#cat /etc/redhat-release
 CentOS Linux release 7.2.1511 (Core)
#vi /etc/yum.conf
proxy = http://192.168.1.188:3128/
#mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
#wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo
#yum makecache
#yum update
#cat /etc/redhat-release
CentOS Linux release 7.3.1611 (Core)

#vi /etc/profile
export http_proxy=http://192.168.1.188:3128/
export ftp_proxy=http://192.168.1.188:3128/