My terminal failed to fetch, and some index files failed to download: W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/precise-security/Release.gpg Something wicked happened resolving 'security.ubuntu.com:http' (-5 - No address associated with hostname) E: Some index files failed to download. They have been ignored, or old ones used instead. please help me to solve the problem in my terminal. I really appreciate the solution.