删除原来的/etc/yum.conf文件和/etc/yum.repo.d目录
生成yum.conf,内容

[root@localhost etc]# cat yum.conf
[main]
cachedir=/var/cache/yum
keepcache=1
debuglevel=2
logfile=/var/log/yum.log
pkgpolicy=newest
distroverpkg=redhat-release
tolerant=1
exactarch=1
obsoletes=1
gpgcheck=1
plugins=1
metadata_expire=1800

# PUT YOUR REPOS HERE OR IN separate files named file.repo
# in /etc/yum.repos.d
[base]
name=centos-5 - Base
baseurl=http://centos.ustc.edu.cn/centos/5/os/i386/
# the other site: [url]http://centos.candishosting.com.cn/centos/5/os/i386/[/url]
# you can find more site in: [url]http://www.centos.org/modules/tinycontent/index.php?id=13[/url]
enabled=1
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5

#released updates
[update]
name=CentOS-5 - Updates
baseurl=http://mirror.centos.org/centos/5/updates/i386/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5

#packages used/produced in the build but not released
[addons]
name=CentOS-5 - Addons
baseurl=http://mirror.centos.org/centos/5/addons/$basearch/
gpgcheck=1
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5

#additional packages that may be useful
[extras]
name=CentOS-5 - Extras
baseurl=http://mirror.centos.org/centos/5/extras/$basearch/
gpgcheck=1
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos4

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5 - Plus
baseurl=http://mirror.centos.org/centos/5/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5

#contrib - packages by Centos Users
[contrib]
name=CentOS-5 - Contrib
baseurl=http://mirror.centos.org/centos/5/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5

#packages in testing
[testing]
name=CentOS-5 - Testing
baseurl=http://mirror.centos.org/centos/5/testing/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://centos.ustc.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5
2008.07.28更新
1. 安装yum
2. 修改.repo文件
ccd /etc/yum.repos.d/
#vi CentOS-Base.repo
写入以下内容:
[base] 2kCLinux联盟
name=CentOS-5 - Base 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever5&arch=$basearch& 2kCLinux联盟
repo=os 2kCLinux联盟
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ 2kCLinux联盟
baseurl=http://ftp.sjtu.edu.cn/centos/5/os/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#released updates 2kCLinux联盟
[update] 2kCLinux联盟
name=CentOS-5 - Updates 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=updates 2kCLinux联盟
baseurl=http://ftp.sjtu.edu.cn/centos/5/updates/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#packages used/produced in the build but not released 2kCLinux联盟
[addons] 2kCLinux联盟
name=CentOS-5 - Addons 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=addons
baseurl=http://ftp.sjtu.edu.cn/centos/5/addons/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#additional packages that may be useful 2kCLinux联盟
[extras] 2kCLinux联盟
name=CentOS-5 - Extras 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=extras
baseurl=http://ftp.sjtu.edu.cn/centos/5/extras/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#additional packages that extend functionality of existing packages 2kCLinux联盟
[centosplus] 2kCLinux联盟
name=CentOS-5 - Plus 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=centosplus 2kCLinux联盟
baseurl=http://ftp.sjtu.edu.cn/centos/5/centosplus/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
enabled=0 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
#contrib - packages by Centos Users 2kCLinux联盟
[contrib] 2kCLinux联盟
name=CentOS-5 - Contrib 2kCLinux联盟
#mirrorlist=http://mirrorlist.centos.org/?release=4&arch=$basearch&repo=contrib 2kCLinux联盟
baseurl=http://ftp.sjtu.edu.cn/centos/5/contrib/$basearch/ 2kCLinux联盟
gpgcheck=1 2kCLinux联盟
enabled=0 2kCLinux联盟
gpgkey=http://mirror.centos.org/centos/RPM-GPG-KEY-centos5
# vi dag.repo2kCLinux联盟
[dag]2kCLinux联盟
name=Dag RPM Repository for RHEL52kCLinux联盟
baseurl=http://ftp.riken.jp/Linux/dag/redhat/el5/en/$basearch/dag/2kCLinux联盟
enabled=12kCLinux联盟
gpgcheck=12kCLinux联盟
gpgkey=http://ftp.riken.jp/Linux/dag/packages/RPM-GPG-KEY.dag.txt2kCLinux联盟
 
2kCLinux联盟
3. 导入key
#rpm --import [url]http://ftp.sjtu.edu.cn/centos/5/os/i386/RPM-GPG-KEY-CentOS-5[/url]
4. 运行,测试
#yum update
***********************************************************************************
让RHEL5也能YUM Centos5.1
2008-04-11 11:02
1.卸载rhel的yum相关的包
[root@virhost ~]# rpm -qa|grep yum
yum-updatesd-3.0.1-5.el5
yum-3.0.1-5.el5
yum-rhn-plugin-0.5.2-3.el5
yum-metadata-parser-1.0-8.fc6
yum-security-1.0.4-3.el5
[root@virhost ~]# yum update
Loading "rhnplugin" plugin
Loading "installonlyn" plugin
Loading "security" plugin
This system is not registered with RHN.
RHN support will be disabled.
Setting up Update Process
Setting up repositories
No Repositories Available to Set Up
Reading repository metadata in from local files
Skipping security plugin, no data
No Packages marked for Update/Obsoletion
[root@virhost ~]# rpm -e yum-3.0.1-5.el5 --nodeps
[root@virhost ~]# rpm -qa|grep yum
yum-metadata-parser-1.0-8.fc6
yum-updatesd-3.0.1-5.el5
yum-rhn-plugin-0.5.2-3.el5
yum-security-1.0.4-3.el5
[root@virhost ~]# rpm -e yum-updatesd-3.0.1-5.el5
[root@virhost ~]# rpm -e yum-rhn-plugin-0.5.2-3.el5
[root@virhost ~]# rpm -e yum-metadata-parser-1.0-8.fc6(后来测试这个不应该卸载)
[root@virhost ~]# rpm -e yum-security-1.0.4-3.el5

