使用RedHat 系统在线安装时提示This system is not registered with RHN.如下：
[root@localhost ~]# yum install httpd
Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
Setting up Install Process
No package httpd available.
Nothing to do
解决方案：
1.卸载redhat自带的yum组件
[root@localhost ~]# rpm -qa|grep yum|xargs rpm -e --nodeps
2.安装CentOS的yum包
[root@localhost ~]# wget http://centos.ustc.edu.cn/centos/5/os/i386/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.i386.rpm
[root@localhost ~]# wget http://centos.ustc.edu.cn/centos/5/os/i386/CentOS/yum-fastestmirror-1.1.16-16.el5.centos.noarch.rpm
[root@localhost ~]# wget http://centos.ustc.edu.cn/centos/5/os/i386/CentOS/yum-3.2.22-37.el5.centos.noarch.rpm
[root@localhost ~]# rpm -ivh *.rpm
warning: yum-3.2.22-37.el5.centos.noarch.rpm: Header V3 DSA signature: NOKEY, key ID e8562897
Preparing... ######################## [100%]
1:yum-metadata-parser ######################### [ 33%]
2:yum-fastestmirror ########################## [ 67%]
3:yum ############################ [100%]
3. 下载更新源，并存放在系统目录中
[root@localhost ~]# wget http://www.linux.com/files/2011/05/06/CentOS-Base.repo
[root@localhost ~]#mv CentOS_Base.repo /etc/yum.repos.d/CentOS-Base.repo
4.生成缓存并进行安装
[root@localhost ~]# yum makecache
[root@localhost ~]# yum install httpd