RHEL下yum更新出现This system is not registered with RHN解决办法       说一下如何改用CentOS的yum软件来升级未注册的RHEL系统：     首先 rpm -qa|grep yum  查看 yum 服务相关的包     # rpm -qa|grep yum  www.2cto.com   yum-security-1.1.16-13.el5_4.1 yum-3.2.22-33.el5 yum-updatesd-0.9-2.el5 yum-metadata-parser-1.1.2-3.el5    # rpm -qa|grep yum|xargs rpm -e --nodeps           强制删除这一些包    下载CentOS yum 服务相关的 rpm 包    wget http://centos.ustc.edu.cn/centos/5/os/x86_64/CentOS/yum-3.2.22-39.el5.centos.noarch.rpm    wget http://centos.ustc.edu.cn/centos/5/os/x86_64/CentOS/yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm    wget http://centos.ustc.edu.cn/centos/5/os/x86_64/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm           # rpm -ivh yum-* warning: yum-3.2.22-39.el5.centos.noarch.rpm: Header V3 DSA signature: NOKEY, key ID e8562897  www.2cto.com   Preparing...                ########################################### [100%]    1:yum-metadata-parser    ########################################### [ 33%]    2:yum-fastestmirror      ########################################### [ 67%]    3:yum                    ########################################### [100%]     哦了，安装完毕，现在进入系统的yum目录     # cd /etc/yum.repos.d/     # mv rhel-debuginfo.repo rhel-debuginfo.repo.bakup     # mv rhel-source.repo rhel-source.repo.bakup     # wget http://docs.linuxtone.org/soft/lemp/CentOS-Base.repo    (下载CentOS的repo包)     # yum makecache     等待 yum 库更新完毕，就可以 yum 安装软件了     作者 fairplay_li