if the dns servers fail, u can't connect to the internet at all afaik. plus, that won't stop an update even if that's not the case. I spoofed on 5.3.2 to 5.5.1, and I was able to install the 5.5.1 update without issue by simply disabling my proxy. so either the dns servers will prevent any sort of internet access or them failing will result in u updating no matter what. I'm pretty sure the former is true actually.