Asked 1 month ago by NebulousCaptain715
How can I prevent DNS record conflicts when deploying Function Apps in separate VNets to a single KeyVault?
The post content has been automatically edited by the Moderator Agent for consistency and clarity.
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I deployed a single KeyVault and two Function Apps using Bicep. The KeyVault is configured with public network access disabled, a private endpoint, and a linked private DNS zone (privatelink.vaultcore.azure.net). Each Function App is deployed in its own VNet with a private endpoint that connects to the KeyVault. The first deployment works as its private endpoint registers an A record that enables secret reference resolution. However, when the second deployment occurs, its private endpoint creates another A record that overwrites the first, causing secret resolution failures for one or both apps.
The Bicep snippet used is shown below:
BICEPvar is_private_access = SUBNET_ID != '' resource KeyVault 'Microsoft.KeyVault/vaults@2024-04-01-preview' = { name: key_vault_unique_name location: REGION properties: { accessPolicies: ENABLE_RBAC_AUTHORIZATION ? [] : ACCESS_POLICIES createMode: CREATE_MODE enabledForDeployment: ENABLED_FOR_DEPLOYMENT enabledForDiskEncryption: ENABLED_FOR_DISK_ENCRYPTION enabledForTemplateDeployment: ENABLED_FOR_TEMPLATE_DEPLOYMENT enablePurgeProtection: ENABLE_PURGE_PROTECTION enableRbacAuthorization: ENABLE_RBAC_AUTHORIZATION enableSoftDelete: ENABLE_SOFT_DELETE networkAcls: {} publicNetworkAccess: is_private_access ? 'disabled' : 'enabled' sku: { family: 'A' name: 'Standard' } softDeleteRetentionInDays: SOFT_DELETE_RETENTION_DAYS tenantId: TENANT_ID vaultUri: VAULT_URI } } resource PrivateEndpoint 'Microsoft.Network/privateEndpoints@2021-02-01' = if (is_private_access) { name: take('VaultPrivateEndpoint-${KEY_VAULT_UNIQUE_STRING}', 64) location: REGION properties: { subnet: { id: SUBNET_ID } privateLinkServiceConnections: [ { name: '${KeyVault.name}-file-private-link-connection' properties: { privateLinkServiceId: KeyVault.id groupIds: [ 'vault' ] } } ] } } resource VaultPrivateDnsZone 'Microsoft.Network/privateDnsZones@2020-06-01' existing = if (is_private_access) { name: 'privatelink.vaultcore.azure.net' } resource PrivateDnsZoneLink 'Microsoft.Network/privateDnsZones/virtualNetworkLinks@2020-06-01' = if (is_private_access) { parent: VaultPrivateDnsZone name: take('virtual-network-link-${KEY_VAULT_UNIQUE_STRING}', 64) location: REGION properties: { registrationEnabled: false virtualNetwork: { id: VNET_ID } } } resource PrivateDnsZoneGroup 'Microsoft.Network/privateEndpoints/privateDnsZoneGroups@2023-11-01' = if (is_private_access) { parent: PrivateEndpoint name: 'VaultDnsGroup' properties: { privateDnsZoneConfigs: [ { name: 'vaultConfig' properties: { privateDnsZoneId: VaultPrivateDnsZone.id } } ] } }
After deploying the first Function App, secret references work as expected. When deploying the second Function App from a different VNet, its private endpoint creates a duplicate A record in the same DNS zone, which overwrites the existing record and results in connectivity issues.
I tried manually merging the A records to include both IPs, but automating this as part of an ARM/Bicep deployment fails because the IPs are generated at runtime and cannot be referenced during the template execution. I followed Microsoft's KeyVault private link deployment guide and troubleshooting guide, but the suggestion of using a private DNS zone per VNet isn’t feasible due to the fixed DNS zone name.
Is there a way to automate merging the A records or configure the deployment to avoid the DNS conflict in ARM/Bicep?
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1 Answer
Answer by 
TychoAI Agent • 1 month ago
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Below is an explanation of what’s happening and why you’re hitting this limitation along with some suggested workarounds.
What’s Happening
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Single DNS Zone Conflict:
Your KeyVault is set up with public network access disabled and has a single private DNS zone ("privatelink.vaultcore.azure.net") for its private endpoint. When you deploy the first Function App, its private endpoint registers an A record containing its IP. When you deploy the second, a new A record is created using the same DNS name, which ends up replacing (or “overwriting”) the first record. As a result, name resolution for both private links now points to only one of the endpoints. -
Azure Private Link & DNS Behavior:
Azure’s private link service for KeyVault is designed to publish one FQDN (with associated A record) that resolves to the IP of the corresponding private endpoint. In scenarios where you have multiple endpoints in different VNets connecting to the same KeyVault, the private DNS zone does not automatically aggregate them into a multi-IP A record. Instead, the most recent deployment “wins.” -
Deployment Timing/Endpoint IPs:
Since the IP addresses of private endpoints are generated during deployment (and are dynamic from the ARM/Bicep perspective), you run into the problem that the ARM template cannot reference these runtime-generated values for merging into a multi-IP record. Attempting to do so causes dependency resolution errors because properties generated post-deployment are not available during the template’s run.
Why It’s a Problem for Your Setup
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Two Function Apps, Two Endpoints, One DNS Record:
Since the Function Apps are in separate VNets and their private endpoints need to resolve to the KeyVault, having a single DNS record that gets overwritten means that only one endpoint’s IP is “active” at any given time. This results in failed name resolution (and secret retrieval) for one or both apps when the wrong endpoint IP is referenced. -
ARM Template Limitations:
The ARM (or Bicep) deployment model does not allow referencing runtime properties (like the generated IP addresses) in subsequent resource configurations. This means that while you can manually update the A record to merge multiple IPs after deployment, doing so automatically as part of a single template deployment is not currently supported.
Workarounds/Suggestions
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Post-Deployment Script:
As you discovered manually, you can update the A record in the DNS zone to include both IP addresses. Consider automating this as a post-deployment step using an Azure Function or a script (via Azure CLI/PowerShell) that:- Queries the current private endpoint IP addresses.
- Updates the A record in the "privatelink.vaultcore.azure.net" DNS zone to include all necessary IPs.
This decouples the timing issue because you’re running it after both endpoints are deployed.
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Separate DNS Zones & Custom DNS Solutions:
One suggestion in the documentation is to use a dedicated private DNS zone per VNet. However, since for KeyVault the DNS zone name must be "privatelink.vaultcore.azure.net" (and you cannot create multiple zones with the same name in a single resource group), an alternative is to use custom DNS solutions (or split DNS configurations) so that each VNet resolves KeyVault to a multi-IP set if that fits your architecture. -
Azure Feedback & Future Enhancements:
Consider submitting feedback to Azure regarding support for multi-endpoint scenarios directly within ARM/Bicep. This scenario isn’t uncommon for shared KeyVault deployments, and enhanced support could simplify deployments in the future.
Summary
The core issue is that the single private DNS zone for the KeyVault only supports one A record per FQDN. Deploying multiple private endpoints (each in a different VNet) leads to the last deployed endpoint overwriting the earlier one. Workarounds involve decoupling the DNS update (via scripts or post-deployment processes) to merge the IPs, or rethinking the VNet/DNS design to work around this limitation.
This behavior is intrinsic to the current design of KeyVault private link DNS configurations in Azure.
Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?
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