Centos7出现yum安装出现系列问题，正在尝试其他镜像。。。或者安装docker后无法使用yum安装。。。
首先查看是否能ping通网络，ping www.baidu.com
不能ping通，查看网络配置，修改DNS
vim /etc/resolv.conf 将域名修改为8.8.8.8
vim /etc/sysconfig/network-scripts/ifcfg-ens32
添加DNS1，并重启网络。再次尝试，是否可以正常使用yum安装。
问题任然不能解决，那么就要修改yum源了
cd /etc/yum.repos.d #此目录存放yum源的文件
1.首先备份旧文件
mv CentOS-Base.repo CentOS-Base.repo.bak
2.下载阿里源的文件
wget -O CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
3.安装epel repo源
wget -O /etc/yum.repos.d/epel.repo http://mirrors.aliyun.com/repo/epel-7.repo
4.清理缓存，重新生成缓存
yum clean all
yum makecache fast
此时就已经修改成了阿里云的yum源，再次尝试。
倘若问题任然没有解决，曾经安装过docker可能是docker镜像的不匹配导致，需要修改docker镜像。
错误提示：
[Errno 12] Timeout on https://download.docker.com/linux/centos/7/x86_64/stable/repodata/repomd.xml
下面修改docker镜像，更改docker-ce.repo中的docker地址
官方的docker-ce.repo内容为：
docker-ce-stable] name=Docker CE Stable - $basearch baseurl=https://download.docker.com/linux/centos/7/$basearch/stable enabled=1 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-stable-debuginfo] name=Docker CE Stable - Debuginfo $basearch baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/stable enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-stable-source] name=Docker CE Stable - Sources baseurl=https://download.docker.com/linux/centos/7/source/stable enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-edge] name=Docker CE Edge - $basearch baseurl=https://download.docker.com/linux/centos/7/$basearch/edge enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-edge-debuginfo] name=Docker CE Edge - Debuginfo $basearch baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/edge enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-edge-source] name=Docker CE Edge - Sources baseurl=https://download.docker.com/linux/centos/7/source/edge enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-test] name=Docker CE Test - $basearch baseurl=https://download.docker.com/linux/centos/7/$basearch/test enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-test-debuginfo] name=Docker CE Test - Debuginfo $basearch baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/test enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-test-source] name=Docker CE Test - Sources baseurl=https://download.docker.com/linux/centos/7/source/test enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-nightly] name=Docker CE Nightly - $basearch baseurl=https://download.docker.com/linux/centos/7/$basearch/nightly enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-nightly-debuginfo] name=Docker CE Nightly - Debuginfo $basearch baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/nightly enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg [docker-ce-nightly-source] name=Docker CE Nightly - Sources baseurl=https://download.docker.com/linux/centos/7/source/nightly enabled=0 gpgcheck=1 gpgkey=https://download.docker.com/linux/centos/gpg
然后通过sed命令，替换download.docker.com等域名：
sed -i 's+download.docker.com+mirrors.tuna.tsinghua.edu.cn/docker-ce+' /etc/yum.repos.d/docker-ce.repo
或者复制下面的内容到docker-ce.repo文件中:
[docker-ce-stable] name=Docker CE Stable - $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/$basearch/stable enabled=1 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-stable-debuginfo] name=Docker CE Stable - Debuginfo $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/debug-$basearch/stable enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-stable-source] name=Docker CE Stable - Sources baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/source/stable enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-edge] name=Docker CE Edge - $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/$basearch/edge enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-edge-debuginfo] name=Docker CE Edge - Debuginfo $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/debug-$basearch/edge enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-edge-source] name=Docker CE Edge - Sources baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/source/edge enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-test] name=Docker CE Test - $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/$basearch/test enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-test-debuginfo] name=Docker CE Test - Debuginfo $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/debug-$basearch/test enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-test-source] name=Docker CE Test - Sources baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/source/test enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-nightly] name=Docker CE Nightly - $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/$basearch/nightly enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-nightly-debuginfo] name=Docker CE Nightly - Debuginfo $basearch baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/debug-$basearch/nightly enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg [docker-ce-nightly-source] name=Docker CE Nightly - Sources baseurl=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/7/source/nightly enabled=0 gpgcheck=1 gpgkey=https://mirrors.tuna.tsinghua.edu.cn/docker-ce/linux/centos/gpg
修改完后，更新yum缓存，再次尝试成功！！！