Monday, July 31, 2017

Testing connectivity to your domain controller

In the distant past there was a useful client side tool for checking connectivity between clients and domain controllers (netdiag.exe). According to microsoft's command line reference guide, it is available in windows 8 and 2012, but in reality the command does not exist on any windows machine I have checked beyond 2003. Trying to run an older version won't work either due to some incompatibility. So, alternatives are required to do checks. One thing you would typically want to check between a client and a domain controller is port connectivity.  Below, I will show a simple script that tests most of the ports.  Some may not be open in your environment (like 636,3269 for ldaps).  Some ports are dynamic, so I haven't included trying to check these.

To begin with, you should know what domain controller your workstation has logged into.  This machine logon establishes the "secure channel" between your machine and the domain.  You can use an old tool that is still around called nltest. 

C:\Windows>nltest /
Trusted DC Name \\
Trusted DC Connection Status Status = 0 0x0 NERR_Success
The command completed successfully
This output shows the status of your secure channel, and the name of the domain controller you are querying.  You will need to provide the name of the domain you are connected to.  FQDN domain name or NETBIOS domain name should work fine.

This script will provide two functions, one port checker and one function to run to test your connection.  Run Test-DomainControllerPorts with your domain name (or leave it blank for auto detect).  The script returns the name of the DC that you are connected to, along with 2 arrays of ports that are open and another of ports that aren't responding.

function tcpt ([string]$serv, [string]$p) {
 $result = $false
 try {
  $conn = new-object$serv,$p)
  if ($conn.connected) { $result = $true } else { result = $false }
 } catch {
  $result =  $false
 $conn = $null
 return $result
function test-DomainControllerPorts {
 param (
  $domainname = (gwmi win32_computersystem).domain
 $secureChannelDC = (nltest /sc_query:$domainname |
  where {$_ -match "Trusted DC Name"}).split("\\") |
  where {$_ -match $domainname}
 $secureChanneldc = $securechanneldc.trim()
 $functionalports = @()
 $nonFunctionalPorts = @()
 $portsToCheck = ("53", "88", "137", "139", "389", "445", "464", "3268", "636", "3269")
 foreach ($port in $portsToCheck) {
  $portstat = tcpt $secureChannelDC $port
  if ($portstat) {
   $functionalports += $port
  } else {
   $nonfunctionalPorts += $port
 $result = new-object PSObject
 add-member -inp $result NoteProperty DomainController $secureChannelDC
 add-member -inp $result NoteProperty OpenPorts $functionalports
 add-member -inp $result NoteProperty UnOpenPorts $nonfunctionalports
 out-default -inp $result

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