ClearOS - Security Gateway
I found a good open source security Gateway OS based on CentOS





It's coll & very easy to configure via the web browser.
SECURITY FOR FREE







http://www.clearfoundation.com/
Features
* Integrated LDAP for User and Group Management
* User Security Certificate Manager
Network Features
* Multi-WAN
* VPN - PPTP, IPsec, OpenVPN
* DMZ and 1-to-1 NAT
* Stateful Firewall
* Local DHCP and DNS Servers
Gateway Features
* Antimalware - Antivirus, Antiphishing, Antispyware
* Antispam
* Bandwidth Management
* Intrusion Protection, Intrusion Prevention, Intrusion Detection
* Protocol Filtering including Peer-to-Peer Detection
* Content Filter
* Web Proxy
* Access Control
Server Features
* Windows Networking with PDC Support
* File and Print Services
* Flexshares
* Groupware with Outlook Connector
* Mail Server - POP, IMAP, SMTP, Webmail, Retrieval
* Mail Filtering - Antispam, Antimalware, Greylisting, Quarantine
* Mail Archiving
* Database with MySQL
* Web Server with PHP Support