WM云建站云友“alifanss”在问答里说使用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 5.11 32位。
过程：
1. 登录到系统，运行 yum update ，会提示：
引用http://mirrors.aliyuncs.com/centos/5/os/i386/repodata/repomd.xml: [Errno 14] HTTP Error 404: Not Found
Trying other mirror.
http://mirrors.aliyun.com/centos/5/os/i386/repodata/repomd.xml: [Errno 14] HTTP Error 404: Not Found
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again
2. 编辑文件 /etc/yum.repos.d/CentOS-Base.repo ，将其中的 mirrors.aliyun.com/centos/$releasever 替换成相应历史源站地址和版本号，如 vault.centos.org/5.11
- [base]
- name=CentOS-$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. 之后，可继续 yum update 了
4. 也可以安装其它软件了，如 yum install httpd
参考：
1. yum fails – https://www.centos.org/forums/viewtopic.php?t=62106
2. Using Vault Repositories – https://www.centos.org/forums/viewtopic.php?t=36628
