解决CentOS 5.5 yum源不能用的问题问题描述
在yum check-update的时候，总是提示错误。按照网上的方法做处理，结果提示各种错误，大致如下：
YumRepo Error: All mirror URLs are notusing ftp, http[s] or file
Error: Cannot retrieve repository metadata(repomd.xml) for repository: addons.
Loaded plugins: fastestmirror,refresh-packagekit, security Loading mirror speeds from cached hostfi
试了网上解决方案，我的问题均未解决。最后自己试出来了，先说明下大概情况：
1.最新版的CentOS系统是7.4.1708，
2.国内的各大镜像站只有最新版本目录有内容，其他版本目录下仅有readme文件
3.各大镜像站下载的CentOS-Base.repo文件，放到/etc/yum.repos.d/下都用不了
解决方案
在readme文件中发现http://vault.centos.org/网站是有各个版本镜像的，所以
1.通过lsb_release -a查询下，系统版本如图
2.根据查询信息，将/etc/yum.repos.d/CentOS-Base.repo中baseurl路径做调整,文件内容如下：
# CentOS-Base.repo
#
# The mirror system uses the connecting IPaddress of the client and the
# update status of each mirror to pickmirrors that are updated to and
# geographically close to the client. You should use this for CentOS updates
# unless you are manually picking othermirrors.
#
# If the mirrorlist= does not work for you,as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-$releasever - Base
baseurl=http://vault.centos.org/5.5/os/i386/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-$releasever - Updates
baseurl=http://vault.centos.org/5.5/updates/i386/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#packages used/produced in the build butnot released
[addons]
name=CentOS-$releasever - Addons
baseurl=http://vault.centos.org/5.5/addons/i386/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
baseurl=http://vault.centos.org/5.5/extras/i386/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#additional packages that extendfunctionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
baseurl=http://vault.centos.org/5.5/centosplus/i386/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
baseurl=http://vault.centos.org/5.5/contrib/i386/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
3.保存文件后，运行yum clean all
4.执行yum check-update测试下，ok
国内的几个yum镜像源
http://ftp.sjtu.edu.cn/centos/，上海交大，但服务器位于北京，中国教育网网络中心，我看像是朝阳区那边的IP，以前在那边上过班，下载速度高达十M。北方用户与教育网用户推荐，速度飞快。
http://centos.ustc.edu.cn，中国科技大学，服务器位于合肥。 南方用户推荐。
http://mirrors.sohu.com/，sohu的开源镜像服务器，服务器位于山东省联通。 速度飞快，全国用户推荐，经常用FireFox下载CD镜像。
http://mirrors.163.com/centos，网易的开源服务器镜像 速度也不错，全国用户推荐.下载CentOS-Base-163.repo, 放入/etc/yum.repos.d/
