redhat替换centos 6.6 yum源
新安装了redhat6.4.安装后，登录系统，使用yum update 更新系统。提示：This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.
无法更新。
redhat 默认自带的 yum 源需要注册，才能更新。我们想不花钱也可以更新，需要替换掉redhat的yum源。
1.检查是否安装yum包
查看RHEL是否安装了yum，若是安装了，那么又有哪些yum包：
# rpm -qa |grep yum
yum-metadata-parser-1.0-8.fc6
yum-3.0.1-5.el5
yum-rhn-plugin-0.4.3-1.el5
yum-updatesd-3.0.1-5.el5
2 删除redhat自带的yum包
卸载上面显示的所有yum包：
# rpm -qa|grep yum|xargs rpm -e --nodeps（不检查依赖，直接删除rpm包）
再用
# rpm -qa |grep yum
#
查看，无信息显示表示已经卸载完成。
3.下载新的yum包。使用Centos6.6的yum包
# wget http://mirrors.163.com/centos/6.5/os/i386/Packages/yum-metadata-parser-1.1.2-16.el6.i686.rpm
# wget http://mirrors.163.com/centos/6.5/os/i386/Packages/yum-3.2.29-40.el6.centos.noarch.rpm
# wget http://mirrors.163.com/centos/6.5/os/i386/Packages/yum-plugin-fastestmirror-1.1.30-14.el6.noarch.rpm
安装yum软件包
注意:单个的安装包可能会依赖其它包（例如yum和yum-fastestmirror会相互依赖），所以我们可以把所有这些包放在一起，用一行命令将它们同时安装即可：
# rpm -ivh yum-metadata-parser-1.1.2-16.el6.i686.rpm yum-3.2.29-40.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.30-14.el6.noarch.rpm
4.更换yum源。使用163的源
# cd /etc/yum.repos.d/
# wgethttp://mirrors.163.com/.help/CentOS6-Base-163.repo
# vi CentOS6-Base-163.repo
编辑文件，把文件里面的$releasever全部替换为版本号，即6.6 最后保存！或者直接把下面的内存拷贝到CentOS6-Base-163.repo文件中即可（已经修改好）
name=CentOS-6.6 - Base - 163.com
baseurl=http://mirrors.163.com/centos/6.6/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=6.6&arch=$basearch&repo=os
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
#released updates
name=CentOS-6.6 - Updates - 163.com
baseurl=http://mirrors.163.com/centos/6.6/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=6.6&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
#additional packages that may be useful
name=CentOS-6.6 - Extras - 163.com
baseurl=http://mirrors.163.com/centos/6.6/extras/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=6.6&arch=$basearch&repo=extras
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
#additional packages that extend functionality of existing packages
name=CentOS-6.6 - Plus - 163.com
baseurl=http://mirrors.163.com/centos/6.6/centosplus/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=6.6&arch=$basearch&repo=centosplus
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
#contrib - packages by Centos Users
name=CentOS-6.6 - Contrib - 163.com
baseurl=http://mirrors.163.com/centos/6.6/contrib/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=6.6&arch=$basearch&repo=contrib
gpgcheck=1
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-6
5.清除原有缓存
1
# yum clean all
重建缓存，以提高搜索安装软件的速度
1
# yum makecache
6.更新系统
1
# yum update
报错内容如下：
Loaded plugins: fastestmirror
Could not retrieve mirrorlisthttp://mirrorlist.centos.org/?release=6&arch=x86_64&repo=oserror was
14: PYCURL ERROR 6 - "Couldn't resolve host'mirrorlist.centos.org'"
Error: Cannot retrieve repository metadata (repomd.xml) forrepository: base. Please verify its path and try again
解决办法
1，执行
echo "nameserver 8.8.8.8" > /etc/resolv.conf
设置dns服务器
即可
找到问题的原因，是由于服务器没设置dns服务器地址导致的上述错误
1，执行
yum clean all
应该执行这两步即可。(第二步要不要执行有待验证)
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