How do I see what OUTBOUND ports are blocked?

Question

I'm trying to plan for getting a SlingBox but where I go to college they restrict the ports to stop file-sharing and spammers (it's a public/open network). I want to be able to watch TV between classes (say while I eat lunch) but I don't want to waste money on a slingbox if it won't work.

Is there any way I can check what outbound ports are open? I know 80, 110, and 443 are allowed and most others are blocked...but not exactley what's what.

My home ISP blocks ports 80, 25, and 110 to stop home-servers from working. I'm already using port 443 for remote TCP/IP printing, so I'd rather not give that up...I haven't had time to try many others because it involves changing my home network, then checking when I get to class, and the next day repeating the process - so I can check one port a day...not very practical.

Is there some program that will scan all the ports (or a lot of them) and tell me what OUTBOUND ports work so I can narrow my search for a port allowed both out of my college and into my home?

Answer 1

Nmap ("Network Mapper") is a free and open source (license) utility for network exploration or security auditing. Many systems and network administrators also find it useful for tasks such as network inventory, managing service upgrade schedules, and monitoring host or service uptime. Nmap uses raw IP packets in novel ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they are running, what type of packet filters/firewalls are in use, and dozens of other characteristics. It was designed to rapidly scan large networks, but works fine against single hosts. Nmap runs on all major computer operating systems, and both console and graphical versions are available.



By scanning a public computer on the internet, you can learn what ports are blocked on your network. Read the documentation further for more information.



Keep in mind that use of this tool may be against the network policies of your school and could possibly get you expelled off the network, so use wisely.



If your looking for an outbound looking in scanner, try the following below:



http://www.hackerwatch.org/probe/

Answer 2

Interesting, one star awarded! Not sure why they would block skype traffic? Anyway, if they are blocking based on destination IP and port (this would be most likely) then my only suggestion would be to use a proxy of some kind. You'd have to find an open public proxy that is fast enough for VOIP which might be difficult. From looking at the Skype website I think this can be done although might be tricky and unreliable. If they are doing some kind of SPI (stateful packet inspection) to check the packets (not quite sure how this would work) and block at this level, the only suggestion I can come up with is to establish a VPN with a friend or another network which has an ISP/link to the internet that allows skype. This way your traffic would be encrypted and your ISP could not see inside to do SPI blocking NOR block on port IP. This pre-supposes they allow VPN of course. When you work it out, can you post a comment to let us know how you did it?

Answer 3

It's inbound ports that are - effectively - blocked. (They're not actually blocked, they're not available unless they're forwarded, and the school doesn't forward any ports to you.) Outbound ports, since they don't have to be forwarded, aren't "blocked".



"My home ISP blocks ports 80, 25, and 110 to stop home-servers from working."

That's inbound, not outbound. If they were blocked outbound you couldn't browse the web or receive or send email.