System Root

While working on an ASP.Net project at work, I found a strange “feature” (I’m more inclined to call it an annoyance) that is built into certain browsers. It seems that Firefox and WebKit have automatic port blocking built into them for specific ports. I was specifically trying to use port 6000 (a seemingly random, easy to remember port) but couldn’t access the web server. After a little searching, I managed to find some information and thought I’d share it.

You can disable the behaviour in Mozilla by adding the config string network.security.ports.banned.override with a list of comma-separated ports you want to unblock. You can access the config section by typing about:config into your browser’s address bar.

And now, without further ado, here are the ports:

Sources:

• Mozilla article on port blocking
• WebKit source code for blocked ports