Ask HN: Making Server with open source?

My organization currently own a server from Fujitsu with cent-OS 6, but Redhat hinder us to install open source software by saying that they will not provide support if we install patches and software's from untested sources. So want to know what is the best list of open source software’s to implement a server with Ubuntu which should have mail server, firewall or watch-guard, ldap and webserver.