Centos Linux operating system can be downloaded from Centos Public Mirrors. Most of the download locations are at US. Current version is 6.3 now. DVD version consist of two files. CentOS-6.3-x86_64-bin-DVD1.iso and CentOS-6.3-x86_64-bin-DVD2.iso.
After registering redhat linux system for rhn, yum update command returned "No Packages marked for Update" # yum updateLoaded plugins: downloadonly, securitySetting up Update ProcessNo Packages marked for Update