Windows Server DHCP and PowerShell

Posted Feb 1, 2023 Updated May 24, 2023

By Karubits 7 min read

I was recently tasked with setting Windows DHCP Server with powershell. Coming from isc-dhcp-server on Linux some basic options such as DNS Search Suffix (113) or setting up scopes in bulk can be tedious through the GUI, so this post details some common tasks that one might go through for setting up DHCP server from scratch in powershell.

Common DHCP Options

DNS Search List

As the DHCP option expects an HEX array we need to convert the search list into a HEX array.

Create the DHCP Option

  
Add-DhcpServerv4OptionDefinition -Name "DNS Search List" -OptionId 119 -Type Byte -MultiValued

Setup the variables for the search list and the scope you want to target.

  
$DNS_SEARCH_LIST = "example1.local;coffee.local;karubits.local"
$TARGET_SCOPE = "10.2.4.0"

Convert the DNS search list and add the DHCP Option to the scope.

  
$splittedDomainSearchList = $DNS_SEARCH_LIST -split "\;"
$DNS_SEARCH_LISTHexArray = @();
   
Foreach ($domain in $splittedDomainSearchList) 
{
    $splittedDomainParts = $domain -split "\."
    Foreach ($domainPart in $splittedDomainParts) 
    {
        $domainPartHexArray = @()
        $domainPartHexArray += $domainPart.Length
        $domainPartHexArray += $domainPart.ToCharArray();
        Foreach ($item in $domainPartHexArray) 
        {
            $DNS_SEARCH_LISTHexArray += [System.Convert]::ToUInt32($item)
        }
    }
    $DNS_SEARCH_LISTHexArray+= 0x00
}
   
# Add the DNS Search list as an hex array to the target scope
Set-DhcpServerv4OptionValue -ScopeId $TARGET_SCOPE -OptionId 119 -Value $DNS_SEARCH_LISTHexArray

TimeZone Options

A common use case for setting the timezone options in DHCP can be for VOIP phones and getting the time to be displayed correctly. There are two DHCP options that can be created and set. Option ID 100 and 101.

To read more about the specifics of the option I recommend reader the following link.

Create the DHCP options for 100 and 101. In this example the timezone is specified for Japan.

  
Add-DhcpServerv4OptionDefinition -Name PCode    -Description "TZ-POSIX String" -OptionId 100    -Type "String" # JST-9
Add-DhcpServerv4OptionDefinition -Name TCode    -Description "TZ-Database String" -OptionId 101    -Type "String"

You can set the default value globally instead of the scope level if your DHCP server resides in a single TZ.

  
Set-DhcpServerv4OptionValue  -OptionId 101    -Value "Asia/Tokyo"
Set-DhcpServerv4OptionValue  -OptionId 100    -Value "JST-9"

Unifi Network Controller (L3 Adoption)

If you have a Unifi Network Controller you can create a vendor specific option and define the IP address of the network controller for automatic discovery of Unifi devices.

First create the vendor class for Ubiquiti

  
Add-DhcpServerv4Class -Name Ubiquiti -data "ubnt" -Type "Vendor"

Create the new DHCP Option

  
Add-DhcpServerv4OptionDefinition -Name "UniFi Controller" -OptionId 1 -Type "BinaryData" -VendorClass "Ubiquiti" -Description "Unifi Controller IP as Hex Object"

We can specify the controller IP address and the DHCP scope you want to add the L3 adoption IP address to.
1 2 $UNIFI_CONTROLLER_IP = "10.5.22.12" $SCOPE = "10.7.5.0"

Convert the Unifi controller into a hex

  
$ip = "$UNIFI_CONTROLLER_IP"
$octets = $ip.Split(".")
$hexOctets = @()
foreach ($octet in $octets) {
    $hexOctets += "0x{0:X2}" -f [int]$octet
}
$hexOctets -join ", "

Add the DHCP Option to the target scope.

# Add the controller IP option to the target scope
Set-DhcpServerv4OptionValue -VendorClass 'Ubiquiti' -OptionId 001 -Value $hexOctets -ScopeId $SCOPE

Example of creating DHCP scopes in Bulk

To create DHCP scopes in bulk one approach is to define the scopes and options in a CSV file. Then use a powershell script to import the CSV and loop through each row.

Here is an example of a CSV.

 Name,Description,ScopeID,StartRange,EndRange,LeaseDuration,SubnetMask,Router,DnsDomain,DNSServer,NTPServers,DNSDomainSearch
 102_EMPLOYEES,Tokyo Employees - VLAN: 102  (172.16.150.0/24),172.16.150.0,172.16.150.100,172.16.150.220,18.00:00:00,255.255.255.0,172.16.150.1,employees.karubits.com,"1.1.1.1,1.0.0.1,8.8.8.8","192.168.88.1,10.2.25.3,10.2.25.10,10.2.25.11,192.168.88.14","employees.karubits.com,global.karubits.com,karubits.local"
 103_GUEST,Tokyo Guests - VLAN: 103  (192.168.244.0/24),192.168.244.0,192.168.244.100,192.168.244.220,18.00:00:00,255.255.255.0,192.168.244.1,guests.karubits.com,"1.1.1.1,1.0.0.1,8.8.8.8","192.168.88.1,10.2.25.3,10.2.25.10,10.2.25.11,192.168.88.14",guests.karubits.com

Then use powershell to import the CSV and loop through each row.

