背景，需要安装cacti，google了很多安装资料，需要先yum安装一些准备lib包，比如snmp以及openssl等等。
[root@mysqlvm2 ~]# yum install net-snmp-devel
Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
Setting up Install Process
No package net-snmp-devel available.
Nothing to do
看到yum不识别，所以需要先在linux服务器上面安装yum软件包。
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RedHat 6.2 Linux修改yum源免费使用CentOS源
YUM 安装及清理
1 下载yum安装包
下载地址：wget
[root@mysqlvm2 ~]# wget
--2014-05-15 20:56:34--
正在解析主机 yum.baseurl.org... 140.211.167.207
Connecting to yum.baseurl.org|140.211.167.207|:80... 已连接。
已发出 HTTP 请求，正在等待回应... 200 OK
长度：1140370 (1.1M) [application/x-gzip]
Saving to: `yum-3.4.3.tar.gz\
2 解压缩安装
tar -xvf yum-3.4.3.tar.gz
cd yum-3.4.3
yum main.py install yum
[root@mysqlvm2 yum-3.4.3]# yum main.py install yum
Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
usage: yum [options] COMMAND
List of Commands:
红帽中出现This system is not registered with RHN这个的原因是因为红帽中没有注册RHN。解决办法：（假定你已安装yum，且网络畅通）更改yum的源，即更换/etc/yum.repos.d /rhel-debuginfo.repo 这个文件。进入/etc/yum.repos.d/目录，终端中输入wget 即可在此目录下得到CentOS- Base.repo文件，这是centos的源文件，只需将其重命名为rhel-debuginfo.repo即可，以前的文件做个备份。
wget
cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.bak
[root@localhost yum.repos.d]# cp rhel-debuginfo.repo rhel-debuginfo.repo.bak
[root@localhost yum.repos.d]# mv CentOS-Base.repo rhel-debuginfo.repo
mv：是否覆盖“rhel-debuginfo.repo”? y
[root@localhost yum.repos.d]#
[root@mysqlvm2 yum-3.4.3]# yum makecache
Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
addons | 1.9 kB 00:00
addons/filelists_db | 570 B 00:00
addons/other_db | 554 B 00:00
addons/primary_db
[root@mysqlvm2 yum-3.4.3]# yum install httpd
Loaded plugins: rhnplugin, security
This system is not registered with RHN.
RHN support will be disabled.
Setting up Install Process
Package httpd-2.2.3-85.el5.centos.x86_64 already installed and latest version