Blocking Malware Domains in ISA 2006

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As in many households, several regular and occasional computer users take advantage of my connection to the outside world. I use ISA Server 2006 running as a virtual Hyper-V instance for firewalling and connection management (I'm not brave enough to upgrade to Forefront yet), and all incoming ports are firmly closed. But these days the risk of picking up some nasty infection is just as great from mistaken actions by users inside the network as from the proliferation of malware distributors outside.

Even when running under limited-permission Windows accounts and with reasonably strict security settings on the browsers, there is a chance that less informed users may allow a gremlin in. So I decided some while ago to implement additional security by blocking all access to known malware sites. I know that the web browser does this to some extent, but I figured that some additional layer of protection - plus logging and alerts in ISA - would be a useful precaution. So far it seems to have worked well, with thankfully only occasional warnings that a request was blocked (and mostly to an ad server site).

The problem is: where do you get a list of malware sites? After some searching, I found the Malware Domain Blocklist site. They publish malware site lists in a range of formats aimed at DNS server management and use in your hosts file. However, they also provide a simple text list of domains called JustDomains.txt that is easy to use in a proxy server or ISA. Blocking all web requests for the listed domains will provide some additional protection against ingress and the effects of malware that might inadvertently find its way into a machine otherwise; and you will see the blocked requests in your log files.

They don’t charge for the malware domain lists, but you decide to use them please do as I did and make a reasonable donation. Also be aware that malware that connects using an IP address instead of a domain name will not be blocked when you use just domain name lists.

To set it up in ISA 2006, you need the domain list file to be in the appropriate ISA-specific format. It's not available in this format, but a simple utility will convert it. You can download a free command line utility I threw together (the Visual Studio 2010 source project is included so you can check the code and recompile it yourself if you wish). It takes the text file containing the list of malware domain names and generates the required XML import file for ISA 2006 using a template. There's a sample supplied but you'll need to export your own configuration from the target node and edit that to create a suitable template for your system. You can also use a template to generate a file in any other format you might need.

ISA 2006 Toolbox

To configure ISA open the Toolbox list, find the Domain Name Sets node, right-click, and select New Domain Name Set. Call it something like "Malware Domains". Click Add and add a dummy temporary domain name to it, then click OK. The dummy domain will be removed when you import the list of actual domain names. Then right-click on your new Malware Domains node, click Export Selected, and save the file as your template for this node. Edit it to insert the placeholders the utility requires to inject the domain names into it as described in the readme file and sample template provided.

Malware Domaind List

After you generate your import file, right-click on your Malware Domains node, click Import to Selected, and locate the import file you just created from the list of domain names. Click Next, specify not to import server-specific information, and then click Finish. Open your Malware Domain set from the Toolbox and you should see the list of several thousand domain names.

 Now you can configure a firewall rule for the new domain name set. Right-click the Firewall Policy node in the main ISA tree view and click New Rule. Call it something recognizable such as "Malware Domains". In the Action tab select Deny and turn on the Log requests matching this rule option. In the Protocols tab, select All outbound traffic. In the From tab, click Add and add all of your local and internal networks. In the To tab click Add and add your Malware Domains domain name set. In the Content Types tab, select All content types. In the Users tab select All users, and in the Schedule tab select Always. Then click OK, click Apply in the main ISA window, and move the rule to the top of the list of rules.

ISA Block Rule

You can test your new rule by temporarily adding a dummy domain to the Domain Name Set list and trying to navigate to it. You should see the ISA server page indicating that the domain is blocked.

If you wish, you can create a list of IP addresses of malware domains and add this set to your blocking rule as well so that malware requests that use an IP address instead of a domain name are also blocked. The utility can resolve each of the domain names in the input list and create a file suitable for importing into a Computer Set in ISA 2006. The process for creating the Computer Set and the template is the same as for the Domain Name Set, except you need to inject the domain name and IP address of each item into your import file. Again, a sample template that demonstrates how is included, but you must create your own version as described above.

Be aware that some domains may resolve to internal or loopback addresses, which may affect operation of your network if blocked. The utility attempts to recognize these and remove them from the resolved IP address list, but use this feature with care and check the resolved IP addresses before applying a blocking rule.

Another issue is the time it takes to perform resolution of every domain name, and investigations undertaken here suggest that only about one third of them actually have a mapped IP address. You'll need to decide if it's worth the effort, but you can choose to have the utility cache resolved IP addresses to save time and bandwidth resolving them all again (though this can result in stale entries). If you do create a Computer Set, you simply add it to the list in the To tab of your blocking rule along with your Domain Name Set. Of course, you need to regularly update the lists in ISA, but this just involves downloading the new list, creating the import file(s), and importing them into your existing Domain Name Set and Computer Set nodes in ISA.

  • has a tool to automate the importing of the Malware Domains list into ISA available at

  • Good to know that your guys are using my tool still. Thanks.

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Blocking Malware Domains in ISA 2006