因为昨晚看到“鬼才”说新创建的ECS实例，运行yum update的命令出错，提示文件未找到。

今天（2017-08-28）新创建的一个网络类型为“经典网络”的1GB ECS实例（未分配公网IP地址），测试结果如下：

a. 内核信息如下：Linux iZbp1e4mchw553gx59aygaZ 3.10.0-514.26.2.el7.x86_64 #1 SMP Tue Jul 4 15:04:05 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

b. 运行 yum update 命令正常

c. /etc/yum.repos.d 目录下有 CentOS-Base.repo 和 epel.repo 两个安装源文件，分别内容如下：

  [base]
  name=CentOS-$releasever - Base
  failovermethod=priority
  baseurl=http://mirrors.aliyuncs.com/centos/$releasever/os/$basearch/
  gpgcheck=1

gpgkey=http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

released updates

[updates]
name=CentOS-$releasever - Updates
failovermethod=priority
baseurl=http://mirrors.aliyuncs.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

additional packages that may be useful

[extras]
name=CentOS-$releasever - Extras
failovermethod=priority
baseurl=http://mirrors.aliyuncs.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

additional packages that extend functionality of existing packages

[centosplus]
name=CentOS-$releasever - Plus
failovermethod=priority
baseurl=http://mirrors.aliyuncs.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

contrib - packages by Centos Users

[contrib]
name=CentOS-$releasever - Contrib
failovermethod=priority
baseurl=http://mirrors.aliyuncs.com/centos/$releasever/contrib/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.aliyuncs.com/centos/RPM-GPG-KEY-CentOS-7

[epel]
name=Extra Packages for Enterprise Linux 7 - $basearch
baseurl=http://mirrors.aliyuncs.com/epel/7/$basearch
failovermethod=priority
enabled=1
gpgcheck=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7

[epel-debuginfo]
name=Extra Packages for Enterprise Linux 7 - $basearch - Debug
baseurl=http://mirrors.aliyuncs.com/epel/7/$basearch/debug
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=0

[epel-source]
name=Extra Packages for Enterprise Linux 7 - $basearch - Source
baseurl=http://mirrors.aliyuncs.com/epel/7/SRPMS
failovermethod=priority
enabled=0
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-EPEL-7
gpgcheck=0