我有一个运行 CentOS 7 的虚拟机，我很久没有使用了。今天我启动了它，并尝试使用 将 CentOS 系统更新到最新版本yum update，但出现了很多错误：
Loaded plugins: fastestmirror, langpacks
http//bay.uchicago.edu/centos/7.0.1406/os/x86_64/repodata/repomd.xml:
[Errno 14] HTTP Error 404 - Not Found Trying other mirror.
http//mirror.cs.pitt.edu/centos/7.0.1406/os/x86_64/repodata/repomd.xml:
[Errno 14] HTTP Error 404 - Not Found Trying other mirror.
http//mirror.anl.gov/pub/centos/7.0.1406/os/x86_64/repodata/repomd.xml:
[Errno 14] HTTP Error 403 - Forbidden Trying other mirror.
http//mirror.pac-12.org/7.0.1406/os/x86_64/repodata/repomd.xml: [Errno
14] HTTP Error 404 - Not Found Trying other mirror.
http//centos.expedientevirtual.com/7.0.1406/os/x86_64/repodata/repomd.xml:
[Errno 14] HTTP Error 404 - Not Found Trying other mirror.
（省略了许多其他类似的错误......）
Trying other mirror. Loading mirror speeds …