I had a similar problem, and the solution is ridiculous, but this is what worked for me...
I use Mandrake 10, so I'm not sure EXACTLY what you'll see, but try this:
Use the K Menu (it's the star button on the taskbar) and go to System/Configuration/Configure Your Computer. (It might be called DrakConf or Mandrake Control Center, also.) It will probably ask for a root password, so you'll have to give it the root password you created when you installed linux.
In there, click "Network and Internet."
Click Remove a Connection. Remove any connections you see in there.
back in "Network and Internet" click "New Connection"
If you're dsl modem is connected directly to your computer, then set the connection up as a dsl connection. After that, I'm not sure what you should do. try the default settings. You might have to set it up with pppOE. I don't have dsl, so I don't know.
If you're dsl modem is connected to a router or switch or hub which then connects to your computer, then set up the connection as a LAN connection. after that, try the default settings again. when it asks you for a hostname (NOT DHCP HOSTNAME!!!) then put whatever you want, so long as it matches the format myhost.mydomain
That means myhost can say whatever you want, so long as it's followed by a period (.) and then a domain. I'm not sure what domains are ok, so I just put whatever.home, because I know that home is an ok domain.
if it asks for a zeroconfhostname, just leave it blank.
finish and then log out of the graphical interface. (you can do this using the taskbar menu and clicking "logout...") then log BACK In and go to the Mandrake Control center I told you to go to before. instead of clicking "Network and Internet" this time, click "System."
then click "Services"
scroll down the list until you get to something called "ShoreWall." it might be all lowercase. I'm not sure. Make sure that the checkbox next to "on boot" is UNchecked. then click the stop button to the right.
exit Mandrake Control Center.
The reason you're having the problem, if you had the same problem as me, is that sometimes linux just decides you need an extra firewall without telling you. The problem is that shorewall is comprised entirely of evil and therefore kills your machine's internet connection. It's very secure, though, because it doesn't let you download ANYTHING.
hopefully this works. if so, enjoy, and thank Alfanut for his post on this thread