DirectAccess Selective Tunneling

DirectAccess Selective TunnelingDirectAccess administrators, and network administrators in general, are likely familiar with the terms “split tunneling” and “force tunneling”. They dictate how traffic is handled when a DirectAccess (or VPN) connection is established by a client. Split tunneling routes only traffic destined for the internal network over the DirectAccess connection; all other traffic is routed directly over the Internet. Force tunneling routes all traffic over the DirectAccess connection.

Force Tunneling

DirectAccess uses split tunneling by default. Optionally, it can be configured to use force tunneling if required. Force tunneling is commonly enabled when DirectAccess administrators want to inspect and monitor Internet traffic from field-based clients.

Note: One-time password user authentication is not supported when force tunneling is enabled. Details here.

Drawbacks

Force tunneling is not without its drawbacks. It requires that an on-premises proxy server be used by DirectAccess clients to access the Internet, in most cases. In addition, the user experience is often poor when force tunneling is enabled. This is caused by routing Internet traffic, which is commonly encrypted, over an already encrypted connection. The added protocol overhead caused by double encryption (triple encryption if you are using Windows 7!) along with using a sub-optimal network path increases latency and can degrade performance significantly. Also, location-based services typically fail to work correctly.

Selective Tunneling

“Selective Tunneling” is a term that I commonly use to describe a configuration where only one or a few specific public resources are tunneled over the DirectAccess connection. A common use case is where access to a cloud-based application is restricted to the IP address of a corporate proxy or firewall.

Using the Name Resolution Policy Table (NRPT) and taking advantage of DirectAccess and its requirement for IPv6, DirectAccess administrators can choose to selectively route requests for public hosts or domains over the DirectAccess connection. The process involves defining the public Fully Qualified Domain Name (FQDN) as “internal” in the DirectAccess configuration and then assigning an on-premises proxy server for DirectAccess clients to use to access that namespace.

Enable Selective Tunneling

While some of the selective tunneling configuration can be performed using the Remote Access Management console, some of it can only be done using PowerShell. For this reason, I prefer to do everything in PowerShell to streamline the process.

Run the following PowerShell commands on the DirectAccess server to enable selective tunneling for the “.example.com” domain.

$namespace = “.example.com” # include preceding dot for namespace, omit for individual host
$dnsserver = Get-ItemPropertyValue –Path HKLM:\\SYSTEM\CurrentControlSet\Services\RaMgmtSvc\Config\Parameters -Name DnsServers

Add-DAClientDnsConfiguration -DnsSuffix $namespace -DnsIpAddress $dnsserver -PassThru

$gpo = (Get-RemoteAccess).ClientGpoName
$gpo = $gpo.Split(‘\’)[1]
$proxy = “proxy.corp.example.net:8080” # this is the FQDN and port for the internal proxy server
$rule = (Get-DnsClientNrptRule -GpoName $gpo | Where-Object Namespace -eq $namespace | Select-Object -ExpandProperty “Name”)

Set-DnsClientNrptRule -DAEnable $true -DAProxyServerName $proxy -DAProxyType “UseProxyName” -Name $rule -GpoName $gpo

If Windows 7 client support has been enabled, run the following PowerShell commands on the DirectAccess server. If multisite is enabled, run these commands on one DirectAccess server in each entry point.

$downlevelgpo = (Get-RemoteAccess).DownlevelGpoName
$downlevelgpo = $downlevelgpo.Split(‘\’)[1]
$proxy = “proxy.corp.example.net:8080” # this is the FQDN and port for the internal proxy server
$downlevelrule = (Get-DnsClientNrptRule -GpoName $downlevelgpo | Where-Object Namespace -eq $namespace | Select-Object -ExpandProperty “Name”)

Set-DnsClientNrptRule -DAEnable $true -DAProxyServerName $proxy -DAProxyType “UseProxyName” -Name $downlevelrule -GpoName $downlevelgpo

To remove a namespace from the NRPT, run the following PowerShell command.

Remove-DAClientDnsConfiguration -DnsSuffix $namespace

Caveats

While selective tunneling works well for the most part, the real drawback is that only Microsoft browsers (Internet Explorer and Edge) are supported. Web sites configured for selective tunneling will not be reachable when using Chrome, Firefox, or any other third-party web browser. In addition, many web sites deliver content using more than one FQDN, which may cause some web pages to load improperly.

Additional Resources

DirectAccess Force Tunneling and Proxy Server Configuration

NetMotion Mobility for DirectAccess Administrators – Split vs. Force Tunneling

Leave a comment

4 Comments

  1. Jonas

     /  June 28, 2018

    First of love your work, you’re life saver!

    What kind of proxy’s would you recommend for selective tunneling?

    You say that only internet explorer and edge is supported. Does that mean that this only support web traffic?

    Reply
    • Thanks Jonas! 🙂 You can use any proxy you like, really. There are many excellent commercial solutions, but I often deploy Squid on Linux as it is open source and free. If you’re not comfortable with Linux there’s a Windows version as well. http://www.squid-cache.org/

      Reply
  2. Wojciech

     /  October 4, 2018

    Hello Richard,

    Following your blog, I have implemented the “selective tunneling” solution in our environment. In general, it seems to work. I do however have one problem, namely with name resolution.

    In my case, I need to route traffic to “domain.com” through our internal network, and at the same time resolve addresses from that domain on the Direct Access server. I have noticed that even if i put “domain.com” in the NRPT table, the DA clients resolve those names on the external DNS servers. Do you know if this can be changed?

    As a workaround, I can explicitly specify hosts in “domain.com” in the NRPT table and have them resolve internally. Though with dozens of hosts, I’m scared that the NRPT table would grow too big and slow down the performance of remote clients.

    Of course if there is no other option, I’ll do it:)

    Kind regards,

    Wojciech

    Reply

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