I have found that NGX sucessfully runs on CentOS 3. CentOS is built on RHEL source code with very little changes. More info on CentOS at
www.centos.org.
Note: CentOS is not a supported OS for NGX. This information is only provided for the purpose of demonstrating and testing NGX in the RHEL environment without having to purchase RHEL. I do not recommend installing NGX on CentOS for your production firewall.