Internet no longer working?

[Arjan Geven]

Hey guys,

For a while, Ubuntu worked like a charm. That is, the last two months
or so. Then, my mainboard crashed, and I installed a new one, this
time with a onboard LAN-connection (eth1). I plugged in my old
ethernet card as well (eth0), to set up a small home network with two
other computers.

However, my internet connection degraded severely. The connection fell
away often, and I have no idea why. Now, the internet seems not to
work at all anymore (probably due to my own troubleshooting,
newbie-style). Help would be greatly appreciated! On Windows XP, I
don't have any problems with my internet by the way, but what's the
point of having Ubuntu if I have to boot windows for internet? ;-)

Anyway, the situation:
I went to System > Networking, and configured eth1 for DHCP lookup and
eth0 for a static IP-address at 192.168.0.1. Eth1 is selected as the
default gateway.

I thought the problem originated from slow DNS-lookups from my
provider, so I added some OpenNIC DNS-servers, as found somewhere on
the Ubuntu-forums.

I also recently installed "dnsmasq", which makes my Ubuntu act more or
less as an internet-sharing device for my other computers. (I know,
every cheap router could do this as well, but I only need internet on
those machines every once in a while, and it should work fine like
this). This worked, I got some internet on my other machines. No
longer :-(.

$sudo ifconfig eth1 up gives me an IP address assigned by my ISP. I
can also ping my ISP, and get normal results. If I try to ping any
other IP-address though, I get a message "sndmessage not permitted".
Any attempts to resolve DNS results in "DNS lookup temporarily
unavailable". If it only were temporary ;-).

I also tried to type in an IP-address instead of a web page address in
Mozilla, but this also results in a nice timeout page, and no data.

I'm getting kind of desperate, because the damn thing worked perfectly
until this mainboard upgrade. Windows gives me internet, so why can't
I get Ubuntu to do that as well? Arghh!

Can you guys help me?

Thanks,
Arjan