在数据中心内是不允许直接连外网的，上外网需要通过代理。那么如何使用外网YUM源呢？
1、设置/etc/profile
在最后添加两行：
http_proxy=代理的IP:端口
export http_proxy
然后保存
source /etc/profile
2、配置/etc/yum.conf文件
里面加一句
proxy=代理的IP:端口
然后保存
3、下载外网的yum源repo文件
例如：下载国内网易对应版本repo文件
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.163.com/.help/CentOS6-Base-163.repo
把文件拷贝到/etc/yum.repos.d目录下
enabled=1 #将原本的0改成1
gpgcheck=0 #将原本的1改成0
4、运行以下命令生成缓存
yum clean all
yum makecache
5、然后就可以用了
vim /etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
#released updates
[updates]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
baseurl=http://mirrors.163.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
baseurl=http://mirrors.163.com/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-6
[root@bastion-IDC ~]# yum clean all
[root@bastion-IDC ~]# yum makecache //将服务器上的软件包信息缓存到本地,以提高搜索安装软件的速度
[root@bastion-IDC ~]# yum update //更新系统
依照上面操作后，yum源就更换成网易的了，并能正常使用。
要特别注意的是：如果要想CentOS使用yum update更新时不升级内核，方法如下：
[root@bastion-IDC ~]# cp /etc/yum.conf /etc/yum.confbak
方法一）修改yum的配置文件
[root@bastion-IDC ~]# vim /etc/yum.conf
在[main]的最后添加 exclude=kernel*
方法二）直接在yum的命令后面加上如下的参数：
[root@bastion-IDC ~]# yum --exclude=kernel* update
如果要更换成阿里云的yum源，只需要将/etc/yum.repos.d/CentOS-Base.repo内容替换成下面内容即可，然后yum clean all、yum makecache、yum update
[base]
name=CentOS-$releasever - Base - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/os/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/os/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-6
#released updates
[updates]
name=CentOS-$releasever - Updates - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/updates/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/updates/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-6
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/extras/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/extras/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-6
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/centosplus/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/centosplus/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-6
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos/$releasever/contrib/$basearch/
http://mirrors.aliyuncs.com/centos/$releasever/contrib/$basearch/
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos/RPM-GPG-KEY-CentOS-6
[root@bastion-IDC ~]# yum clean all
[root@bastion-IDC ~]# yum makecache
如果出现下面报错：
https://mirrors.aliyun.com/centos/$releasever/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
原因分析：
$releasever这个变量在CentOS-Base.repo文件中没有被识别出来。可以去aliyun的镜像官网上查看下正确地址：
即正确地址是：https://mirrors.aliyun.com/centos/6.9/os/x86_64/repodata/repomd.xml解决办法：将$releasever换成6.9[root@bastion-IDC ~]# sed -i 's/$releasever/6.9/g' CentOS-Base.repo
