What configuration will allow another computer in the internet to probe my computer's ports?

Question

I've just tested my computer with Gibson's port scan test:

The test results stated that my system is "uncommon". It also said that my computer deliberately chose not to return. But I did not do any settings with my computer so I was wondering what is the reason that the probing tests had passed?

Or rather, what settings do I have to do to my computer to allow the probing tests to fail?

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Windows Vista Home Premium sp2

Answer 1

According to ye olde standards, a computer would respond to every connection attempt with, typically, one of three responses:

1. Application banner indicating that a service is listening and which service
2. ICMP Port Unreachable - indicates that nothing is exists on this port
3. ICMP Administratively Prohibited - indicates that your system is not allowed to access this port

By those rules of etiquette, one would expect firewalls to respond with the 3rd kind of message. However, nowadays one typically expects firewalls to drop packets without sending a response. This is what Gibson is referring to as "stealth mode".

Unfortunately, the Shield's Up website is fairly dated. The advice and analysis makes assumptions based on the late 1990's Internet. At that time it may have been a reasonable assessment, these days I wouldn't give it much credence.

A host based firewall has been included and enabled since Windows XP SP2. So, whether you realize it or not, you do have a firewall running. However, the more likely culprit is your home router. Those require special configuration, called port forwarding, to pass unsolicited traffic from the Internet to your computer.

Answer 2

Gibson's scanner is outdated and not a good source of information on computer security. I would ignore the message as a relic of ancient history.

Answer 3

Port 139 could be dropped anywhere along the route from the Gibson scanner

• ISP - Because NetBIOS is a local protocol, and not intended for the Internet, more and more ISP's are dropping this by default.
• Your router - it may have a firewall built in
• your computer - although you say you have disabled firewall, you may actually have NetBIOS turned off - check in your network properties

Unfortunately I think the comment from Gibson's software is misleading. It may have been true at some point that port 139 was common on the internet, but it is much less common now. I also wouldn't take any comfort from the comment that "this computer appears VERY SECURE" :-)

Answer 4

Most likely your software or hardware firewall is "hiding" those ports by default. The firewall prevents responses to any connection attempts on those ports, so that any potential attackers won't get any information that might help them use any vulnerabilites in your system.