Introduction

resource legacyDns 'Microsoft.Network/privateDnsZones@2020-06-01' = {
name: 'legacy.local'
location: 'global'
}Diagram:

  • Hard-coded static IPs
  • Manual DNS or HOSTS file entries
  • Application-layer dependencies on specific ports
  • Rigid firewall rules
  • Lack of support for dynamic addressing or name resolution

# Allow TCP 1433 for SQL legacy app
New-AzLocalNetworkSecurityRuleConfig -Name "AllowSQL1433" -Protocol "Tcp" -Direction "Inbound" -Priority 100 `
-SourceAddressPrefix "*" -SourcePortRange "*" -DestinationAddressPrefix "*" -DestinationPortRange "1433" -Access "Allow"

B. Port Forwarding via SDN Load Balancer

Modernizing legacy workloads with Azure Local SDN demands creative workarounds, automation, and careful planning. By leveraging SDN IP reservations, NAT overlays, custom DNS, and explicit port handling, network architects and sysadmins can shape robust, cloud-aligned networks even for the most stubborn legacy applications. These approaches reduce risk, streamline migrations, and set the foundation for future cloud-native evolution.

Solutions

A. Preserving Original IPs in Azure Local SDN

#image_title

PowerShell Example:

PowerShell Example:

PowerShell Example:

PowerShell Example:

Diagram:

5. Automating Workarounds with PowerShell, Bicep, and JSON

  • Use ARM/Bicep templates to create SDN subnets, NSGs, and static IP pools matching legacy needs.
  • Automate host file, DNS, and firewall rule injection via PowerShell DSC or VM extensions.
  • Leverage SDN APIs for bulk changes.

Define explicit NSG rules to allow only the required legacy ports.

$vmList = Get-AzVM -ResourceGroupName "LegacyRG"
foreach ($vm in $vmList) {
Invoke-AzVMRunCommand -ResourceGroupName $vm.ResourceGroupName -VMName $vm.Name -CommandId 'RunPowerShellScript' -ScriptPath 'Update-Hosts.ps1'
}

Disclaimer: The views expressed in this article are those of the author and do not represent the opinions of Microsoft, my employer or any affiliated organization. Always refer to the official Microsoft documentation before production deployment.

  • Reserved those same IPs in Azure Local SDN’s static IP pools
  • Used PowerShell scripts to automate reservation and network attachment
  • Updated DNS and firewall rules to map to SDN-assigned resources

Post-Migration:

Case Study 2: Manufacturing Plant App With Legacy Port Dependencies

Post-Migration:

7. Comparison Table: Legacy vs SDN Approaches

Challenge Legacy On-Prem Solution Azure Local SDN Solution
Hard-coded IPs Manual static IP IP reservation, NAT overlays
Static DNS HOSTS file or static server Private DNS zones, host file injection
Static ports Manual firewall, open port NSG, SLB, App Gateway rules
Rigid config Siloed network, manual management Automated policy, scripted enforcement
Change resistance Manual edits, high outage risk Automated scripts, minimal downtime

8. Checklists for Migration

# Reserve a specific IP for a VM NIC
$ipPool = Get-SdnIpPool -Name "LegacySubnet"
$staticIP = "10.10.10.50"
Add-SdnIpReservation -IpPool $ipPool -IpAddress $staticIP -ResourceId "/subscriptions/xxxx/resourceGroups/LegacyRG/providers/Microsoft.Network/virtualNetworks/LegacyVNet"

B. Network Address Translation (NAT) Overlays

#image_title

  • Script IP reservations in Azure Local SDN
  • Implement custom DNS solutions as needed
  • Create NSG/SLB rules for static ports

{
"name": "legacyStaticRoute",
"properties": {
"addressPrefix": "10.10.10.50/32",
"nextHopType": "VirtualAppliance",
"nextHopIpAddress": "10.1.2.5"
}
}

3. DNS Workarounds for Non-Compliant Applications

Challenge

Applications reference fixed IP addresses, either in code, configuration, or third-party integrations.

PowerShell Example:

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