Windows Server 2012 R2 DirectAccess on Amazon Web Services

A while back I wrote about configuring Windows Server 2012 R2 DirectAccess in Microsoft Azure. Running DirectAccess in Azure suffered from a few complications, and ultimately it is not supported. That doesn’t mean Azure wouldn’t be useful to stand up a test environment with which to evaluate DirectAccess, however. That’s not the case for DirectAccess running on Amazon though. DirectAccess is a fully supported workload running in the Amazon public cloud. Recently Amazon released a whitepaper demonstrating the use of Windows Server 2012 R2 DirectAccess running on AWS. Although this first whitepaper documents the use of DirectAccess in single-NIC mode supporting only Windows 8 clients, look for future whitepapers to cover topics like support for Windows 7 clients and high availability using Amazon Elastic Load Balancing (ELB). For more information, visit the Amazon whitepapers site and select the Partner Whitepapers tab.

Configure Windows Server 2012 R2 DirectAcess on Amazon Web Services (AWS)

Or, if you prefer, you can download the document directly here.

DirectAccess IPv6 Transition Protocols Explained

Introduction

From a client perspective, DirectAccess is an IPv6-only solution. The DirectAccess client communicates with the DirectAccess server exclusively using IPv6. However, IPv6 is not widely deployed, so the most common scenario will find your DirectAccess clients and servers on the IPv4 Internet.

To facilitate DirectAccess client to server communication with IPv6 when the client is on the IPv4 Internet, IPv6 transition protocols are employed. These protocols effectively tunnel IPv6 packets in IPv4 packets. DirectAccess makes use of three IPv6 transition protocols for client to server connections – 6to4, Teredo, and IP-HTTPS.

DirectAccess Transition Protocols

6to4 – The 6to4 IPv6 transition protocol works by encapsulating IPv6 packets in IPv4 packets using IP protocol 41. 6to4 does not work when the client or the server is behind a NAT, so this IPv6 transition protocol is only used when the client and server are assigned public IPv4 addresses. DirectAccess clients with public IPv4 addresses aren’t common though, and there are some challenges with the stability of 6to4. From experience I can tell you that 6to4 often fails when clients use a cellular Wi-Fi hotspot, for example. For this reason it is generally recommended that you proactively disable this transition protocol to avoid potential issues in the future.

TeredoTeredo is an IPv6 transition protocol that is designed to work when a DirectAccess client (but not the DirectAccess server) is behind a NAT. It works by encapsulating IPv6 packets in IPv4 packets using UDP on port 3544. Teredo will be used any time the DirectAccess client has a private IPv4 address, or when the client has a public IPv4 address and the 6to4 protocol is unavailable (e.g. 6to4 is disabled, or outbound access to IP protocol 41 is restricted by firewall policy). To support Teredo, the DirectAccess server must be configured with two consecutive public IPv4 addresses. In addition, Teredo uses ICMP for NAT detection (e.g. cone, restricted, symmetric), so ICMPv4 echo requests must be allowed inbound to any host with which the DirectAccess client communicates.

IP-HTTPSIP-HTTPS is an IPv6 transition protocol that works by encapsulating IPv6 packets in IPv4 packets using HTTP with SSL/TLS. It is the IPv6 transition protocol of last resort, and will be used any time that 6to4 or Teredo aren’t available. The advantage to using IP-HTTPS is ubiquitous firewall access. Any network with access to the public Internet should, at a minimum, allow outbound HTTP and HTTPS. In some deployment scenarios, IP-HTTPS can be disadvantageous. For example, when Windows 7 DirectAccess clients leverage this IPv6 transition protocol, IPsec-encrypted traffic is encrypted again using SSL/TLS. This double encryption results in high processing overhead and often translates to poor performance and limited scalability. Windows 8 and later clients do not suffer this limitation, as they support null encryption which eliminates the negative effects imposed by double encryption. For the best results using IP-HTTPS, use an application delivery controller to offload SSL, or deploy Windows 8 or later clients. In any case, do not collocate the client-based VPN role on the DirectAccess server, as doing so will remove support for null encryption completely and force even Windows 8 and later clients to perform double encryption for IP-HTTPS traffic.

DirectAccess Server Configuration

To support the 6to4 and Teredo IPv6 transition protocols, the DirectAccess server must be configured with two network interfaces; one internal and one external. The DirectAccess server must have public IPv4 addresses assigned to its external network interface. For Teredo in particular, the DirectAccess server requires two consecutive public IPv4 addresses. Beginning with Windows Server 2012, DirectAccess provides support for DMZ/perimeter network deployment behind a NAT device using RFC1918 private IPv4 addresses with either one or two network interfaces. In this deployment scenario, the DirectAccess server only supports the use of the IP-HTTPS IPv6 transition protocol. 6to4 and Teredo are not available when the DirectAccess server is located behind a NAT device and these IPv6 transition protocols should be disabled on all DirectAccess clients.

