最近安装了Centos5.6，已经算是老系统了，官方不在维护更新了。
安装完毕，着手装软件，发现yum罢工了：

[root@ Robinson]# yum update
Loaded plugins: fastestmirror,security  
Loading mirror speeds from cached hostfile  
YumRepo Error: All mirror URLs are not using ftp,http[s] or file.  
 Eg. Invalid release/  
removing mirrorlist with no valid mirrors: /var/cache/yum/base/mirrorlist.txt  
Error: Cannot find a valid baseurl for repo: base

1、将/etc/yum.repos.d/CentOS-Base.repo文件内容修改：
保险一点，可以先把这个文件备份一下

cp /etc/yum.repos.d/CentOS-Base.repo  /etc/yum.repos.d/CentOS-Base.repo.backup

将文件内容替换为：

# CentOS-Base.repo
#  
# The mirror system uses the connecting IP address of the client and the  
# update status of each mirror to pick mirrors that are updated to and  
# geographically close to the client.  You should use this for CentOS updates  
# unless you are manually picking other mirrors.  
#  
# If the mirrorlist= does not work for you,as a fall back you can try the   
# remarked out baseurl= line instead.  
#  
#  

[base]  
name=CentOS-$releasever - Base  
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os  
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/  
baseurl=http://vault.centos.org/5.6/os/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5  

#released updates   
[updates]  
name=CentOS-$releasever - Updates  
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates  
#baseurl=http://mirror.centos.org/centos/$releasever/updates/$basearch/  
baseurl=http://vault.centos.org/5.6/updates/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5  

#additional packages that may be useful  
[extras]  
name=CentOS-$releasever - Extras  
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras  
#baseurl=http://mirror.centos.org/centos/$releasever/extras/$basearch/  
baseurl=http://vault.centos.org/5.6/extras/$basearch/  
gpgcheck=1  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5  

#additional packages that extend functionality of existing packages  
[centosplus]  
name=CentOS-$releasever - Plus  
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=centosplus  
#baseurl=http://mirror.centos.org/centos/$releasever/centosplus/$basearch/  
baseurl=http://vault.centos.org/5.6/centosplus/$basearch/  
gpgcheck=1  
enabled=0  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5   

#contrib - packages by Centos Users  
[contrib]  
name=CentOS-$releasever - Contrib  
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=contrib  
#baseurl=http://mirror.centos.org/centos/$releasever/contrib/$basearch/  
baseurl=http://vault.centos.org/5.6/contrib/$basearch/  
gpgcheck=1  
enabled=0  
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-5

我安装的是5.6，如果你是别的版本可将5.6全文替换为你自己的版本。

2、更新源

yum update

这样就好了。

本文参考：http://www.jb51.cc/article/p-alxhcpmb-bps.html