参照别人的文档，设置centos7 镜像源为阿里云

安装 wget
yum install -y wget

进入目录
cd /etc/yum.repos.d/

创建目录
mkdir backup

默认源配备份
mv C* backup/

下载阿里云yum源
wget -O /etc/yum.repos.d/CenOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo

清除旧缓存
yum clean all

创建新缓存
yum makecache

更新yum
yum update -y

可是到yum makecache的时候，总是提示'Connection timed out after 30002 milliseconds') Trying other mirror，试了很多方法都不好靠谱

错误内容如下：

http://mirrors.aliyun.com/centos/7/extras/x86_64/repodata/60d96b8cca8dc5c208506be688a654ea5945eae2dd0b132a3f9e1b3a363f918a-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyun.com/centos/7/extras/x86_64/repodata/60d96b8cca8dc5c208506be688a654ea5945eae2dd0b132a3f9e1b3a363f918a-primary.sqlite.bz2: (28, 'Operation too slow. Less than 1000 bytes/sec transferred the last 30 seconds')
Trying other mirror.
(6/10): updates/7/x86_64/filelists_db                                                                            | 2.1 MB  00:01:50     
(7/10): updates/7/x86_64/other_db                                                                                | 314 kB  00:00:16     
extras/7/x86_64/primary_db     FAILED                                          =========              ]  26 kB/s |  15 MB  00:04:47 ETA 
http://mirrors.aliyuncs.com/centos/7/extras/x86_64/repodata/60d96b8cca8dc5c208506be688a654ea5945eae2dd0b132a3f9e1b3a363f918a-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/7/extras/x86_64/repodata/60d96b8cca8dc5c208506be688a654ea5945eae2dd0b132a3f9e1b3a363f918a-primary.sqlite.bz2: (28, 'Connection timed out after 30000 milliseconds')
Trying other mirror.
extras/7/x86_64/primary_db     FAILED                                          =========-             ]  17 kB/s |  16 MB  00:07:10 ETA 
http://mirrors.cloud.aliyuncs.com/centos/7/extras/x86_64/repodata/60d96b8cca8dc5c208506be688a654ea5945eae2dd0b132a3f9e1b3a363f918a-primary.sqlite.bz2: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; Unknown error"
Trying other mirror.
(8/10): extras/7/x86_64/primary_db                                                                               | 206 kB  00:00:08     
updates/7/x86_64/primary_db    FAILED                                          ===========-           ]  35 kB/s |  17 MB  00:02:52 ETA 
http://mirrors.aliyun.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 14] curl#18 - "transfer closed with 1485553 bytes remaining to read"
Trying other mirror.
updates/7/x86_64/primary_db    FAILED                                          ========               ]  19 kB/s |  15 MB  00:06:55 ETA 
http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
updates/7/x86_64/primary_db    FAILED                                          ========-              ]  12 kB/s |  15 MB  00:11:06 ETA 
http://mirrors.cloud.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 14] curl#6 - "Could not resolve host: mirrors.cloud.aliyuncs.com; Unknown error"
Trying other mirror.
updates/7/x86_64/primary_db    FAILED                                          =========-             ] 7.9 kB/s |  16 MB  00:15:14 ETA 
http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
updates/7/x86_64/primary_db    FAILED                                          ==========-            ] 7.1 kB/s |  16 MB  00:15:44 ETA 
http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: (28, 'Connection timed out after 30001 milliseconds')
Trying other mirror.
updates/7/x86_64/primary_db    FAILED                                          ===========            ] 6.8 kB/s |  16 MB  00:15:30 ETA 
http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: [Errno 12] Timeout on http://mirrors.aliyuncs.com/centos/7/updates/x86_64/repodata/1c1cef9b9757d3f2013951752cad3bf93417ab30c1b8144b4d465df77510c502-primary.sqlite.bz2: (28, 'Connection timed out after 30002 milliseconds')
Trying other mirror.

解决办法：

DNS223.5.5.5是由阿里巴巴提供的免费DNS服务器的IP地址

设置网络的DNS地址为这个223.5.5.5就解决了。

vi /etc/sysconfig/network-scripts/ifcfg-ens33


# 设置最后的DNS为

TYPE=Ethernet
PROXY_METHOD=none
BROWSER_ONLY=no
BOOTPROTO=static
DEFROUTE=yes
IPV4_FAILURE_FATAL=no
IPV6INIT=yes
IPV6_AUTOCONF=yes
IPV6_DEFROUTE=yes
IPV6_FAILURE_FATAL=no
IPV6_ADDR_GEN_MODE=stable-privacy
NAME=ens33
UUID=b92f449c-bfdb-4884-be6d-95e95ba4a648
DEVICE=ens33
ONBOOT=yes
IPADDR=192.168.9.215
NETMASK=255.255.255.0
GATEWAY=192.168.9.1
DNS1=114.114.114.114
DNS2=223.5.5.5

#重启网络
service network restart