In Microsoft Direct Access Deployments, you need to configure a server (or server cluster) on your network to act as a Network Location Server (NLS). The NLS is simply a Web Server with a HTTPS server certificate... see the following TechNet article on how to configure an NLS:
https://technet.microsoft.com/en-us/library/ee649252(v=ws.10).aspx
If the DirectAccess client can establish a connection to the NLS, it will believe that it is on the internal network and the DirectAccess tunnels will not be established. If a connection to the NLS cannot be established, the DirectAccess client believes it is outside the corporate network and will attempt to establish DirectAccess connectivity. It is for this reason that the NLS should not be resolvable in public DNS and should not be reachable externally. If it is, DirectAccess clients will always think they are inside the corporate network, and DirectAccess will never be established. Also, it is extremely important that the NLS be highly available. If the NLS server is offline or unreachable for any reason, DirectAccess clients that are on the internal network will suddenly think they are outside the corporate network and attempt to establish DirectAccess connectivity. This will fail, leaving DirectAccess clients unable to connect to any internal resources until the NLS is once again available. For this reason it is strongly recommended that you implement at least two NLS servers in a cluster, using either Network Load Balancing (NLB) or an external hardware load balancer.
However for smaller deployments of Direct Access, a NLS cluster for redundancy is not always practical and sometimes only a single NLS server is deployed. This is often the case with all in one Direct Access Deployments running Windows Server 2012 R2 when a single Direct Access server is deployed to act as both the NLS and Direct Access endpoint. If the all in one Direct Access server was to fail, all clients on the Internal network will no longer think they are on the internal network as they cannot contact the NLS and as a result attempt to establish a Direct Access connection which will also fail leaving them in a state of no connectivity to any domain names configured with a Name Resolution Policy Table (NRPT) in the Direct Access group policy. Most Direct Access deployments have the Active Directory domain namespace configured as an NRPT policy and sometimes additional domain names. To understand what NRPT is, please see the following blog post:
http://clintboessen.blogspot.com.au/2014/01/what-is-name-resolution-policy-table.html
What Happens if Your In this Situation?
What happens if your in this situation when you have a single Direct Access server which has failed and as a result all Direct Access configured clients on the internal network now have no network connectivity?
The NRPT Policies which are pushed out via Group Policy are located under the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\DnsClient\DnsPolicyConfig
You can manually connect to the direct access computers and remove these registry keys by IP address using the "Connect Network Registry" option in regedit.exe. Once you are connected to the remote computer, you can delete the two NRPT policies named DA-{GUID].
After you have deleted the two policies remotely, simply reboot the workstation and it will regain connectivity.
I know this is a very manual process and can be very time consuming if there are hundreds of computers experiencing the issue - I would like to hear from Microsoft for a better solution in this scenario. I sent of an email regarding this and hopefully will hear back soon!
It is also important to note, for my customer who's Direct Access server failed, rebuilding a new server with the same IP address, hostname and configuration did not resolve this issue. I still needed to delete the NRPT policies from the registry from the effected computers and reboot. After this, the clients had restored connectivity to the internal network and were able to refresh group policy again in which they downloaded the new Direct Access Client Settings containing the new NRPT policies.
