Microsoft Forefront TMG – Best Practices Firewall policy rules

Let’s begin

Forefront TMG uses many configuration elements for creating Firewall policy rules based on several objects.

The first configuration part is to create the required network using Forefront TMG. Basically, you must create networks and network relationships using type ROUTE or NAT.

After a Standard Forefront TMG installation there are several predefined network objects. For example, there are the following networks:

  • Internal – contains all IP addresses of the Internal IP address ranges
  • External – represents the “Internet” – the untrusted network and contains all IP addresses which are not part of other TMG networks
  • VPN-Clients – A dynamic network object which contains all clients which connects via VPN to the TMG Server
  • LocalHost – The TMG Server itself with all connected network adapters.

The network objects must be linked with a network relationship using type ROUTE or NAT depending on your internal IP address environment. Typically you will use NAT when you want to allow clients with private IP addresses access to the Internet and ROUTE when you have a DMZ. For example, when webservers are located within public IP addresses.

After a Standard Forefront TMG installation, several network rules are created by Forefront TMG. For example:

  • A ROUTE relationship from LocalHost to the Internal network
  • A NAT relationship from the Internal network to the External network
Figure 1: Forefront TMG network elements
Figure 1: Forefront TMG network elements

After the network objects have been defined and associated with network rules, it is possible to create Firewall Policy rules to allow or deny access through the TMG Server based on several elements using the TMG toolbox.

Firewall Policy rules

Following a Standard Forefront TMG installation, predefined Firewall Policy rules and Firewall Policy rule elements are set. Every Forefront TMG installation comes with three types of Firewall Policies and Firewall Policy rule sets:

  • System Policy rule set
  • User defined Firewall Policy rule set
  • The “Deny all” rule or sometimes called cleanup rule

The System Policy rule set contains a number of predefined Firewall policy rules which exist in every Forefront TMG installation. System Policy rules allows or denies traffic for daily operations from the TMG server to the internal network and to some destinations on the External network. For example:

  • Allow Active Directory services access from LocalHost to Internal
  • Allow access to Windows update services from Localhost to Microsoft update sites

System Policy rules will be dynamically activated and deactivated by Forefront TMG. For example, when you enable Forefront TMG as a VPN Server, the appropriate TMG System policy rules will also be activated.

Figure 2: Forefront TMG System Policy rules
Figure 2: Forefront TMG System Policy rules

The recommended way to configure the System Policy is to use the System Policy Editor as shown in the following screenshot.

Figure 3: Forefront TMG System Policy editor
Figure 3: Forefront TMG System Policy editor

User defined Firewall Policy rules

As part of the daily operations, Forefront TMG Administrators must create Firewall Policy rules to allow or deny access from internal clients to the Internet and from the Internet to internal resources with the help of Server and Webserver publishing rules. You can use the Forefront TMG Management console to create the required Firewall policy rules.

Figure 4: Forefront TMG user defined Firewall Policy rules
Figure 4: Forefront TMG user defined Firewall Policy rules

Toolbox

All required elements for creating Firewall Policy rules can be found in the Toolbox of the management console. The Toolbox consists of several predefined protocol definitions, network objects, computer and URL sets. You are able to create your own protocol definitions.

Figure 5: Forefront TMG toolbox and Server protocol properties
Figure 5: Forefront TMG toolbox and Server protocol properties

Firewall Policy rules best practice

Forefront TMG checks Firewall policy rules in order, from top to down with first match.

If the Forefront TMG Standard edition is used, Forefront TMG will evaluate the requests in the following order:

  • Network rules
  • System Policy rule set
  • User defined Firewall Policy rule set
  • Deny All (Cleanup rule)

If an allow Firewall policy rule applies to the request, Forefront TMG will allow the request. Specifically, Forefront TMG applies a rule if the request matches the following rule conditions, checking the rule elements in this order:

  • Protocol
  • From/source address and port
  • Schedule
  • To/destination addresses, names, URL
  • Users
  • Content groups

After a matched Firewall policy rule has been found, Forefront TMG stops Firewall policy rule evaluation and then it checks the network rules again to determine how the networks are connected. Forefront TMG checks for existing Web chaining rules if a Web proxy client tries to open the object.

The evaluation order of Forefront TMG Firewall Policy rules is very important for an efficient rule set and also for some performance improvements of daily operations.

General Firewall Policy rule order guidelines

The performance of Forefront TMG may be related to the type of information it requires to evaluate the rules. Because Firewall policy rules are evaluated in order, it may be helpful to place the often used Firewall policy rules near the top of the rule set, if this order doesn’t conflict for example, with Firewall Policy rules which denies access to some destinations.

Simple Firewall Policy rule elements

Some objects of the Forefront TMG toolbox are easy to evaluate without an additional overhead to do some authentication against your internal Active Directory for example:

  • Protocol definitions
  • Schedules
  • All network elements like Computers, Computer sets, IP Subnets and more

Microsoft recommends that you should place Firewall Policy rules with these elements at the top of the rule set.

Complex Rule Elements

The following Firewall Policy rule elements require additional networking information like DNS name resolution, Active Directory (LDAP, GC) lookup and should be placed at the bottom of the Firewall Policy rule set:

  • Domain name sets
  • URL sets
  • Users
  • Content type

Firewall Policy rules which use Application filters

Forefront’s Secure Web Access Gateway comes with a lot of additional Application and Web filters which allow the filtering of different protocols. Some examples of those filters are:

  • SMTP filter
  • HTTP filter
  • Malware inspection filter
  • Outgoing HTTPS inspection
  • FTP Access filter
  • GAPA (NIS) filter

Firewall Policy rules which uses these filters will take longer to be processed than those rules without “intelligent” application filter.

General rule order recommendations

Based on this information Microsoft recommends organizing Firewall Policy rules in the following order:

  • Global deny rules
  • Global allow rules
  • Rules for specific computers
  • Rules for specific users
  • Other allow rules

Global deny rules

A Global deny rule should be used when you want to deny all users access to specific protocols for example you would like to deny access the use of the SIP protocol from Internal to External for all users.

Global allow rules

A global allow rule for example to allow all users to access FTP servers on the Internet.

Rules for specific computers

Rules that allow or deny access for specific computers, For example you only want to allow you Administration PC to access one Server in the Internet with the RDP protocol.

Rules for specific users, URLs, and MIME types

Firewall policy rules for specific users, or for specific URL or MIME types and with advanced filtering like Malware Inspection, HTTP filtering, Network Inspection System (NIS).

Other allow rules

At least you should place other allow Firewall policy rule at the end of the Firewall Policy rule set when they doesn’t match the other criteria’s for placing Firewall policy rules.

Attention:
Webserver and Server publishing rules can be placed anywhere in the Firewall Policy rule set but I recommend grouping these publishing rules to have a better overview about the Firewall policy rules.

Conclusion

In this article we started with a short overview about Forefront TMG networks, network rule, and Firewall policy rule elements to give you an overview of these essential daily administration tasks. After that I discussed some guidelines for best practices on how to create an efficient Firewall policy with Forefront TMG and how important the order of Firewall Policy rule may be.

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