阿里云ESC云主机centos6系统yum更新时，会遇到类似如下404等错误：

http://mirrors.aliyun.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found" Trying other mirror.
http://mirrors.aliyuncs.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 7 - "couldn't connect to host" Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again

处理方法：修改/etc/yum.repos.d/目录下的CentOS-Base.repo、epel.repo文件

把CentOS-Base.repo文件中的以下网址
http://mirrors.aliyun.com/centos/
http://mirrors.aliyuncs.com/centos/
http://mirrors.cloud.aliyuncs.com/centos/
修改成
http://mirrors.aliyun.com/centos-vault/centos/
 
把epel.repo文件中的
enabled=1
修改成
enabled=0

修改后CentOS-Base.repo文件

# 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-$releasever - Base - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/centos/$releasever/os/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/os/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/centos/RPM-GPG-KEY-CentOS-6
 
#released updates 
[updates]
name=CentOS-$releasever - Updates - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/centos/$releasever/updates/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/updates/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/centos/RPM-GPG-KEY-CentOS-6
 
#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/centos/$releasever/extras/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/extras/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyun.com/centos-vault/centos/RPM-GPG-KEY-CentOS-6
 
#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/centos/$releasever/centosplus/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/centosplus/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/centos/RPM-GPG-KEY-CentOS-6
 
#contrib - packages by Centos Users
[contrib]
name=CentOS-$releasever - Contrib - mirrors.aliyun.com
failovermethod=priority
baseurl=http://mirrors.aliyun.com/centos-vault/centos/$releasever/contrib/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/contrib/$basearch/
        http://mirrors.aliyun.com/centos-vault/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyun.com/centos-vault/centos/RPM-GPG-KEY-CentOS-6

修改后epel.repo文件

[epel]
name=Extra Packages for Enterprise Linux 6 - $basearch
baseurl=http://mirrors.aliyun.com/epel/6/$basearch
failovermethod=priority
enabled=0
gpgcheck=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
 
[epel-debuginfo]
name=Extra Packages for Enterprise Linux 6 - $basearch - Debug
baseurl=http://mirrors.aliyun.com/epel/6/$basearch/debug
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
gpgcheck=0
 
[epel-source]
name=Extra Packages for Enterprise Linux 6 - $basearch - Source
baseurl=http://mirrors.aliyun.com/epel/6/SRPMS
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-6
gpgcheck=0

安装阿里云yum源

1、备份
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
2、下载新的CentOS-Base.repo 到/etc/yum.repos.d/
CentOS 5
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-5.repo
CentOS 6
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-6.repo
CentOS 7
wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
3、之后运行yum makecache生成缓存
yum clean all
yum makecache

参考文档

• 阿里云centos系统yum源错误解决方法