  
 # Import the CSV file
 $data = Import-Csv -Path 'dhcp_scopes.csv'
   
 # Loop through each row in the CSV file
 foreach ($row in $data) {
     # Create a new DHCP scope
     $ScopeID = $row.ScopeID
     $DnsServer = $row.DNSServer.Split(',')
     $Router = $row.Router
     $DnsDomain = $row.DnsDomain
     $NTPServers =  $row.NTPServers.Split(',')
     $DomainSearchList = $row.DNSDomainSearch
     $Name = $row.Name
   
     # Check if scope already exists
     $ScopeExists = Get-DhcpServerv4Scope | Where-Object { $_.ScopeId -eq $ScopeID }
   
     # If scope does not exist, create it
     if (!$ScopeExists) {
         Add-DhcpServerv4Scope -Name $Name -Description $row.Description -SubnetMask $row.SubnetMask -StartRange $row.StartRange -EndRange $row.EndRange -LeaseDuration $row.LeaseDuration
     } else {
         Write-Host "Scope with ID $ScopeID already exists, skipping creation"
     }
   
     # Add DNS servers, DNS Domain, and Default Gateway to the DHCP scope
     Set-DhcpServerv4OptionValue -ScopeId $ScopeID -DNSServer $DnsServer -DnsDomain $DnsDomain -Router $Router 
   
     # Add NTP servers to the DHCP scope
     Set-DHCPServerv4optionvalue -ScopeId $ScopeID -OptionId 042 -Value $NTPServers
   
     # Convert the domain search list into a HEX array
     $splittedDomainSearchList = $DomainSearchList -split "\;"
     $domainSearchListHexArray = @();
   
     # Convert the domain search list into a hex array
     Foreach ($domain in $splittedDomainSearchList) 
     {
         $splittedDomainParts = $domain -split "\."
         Foreach ($domainPart in $splittedDomainParts) 
         {
             $domainPartHexArray = @()
             $domainPartHexArray += $domainPart.Length
             $domainPartHexArray += $domainPart.ToCharArray();
             Foreach ($item in $domainPartHexArray) 
             {
                 $domainSearchListHexArray += [System.Convert]::ToUInt32($item)
             }
         }
         $domainSearchListHexArray+= 0x00
     }
   
     # Set the domain search list with its HEX value 
     Set-DhcpServerv4OptionValue -ScopeId $ScopeID -OptionId 119 -Value $domainSearchListHexArray
   
 }

Migrating DHCP reservations from pfsense

This python script will read the config.xml of a pfsense configuration and create a CSV file containg the MAC Address, IP Address, and description.

  
 ## Static Leases from pfsense
   
 # Python to read the config.xml and write the static leases out to csv
 import xml.etree.ElementTree as ET
 import csv
   
 # parse the xml file
 tree = ET.parse('config.xml')
 root = tree.getroot()
   
 # open a csv file for writing
 with open('static-dhcp-entries.csv', 'w', newline='') as csvfile:
     # create a csv writer
     csvwriter = csv.writer(csvfile)
     # write the header row
     csvwriter.writerow(['mac', 'ipaddr', 'descr'])
     # iterate over all staticmap elements
     for staticmap in root.findall('.//staticmap'):
         # extract the values of mac, ipaddr, and descr
         mac = staticmap.find('mac').text
         ipaddr = staticmap.find('ipaddr').text
         descr = staticmap.find('descr').text
         # write the values to the csv file
         csvwriter.writerow([mac, ipaddr, descr])

Windows Server DHCP requires a name for the dhcp reservation and in this case they were manually added to the CSV.

Then to import the DHCP reservations we can use powershell to read the csv file.

  
 # DHCPv4 Reservations Creation Script
 # This script reads a CSV file containing DHCP reservations information and creates the reservations in the DHCP server.
 # It handles the following:
 # - CSV headers match the same name as the PowerShell parameters
 # - The scope ID uses the IP address, but changes the last octet to 0 (e.g. 192.168.2.23 becomes 192.168.2.0)
 # - The MAC address format is 00:17:fc:6f:3c:78, which is converted to 00-17-fc-6f-3c-78
 # - Assumes the -Type is set to "Both" for all reservations
   
 # Check to see if the CSV file exists. 
 $file = "pfsense-static-leases.csv"
 if (!(Test-Path $file -PathType Leaf)) {
   Write-Error "Error: File $file not found"
   return
 }
   
 # Import the DHCP reservations data from the CSV file
 Import-Csv $file | ForEach-Object {
     # Assign the values from the CSV to variables
     $ip = $_.'IP Address'
     $name = $_.'Name'
     $mac = $_.'Mac Address' -replace ':', '-'
     $description = $_.'Description'
     $scopeid = ($_.'IP Address'.Split('.')[0..2] -join '.') + '.0'
   
     # Check if the reservation already exists
     if (!(Get-DhcpServerv4Reservation -ScopeId $scopeid -ClientId $mac -ErrorAction SilentlyContinue)) {
         # If the reservation does not exist, display a message and create the reservation
         Write-Host -ForegroundColor Green "Creating reservation for $name with IP address $ip and MAC address $mac"
         Add-DhcpServerv4Reservation -ScopeId $scopeid -IPAddress $ip -ClientId $mac -Name $name -Type "Both" -Description $description
     } else {
         # If the reservation already exists, display a message and skip creating it
         Write-Host -ForegroundColor Green "Updating reservation for $name with IP address $ip and MAC address $mac as it already exists."
 	Set-DhcpServerv4Reservation -IPAddress $ip -ClientId $mac -Name $name -Type "Both" -Description $description
     }
 }

windows server

This post is licensed under CC BY 4.0 by the author.