Change vNet DNS servers with Powershell

Question

I have several hundred vNets over several hundred subscriptions.
We have updated the IP of our DNS server that we use in the vNet of each subscription.
I would like to use Powershell to check each vNet in each Subscription for the list of DNS servers and if one of the IP's in the list matches, then change it to the new IP.

Im able to use powershell to get the DNS servers with:
$vnet = Get-AzVirtualNetwork -resourcegroup "ResGroup" -name "VNETNAME"
$vnet.DhcpOptions.DnsServers

I can change the DNSservers in the object with $vnet.DhcpOptions.DnsServers += "IP_address"

But how do I push the change to Azure ?

Answer 1

Hi @Catalyph ,

to set the DNS server config of a vNet in Azure you can use this script:

$vNetRGname = "<name of resourcegroup>"
$vNet = "<name of vNet>"
$vNet = Get-AzVirtualNetwork -ResourceGroupName $vNetRGname -name $vNet
# Replace the IPs with your DNS server IPs here
$array = @("10.0.0.4" "10.0.0.5")
$newObject = New-Object -type PSObject -Property @{"DnsServers" = $array}
$vNet.DhcpOptions = $newObject
$vNet | Set-AzVirtualNetwork

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(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)

Regards
Andreas Baumgarten

Answer 2

@Catalyph, to modify the existing DNS server values and replace them with new values, use the below script,

$subName = "<Subscription Name>"  
$rgName = "<Resource Group Name>"  
$vNetName = "<vNet Name>"  
$DNSIPs = "192.168.1.10", "192.168.1.11", "192.168.1.12" #Modify as necessary.  
     Login-AzureRmAccount  
Select-AzureRmSubscription -SubscriptionName $subName  
     $vnet = Get-AzureRmVirtualNetwork -ResourceGroupName $rgName -name $vNetName  
     $vnet.DhcpOptions.DnsServers = $null  
     foreach ($IP in $DNSIPs)  
{  
$vnet.DhcpOptions.DnsServers += $IP  
}  
     Set-AzureRmVirtualNetwork -VirtualNetwork $vnet  

Disclaimer: The script sets the DNS server to NULL before applying the new list of DNS servers. I would recommend you to use it in test envoronment first and then deploy to production accordingly.

Source Credit: https://sydsachar.wordpress.com/2016/05/26/azure-powershell-arm-add-multiple-dns-servers-to-azure-rm-vnet-via-powershell/

Answer 3

I updated the script with loops:

Import-module Az.Resources
Import-module Az.Compute
$path = "vnet-list.csv" 
$csv = Import-Csv -path $path
$New_DNS_Servers = @("10.10.10.10","10.20.30.40")
$New_CustomDNS_Obj = New-Object -type PSObject -Property @{"DnsServers" = $New_DNS_Servers}
# Iterate through each subscription:
foreach ($line in $csv) { 
    $properties = $line | Get-Member -MemberType Properties
    $rg_name = $line | Select -ExpandProperty $properties[0].Name
    $subscription_id = $line | Select -ExpandProperty $properties[1].Name
    $vnet_name = $line | Select -ExpandProperty $properties[2].Name
	Set-AzContext -SubscriptionId $subscription_id | Out-null
	$VNet_Obj = Get-AzVirtualNetwork -ResourceGroupName $rg_name -name $vnet_name
	$VNet_Obj.DhcpOptions = $New_CustomDNS_Obj
	$result = Set-AzVirtualNetwork -VirtualNetwork $VNet_Obj
    if ($result -eq "") {
		Write-Host "Error while updating Custom DNS-Server for $($vnet_name)"
	} 
	else {
		Write-Host "Updated Custom DNS-Server for $($vnet_name)"
	}
	$VNet_Obj = ""
}