最近在ubuntu 12.10上执行sudo apt-get update 命令后出现了如下错误：
Ign http://extras.ubuntu.com natty/main TranslationIndex
Err http://extras.ubuntu.com natty/main Sources
404 Not Found
Err http://extras.ubuntu.com natty/main i386 Packages
404 Not Found
Ign http://extras.ubuntu.com natty/main Translation-en_US
Err http://security.ubuntu.com natty-security/universe Sources
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/multiverse Sources
404 Not Found [IP: 91.189.92.200 80]
Ign http://extras.ubuntu.com natty/main Translation-en
Err http://security.ubuntu.com natty-security/universe i386 Packages
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/multiverse i386 Packages
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/main Sources
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/restricted Sources
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/main i386 Packages
404 Not Found [IP: 91.189.92.200 80]
Err http://security.ubuntu.com natty-security/restricted i386 Packages
404 Not Found [IP: 91.189.92.200 80]
Ign http://security.ubuntu.com natty-security/main Translation-en_US
Ign http://security.ubuntu.com natty-security/main Translation-en
Ign http://security.ubuntu.com natty-security/multiverse Translation-en_US
Ign http://security.ubuntu.com natty-security/multiverse Translation-en
Ign http://security.ubuntu.com natty-security/restricted Translation-en_US
Ign http://security.ubuntu.com natty-security/restricted Translation-en
Ign http://security.ubuntu.com natty-security/universe Translation-en_US
Ign http://security.ubuntu.com natty-security/universe Translation-en
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/main/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/restricted/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/universe/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/multiverse/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/main/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/restricted/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/universe/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty/multiverse/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/main/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/restricted/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/universe/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/multiverse/source/Sources 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/main/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/restricted/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/universe/binary-i386/Packages 404 Not Found
W: Failed to fetch http://cn.archive.ubuntu.com/ubuntu/dists/natty-updates/multiverse/binary-i386/Packages 404 Not Found
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/universe/source/Sources 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/multiverse/source/Sources 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/universe/binary-i386/Packages 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/multiverse/binary-i386/Packages 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/main/source/Sources 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/restricted/source/Sources 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/main/binary-i386/Packages 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/natty-security/restricted/binary-i386/Packages 404 Not Found [IP: 91.189.92.200 80]
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/main/source/Sources 404 Not Found
W: Failed to fetch http://extras.ubuntu.com/ubuntu/dists/natty/main/binary-i386/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
chaihuasong@chaihuasong:~$ sudo apt-get install git
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package git
发现很多源找不到，需要重新寻找更新源，最后把/etc/apt/sources.list文件中的内容直接替换成:
# deb cdrom:[Ubuntu 15.04 _Vivid Vervet_ - Release i386 (20150422)]/ vivid main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ vivid main restricted
deb-src http://us.archive.ubuntu.com/ubuntu/ vivid main restricted
## Major bug fix updates produced after the final release of the
## distribution.
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ vivid universe
deb-src http://us.archive.ubuntu.com/ubuntu/ vivid universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ vivid multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ vivid multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ vivid-backports main restricted universe multiverse
deb-src http://us.archive.ubuntu.com/ubuntu/ vivid-backports main restricted universe multiverse
deb http://security.ubuntu.com/ubuntu vivid-security main restricted
deb-src http://security.ubuntu.com/ubuntu vivid-security main restricted
deb http://security.ubuntu.com/ubuntu vivid-security universe
deb-src http://security.ubuntu.com/ubuntu vivid-security universe
deb http://security.ubuntu.com/ubuntu vivid-security multiverse
deb-src http://security.ubuntu.com/ubuntu vivid-security multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu vivid partner
# deb-src http://archive.canonical.com/ubuntu vivid partner