Using telnet, you can make an educated guess whether a port on a remote host is open, closed or blocked by a firewall by the error message it throws. If you get no error message, the port is open. If you get an error like the following: Unable to connect to remote host: No route to host you are almost certainly seeing a blocked (by firewall or whatever) port. If you get an error like the following: Unable to connect to remote host: Connection timed out the port is almost certainly a closed port.
In a recent issue of the MSDN magazine (Yes I do read Microsoft publications!), I read a short but very interesting article by David Platt entitled When Security Doesn’t Make Sense. Here is an excerpt from that article: According to Herley, users who ignore our security instructions are being rational from their point of view. They subconsciously calculate that the constant efforts we demand of them are greater than the infrequent (albeit larger) losses to them if they don’t follow our instructions. They then rationally decide to ignore us. Herley writes: “Consider an exploit that affects 1 percent of users