There are two parts to your question:
- fixing temporary resolve messages
- fixing the package management issues
Temporary resolve
It is likely that this issue is either:
- temporary due to your Internet Service Provider not correctly forwarding internet naming (DNS) to either its or external DNS servers, or
- due to a change in your network has similarly blocked this naming - for example, new router/modem, reconfiguring a switch with a new configuration.
Lets look at the possible DNS resolving issues.
First, temporarily add a known DNS server to your system.
echo "nameserver 8.8.8.8" | sudo tee /etc/resolv.conf > /dev/null
Then run sudo apt-get update.
If this fixes your temporary resolving messages then either wait for 24 hours to see if your ISP fixes the issue for you (or just contact your ISP) - or you can permanently add a DNS server to your system:
echo "nameserver 8.8.8.8" | sudo tee /etc/resolvconf/resolv.conf.d/base > /dev/null
8.8.8.8 is Google‘s own DNS server.
Another example DNS server you could use is OpenDNS - for example:
echo "nameserver 208.67.222.222" | sudo tee /etc/resolvconf/resolv.conf.d/base > /dev/null
package-management issues
In addition to the temporary resolve issues - you have a few package management issues that need to be corrected - I‘m assuming you have tried recently to upgrade from one Ubuntu version to the next recommended version - in your case from Natty (11.04) to Oneiric (11.10)
Open a terminal and type
sudo nano /etc/apt/sources.list
Look for lines that have your a different distribution name in the list than you were expecting - in your case - you have upgraded to oneiric but you have another release name natty
For example, look for lines that look like deb http:/archive.canonical.com/ natty backports
Add a # to the beginning of the line to comment it out - for example
#deb http:/archive.canonical.com/ natty backports
Save and re-run:
sudo apt-get update && sudo apt-get upgrade
You should not have any more release naming errors.
At the time of writing this, possible common release names include lucid, maverick, natty, oneiric, precise and quantal