【运行报错】yum 在 Downloading packages 时不停的在换镜像源

1. 报错信息：

Downloading packages:
No Presto metadata available for extras
No Presto metadata available for centos-openstack-queens
libev-4.15-7.el7.x86_64.rpm    FAILED                                          
/centos/7/extras/x86_64/Packages/libev-4.15-7.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"                                ]  0.0 B/s |    0 B  --:--:-- ETA 
正在尝试其它镜像。
openpgm-5.2.122-2.el7.x86_64.r FAILED                                          
/centos/7/cloud/x86_64/openstack-queens/Packages/o/openpgm-5.2.122-2.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"         ]  0.0 B/s |    0 B  --:--:-- ETA 
正在尝试其它镜像。
openstack-neutron-common-12.1. FAILED                                          
/centos/7/cloud/x86_64/openstack-queens/Packages/o/openstack-neutron-common-12.1.1-1.: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"B/s |    0 B  --:--:-- ETA 
正在尝试其它镜像。
openstack-neutron-linuxbridge- FAILED                                          
/centos/7/cloud/x86_64/openstack-queens/Packages/o/openstack-neutron-linuxbridge-12.1.1-1.: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"    0 B  --:--:-- ETA 
正在尝试其它镜像。
libev-4.15-7.el7.x86_64.rpm    FAILED                                          
/centos/7/extras/x86_64/Packages/libev-4.15-7.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"                ]  0.0 B/s |    0 B  --:--:-- ETA 
正在尝试其它镜像。
openvswitch-2.11.0-4.el7.x86_6 FAILED                                          
/centos/7/cloud/x86_64/openstack-queens/Packages/o/openvswitch-2.11.0-4.el7.x86_64.rpm: [Errno 14] curl#6 - "Could not resolve host: ; Unknown error"      ]  0.0 B/s |    0 B  --:--:-- ETA 
正在尝试其它镜像。

2. 原因分析：

3. 解决方法：

1. 将yum缓存清理一下

   yum clean all
   

2. 或者 重启 network 服务

   service restart network