App to spot a hurdle between a user and an http server

Question

I'd like to know if there is a tool that can be placed on a user's computer and given a url, can diagnose if and where a block is situated in accessing a web page (with a specified port). Sort of a suped-up Traceroute/Telnet hybrid.

The context: Heavily protected (all ports blocked by default, VPN, etc) intranet with an installation of Apache. A DNS which does resolve the url succesfully, but Apache logs no access attempts. There is another web service installed on another port (800), and it is opened and working. The server can reach itself with the URL, but users on other machines can't. Telnetting from user machine to server times out.

With a tool like this I could say if the blockage to port 80 is caused by:

• Some browser plugin or site blocker
• Firewall
• Proxy
• etc.

Answer 1

Use the well-known nmap utility to list open/closed ports on the remote server. If the port you are attempting to connect to is listed as closed, most probably a firewall is blocking it.

I recommand the use of Zenmap for beginners.

Answer 2

You can use a combination of client side tools like Fiddler (IE) or Firebug (FF) and the web server's logs to detect access. Blocked access will be noted on the client. HTTP errors will be noted on both sides.