Open

[Sean Knox]

To check if a port is accessible from the outside world, just telnet to 
the port. (i.e. telnet host 25)

To see what ports your machine is listening to, the unix command is 
"netstat tlu" to display all TCP and UDP listening ports. In the nagios 
world, you can use "check_tcp" or check_udp to see if a TCP/UDP port is open

Sean



Jake Colman wrote:

>My ISP has threatened to close the smtp port (25) to prevent commercial users
>from using the residential cable.  I am running sendmail and can use
>check_smtp to ensure that the server is up and running on my host. Is there
>anything I can monitor to ensure that sendmail can be accessed from the
>outside world?
>
>More generally, is there I can use to monitor whether specific ports are open
>to the outside and have nagios flag it if server on that port cannot be
>reached?
>
>
>  
>



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