SharePoint 2007: The Definitive Guide

Protecting SharePoint Sites with Microsoft ISA Server

If your SharePoint site is accessible from the Internet, a great way to provide another layer of security is to use a reverse web proxy. A reverse proxy accepts the HTTP or HTTPS traffic from the Internet, inspects it, and then forwards it on to the actual web server. It can even inspect HTTPS traffic because the SSL certificate resides on the proxy itself. It decrypts and inspects the traffic, and can then re-encrypt it and pass it onto the web server for end-to-end encryption, with application-layer inspection in between.

The ideal choice for reverse proxy with Office SharePoint Server is Microsoft Internet Security and Acceleration (ISA) Server. With ISA Server, users can first be forced to authenticate successfully to Active Directory via web form authentication, prior to any traffic passing through ISA Server to the Office SharePoint Server. This provides the benefit of not allowing traffic from the Internet to touch your SharePoint Server until after the user authenticates.

The following TechNet article describes this involved process in detail. Though it focuses on Windows SharePoint Services, it will work the same with Office SharePoint Server:

http://www.microsoft.com/technet/isa/2004/plan/isawss.mspx