1,删除redhat原有的yum，

rpm -qa | grep yum | xargs rpm -e --nodeps；

2,下载yum安装文件

wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-3.2.27-14.el6.centos.noarch.rpm

wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-metadata-parser-1.1.2-14.1.el6.x86_64.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/yum-plugin-fastestmirror-1.1.26-11.el6.noarch.rpm
wget http://mirrors.163.com/centos/6/os/x86_64/Packages/python-iniparse-0.3.1-2.1.el6.noarch.rpm

3,进行安装yum

rpm -ivh python-iniparse-0.3.1-2.1.el6.noarch.rpm

rpm -ivh yum-metadata-parser-1.1.2-14.1.el6.x86_64.rpm
rpm -ivh yum-3.2.27-14.el6.centos.noarch.rpm yum-plugin-fastestmirror-1.1.26-11.el6.noarch.rpm  
注意最后两个包必需同时安装，否则会相互依赖

4,下载yum公钥导入证书

 wget http://vault.centos.org/6.3/os/x86_64/RPM-GPG-KEY-CentOS-6  

rpm --import RPM-GPG-KEY-CentOS-6

5,下载CentOS-Base.repo文件到/etc/yum.repos.d中;

wget http://mirrors.163.com/.help/CentOS6-Base-163.repo -O /etc/yum.repos.d/CentOS6-Base.repo

6,vim 修改下载来的文件，把其中的$releasever全部替换成6.3，保存，参考命令:s/$releasever/6.3/g或sed -i 's/\$releasever/6.3/g' /etc/yum.repos.d/CentOS6-Base.repo

7,yum clean all 清除yum的缓存，

8，yum update -y 更新软件包。

RedHat5.6使用CentOS yum源更新的方法
================================================================================

由于RedHat的yum在线更新是收费的，我们的RedHat没有注册，不能在线更新下载rpm包。

需将RedHat的yum卸载后，重启安装Centos的yum,再配置其他yum源。

先进入yum源配置目录

cd /etc/yum.repos.d
备份系统自带的yum源
mv CentOS-Base.repo CentOS-Base.repo.save
下载其他更快的yum源
中科大的yum源：（现在打不开了 ）
wget http://centos.ustc.edu.cn/CentOS-Base.repo
163的yum源：
wget http://mirrors.163.com/.help/CentOS-Base-163.repo
修改CentOS-Base.repo  （不修改会报错），下载到本地把“$releasever”，替换成“5”
sohu的yum源
wget http://mirrors.sohu.com/help/CentOS-Base-sohu.repo
更新玩yum源后，建议更新一下，使操作立即生效
yum makecache

一、确认RedHat的版本

cat /etc/redhat-release
uname -m
二、删除redhat原有的yum源
rpm -aq|grep yum|xargs rpm -e --nodeps
三、下载CentOS的yum安装包（163源）  (可以在这里手工找rpm包：http://mirrors.163.com/centos/)
32位系统：
wget http://mirrors.163.com/centos/5/os/i386/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.i386.rpm
wget http://mirrors.163.com/centos/5/os/i386/CentOS/yum-3.2.22-39.el5.centos.noarch.rpm
wget http://mirrors.163.com/centos/5/os/i386/CentOS/yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm
64位系统：
wget http://mirrors.163.com/centos/5/os/x86_64/CentOS/yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm
wget http://mirrors.163.com/centos/5/os/x86_64/CentOS/yum-3.2.22-39.el5.centos.noarch.rpm
wget http://mirrors.163.com/centos/5/os/x86_64/CentOS/yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm
还依赖python-iniparse-0.2.3-4.el5.noarch.rpm，
但我的系统里已安装了，是系统默认Python2.4中自带的。
如果你的系统没有安装，请安装吧。
四、安装yum软件包
32位系统：
rpm -ivh yum-metadata-parser-1.1.2-3.el5.centos.i386.rpm
rpm -ivh yum-3.2.22-39.el5.centos.noarch.rpm  yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm
注意：最后两个安装包要放在一起同时安装，否则会提示相互依赖，安装失败。
64位系统：
rpm -ivh yum-metadata-parser-1.1.2-3.el5.centos.x86_64.rpm
rpm -ivh yum-3.2.22-39.el5.centos.noarch.rpm  yum-fastestmirror-1.1.16-21.el5.centos.noarch.rpm
注意：最后两个安装包要放在一起同时安装，否则会提示相互依赖，安装失败。
五、更改yum源  #我们使用网易的CentOS镜像源
cd /etc/yum.repos.d/
vi CentOS6-Base-163.repo
# CentOS-Base.repo
#
# The mirror system uses the connecting IP address of the client and the
# update status of each mirror to pick mirrors that are updated to and
# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
#
# If the mirrorlist= does not work for you, as a fall back you can try the
# remarked out baseurl= line instead.
#
#
[base]
name=CentOS-5 - Base - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/5/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-5 - Updates - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/5/updates/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#packages used/produced in the build but not released
[addons]
name=CentOS-5 - Addons - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=addons
baseurl=http://mirrors.163.com/centos/5/addons/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#additional packages that may be useful
[extras]
name=CentOS-5 - Extras - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/5/extras/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5 - Plus - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=centosplus
baseurl=http://mirrors.163.com/centos/5/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
#contrib - packages by Centos Users
[contrib]
name=CentOS-5 - Contrib - 163.com
mirrorlist=http://mirrorlist.centos.org/?release=5&arch=$basearch&repo=contrib
baseurl=http://mirrors.163.com/centos/5/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5
六、清理
yum clean all
yum makecache  
如果报错：imary.sqlite.bz2 from base: [Errno 256] No more mirrors to try.
执行：yum makecache  

七、更新

cd /etc/pki/rpm-gpg

wget http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5

yum update

很长时间的下载后，报错：

warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID e8562897

GPG key retrieval failed: [Errno 5] OSError: [Errno 2] No such file or directory: '/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5'
这是因为：指定的文件/etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5在本地不存在导致的。
解决：
去官网http://mirror.centos.org/centos/下载相应文件
cd /etc/pki/rpm-gpg
wget http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-5