Recently, I was testing a PC-based NVR that supports Dual-LAN and uses Ubuntu.

The testing environment was very simple: One NIC was connected to an isolated network that has a switch connected to a lot of cameras. And the other NIC was connected to internet.

And not surprisingly, I could clearly see the camera screens via embedded VMS viewer. Everything seemed working well.

But I soon realized that I couldn't use internet at all. That didn't make any sense for me since I used internet about just few minutes ago.

Since connecting to internet is very important for the product(It allows us debug remotely when issues happen in sites.), I had to deal with this issue.

First, I tried the simplest method : I disabled network interface that was connected to internet and enabled it. Result? I could access internet again. But only for about 4 minutes.

I repeated the same treatment several times, but the results were the same. I could access internet only for a few minutes. Every single time.…