Windows Server 2012 Remote Access Management Service Memory Leak

When Windows Server 2012 is configured for DirectAccess or client-based remote access Virtual Private Networking (VPN), a memory leak may occur in the Remote Access Management service when remote clients access the Internet using the DirectAccess or VPN connection. Microsoft knowledgebase article KB2895930 describes the issue in detail and includes a link to the hotfix to resolve this issue.

Microsoft Windows Server 2012/R2 DirectAccess

Troubleshooting Name Resolution Issues on DirectAccess Clients

When troubleshooting name resolution issues on a Windows client, NSlookup is an essential tool. However, it is important to understand that using NSlookup on a DirectAccess client might not always work as you expect. Although using NSlookup on a DirectAccess client will work normally when the client is on the corporate network, it will not provide the correct results to queries for internal hostnames when the DirectAccess client is outside of the corporate network without taking additional steps. This is because when a DirectAccess client is outside the corporate network, the Name Resolution Policy Table (NRPT) is enabled. The NRPT provides policy-based name resolution routing for DirectAccess clients, sending name resolution requests for certain namespaces to specific DNS servers. You can view the NRPT on a Windows 8.x DirectAccess client by issuing the following PowerShell command:

Get-DnsClientNrptPolicy

Troubleshooting Name Resolution Issues on DirectAccess Clients

You can view the NRPT on a Windows 7 DirectAccess client by issuing the following netsh command:

netsh namespace show policy

Troubleshooting Name Resolution Issues on DirectAccess Clients

Here you’ll notice that the namespace .lab.richardhicks.net is configured to use the DNS64 service running on the DirectAccess server at 2002:62bd:d898:3333::1. Notice also that the host nls.lab.richardhicks.net is not configured to use a DNS server. This effectively exempts this host from the NRPT, forcing name resolution requests for this Fully-Qualified Domain Name (FQDN) to be delivered to the DNS servers configured on the network adapter.

A Working Example

With the NRPT enabled, which occurs whenever the DirectAccess client is outside of the corporate network, a name resolution request for app1.lab.richardhicks.net would be sent to the DNS64 service on the DirectAccess server. A name resolution request for technet.microsoft.com would be sent to the DNS servers assigned to the network adapter because the NRPT contains no entry for this namespace. And even though the host nls.lab.richardhicks.net is a part of the internal namespace, a name resolution request for this host would also be sent to the DNS servers assigned to the network adapter because it has been specifically exempted from the NRPT.

NSlookup

The NSlookup utility is unaware of the NRPT. Whenever you use NSlookup it will, by default, automatically send queries directly to the DNS servers configured on the network adapter, regardless of the NRPT. If you wish to use NSlookup to test name resolution for external hostnames, use it as you normally would. However, if you wish to use NSlookup to resolve internal hostnames over the DirectAccess connection, you will need to tell NSlookup to use the DNS64 service running on the DirectAccess server. You can do this by running NSlookup interactively and using the server command to point it to the IPv6 address of the DNS64 service, which you can find in the NRPT.

Troubleshooting Name Resolution Issues on DirectAccess Clients

This also applies to the PowerShell cmdlet Resolve-DNSname. Here you’ll use the -Server switch to specify the DNS64 server’s IPv6 address.

Resolve-DNSName –Server <DNS64_IPv6_Address> app1.lab.richardhicks.net

Troubleshooting Name Resolution Issues on DirectAccess Clients

Configuration Guidance for DirectAccess Security Advisory KB2862152

Introduction

Since Microsoft released security advisory KB2862152, there has been much confusion surrounding where the associated update should be installed, in what deployment scenarios it needs to be installed, and what the best way to configure it is. Recently my colleague and good friend Jason Jones and I worked together to research and answer these questions.

Overview

Microsoft security advisory KB2862152 addresses a vulnerability in IPsec that could allow an attacker to perform a man-in-the-middle attack by spoofing a DirectAccess server to intercept network traffic and potentially capture encrypted domain credentials. The associated update is designed to allow security administrators to configure DirectAccess clients to perform more rigorous validation checks when establishing the DirectAccess IPsec tunnels. It’s important to understand that without additional client-side configuration, this security update does nothing.

