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Planning Server Isolation Zones

Updated: January 27, 2010

Applies To: Windows Server 2008, Windows Server 2008 R2

Sometimes a server hosts data that is sensitive. If your servers host data that must not be compromised, you have several options to help protect that data. One was already addressed: adding the server to the encryption zone. Membership in that zone prevents the server from being accessed by any computers that are outside the isolated domain, and encrypts all network connections to server.

The second option is to additionally restrict access to the server, not just to members of the isolated domain, but to only those users or computers who have business reasons to access the resources on the server. You can specify only approved users, or you can additionally specify that the approved users can only access the server from approved computers.

To grant access, you add the approved user and computer accounts to network access groups (NAGs) that are referenced in a firewall rule on this server. When the user sends a request to the server, the standard domain isolation rules are invoked. This causes IKE to use Kerberos V5 to exchange credentials with the server. The additional firewall rule on the server causes Windows to check the provided computer and user accounts for group membership in the NAGs. If either the user or computer is not a member of a required NAG then the network connection is refused.

If you are using an isolated domain, the client computers already have the IPsec rules to enable them to authenticate traffic when the server requires it. If you add an isolated server, it must have a GPO applied to its group with the appropriate connection security and firewall rules. The rules enforce authentication and restrict access to only connections that are authenticated as coming from an authorized computer or user.

If you are not using an isolated domain, but still want to isolate a server that uses IPsec, you must configure the client computers that you want to access the server to use the appropriate IPsec rules. If the client computers are members of an Active Directory domain, you can still use Group Policy to configure the clients. Instead of applying the GPO to the whole domain, you apply the GPO to only members of the NAG.

Each set of servers that must be accessed by different sets of users should be set up in its own isolated server zone. After one set of GPOs for one isolated server zone has been successfully created and verified, you can copy the GPOs to a new set. You must change the GPO names to reflect the new zone, the name and membership of the isolated server zone group to which the GPOs are applied, and the names and membership of the NAG groups that determine which clients can access the servers in the isolated server zone.

Creation of the groups and how to link them to the GPOs that apply the rules to members of the groups are discussed in the Planning Group Policy Deployment for Your Isolation Zones section.

An isolated server is often a member of the encryption zone. Therefore, copying that GPO set serves as a good starting point. You then modify the rules to additionally restrict access to only NAG members.

GPOs for computers running Windows Server 2008 R2 or Windows Server 2008 should include the following:

noteNote
The connection security rules described here are identical to the ones for the encryption zone. If you do not want to encrypt access and also restrict access to NAG members, you can use connection security rules identical to the main isolated domain. You must still add the firewall rule described at the end of this list to change it into an isolated server zone.

  • IPsec default settings that specify the following options:

    1. Exempt all ICMP traffic from IPsec.

    2. Key exchange (main mode) security methods and algorithm. We recommend that you do not include Diffie-Hellman Group 1, DES, or MD5 in any setting. They are included only for compatibility with previous versions of Windows. Use the strongest algorithm combinations that are common to all your supported operating systems.

    3. Data protection (quick mode) algorithm combinations. Check Require encryption for all connection security rules that use these settings, and then specify one or more integrity and encryption combinations. We recommend that you do not include DES or MD5 in any setting. They are included only for compatibility with previous versions of Windows. Use the strongest algorithm combinations that are common to all your supported operating systems.

      If any NAT devices are present on your networks, do not use AH because it cannot traverse NAT devices. If isolated servers must communicate with hosts in the encryption zone, include an algorithm that is compatible with the requirements of the encryption zone GPOs.

    4. Authentication methods. Include at least computer-based Kerberos V5 authentication for compatibility with the rest of the isolated domain. If you want to restrict access to specific user accounts, also include user-based Kerberos V5 authentication as an optional authentication method. Do not make the user-based authentication method mandatory, or else computers that cannot use AuthIP instead of IKE, including Windows XP and Windows Server 2003, cannot communicate. Likewise, if any of your domain isolation members cannot use Kerberos V5, include certificate-based authentication as an optional authentication method.

  • The following connection security and firewall rules:

    • A connection security rule that exempts all computers on the exemption list from authentication. Be sure to include all your Active Directory domain controllers on this list. Enter subnet addresses, if applicable in your environment.

    • A connection security rule, from Any IP address to Any IP address, that requires inbound and requests outbound authentication by using Kerberos V5 authentication.

      ImportantImportant
      Be sure to begin operations by using request in and request out behavior until you are sure that all the computers in your IPsec environment are communicating successfully by using IPsec. After confirming that IPsec is operating as expected, you can change the GPO to require in, request out.

    • A firewall rule that specifies Allow only secure connections, Require encryption, and on the Users and Computers tab includes references to both computer and user network access groups.

