======Microsoft - Scripting - PowerShell - Snippets - Disable/Enable LLMNR======

A snippet to disable LLMNR at the start of a script, so a DNS name resolution call would not get resolved with LLMNR [System.Net.Dns]::GetHostAddresses("SERVERNAME") , and re-enable LLMNR at the end of the script.

See [[microsoft:scripting:powershell:snippets:logging]] for the Log-* funtions.


=====Start with Disable=====

<code powershell>
$bLLMNRPolicyState = 0

Log-Info "Disabling LLMNR"

# Disable LLMNR.
# If the DNSClient is not present, create it and the EnableMulticast value.
if (-Not (Test-Path -Path "HKLM:\Software\policies\Microsoft\Windows NT\DNSClient")) {

	Log-Info "DNSClient key not present, creating it and the EnableMulticast value."

	$bLLMNRPolicyState = 1	# Value not present.

	try {

		Log-Info "Creating the DNSClient policy key."

		# Create the DNSClient key.
		New-Item -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" -ErrorAction Stop

		Log-Info "Creating the EnableMulticast property with value."

		# Create the EnableMulticast value set to 0.
		New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" `
			-Name "EnableMulticast" -PropertyType "DWORD" -Value 0 -Force | Out-Null

		Log-Info "Restarting the DNS Client service."

		# Restart the DNS Client service to enforce the change.
		Restart-Service -Name "dnscache" -Force

	} catch {

		Log-Error "Could not disable LLMNR on the local system."
		Log-Error $Error[0]
		Exit

	}

# If the DNSClient is present, check and correct the EnableMulticast value.
} else {

	Log-Info "DNSClient key is already present, checking EnableMulticast value."

	$objEnableMulticast = Get-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" -PSProperty "EnableMulticast" -ErrorAction SilentlyContinue

	# If it's set to 0 it is already disabled.
	if ($objEnableMulticast.EnableMulticast -eq 0) {

		Log-Info "LLMNR is already disabled. Not changing this."

		$bLLMNRPolicyState = 2	# Already disabled.

	} elseif ($objEnableMulticast.EnableMulticast -eq 1) {

		Log-Info "LLMNR is explicitly enabled. Setting it to disabled."

		try {

			# Overwrite the value.
			New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" `
				-Name "EnableMulticast" -PropertyType "DWORD" -Value 0 -Force | Out-Null

			# Restart the DNS Client service to enforce the change.
			Restart-Service -Name "dnscache" -Force

		} catch {

			Log-Error "Could not disable LLMNR on the local system."
			Log-Error $Error[0]
			Exit

		}

		$bLLMNRPolicyState = 3	# Disabled by this script.

	} else {

		Log-Info "The EnableMulticast value is not present. Creating it."

		try {

			# Create the policy property with value.
			New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" `
				-Name "EnableMulticast" -PropertyType "DWORD" -Value 0 -Force | Out-Null

			# Restart the DNS Client service to enforce the change.
			Restart-Service -Name "dnscache" -Force

		} catch {

			Log-Error "Could not disable LLMNR on the local system."
			Log-Error $Error[0]
			Exit

		}

		$bLLMNRPolicyState = 1	# Value not present.

	}

	Clear-Variable objEnableMulticast

}
</code>


=====End with Enable=====

<code powershell>
Log-Info "Restoring LLMNR settings."

# Restore the LLMNR setting to the value it had before starting the script.
switch ($bLLMNRPolicyState) {

	# LLMNR key was not present
	1 {

		try {

			# Delete the key.
			Remove-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" -Name "EnableMulticast" -Force -ErrorAction Stop

			# Restart the DNS Client service to enforce the change.
			Restart-Service -Name "dnscache" -Force -ErrorAction Stop

		} catch {

			Log-Error "Failed to delete the EnableMulticast value."
			Log-Error $Error[0]

		}

	}

	# LLMNR was already set to disabled.
	2 {

		Log-Info "LLMNR was already set to disabled. Not changing."

	}

	# LLMNR was disabled by this script.
	3 {

		try {

			# Overwrite the value.
			New-ItemProperty -Path "HKLM:\SOFTWARE\Policies\Microsoft\Windows NT\DNSClient" `
				-Name "EnableMulticast" -PropertyType "DWORD" -Value 0 -Force -ErrorAction Stop | Out-Null

			# Restart the DNS Client service to enforce the change.
			Restart-Service -Name "dnscache" -Force -ErrorAction Stop

		} catch {

			Log-Error "Failed to restore the LLMNR setting to enabled."
			Log-Error $Error[0]

		}

	}

	Default {

		Log-Warning "Unexpected value of $bLLMNRPolicyState for bLLMNRPolicyState. Not changing LLMNR registry setting."

	}

}
</code>