Troubleshooting Azure VPN Gateway Connections
This article provides guidance on diagnosing and resolving common issues encountered with Azure VPN Gateway connections. We'll cover frequently seen problems and offer step-by-step solutions.
Common VPN Gateway Issues
1. Connection Established, but No Traffic Flow
This is a common scenario where the tunnel appears healthy, but data isn't being transmitted correctly. This can often be due to routing or firewall configurations.
Possible Causes and Solutions:
- Incorrect Route Propagation: Ensure that routes from your on-premises network are being advertised to the Azure VPN Gateway and vice-versa.
- Check the effective routes on your Azure VM or subnet.
- Verify User Defined Routes (UDRs) are not blocking traffic.
- Ensure BGP is correctly configured if used.
- On-Premises Firewall Rules: Your local firewall might be blocking traffic from the Azure VPN subnet.
- Review firewall logs for dropped packets originating from the Azure VPN IP address range.
- Add explicit rules to allow traffic from the Azure network.
- Network Security Groups (NSGs): NSGs applied to your Azure subnet might be too restrictive.
- Check inbound and outbound rules for the relevant subnets.
- Ensure protocols and ports required for your application traffic are allowed.
2. VPN Tunnel Disconnecting Frequently
Intermittent disconnections can be frustrating. This often points to network instability or configuration mismatches.
Possible Causes and Solutions:
- IPsec/IKE Policy Mismatch: Ensure the encryption, integrity, and Diffie-Hellman group settings are identical on both your Azure VPN Gateway and your on-premises VPN device.
- Refer to the supported IPsec/IKE parameters for Azure VPN Gateway.
- Use the
Az.NetworkPowerShell module or Azure CLI to view and set policies. Example:Get-AzVirtualNetworkGateway -ResourceGroupName "MyResourceGroup" -Name "MyVpnGateway" | Select-Object -ExpandProperty IpsecPolicies
- Network Latency or Packet Loss: High latency or packet loss on the underlying internet connection can cause tunnels to drop.
- Use `ping` and `tracert` (or `traceroute`) from both ends to assess network path quality.
- Consider using a VPN device that supports Dead Peer Detection (DPD) with appropriate timeouts.
- NAT Traversal (NAT-T) Issues: If your on-premises VPN device is behind a NAT, ensure NAT-T is enabled and compatible.
- Most modern VPN devices support NAT-T.
- Ensure UDP port 500 and UDP port 4500 are open if NAT-T is used.
3. Unable to Establish a VPN Connection
If the tunnel never establishes, the issue is likely with the initial handshake or authentication.
Possible Causes and Solutions:
- Incorrect Pre-Shared Key (PSK): The PSK must be exactly the same on both sides.
- Double-check the PSK for typos or case sensitivity.
- Regenerate the PSK if unsure.
- Public IP Address Mismatch: The public IP address of your on-premises VPN device must be correctly configured in the Azure VPN Gateway settings.
- Verify the static public IP address configured for the Azure VPN Gateway matches the address seen by Azure.
- Firewall Blocking VPN Protocols: Intermediate firewalls or your on-premises firewall might be blocking ESP (IP Protocol 50), AH (IP Protocol 51), or IKE (UDP Port 500).
- Ensure these protocols and ports are permitted through all network devices between the Azure VPN Gateway and your on-premises device.
Tip:
Leverage Azure Network Watcher's VPN Troubleshoot feature for automated diagnostics. It can help identify common configuration errors and connectivity issues.
Advanced Troubleshooting Steps
Using Azure Network Watcher
Network Watcher provides powerful tools for diagnosing network issues in Azure.
- Connection Troubleshoot: Use the Connection Troubleshoot feature to test connectivity between two endpoints.
- Packet Capture: Capture network traffic on your VPN Gateway or target VMs to analyze packet flows and identify errors.
- IP Flow Verify: Determine if traffic is allowed or denied by NSGs and UDRs.
Logs and Monitoring
Regularly review logs for both Azure VPN Gateway and your on-premises VPN device.
- Azure Activity Log: Provides insights into gateway operations and configuration changes.
- Azure Diagnostic Logs: Detailed logs from the VPN Gateway service itself. Configure diagnostic settings to send these to Log Analytics or Storage Accounts.
- On-Premises VPN Device Logs: Essential for correlating events and understanding the handshake process from the other side.
Important:
When troubleshooting IPsec/IKE policies, ensure you are using compatible algorithms and key strengths as outlined in the Azure VPN Gateway documentation. Incompatible settings are a frequent cause of connection failures.