centos yum 无法使用

[root@fm3 yum.repos.d]# yum update yum
Setting up Update Process
Setting up repositories
not using ftp, http[s], or file for repos, skipping - 4 is not a valid release or hasnt been released yet
Cannot find a valid baseurl for repo: update
Error: Cannot find a valid baseurl for repo: update

解决方法：

1.查看系统版本

 lsb_release -a

访问 http://vault.centos.org/4.9/ （4.9是系统版本号）

下载repo： wget http://vault.centos.org/4.8/CentOS-Base.repo

覆盖 /etc/yum.repos.d/CentOS-Base.repo 文件

再试试，应该ok！