How to find open ports on a network?

[PranksterO]

For the moment I am hosting my own web page from my house on a recycled computer. It works great! The only thing is in the fall ill be living in a dorm and I wont have access to the firewall like I have here at my house to control what ports are open. My question is, is there a way I can get by this and still host my page from the dorm? How could I look and see what ports the firewall has opened at the college? Can anyone thik of any ideas of what I could do? Thanks!

[cowanrl]

One of the best tools for scanning for open ports in Linux is nmap. I use it at work as a security check by checking what ports are open on the network. It can be used for other things like checking to see if a service is running on a server.

If you wanted to scan the IP address of the firewall at the college the command would look like this:

nmap -sTU -p 1-1024 -P0 <ipaddress>

That would scan all TCP and UDP ports below 1024 for the specified IP address. The -P0 would stop it from sending a ping first to see if the IP address is active.

It would return the status of all of the specified ports. If the list is long, you can always redirect the output to a file and view the file with more or less.

There's a lot more options you can use with the command. You would need to see the man page for details.

On any install of Red Hat I've ever done, nmap has always been there. If you don't have it on your Linux machine, check out:

http://www.insecure.org/nmap

[Gertrude]

There is a good chance your not going to be able to do this if your living in a dorm. The first thing that could be a issue is if they give out non-routable, local IP addresses to the student or dorm subnet, like 192.168.0.X, or 10.X.X.X, . The reason you wouldnt be able to host your page if this was the case would be because the admin would have to forward port 80 to your computer and that is most likely not going to happen, unless you buy him a few cases of beer, and hang out with him on friday nights and play pool with each other, there would also be issues with the domain name of your site and again you you would need to talk to the admin about that.

If you do get a public IP assigned to your computer there could still be problems. Alot of schools, and ISPs like to block incomming connections on port 80 to restrict people from doing what your trying to do. So in that case you would need to change the port that your webserver is running at, but thats kinda crappy because then anyone who wants to access your server would to to specify the port after the URL. Your best bet to do this would be to either buy web hosting from someone, or keep that server running at home or some other location and manage it via SSH.

EDIT:

You also wanna be careful about trying to circumvent any of the schools security policies because just as fast as you find a way around something, you may find yourself with no internet connection just as fast if you get caught.