创建/etc/yum.repos.d/CentOS-Base.repo文件，定义yum更新源，www.linuxidc.com这里使用的是上海交大的CentOS更新源
[root@linux-a ~]# vi /etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-5-Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever5&arch=$basearch&repo=os
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
baseurl=http://ftp.sjtu.edu.cn/centos/5/os/$basearch/
gpgcheck=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#released updates
[update]
name=CentOS-5-Updates
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=updates
baseurl=http://ftp.sjtu.edu.cn/centos/5/updates/$basearch/
gpgcheck=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#packages used/produced in the build but not released
[addons]
name=CentOS-5-Addons
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=addons
baseurl=http://ftp.sjtu.edu.cn/centos/5/addons/$basearch/
gpgcheck=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#additional packages that may be useful
[extras]
name=CentOS-5-Extras
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=extras
baseurl=http://ftp.sjtu.edu.cn/centos/5/extras/$basearch/
gpgcheck=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5-Plus
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=centosplus
baseurl=http://ftp.sjtu.edu.cn/centos/5/centosplus/$basearch/
gpgcheck=0
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#contrib - packages by Centos Users
[contrib]
name=CentOS-5-Contrib
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=contrib
baseurl=http://ftp.sjtu.edu.cn/centos/5/contrib/$basearch/
gpgcheck=0
enabled=0
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
# vi dag.repo
[dag]
name=Dag RPM Repository for RHEL5
baseurl=http://ftp.riken.jp/Linux/dag/redhat/el5/en/$basearch/dag/
enabled=1
gpgcheck=0
gpgkey=http://ftp.riken.jp/Linux/dag/packages/RPM-GPG-KEY.dag.txt
导入key
[root@linux-a ~]# rpm --import http://ftp.sjtu.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5