Checking if a Port is Open

[Sean Knox]

Ooops, the syntax for netstat is "netstat -tlu".

sk

Sean Knox wrote:

> 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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