搭建环境时碰到yum更新出问题了，报如下错误：

YumRepo Error: All mirror URLs are not using ftp, http[s] or file.
 Eg. $releasever is not a valid release or hasnt been released yet/
removing mirrorlist with no valid mirrors: /var/cache/yum/base/mirrorlist.txt
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again

分析一般有三种原因：

1.有可能是防火墙阻碍了出网http请求或是ftp请求，service iptables status

2.有可能是dns不对，无法正常解析，换成8.8.8.8

3.系统取不到Release号

测试环境时使用ping om 显示可以ping通，说明DNS没有问题，那可能就是第三中原因了

需要修改*.repo源文件中将$releasever变量全替换成6即可正常yum了

vim  /etc/yum.repos.d/CentOS6-Base-163.repo

【

[base]

name=CentOS-6 - Base - 163.combaseurl=http://mirrors.163.com/centos/6/os/$basearch/#mirrorlist=http://mirrorlist.centos.org/?release=6&arch=$basearch&repo=osgpgcheck=1gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6

#released updates

[updates]name=CentOS-6 - Updates - 163.combaseurl=http://mirrors.163.com/centos/6/updates/$basearch/#mirrorlist=http://mirrorlist.centos.org/?release=6&arch=$basearch&repo=updatesgpgcheck=1gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6

#additional packages that may be useful

[extras]name=CentOS-6 - Extras - 163.combaseurl=http://mirrors.163.com/centos/6/extras/$basearch/#mirrorlist=http://mirrorlist.centos.org/?release=6&arch=$basearch&repo=extrasgpgcheck=1gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6

#additional packages that extend functionality of existing packages

[centosplus]name=CentOS-6 - Plus - 163.combaseurl=http://mirrors.163.com/centos/6/centosplus/$basearch/#mirrorlist=http://mirrorlist.centos.org/?release=6&arch=$basearch&repo=centosplusgpgcheck=1enabled=0gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6

#contrib - packages by Centos Users

[contrib]name=CentOS-6 - Contrib - 163.combaseurl=http://mirrors.163.com/centos/6/contrib/$basearch/#mirrorlist=http://mirrorlist.centos.org/?release=6&arch=$basearch&repo=contribgpgcheck=1enabled=0gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6

】

最后更新缓存：yum makecache