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  1. Microsoft ISA Server 2004 Firewall Microsoft ISA Server 2004 is a hybrid stateful packet-inspecting, circuit-filtering, and application layer proxy firewall. By hybrid, we mean that it can provide any of those functionalities at any given time based on the traffic it is receiving. If it has an application filter for the given protocol or application, it will function as an application proxy firewall for that traffic. If it does not, it will resort to either stateful packet inspecting or circuit filtering as required. In addition, ISA Server 2004 includes virtual private networking (VPN) and caching capabilities, allowing it to function as an all-in-one device that, as one would expect, integrates pretty cleanly with Microsoft-centric environments. Before we look at the features of Microsoft ISA Server 2004, let's talk about the elephant in the room, namely the perception that ISA Server 2004 is not a "real" firewall. This perception is largely the result of misinformation, lack of education regarding the product, and simple dislike/disregard of anything Microsoft being remotely considered as a security solution. When you look at ISA Server 2004 with an honest and skeptical eye, it is relatively easy to cut through many of the fallacies and realize that Microsoft ISA Server 2004 is an effective and practical firewall solution. First on the list of misconceptions is the statement that any firewall running on a Windows platform cannot be secure. This is just not factually accurate. All firewalls run on some operating system. In the case of firewalls such as the Cisco PIX Firewall or Check Point SecurePlatform, the operating system is specialized and hardened for use on a firewall. Windows, out of the box, is not designed to be run on a firewall, but it can be effectively secured and hardened following the principles of running the minimum required services and functionality necessary to operate as a firewall alone. Some excellent resources detail how to effectively secure the underlying Windows operating system: • NSA Security Configuration Guides http://www.nsa.gov/snac/downloads_all.cfm?MenuID=scg10.3.1 • Hardening the Windows Infrastructure on the ISA Server 2004 Computer http://www.microsoft.com/technet/prodtechnol/isa/2004/plan/hardeningwindows. mspx • Windows Server 2003 Security Guide http://www.microsoft.com/technet/security/prodtech/windowsserver2003/w2003h g/sgch00.mspx
  2. • ISA Server 2004 Security Hardening Guide http://www.microsoft.com/technet/prodtechnol/isa/2004/plan/securityhardeninggui de.mspx Note Keep in mind that many of the procedures for Windows 2000 are applicable to Windows 2003 and vice versa, so do not hesitate using both the Windows 2000 and 2003 guides regardless of your actual operating system Another frequent misconception is that ISA Server 2004 is "just" an upgrade to Microsoft Proxy Server 2.0. Although ISA Server 2004 is indeed the logical upgrade to Proxy Server 2.0 (technically, ISA Server 2000 is the direct upgrade to Proxy Server 2.0), that is not to say that ISA Server 2004 is just a proxy server. Proxy Server 2.0 had absolutely no advanced firewall features. It was primarily a caching engine with basic packet-filtering capabilities. Microsoft ISA Server 2004 is a fully featured firewall, capable of performing stateful packet inspection as well as application layer filtering and proxying. In addition, it can function as a caching engine. Simply put, trying to claim that because ISA Server 2004 is an upgrade to Proxy Server it is therefore not a "real" firewall has absolutely no technical merit. Microsoft ISA Server 2004 Features Microsoft ISA Server 2004 consists of two editions: Standard Edition and Enterprise Edition. The predominant differences between the Standard and Enterprise editions relate to scalability. Table 8-1 summarizes the differences between the Standard and Enterprise editions. Table 8-1. Comparison of ISA Server 2004 Standard and Enterprise Editions Feature Standard Edition Enterprise Edition Networks Unlimited Unlimited, with the addition of enterprise networks (networks that can be applied to any firewall array anywhere in the enterprise) Scale up Up to 4 CPUs and 2-GB RAM Unlimited (per operating system) Scale out Single server Up to 32 nodes using
  3. Table 8-1. Comparison of ISA Server 2004 Standard and Enterprise Editions Feature Standard Edition Enterprise Edition Microsoft Network Load Balancing (NLB) Caching Single server store Unlimited (through the use of Cache Array Routing Protocol (CARP)) High None Yes (using NLB) availability Management Local management and configuration Array and enterprise-level configuration Underlying Microsoft Windows Server 2003 Microsoft Windows Server operating (Standard or Enterprise Edition), 2003 (Standard or Enterprise system Microsoft Windows 2000 Server or Edition) Advanced Server with Service Pack 4 (SP4) or later, or Windows 2000 Datacenter Server As you can see, if you need multiple ISA servers working in tandem, or need additional memory and processors, you need to use the Enterprise Edition. Similarly, if you need a high-availability solution, use Enterprise Edition. In general, Microsoft ISA Server 2004's firewall features can be categorized as follows: • Security and filtering features • Firewall clients and authentication • Web caching server functionality • Network services publishing • VPN functionality • Management and administration features • Miscellaneous features Security and Filtering Features The Microsoft ISA Server 2004 firewall is a hybrid firewall capable of performing the following: • Stateful packet inspection ISA Server 2004 can perform full stateful packet inspection and filtering of all traffic passing through the firewall.
  4. • Circuit filtering ISA Server 2004 can perform application-transparent circuit filtering to a host of protocols, including Telnet, RealAudio, Windows Media technologies, and Internet Relay Chat (IRC). This filtering occurs at the transport or session layer as opposed to the application layer. This proxy functionality works in conjunction with the stateful packet-inspection functionality. • Application filtering ISA Server 2004 can act as an application proxy and filter traffic for a number of protocols including HTTP, FTP, and Gopher. This allows Microsoft ISA Server 2004 to act on behalf of clients, hiding and protecting the client from external resources and threats. Microsoft ISA Server 2004 can perform these filtering functions in a multidirectional method, supporting as many network interfaces as the physical hardware can contain. This allows for the creation of multiple demilitarized (DMZ) segments, allowing for the creation of unique filtering rulesets on a per-DMZ segment basis. Of course, filtering of traffic for/from the internal and external networks is also available. Microsoft ISA Server 2004 also supports basic intrusion detection functionality, although full intrusion detection system (IDS) functionality is best provided through the integration of third-party products such as ISS RealSecure or similar products. Currently, ISA Server 2004 can natively detect the following intrusion/attack attempts: • WinNuke • Ping of death • Land attack • IP half scan • Port scan • UDP bomb • POP3 buffer overflow • SMTP buffer overflow • DNS zone transfer • DNS length overflow • DNS host name overflow Firewall Clients and Authentication Microsoft ISA Server 2004 supports the following three types of firewall clients for systems that are attempting to access resources outside the protected network: • SecureNAT client • Firewall client • Web proxy client