为什么Centos 7的镜像源都不能用了？

http://mirrors.aliyun.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
 

[root@k8s-master ~]# yum install -y tree
Loaded plugins: fastestmirror
Repository base is listed more than once in the configuration
Repository updates is listed more than once in the configuration
Repository extras is listed more than once in the configuration
Loading mirror speeds from cached hostfile
http://mirrors.aliyun.com/centos/7/os/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
Trying other mirror.
https://mirrors.aliyun.com/docker-ce/linux/centos/7/x86_64/stable/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
Trying other mirror.
http://mirrors.aliyun.com/centos/7/extras/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
Trying other mirror.
https://mirrors.aliyun.com/kubernetes/yum/repos/kubernetes-el7-x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
Trying other mirror.
http://mirrors.aliyun.com/centos/7/updates/x86_64/repodata/repomd.xml: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"
Trying other mirror.
Resolving Dependencies
--> Running transaction check
---> Package tree.x86_64 0:1.6.0-10.el7 will be installed
--> Finished Dependency Resolution

Dependencies Resolved

============================================================================================================================================================================================
 Package                                   Arch                                        Version                                              Repository                                 Size
============================================================================================================================================================================================
Installing:
 tree                                      x86_64                                      1.6.0-10.el7                                         base                                       46 k

Transaction Summary
============================================================================================================================================================================================
Install  1 Package

Total download size: 46 k
Installed size: 87 k
Downloading packages:
tree-1.6.0-10.el7.x86_64.rpm   FAILED                                          
http://mirrors.aliyun.com/centos/7/os/x86_64/Packages/tree-1.6.0-10.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: mirrors.aliyun.com; Unknown error"s |    0 B  --:--:-- ETA 
Trying other mirror.


Error downloading packages:
  tree-1.6.0-10.el7.x86_64: [Errno 256] No more mirrors to try.