阿里云CentOS 5 的系统，无法用yum来安装应用软件。 
 
原因：CentOS 5 在2017-03-31日已经结束支持，不再提供维护更新，所以包括阿里云镜像站的文件可能都是过时或已经有部分文件缺失。 
 
环境：CentOS 5. X

方法

1.只保留/etc/yum.repos.d/CentOS-Base.repo 其它文件 要么删除要么改后缀

2.编辑文件 /etc/yum.repos.d/CentOS-Base.repo ，将其中的 mirrors.aliyun.com/centos/$releasever 替换成相应历史源站地址和版本号，如 vault.centos.org/5.11

完整文件下载http://tadown.com/fs/6x1idado5g5g9vdip0/

[base]
name=CentOS-$releasever - Base
failovermethod=priority
baseurl=http://vault.centos.org/5.11/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-5
http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-$releasever - Updates
failovermethod=priority
baseurl=http://vault.centos.org/5.11/updates/$basearch/http://mirrors.aliyuncs.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-5
http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
failovermethod=priority
baseurl=http://vault.centos.org/5.11/extras/$basearch/http://mirrors.aliyuncs.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-5
http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
failovermethod=priority
baseurl=http://vault.centos.org/5.11/centosplus/$basearch/http://mirrors.aliyuncs.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-5
http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-5
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
failovermethod=priority
baseurl=http://vault.centos.org/5.11/contrib/$basearch/http://mirrors.aliyuncs.com/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-5
http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-5

3. 确认在 /etc/yum.repos.d/目录没有除了CentOS-Base.repo之外其他以repo结尾的文件，如果有的话需要删除，然后执行 yum clean all && yum makecache 更新和启用新的yum源。