At work we have to use Sonicwall Netextender to connect over VPN. The NetExtender client works fine on Windows x64 for me, but when using the same settings in my Ubuntu , the connection seems to work fine at first - But after just some packages have been sent, it slows down to almost 0 or 0 ( the connection is still up and seems fine)
Also tried with latest NetExtenderPackage "NetExtender.Linux.7.0.741.x86_64" and it's the same symptoms - i can connect successfully, fetch about one webpage, and then everything just seem to stall. I also upgraded to Ubuntu 13.04.
Not sure if you got a solution to this or not, but Dell Sonicwall just released a firmware update to the NSA series that somewhat addresses this type of issue. I was having similar "trailing off" issues with my windows client. The Global VPN client would work fine, but the Netexender would slow to a crawl after a couple minutes. This firmware update has increased the throughput by a factor of at least eight and the speed stays constant!
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