2.安装centos的源
1.el5.centos.5.noarch.rpm
[root@virhost ~]# rpm -ivh yum-3.0.5-1.el5.centos.5.noarch.rpm
warning: yum-3.0.5-1.el5.centos.5.noarch.rpm: Header V3 DSA signature: NOKEY, key ID e8562897
error: Failed dependencies:
        yum-metadata-parser is needed by yum-3.0.5-1.el5.centos.5.noarch
[root@virhost ~]# wget [url]http://192.168.1.250/yum-metadata-parser-1.0-8.fc6.i386.rpm[/url]
--07:48:55-- [url]http://192.168.1.250/yum-metadata-parser-1.0-8.fc6.i386.rpm[/url]
Connecting to 192.168.1.250:80... 已连接。
已发出 HTTP 请求,正在等待回应... 200 OK
长度:22950 (22K) [text/plain]
Saving to: `yum-metadata-parser-1.0-8.fc6.i386.rpm'
100%[==================================================================>] 22,950      --.-K/s   in
0.003s
07:48:55 (8.57 MB/s) - `yum-metadata-parser-1.0-8.fc6.i386.rpm' saved [22950/22950]
[root@virhost ~]# rpm -ivh yum-metadata-parser-1.0-8.fc6.i386.rpm
warning: yum-metadata-parser-1.0-8.fc6.i386.rpm: Header V3 DSA signature: NOKEY, key ID 37017186
Preparing...                ########################################### [100%]
   1:yum-metadata-parser    ########################################### [100%]
[root@virhost ~]# rpm -ivh yum-3.0.5-1.el5.centos.5.noarch.rpm
warning: yum-3.0.5-1.el5.centos.5.noarch.rpm: Header V3 DSA signature: NOKEY, key ID e8562897
Preparing...                ########################################### [100%]
   1:yum                    ########################################### [100%]
由此看来yum-metadata-parser-1.0-8.fc6.i386.rpm这个包不应该删除那

[root@virhost ~]# cd /etc/yum.repos.d/
[root@virhost yum.repos.d]# ls
rhel-debuginfo.repo
[root@virhost yum.repos.d]# vi CentOS-Base.repo这个文件内如是下面的
[base]
name=CentOS-5.1 - Base
baseurl=http://mirror.tini4u.net/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5
#released updates
[updates]
name=CentOS-5.1 - Updates
baseurl=http://centos.ustc.edu.cn/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5

#packages used/produced in the build but not released
[addons]
name=CentOS-5.1 - Addons
baseurl=http://centos.ustc.edu.cn/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5

#additional packages that may be useful
[extras]
name=CentOS-5.1 - Extras
baseurl=http://centos.ustc.edu.cn/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-5.1 ¨C Plus
baseurl=http://centos.ustc.edu.cn/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5
#contrib - packages by Centos Users
[contrib]
name=CentOS-5.1 - Contrib
baseurl=http://centos.ustc.edu.cn/centos/5.1/os/$basearch/
[url]http://centos.ustc.edu.cn/centos/5.1/os/$basearch/[/url]
gpgcheck=0
gpgkey=http://centos.ustc.edu.cn/centos/5.1/os/i386/RPM-GPG-KEY-CentOS-5
然后随便测试了一个命令
[root@virhost yum.repos.d]# rpm -qa|grep postfix
刚好系统没有装这个包
[root@virhost yum.repos.d]# yum install postfix
Loading "installonlyn" plugin
Setting up Install Process
Setting up repositories
updates                   100% |=========================| 1.1 kB    00:00    
contrib                   100% |=========================| 1.1 kB    00:00    
centosplus                100% |=========================| 1.1 kB    00:01    
addons                    100% |=========================| 1.1 kB    00:00    
extras                    100% |=========================| 1.1 kB    00:00    
Reading repository metadata in from local files
primary.xml.gz            100% |=========================| 834 kB    00:03    
################################################## 2400/2400
primary.xml.gz            100% |=========================| 834 kB    00:08    
################################################## 2400/2400
primary.xml.gz            100% |=========================| 834 kB    00:08    
################################################## 2400/2400
primary.xml.gz            100% |=========================| 834 kB    00:04    
################################################## 2400/2400
primary.xml.gz            100% |=========================| 834 kB    00:08    
################################################## 2400/2400
Parsing package install arguments
Resolving Dependencies
--> Populating transaction set with selected packages. Please wait.
---> Downloading header for postfix to pack into transaction set.
postfix-2.3.3-2.i386.rpm 100% |=========================| 41 kB    00:00    
---> Package postfix.i386 2:2.3.3-2 set to be updated
--> Running transaction check
Dependencies Resolved
=============================================================================
Package                 Arch       Version          Repository        Size
=============================================================================
Installing:
postfix                 i386       2:2.3.3-2        extras            3.6 M
Transaction Summary
=============================================================================
Install      1 Package(s)        
Update       0 Package(s)        
Remove       0 Package(s)        
Total download size: 3.6 M
Downloading Packages:
(1/1): postfix-2.3.3-2.i3 100% |=========================| 3.6 MB    00:43    
Running Transaction Test
warning: postfix-2.3.3-2: Header V3 DSA signature: NOKEY, key ID e8562897
Finished Transaction Test
Transaction Test Succeeded
Running Transaction
Installing: postfix                      ######################### [1/1]
Installed: postfix.i386 2:2.3.3-2
Complete!