  • A registry policy that includes the following values:

    1. Enable PMTU discovery. Enabling this setting allows TCP/IP to dynamically determine the largest packet size supported across a connection. The value is found at HKLM\System\CurrentControlSet\Services\TCPIP\Parameters\EnablePMTUDiscovery (dword). The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets the value to 1.

    2. Enforce the default IPsec protocol exemptions of ISAKMP only. This setting is documented in Knowledge Base article 810207 at http://go.microsoft.com/fwlink/?linkid=110516. The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets this value to 3.

    3. If required by the organization, enable IPsec over NAT-T. This setting is documented in Knowledge Base article 120492 at http://go.microsoft.com/fwlink/?linkid=120492. The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets this value to 0 (the default). To enable IPsec over NAT-T, you must change this value to either 1 or 2, as required by your environment.

    noteNote
    For a sample template for these registry settings, see Appendix A: Sample GPO Template Files for Settings Used in this Guide.

You must create a new IPsec policy instead of modifying an existing IPsec policy in a copied GPO. Because all GPOs share a common store of IPsec policies, if you modify an IPsec policy in a copied GPO, you are modifying the shared one used by other GPOs. Make sure that your newly created IPsec policy is the one assigned in the GPO.

The GPOs for computers that are running Windows 2000 or Windows Server 2003 should include the following:

noteNote
This IPsec policy is identical to the one for the encryption zone setting, except for the addition of the User Rights Assignment setting. If you do not want to encrypt access and also restrict access to NAG members, you can use an IPsec policy identical to the main isolated domain.

  • An IPsec policy that includes the following settings and security rules:

    1. Key exchange settings that specify main mode security methods and algorithms. We recommend that you do not include Diffie-Hellman Group 1, DES, or MD5 in any setting. They are included only for compatibility with previous versions of Windows. Use the strongest algorithm combinations that are common to all your supported operating systems.

    2. A permit rule for all ICMP traffic using My IP Address to Any IP Address.

    3. A permit rule for all computers on the exempted list. Be sure to include all your Active Directory domain controllers on this list. Take advantage of the ability to enter subnet addresses, if applicable in your environment.

    4. A negotiate rule for all network addresses using My IP Address to subnet addresses that make up the network address space. The filter action should specify Negotiate security, and then specify the encryption protocols to be required by members of the zone. We recommend that you do not include DES or MD5 in any setting. They are included only for compatibility with previous versions of Windows. Use the strongest algorithm combinations that are common to all your supported operating systems. If any NAT devices are present on your networks, do not use AH because it cannot traverse NAT devices.

      To initially make the GPO request authentication for inbound and outbound traffic, select both the Accept unsecured communication and Allow fallback to unsecured communication check boxes. After you have confirmed that all your computers are successfully using IPsec as designed, come back to this setting, and then clear the Accept unsecured communication check box to require inbound authentication.

  • A User Rights Assignment setting that sets the Access this computer from the network user right to only include the computer NAG, the user NAG, and the IPsec exempt computers. Ensure that users who must administer the server, and the computers from which they work are members of the NAG groups.

  • Also under User Rights Assignment, you can consider adding groups for users or computers that must be prevented from accessing the servers in the isolation zone. They are added to the Deny access to this computer from the network user right. This might include the CG_DOMISO_Boundary group, because they are computers at a greater risk of being compromised and therefore pose a higher risk to your isolated servers.

  • A registry policy that includes the following values:

    1. Enable PMTU discovery. Enabling this setting allows TCP/IP to dynamically determine the largest packet size supported across a connection. The value is found at HKLM\System\CurrentControlSet\Services\TCPIP\Parameters\EnablePMTUDiscovery (dword). The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets the value to 1.

    2. Enforce the default IPsec protocol exemptions. This setting is documented in Knowledge Base article 810207 at http://go.microsoft.com/fwlink/?linkid=110516. The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets this value to 3 on Windows Server 2003, and sets the value to 1 on Windows XP and Windows 2000.

    3. If required by the organization, enable IPsec over NAT-T. This setting is documented in Knowledge Base article 120492 at http://go.microsoft.com/fwlink/?linkid=120492. The sample GPO preferences XML file in Appendix A: Sample GPO Template Files for Settings Used in this Guide sets this value to 0 (the default). To enable IPsec over NAT-T, you must change this value to either 1 or 2, as required by your environment.

    4. Set the Simplified IPsec Policy registry entry to a value of 0x14 to improve the 'fall back to clear' behavior in Windows XP and Windows Server 2003. To use this, you must have already deployed the update available for free download (for Windows Server 2003) from the Knowledge Base article 914841 at http://go.microsoft.com/fwlink/?linkid=110514.

    noteNote
    For a sample template for these registry settings, see Appendix A: Sample GPO Template Files for Settings Used in this Guide.

Make sure that your GPOs for stationary computers, such as desktop and server computers, assign all rules to all profiles. For portable computers, you might want to allow more profiles to enable users to communicate successfully when they are not connected to the organization's network.

Next:  Planning Certificate-based Authentication

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