Windows 8 Clients

For DirectAccess deployments that use Kerberos authentication (Kerberos Proxy), the update needs to be installed on all Windows 8.x clients. No updates are required for Windows 7 clients as they are not supported using this deployment model. To enforce additional validation checks, configure the registry on the Windows 8.x DirectAccess clients with the IP addresses and Service Principal Name (SPN) of the DirectAccess server as outlined here.

Windows 7 Clients

For DirectAccess deployments that use certificate-based authentication, the update needs to be installed on all Windows 7 clients. No updates are required for Windows 8.x client using this deployment model. To enforce additional validation checks, configure the registry on the Windows 7 DirectAccess clients with the IP addresses and either the fully-qualified domain name (FQDN) of the DirectAccess server (not recommended) or the Object Identifier (OID) of the computer certificated used for IPsec authentication (recommended, with custom OID).

The choice between using FQDN or OID is a challenging one, however. Choosing to validate the DNS name is simple and straightforward, but this information may be known to an attacker, or perhaps discoverable, allowing them to spoof it. In addition, there is a limit of 10 DNS names supported using this method, which can be potentially limiting, especially in large, multi-site deployments. Using the certificate OID is even more problematic, because by default it uses a well-known Server Authentication EKU OID (1.3.6.1.5.5.7.3.1) common to many Microsoft Active Directory Certificate Services (AD CS) certificate templates which, of course, could be spoofed by an attacker even easier.

The most effective implementation of this security update for DirectAccess deployments that use certificate-based authentication is to use the OID option with a certificate configured with a custom OID. Custom OIDs are unique to your organization and will help prevent spoofing by using a unique value that is much harder to guess or determine. The remainder of this article will outline how to configure and deploy a certificate with a custom OID along with implementation details for configuring the appropriate client-side registry settings via group policy to enforce the additional validation checks.

Server Configuration

To implement this, it will require creating and deploying a new certificate template. In the Certificate Services management console, right-click Certificate Templates and choose Manage. Right-click the Computer certificate template and choose Duplicate Template. Select the General tab and give the template a descriptive name.

DirectAccess KB2862152 Implementation Guidance

Select the Extensions tab, highlight Application Policies and click Edit. Click Add and then New, and then provide a descriptive name. Leave the OID as is and click Ok to continue.

DirectAccess KB2862152 Implementation Guidance

Right-click once again on Certificate Templates and choose New and then Certificate Template to Issue. Select the certificate template you just created and click Ok.

DirectAccess KB2862152 Implementation Guidance

Once complete you can request a new certificate for each of your DirectAccess servers using this new template.

DirectAccess KB2862152 Implementation Guidance

After you have successfully installed the computer certificate using this new template, be sure to delete the old computer certificate on each DirectAccess server. No further changes are required on the DirectAccess server.

Note: If you are assigning a computer certificate to the DirectAccess server via group policy auto enrollment, the certificate will be reinstalled again after it is deleted, once group policy refreshes. To avoid this situation you will need to deny access to this GPO to ensure that only a single computer certificate with the custom OID is installed on the DirectAccess server.

Client Configuration

To instruct the client to validate the tunnel endpoint IPv6 address and the OID of the DirectAccess server certificate before initiating IPsec tunnels we’ll need to configure registry settings on our DirectAccess clients. Jason Jones’ article describes which settings need to be made and when, so I won’t duplicate his efforts here. However, it is recommended that you deploy these settings using group policy, which I will cover.

To create a Group Policy Object (GPO) to deploy these registry settings, open the Group Policy Management Console, expand the target domain, right-click Group Policy Objects and select New. Give the new GPO a descriptive name and click Ok. Right-click the newly created GPO and choose Edit. Expand Computer Configuration, Preferences, and Windows Settings. Right-click Registry and choose New and then Registry Item. Select Update for the action and HKEY_LOCAL_MACHINE for the hive. Enter

SYSTEM\CurrentControlSet\Services\IKEEXT\Parameters\IPsecTunnelConfig\AuthIP\Cert

for the Key Path and enter DTE1 for the value. Select REG_MULTI_SZ for the Value Type and in the Value Data enter the IPv6 address of the first DTE. On the next line enter EKU:<OID> and click Ok.

DirectAccess KB2862152 Implementation Guidance

Repeat this procedure for each tunnel endpoint. Finally, highlight the GPO and change the Security Filtering from Authenticated Users to the security group for your DirectAccess clients and link the GPO to the domain.

DirectAccess KB2862152 Implementation Guidance

Exercise extreme caution when creating and implementing these GPOs to enforce additional validation checks. If there’s a typo somewhere or you forget a DTE, you could potentially orphan your DirectAccess clients. I recommend testing your changes by manually adding the registry entries required and then copying/pasting those settings to the GPO in Active Directory when you’re ready to deploy globally. Also, don’t forget that you’ll need to update GPOs each time you add a cluster node or multisite entry point.

