[lug] How does ping work? How to diagnose firewall problem?

D. Stimits stimits at comcast.net
Wed Jul 26 21:03:34 MDT 2006

siegfried wrote:

>I went to a friend's house and thought I opened up port 22 and 1194 on his
>router. Since he does not have a fixed IP address, I went to
>http://www.grc.com/x/ne.dll?rh1dkyd2 and wrote down the IP address while it
>did a port scan on us. 
>Unfortunately, the port scan stopped at 1056 and I never could verify that
>port 1194 was open. 
>So now I am home and I'm trying to ping his IP address. I suppose I could
>have become very unlucky and he got a new IP address from his DHCP server
>while I was driving home.
>What is required for ping to work? Does it need a port open? Which port?

It has its own protocol, ICMP. His machine must respond to ICMP echo 
requests, no port required.

If the port you're wondering about is TCP and not UDP, you can just 
telnet to it. Should the port not have a listener, it'll dump you and 
say connection refused. If the port accepts you in any other way, you 
found an open port. You can test it for example on a web server. For 
example, if you have a localhost web server:
telnet localhost 80
(for port 80)

After that you don't care, you can close the terminal if you don't know 
the language/protocol used on the port.

D. Stimits, stimits AT comcast DOT net

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