Select a mirror server host with the following characteristics:
This server runs on either CentOS/RHEL/Oracle Linux (5.x or 6.x) and has several GB of storage available. [5.x not supported for Beta]
The firewall allows all cluster nodes (the servers on which you want to install HDP) to access this server, and allows this server to access the Internet (at least those Internet servers for the repositories to be proxied).