今天，维护自己ubuntu服务器，使用apt-get upgrade更新源，但是出现下面的失败信息：
Err http://archive.canonical.com natty InRelease
Err http://security.ubuntu.com oneiric-security InRelease
Err http://extras.ubuntu.com natty InRelease
Err http://security.ubuntu.com oneiric-security Release.gpg
Temporary failure resolving ‘security.ubuntu.com’
Err http://archive.canonical.com natty Release.gpg
Temporary failure resolving ‘archive.canonical.com’
Err http://extras.ubuntu.com natty Release.gpg
Temporary failure resolving ‘extras.ubuntu.com’
Err http://gb.archive.ubuntu.com oneiric InRelease
Err http://gb.archive.ubuntu.com oneiric-updates InRelease
Err http://gb.archive.ubuntu.com natty-backports InRelease
Err http://gb.archive.ubuntu.com oneiric Release.gpg
Temporary failure resolving ‘gb.archive.ubuntu.com’
Err http://gb.archive.ubuntu.com oneiric-updates Release.gpg
Temporary failure resolving ‘gb.archive.ubuntu.com’
Err http://gb.archive.ubuntu.com natty-backports Release.gpg
Temporary failure resolving ‘gb.archive.ubuntu.com’
Reading package lists... Done
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/oneiric/InRelease
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/oneiric-updates/InRelease
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/natty-backports/InRelease
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/InRelease
W: Failed to fetch http://archive.canonical.com/ubuntu/dists/natty/InRelease
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/InRelease
W: Failed to fetch http://archive.canonical.com/ubuntu/dists/natty/Release.gpg Temporary failure resolving ‘archive.canonical.com’
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/oneiric-security/Release.gpg Temporary failure resolving ‘security.ubuntu.com’
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/Release.gpg Temporary failure resolving ‘extras.ubuntu.com’
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/oneiric/Release.gpg Temporary failure resolving ‘gb.archive.ubuntu.com’
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/oneiric-updates/Release.gpg Temporary failure resolving ‘gb.archive.ubuntu.com’
W: Failed to fetch http://gb.archive.ubuntu.com/ubuntu/dists/natty-backports/Release.gpg Temporary failure resolving ‘gb.archive.ubuntu.com’
W: Some index files failed to download. They have been ignored, or old ones used instead.
看了好几个外文网站，找到了解决方案：
最简单的解决方案：