SMB3 PowerShell changes in Windows Server 2012 R2: SMB Delegation

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Introduction 

Windows Server 2012 R2 introduced a new version of SMB. Technically it’s SMB version 3.02, but we continue to call it just SMB3. The main changes are described at https://technet.microsoft.com/en-us/library/hh831474.aspx.

With this new release, we made a few changes in SMB PowerShell to support the new scenarios and features. This includes a few new cmdlets and some changes to existing cmdlets, with extra care not break any of your existing scripts.

This blog post outlines one of the 7 set of changes related to SMB PowerShell in Windows Server 2012 R2.

 

The need for SMB Delegation

 

When you configure Hyper-V over SMB and you manage your Hyper-V hosts remotely using Hyper-V Manager, you will might run into access denied messages. This is because you’re using your credentials from the remote machine running Hyper-V Manager in the Hyper-V host to access a third machine (the file server). This is what we call a “double-hop”, and it’s not allowed by default for security reasons.The main problem with the scenario is that an intruder that compromises one computer in your environment could then connect to other systems in your environments without the need to provide a username and password. One way to work around this issue is to connect directly to the Hyper-V host and providing your credentials at that time, avoiding the double-hop.

You can also address this by configuring Constrained Delegation for SMB shares, which is a process that involves changing properties in Active Directory. The security risk is reduced here because a potential intruder double-hop would be limited to that specific use case (using SMB shares on the specified servers). The constrained delegation process was greatly simplified in Windows Server 2012 when the the Active Directory team introduced resource-based Kerberos constrained delegation, as explained at https://technet.microsoft.com/library/hh831747.aspx. However, even with  this new resource-based constrained delegation, there are still quite a few steps to enable it.

 

Requirements for SMB Delegation

 

Before you use the new SMB Delegation cmdlets, you must meet two specific requirements.

 

First, the new cmdlets do rely on Active Directory PowerShell to perform their actions. For this reason, you need to install the Active Directory cmdlets before using the SMB delegation cmdlets. To install the Active Directory cmdlets, use:

  • Install-WindowsFeature RSAT-AD-PowerShell

 

Second, these cmdlets rely on the the new resource-based delegation in Active Directory. Since that AD feature was introduced in Windows Server 2012, the Active Directory forest must be in “Windows Server 2012” functional level. To check the Active Directory Forest Functional level, use:

  • Get-ADForest

 

The new SMB Delegation cmdlets

 

For Hyper-V over SMB in Windows Server 2012, we provided TechNet and blog-based guidance on how to automate constrained delegation. In Windows Server 2012 R2, SMB has a new set of cmdlets to simplify the configuration of resource-based constrained Delegation in SMB scenarios.

 

Here are the new cmdlets introduced:

  • Get-SmbDelegation –SmbServer X

  • Enable-SmbDelegation –SmbServer X –SmbClient Y

  • Disable-SmbDelegation –SmbServer X [–SmbClient Y] [-Force]

 

Notes

 

1) For the Disable-SmbDelegation cmdlet, if no client is specified, delegation will be removed for all clients.

2) System Center Virtual Machine Manager uses a different method to remote into the Hyper-V host and configure SMB shares. When using VMM, constrained delegation is not required for management of Hyper-V over SMB.

3) This blog post is an updated version of the September 2013 post at  https://blogs.technet.com/b/josebda/archive/2013/09/03/what-s-new-in-smb-powershell-in-windows-server-2012-r2.aspx focused on a single topic.