Troubleshooting Azure Private Link

This document provides guidance on common issues encountered when using Azure Private Link and offers solutions and best practices for resolving them.

Important Note: Ensure you have the necessary permissions and have reviewed the Private Link conceptual documentation before proceeding with troubleshooting.

Common Connectivity Issues

Connectivity problems are the most frequent issues users face. Here are some common scenarios and how to address them:

1. Inability to Access Private Endpoint Resources

Check Network Security Groups (NSGs): Ensure that NSGs applied to your virtual network subnet, or any intermediary subnets, allow outbound traffic to the private endpoint's IP address and the necessary ports (usually 443 for HTTPS, but depends on the service). Also, ensure inbound traffic is allowed to the private endpoint from your client subnet.
Verify Private DNS Zone Configuration: The Private DNS zone must be correctly linked to your virtual network, and the DNS records (A or CNAME) for the service FQDN must resolve to the private endpoint's IP address. Test DNS resolution from your client machine.
Subnet IP Allocation: Confirm that the subnet containing the private endpoint has sufficient IP address space.
Service-Specific Endpoints: Some Azure services have specific port requirements. Consult the documentation for the target service (e.g., Azure Storage, Azure SQL Database).

2. DNS Resolution Failures

Incorrect DNS configuration is a common cause of Private Link issues.

Private DNS Zone Linking: Ensure the Private DNS zone (e.g., privatelink.blob.core.windows.net) is correctly linked to the virtual network from which you are trying to access the resource.
Record Type and Value: Verify that the DNS record (usually an 'A' record) within the Private DNS zone points to the correct IP address of the private endpoint.
On-Premises DNS: If you are accessing the private endpoint from an on-premises network via VPN or ExpressRoute, ensure your on-premises DNS servers are configured to forward queries for the relevant Private DNS zones to Azure DNS.
Client DNS Settings: Confirm that your client machines are using DNS servers that can resolve the private endpoint's FQDN.

3. Private Endpoint Status (Provisioning/Failed)

If your private endpoint is stuck in a provisioning state or shows as 'Failed', investigate the following:

Resource ID Mismatch: Ensure the correct Azure resource ID for the target service was provided during private endpoint creation.
Permissions: The identity creating the private endpoint needs appropriate permissions on the target resource.
VNet and Subnet Validation: Verify that the chosen virtual network and subnet exist and are correctly configured.
Service Limits: Check if you have reached any Azure service limits related to private endpoints.

Troubleshooting Tools and Techniques

Leverage Azure's built-in tools and standard networking utilities:

Azure Network Watcher: Use tools like Connection Troubleshoot, IP Flow Verify, and Packet Capture to diagnose network path issues.
nslookup / dig: Test DNS resolution from your client to the service's FQDN.
telnet / nc (netcat): Test TCP connectivity to the private endpoint's IP address and the required port.
Azure Monitor: Examine metrics and logs for both the private endpoint and the target service. Look for rejected connections, errors, or performance anomalies.
Private Endpoint Diagnostic Settings: Configure diagnostic settings on your private endpoint to send logs to Log Analytics, Storage Accounts, or Event Hubs for detailed analysis.

Firewall Rules on Target Service

Many Azure services have their own firewall rules that can block traffic even with a private endpoint. Remember to configure these correctly.

Azure Storage: For Azure Storage accounts, ensure that the "Networking" settings allow access from "All networks" to Private Endpoints, or specifically configure allowed VNets/IPs if you have hybrid access requirements.
Azure SQL Database: When using a private endpoint for Azure SQL Database, you typically don't need to configure firewall rules for the private endpoint itself, but ensure any server-level firewall rules don't inadvertently block traffic from your client's original IP if you are also using public access.
Other Services: Refer to the specific documentation for the Azure service you are connecting to, as each service may have its own network security configurations.

Best Practice: Always test connectivity from the exact client environment (e.g., VM in the VNet, on-premises machine) that will be consuming the private endpoint.

Performance Concerns

If you experience slow performance, consider the following:

Private Endpoint NIC Throughput: Ensure the network interface associated with the private endpoint has sufficient throughput.
Network Path: Analyze the network path between your client and the private endpoint. Latency can be affected by peering, VPN/ExpressRoute, or complex routing.
Service Capacity: Verify that the target Azure service itself is not experiencing performance bottlenecks (e.g., insufficient scaling, resource saturation).
Client Network: Ensure the client's network is not the bottleneck.

Common Pitfalls and How to Avoid Them

Forgetting Private DNS: The most common mistake. Always set up and link the correct Private DNS zone.
NSG Misconfigurations: Overly restrictive NSGs can block legitimate traffic.
Confusing Private Endpoint IP with Service IP: The private endpoint gets its own IP address within your VNet.
Service-Level Firewalls: Not configuring firewalls on the target Azure resource can lead to access denied errors.