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.

source

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

ubuntu DNS 导致apt-get install 无效问题