powershellwmidhcpnameserversnetwork-interface

Getting interfaces and their DNS servers that are STATIC (not dhcp allocated)

I'm trying to ge the DNS servers of network interfaces via WMI that are static (placed by the user). I have this script that works, except for the static part of course:

Get-WmiObject Win32_NetworkAdapterConfiguration | Where-Object {$_.DNSServerSearchOrder -ne $null} | Select DnsServerSearchOrder,Index,InterfaceIndex

This leads to an output like so:

DnsServerSearchOrder   Index InterfaceIndex
--------------------   ----- --------------
{192.168.122.1}            1              6
{1.1.1.1}                  2             10

The interface with 192.168.122.1 has it's DNS setup to DHCP so that value is not good for me. How do I filter out interfaces where dns is not static? Any ideas?

netsh interface ip show config:

Configuration for interface "Ethernet"
    DHCP enabled:                         Yes
    IP Address:                           192.168.122.130
    Subnet Prefix:                        192.168.122.0/24 (mask 255.255.255.0)
    Default Gateway:                      192.168.122.1
    Gateway Metric:                       0
    InterfaceMetric:                      35
    DNS servers configured through DHCP:  192.168.122.1
    Register with which suffix:           Primary only
    WINS servers configured through DHCP: None

Configuration for interface "Ethernet 2"
    DHCP enabled:                         Yes
    IP Address:                           10.0.0.17
    Subnet Prefix:                        10.0.0.0/24 (mask 255.255.255.0)
    Default Gateway:                      10.0.0.1
    Gateway Metric:                       0
    InterfaceMetric:                      35
    Statically Configured DNS Servers:    1.1.1.1
    Register with which suffix:           Primary only
    WINS servers configured through DHCP: None

Notice the difference in terms between Statically Configured DNS Servers and DNS servers configured through DHCP. I figured I might parse this output but I'm not sure if I can rely on this text if windows language/locale is changed and I'd rather use the WMI interface if possible.

Solution

  • I thought this might be available in the System.Net.NetworkInformation namespace, but evidently not. I looked through a few lower-level Windows networking APIs thinking certainly it must exist somewhere in there, but no such luck. After running dumpbin on netsh.exe to see what kinds of libraries/functions it's consuming, I did get an idea of one other place to look: the registry.

    As it happens, if you look in the registry under the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\ key you will see a key for each network interface with the name in GUID format. Within an interface's key you will find two values of interest: DhcpNameServer and NameServer. On my client system where the DNS server is set by DHCP, DhcpNameServer contains that DNS server's IP address and NameServer contains an empty [String]. If I manually set a DNS server while keeping the automatically-assigned address for the interface itself, NameServer then contains that manually-set DNS server address while DhcpNameServer still contains the same DNS server specified by DHCP.

    Based on these observations, it would seem that...

    Thus, given an interface with ID $interfaceID, you can build a path to its registry key like this...

    $interfaceKeyPath = "HKLM:\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\$interfaceID"

    ...and then retrieve the dynamically-assigned and manually-assigned nameserver values like this...

    Get-ItemProperty -Path $interfaceKeyPath -Name 'DhcpNameServer', 'NameServer'

    That just leaves the matter of from where you get the value for $interfaceID, and there are numerous sources although the trick will be excluding undesirable interfaces (for example, on my Windows 10 system I have a packet capture loopback adapter and a hypervisor adapter that I'd want to be excluded from such a query). The most compatible way (dating back to .NET 2.0) would be the Id property of the NetworkInterface class...

    [System.Net.NetworkInformation.NetworkInterface]::GetAllNetworkInterfaces() `
    | Select-Object -ExpandProperty 'Id'

    ...although the only useful properties on which to filter are Name and NetworkInterfaceType.

    On Windows Vista and above the Win32_NetworkAdapter class provides a GUID property...

    Get-WmiObject -Class 'Win32_NetworkAdapter' -Property 'GUID' -Filter 'PhysicalAdapter = true'

    ...although even when filtering on PhysicalAdapter it still returns the loopback and hypervisor adapters and I'm not seeing any definitive property or class relation that can be used to select only hardware adapters.

    The Win32_NetworkAdapterConfiguration class is much the same...

    Get-WmiObject -Class 'Win32_NetworkAdapterConfiguration' -Property 'SettingID'

    ...with no properties to filter out non-hardware or even non-physical adapters.

    On (I think) Windows 8 and above there's the Get-NetConnectionProfile cmdlet...

    Get-NetConnectionProfile | Select-Object -ExpandProperty 'InstanceID'

    which is documented to get "a connection profile associated with one or more physical network adapters" and, on my system, it does only return my physical adapter.

    There is also the Get-NetAdapter cmdlet...

    Get-NetAdapter -Physical `
    | Where-Object -Property 'EndPointInterface' -NE -Value $true `
    | Select-Object -ExpandProperty 'InterfaceGuid'

    I found that passing the -Physical parameter excluded the hypervisor adapter but not the loopback adapter, so filtering out where EndPointInterface is $true was necessary to eliminate that. The HardwareInterface and Virtual properties might also be of interest.

    Another option would be to invoke the Get-NetAdapterHardwareInfo cmdlet, which seems to know how to distinguish true hardware adapters, and let that determine which adapters are retrieved by Get-NetAdapter...

    Get-NetAdapterHardwareInfo `
    | Get-NetAdapter `
    | Select-Object -ExpandProperty 'InterfaceGuid'

    The Get-Net* cmdlets above return CIM instances so, for example, instead of Get-NetAdapter -Physical you could use something like...

    Get-WmiObject -Namespace 'Root\StandardCimv2' -Class 'MSFT_NetAdapter' `
    -Property 'InterfaceGuid' -Filter 'HardwareInterface = true AND EndPointInterface = false'

    to retrieve the MSFT_NetAdapter instances just the same. I'm not really sure what the guidance is on using one versus the other. It would seem like one should prefer the cmdlets, and yet, unlike WMI/CIM, they offer limited/no parameters for efficiently filtering the output or specifying which properties are desired so you have to do that in the pipeline. I think it's noteworthy, though, that I wasn't able to find any current documentation for these MSFT_* class; they all say they're no longer updated, except for the MSFT_NetConnectionProfile class for which I couldn't find any documentation page at all. That says to me Microsoft doesn't want you relying on any definite structure of these classes, and yet if the cmdlets just pass along those class instances...I'm not sure how can you meaningfully and reliably interact with them if nothing is documented.

    Also, keep in mind that you'll want to prefer Get-CimInstance and its ilk over Get-WmiObject, where possible. I don't think I've yet encountered an instance where it was any more complicated than changing Get-WmiObject to Get-CimInstance, although there are more differences (not necessarily bad) than the name.