How to Install and Configure KB2862152 for DirectAccess

Microsoft recently released security advisory 2862152 to address a vulnerability in IPsec that could allow DirectAccess security feature bypass. The associated update addresses an issue with how the DirectAccess client authenticates with a DirectAccess server. Without the update, it is possible for an attacker to launch a man-in-the-middle attack to intercept DirectAccess communication.

The update itself does not resolve the issue directly, however. The update simply allows administrators to configure DirectAccess clients using specific registry settings to enforce more stringent checks during IPsec negotiation after the update is installed. The challenge with this update is that the documentation contained within the knowledge base article is extremely detailed and includes information that pertains to many different remote access scenarios, not just DirectAccess. This has led to much confusion, and many administrators are unclear for which clients and deployment scenarios the registry changes are required.

For DirectAccess deployments, the update needs to be applied to all of your DirectAccess clients. The update does NOT need to be applied to the DirectAccess server. The registry settings required on the client will be dictated based on the configured authentication method for your DirectAccess deployment. If you have configured DirectAccess to use certificate-based authentication by checking selecting the Use computer certificates option as shown below, you’ll only need to make registry settings changes on your Windows 7 clients. Windows 8/8.1 clients DO NOT require any changes be made to the registry when DirectAccess is configured to use certificate-based authentication.

Microsoft Security Update KB2862152 for DirectAccess

If you are NOT using computer certificates for authentication, then you must make registry changes to all of your Windows 8/8.1 clients. For detailed, prescriptive guidance on implementing the client-side registry changes required to support this update and mitigate this vulnerability, Jason Jones has done a wonderful job documenting those steps specifically, so I’ll refer you to his post here.

You can find the update for KB2862152 for all supported clients here.

Vulnerability in DirectAccess Could Allow Security Feature Bypass

With the November 2013 security bulletin release, Microsoft advises that DirectAccess includes a vulnerability that could allow security feature bypass. This update affects all supported versions of Microsoft Windows and addresses an issue with how the DirectAccess server authenticates connections with DirectAccess clients. The vulnerability could be leveraged by an attacker to pose as a man-in-the-middle and intercept their communication. For more details, please review Microsoft Security Advisory 2862152.

Windows 8 DirectAccess Client Quick Tip

On a Windows 8 or 8.1 DirectAccess client, issuing a Get-DAConnectionStatus may return the following error:

Get-DAConnectionStatus : Network Connectivity Assistant service is stopped or not responding.
At line:1 char:1
+ Get-DAConnectionStatus
+ ~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (MSFT_DAConnectionStatus:root/StandardCi...onnectionStatus) [Get-DAConnect
   ionStatus], CimException
    + FullyQualifiedErrorId : Windows System Error 1753,Get-DAConnectionStatus

DirectAccess Connectivity Assistant Error

This issue is easily resolved by starting the Network Connectivity Assistant service by issuing the following PowerShell command:

Start-Service ncasvc

Get-DAConnectionStatus should now respond normally.

DirectAccess Connectivity Assistant Error

DirectAccess Webinar on October 3, 2013

Are you interested in learning about Microsoft DirectAccess, the always-on, seamless and transparent remote access feature in Windows Server 2012? Then join me on Thursday, October 3, 2013 for a webinar where I’ll describe in detail what DirectAccess is, how it functions, what the benefits are for deploying DirectAccess in terms of security and ease of use, and much more. I’ll also provide you with information about how Iron Networks can assist you with deploying DirectAccess quickly and effectively by leveraging our advanced hardware appliance platform and professional services. You can register for the webinar here.

direct_access

Windows Server 2012

Microsoft Security Update MS13-064 and DirectAccess

With the August security update release cycle, Microsoft issued security bulletin MS13-064 to address a vulnerability in the Windows NAT driver that could result in a denial of service. The vulnerability could be exploited by an attacker who sends a specially crafted ICMP packet to the server running the Windows NAT Driver service. The vulnerability exists only on Windows Server 2012 and the affected driver, winnat.sys, is present when the DirectAccess role is installed. This vulnerability only affects only full installations of Windows Server 2012. Windows Server 2012 Core is not affected. If you are running DirectAccess on a full installation of Windows Server 2012, make sure you install this update as soon as possible to be protected from potential denial of service attacks. For more information about this update, click here. For a comprehensive list of updates that apply to DirectAccess on Windows Server 2012 as well as previous versions, please refer to Jason Jones’ DirectAccess hotfix summary page.

%d bloggers like this: