系统版本: centos 5.6 由于老生产环境使用的 centos 5 ,但是centos5已经停止更新。于是导致yum源也不能用了。 例如安装建立缓存的时候如下图提示

解决方法：

通过修改CentOS-Base.repo文件中的源地址来处理

vi /etc/yum.repos.d/CentOS-Base.repo

将原来的内容全部删掉，粘贴如下内容 最终更改CentOS-Base.repo文件内容如下，加粗部分为新加的内容，斜体部分为注释掉的内容。  

#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#

[base]
name=CentOS-$releasever - Base#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/baseurl=http://vault.centos.org/5.11/os/$basearch/gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

#released updates
[updates]
name=CentOS-$releasever - Updates#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/baseurl=http://vault.centos.org/5.11/updates/$basearch/gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/baseurl=http://vault.centos.org/5.11/extras/$basearch/gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/baseurl=http://vault.centos.org/5.11/centosplus/$basearch/gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/baseurl=http://vault.centos.org/5.11/contrib/$basearch/gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

  文件修改完毕后，执行清空yum源缓存。完成后即可正常使用yum命令了。

yum clean all
yum makecache