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I have something similar on a Xubuntu Trusty upgraded to 14.04 and then to Utopic and also on a fresh install of a Kubuntu Utopic daily image of 02 May. I get "connecting to security.ubuntu.com. Could not resolve gb.archive.ubuntu.com" and it does not mater if I disable security updates (which I could do in Xubuntu but not in Kubuntu) or change the server to Main server or another server in the UK or even US. The problem remains = cannot update/upgrade.
I do not get this in Ubuntu Utopic not even when it is on the devel repositories. In Ubuntu nslookup gb.archive.ubuntu.com gives
But in these two Utopic installs of Xubuntu and Kubuntu I get "Connection timed out: No servers could be resolved." And it does not matter what mirror I have in software sources.Non-authoritative answer:
EDIT: Just install Lubuntu Utopic from 5th May daily image. Installed updates at the same time and with an ethernet connection to the router. And Lubuntu Utopic has the same problem. Firefox will not load its start page or any of the pre-defined bookmarks or any web page. The Network Manager icon is missing. Edit network connections says I am connected by wired. But cannot find any useful way of making a wireless connection.
And Yes, the router is connected to the ISP as proved by posting this from Ubuntu Uptopic. I have never had this problem with any install of the flavours in previous development cycles.