/etc/yum.repos.d/CentOS-Base.repo
#baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/, to this: baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/ (remove the #).
/etc/yum.conf
Add this following line in the main section of the file /etc/yum.conf file.
proxy=http://xxx.xxx.xx.xx:8080
ie, substituting the proxy address with actual proxy. Also change the port number if it is not 8080.
If proxy requires authentication, add this also to the file
proxy_username=username
proxy_password=password
dhclient
