- /etc/yun.repos.d
清華源：
https://mirror.tuna.tsinghua.edu.cn/help/centos/
yum clean all
yum makecache
- /etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
source: https://www.centos.org/keys/RPM-GPG-KEY-CentOS-7
rpm --import RPM-GPG-KEY-CentOS-7
//test
yum install vim
opts：
- proxy
- /etc/profile
vim /etc/profile
export http_proxy=xxx
export https_proxy=xxx
// :!wq
source /etc/profile
- /etc/yum.conf
vim /etc/yum.conf
proxy=xxx
//:!wq
