Add Scripts/Powershell/General Purpose/DNS Hierarchy Correction.md

Scripts/Powershell/General Purpose/DNS Hierarchy Correction.md

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+## Purpose
+When it comes to best-practices with Windows-based DNS servers, you never want to have `127.0.0.1` or the IP of the server itself as the primary DNS server, you want to have a *different* DNS server as primary, and `127.0.0.1` as the secondary or tertiary DNS server instead.
+
+The following script will automatically detect which network interface has a default gateway (there should only ever be one default gateway on a server's networking).  Then it will check if the primary DNS server is the same IP as the localhost.  If it is, it checks for a secondary DNS server, if it finds one, it performs an `nslookup` on the secondary DNS server, and if it succeeds, it swaps the secondary DNS server as the primary, and the primary becomes the secondary (loopback).
+
+```powershell
+<#
+    Section: Information Gathering
+    - Gather the adapter(s) with an IP, DNS servers, AND a default gateway set via WMI.
+#>
+$adapters = Get-WmiObject -Class Win32_NetworkAdapterConfiguration | Where-Object {
+    $_.IPAddress -ne $null -and
+    $_.DNSServerSearchOrder -ne $null -and
+    $_.DefaultIPGateway -ne $null -and
+    $_.DefaultIPGateway.Count -gt 0
+}
+
+foreach ($adapter in $adapters) {
+    Write-Host "-----------------------------------------------------------"
+    Write-Host "Adapter Name: $($adapter.Description)"
+    Write-Host "IP Address: $($adapter.IPAddress -join ', ')"
+    Write-Host "Default Gateway: $($adapter.DefaultIPGateway -join ', ')"
+    Write-Host "DNS Server(s): $($adapter.DNSServerSearchOrder -join ', ')"
+
+    $localIPs = $adapter.IPAddress + "127.0.0.1"
+
+    <#
+        Section: Information Analysis
+        - Identify primary and secondary DNS.
+        - Check if primary DNS matches any local IP.
+    #>
+    $primaryDNS = $adapter.DNSServerSearchOrder[0]
+    $secondaryDNS = $null
+    if ($adapter.DNSServerSearchOrder.Count -ge 2) {
+        $secondaryDNS = $adapter.DNSServerSearchOrder[1]
+    }
+
+    $isPrimaryLocal = $false
+    foreach ($local in $localIPs) {
+        if ($primaryDNS -eq $local) {
+            $isPrimaryLocal = $true
+            break
+        }
+    }
+    if ($isPrimaryLocal) {
+        Write-Host "Primary DNS matches local IP: Yes"
+    } else {
+        Write-Host "Primary DNS matches local IP: No"
+    }
+
+    <#
+        Section: Information Processing
+        - If the primary DNS is a local IP and a secondary exists:
+            a. Test the secondary DNS with nslookup on google.com.
+            b. Only swap if nslookup is successful.
+    #>
+    if ($isPrimaryLocal -and $secondaryDNS) {
+        Write-Host "Testing nslookup on secondary DNS ($secondaryDNS)..."
+        $nslookupResult = nslookup google.com $secondaryDNS 2>&1
+
+        # Simple check for nslookup success
+        $nslookupSuccess = $false
+        if ($nslookupResult -match "Name:\s*google\.com") { $nslookupSuccess = $true }
+        if ($nslookupResult -match "Non-authoritative answer:") { $nslookupSuccess = $true }
+        if ($nslookupResult -match "Address:") { $nslookupSuccess = $true }
+
+        if ($nslookupSuccess) {
+            Write-Host "NSlookup via secondary DNS: SUCCESS"
+            # Swap
+            $newDnsServers = @($secondaryDNS, $primaryDNS)
+            if ($adapter.DNSServerSearchOrder.Count -gt 2) {
+                $newDnsServers += $adapter.DNSServerSearchOrder[2..($adapter.DNSServerSearchOrder.Count - 1)]
+            }
+            $result = $adapter.SetDNSServerSearchOrder($newDnsServers)
+            if ($result.ReturnValue -eq 0) {
+                Write-Host "DNS servers swapped. New primary: $secondaryDNS, New secondary: $primaryDNS"
+            } else {
+                Write-Host "Failed to set new DNS order. Return code: $($result.ReturnValue)"
+            }
+        } else {
+            Write-Host "NSlookup via secondary DNS: FAILED"
+            Write-Host "DNS servers NOT swapped."
+        }
+    } elseif ($isPrimaryLocal -and -not $secondaryDNS) {
+        Write-Host "No secondary DNS set. No changes made."
+    } else {
+        Write-Host "DNS servers are correct. No changes needed."
+    }
+
+    Write-Host "-----------------------------------------------------------"
+}
+
+Write-Host "DNS check and correction completed for adapters with a default gateway."
+```