Linux：将rhel yum 切换到centos yum

Red Hat Enterprise Linux Server(RHEL) yum安装软件时This system is not registered with RHN. RHN support will be disabled.

Red Hat Enterprise Linux Server(RHEL) 的yum服务是付费的，因为没有付费，所以无法使用yum安装软件，如RHEL想安装LNMP必须要先按此教程设置好yum，并可以正常使用后才可以安装LNMP一键安装包，会提示如下错误：This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register. 或如下错误信息：This system is not registered with RHN. RHN support will be disabled.

解决方法：
执行：rpm -qa|grep yum|xargs rpm -e --nodeps  将yum卸载掉
cat /etc/issue && uname -a 看一下系统版本号和系统位数，对于 redhat 7可以通过命令：cat /etc/redhat-release 来查看小版本号


到 http://mirrors.kernel.org/centos/ 或

网易（ http://mirrors.163.com/centos/ ） ，

阿里云（https://mirrors.aliyun.com/centos/7/os/x86_64/Packages/），

浙大（http://mirrors.zju.edu.cn/centos/7.3.1611/os/x86_64/）

清华（http://mirrors.zju.edu.cn/centos/7.3.1611/os/x86_64/）

找对应系统版本号

如果系统是5.* 64位，则打开http://mirrors.kernel.org/centos/5/os/x86_64/CentOS/
如果系统是5.* 32位，则打开http://mirrors.kernel.org/centos/5/os/i386/CentOS/
如果系统是6.* 32位：http://mirrors.kernel.org/centos/6/os/i386/Packages/
如果系统是6.* 32位：http://mirrors.kernel.org/centos/6/os/i386/Packages/

如果系统是7.* 64位    http://mirrors.kernel.org/centos/7/os/x86_64/Packages/

找到yum 、yum-fastestmirror、yum-metadata-parser 这3个软件包的地址，然后执行：以下命令下载地址可能会无法下载，到上面网址找对应的下载文件！下面是以RHEL 5.* 64位 为例，下面下载地址里的链接可能会变，自行更换下载地址

wget http://mirrors.kernel.org/centos/5/os/x86_64/CentOS/yum-3.2.22-39.el5.centos.noarch.rpm
wget http://mirrors.kernel.org/centos/5/os/x86_64/CentOS/yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm
wget http://mirrors.kernel.org/centos/5/os/x86_64/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm

下载下这3个软件包，再执行：
rpm -ivh yum-*
再执行：（如果是6.*的版本，则将下面内容里面的5改成6，同时将下面文件里的$releasever全部替换为版本号5或6后再执行。）

cat >/etc/yum.repos.d/CentOS-Base.repo<<eof
[base]
name=CentOS-$releasever - Base
baseurl=http://mirrors.kernel.org/centos/$releasever/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#released updates 
[updates]
name=CentOS-$releasever - Updates
baseurl=http://mirrors.kernel.org/centos/$releasever/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

#packages used/produced in the build but not released
[addons]
name=CentOS-$releasever - Addons
baseurl=http://mirrors.kernel.org/centos/$releasever/addons/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=addons
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

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

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

#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
baseurl=http://mirrors.kernel.org/centos/$releasever/contrib/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5
eof

Note:

1) 如果是 7，没有addons，可以将[addons]区域的配置移除。

2) gpgkey 的版本号要与下载的版本是一致的。

3）使用上述脚本时，要将变量$releaserver 代表的是系统版本号，$basearch 代表的是 arch 版本（例如x86_64）