默认安装的redhat会报如下错误：
to enable red hat subscription management repositories: subscription-manager

1. 删除旧的yum
rpm -qa | grep yum
rpm -qa|grep yum|xargs rpm -e --nodeps

2. 下载yum相关的包
下载地址：http://mirrors.163.com/centos/7/os/x86_64/Packages/
epel-release-latest-7.noarch.rpm
python-iniparse-0.4-9.el7.noarch.rpm
python-urlgrabber-3.10-9.el7.noarch.rpm
rpm-4.11.3-35.el7.x86_64.rpm
yum-3.4.3-161.el7.centos.noarch.rpm
yum-metadata-parser-1.1.4-10.el7.x86_64.rpm
yum-plugin-fastestmirror-1.1.31-50.el7.noarch.rpm
yum-updateonboot-1.1.31-50.el7.noarch.rpm
yum-utils-1.1.31-50.el7.noarch.rpm

3. 安装yum
rpm -ivh yum-*
# 报错, 依赖包的版本不符：
rpm >= 0:4.11.3-22 is needed by yum-3.4.3-161.el7.centos.noarch
则先安装：
rpm -Uvh rpm-4.11.3-25.el7.x86_64.rpm --nodeps
rpm -Uvh python-urlgrabber-3.10-8.el7.noarch.rpm --nodeps
再执行：rpm -ivh yum-*
则安装成功

4. 更换yum源
rm /etc/yum.repos.d/redhat.repo
然后编辑文件 (没有则是新建)
vim /etc/yum.repos.d/CentOS-Base.repo

使用如下内容：

#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-$7 - Base - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$7&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/7/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7
 
#released updates
[updates]
name=CentOS-$7 - Updates - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$7&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/7/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7
 
#additional packages that may be useful
[extras]
name=CentOS-$7 - Extras - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$7&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/7/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7
 
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$7 - Plus - 163.com
baseurl=http://mirrors.163.com/centos/7/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

5. 清除yum缓存
yum clean all

6. 生成yum缓存
yum makecache