networking - How to fix very slow network access through proxy after upgrading to 12.04

I'm running a lab that sits behind a proxy server. We have been using Ubuntu 10.04.4, and getting reasonable internet transfer rates.

I'd like to upgrade to 12.04, but when I test it, attempts to connect to any site past the proxy take many minutes if they connect at all. Loading a home page routinely takes 30 minutes. It is getting an IP address from the DHCP server here in the lab, which is also the gateway. I can ping other machines in the lab.

If I connect this same machine to an unproxied connection it works fine. If I install 10.04 on this same machine, it works fine. Using 12.04 (and 13.04, btw) on several other machines also doesn't work

This problem exists whether I use a web site hostname, or an IP address. It happens on FTP as well as HTTP.

Things I've tried:

• Upgraded BIOS firmware (Dell Optiplex 745 v2.6.6)


• Built and installed newer tg3 NIC driver (3.133d)


• Restarted the proxy


• Disabled dnsmasq


• Set static IP address/gateway


• Added resolv.conf IPs to /etc/hosts file (to fix reports of nslookup failing)

Can anybody suggest how I can fix this, or at least begin to diagnose what the point of failure in this setup might be?