[root@server mysql-5.0.56]# yum install gcc
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=5&arch=i386&repo=addons error was
[Errno 4] IOError:
Error: Cannot find a valid baseurl for repo: addons
发现原来是没有配置resolv.conf，于是到/etc目录下配置resolv.conf加入nameserver IP：
[root@server mysql-5.0.56]# vi /etc/resolv.conf
nameserver 218.85.157.99
search localdomain
保存resolv.conf，再次运行上面命令就可以。
[root@server mysql-5.0.56]# yum install gcc
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
 * base: centos.ustc.edu.cn
 * extras: centos.ustc.edu.cn
 * updates: centos.ustc.edu.cn
addons                                                   |  951 B     00:00    
addons/primary                                           |  202 B     00:00    
base                                                     | 1.1 kB     00:00    
base/primary                                             | 961 kB     00:06    
base                                                                  2705/2705
extras                                                   | 2.1 kB     00:00    
extras/primary_db                                        | 171 kB     00:00    
updates                                                  | 1.9 kB     00:00     
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