redhat的yum在线更新是收费的，未注册则不能使用，解决办法是更换为centos的yum，以下为详细过程：
1.卸载redhat原有的yum
rpm -e `rpm -qa |grep yum ` --nodeps
2.到网易163镜像源下载centos的yum安装包
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-3.2.29-60.el6.centos.noarch.rpm
wget
http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-metadata-parser-1.1.2-16.el6.x86_64.rpm
wget
http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.30-30.el6.noarch.rpm
3.安装CentOS yum
rpm -ivh yum*
4.更新repo文件
备份原先的repo文件，
wget http://mirrors.163.com/.help/CentOS6-Base-163.repo
mv http://mirrors.163.com/.help/CentOS6-Base-163.repo CentOS-Base.repo
5.yum makecache的时候报错，需要下载安装Centos的release和indexhtml包
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/centos-indexhtml-6-2.el6.centos.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/centos-release-6-6.el6.centos.12.2.x86_64.rpm
安装上面两个rpm包即可。
5、yum clean all ; yum cakecache
6、yum install vnc*
