Server did not respond properly to VPN control packets. Session State: key Material Sent

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The error message "Server did not respond properly to VPN control packets. Session State: Key Material Sent" indicates an issue with the VPN authentication or communication between your client and the Azure VPN Gateway. Here are the possible things you should check and troubleshoot:

1. Certificate Issues

Since you're using a certificate for authentication, verify the following:

• Ensure that the Root and Client Certificates are correctly uploaded in the Azure VPN Gateway.
• The client certificate must be generated from the same root certificate that was uploaded to Azure.
• If you’re using Azure Point-to-Site (P2S) VPN, ensure the client certificate is installed on your machine under Current User > Personal > Certificates.
• Check that the certificate is not expired.

2. VPN Gateway Configuration

• Ensure that the VPN Gateway SKU supports the type of authentication you’re using.
• If using IKEv2, ensure it is enabled on the VPN Gateway.
• If using OpenVPN, ensure the correct profile is being used.

3. Client-Side VPN Configuration

• Make sure you are using the correct VPN client package downloaded from the Azure portal.
• If using Windows, check the VPN configuration under Windows Settings and verify that it is set to use the correct VPN protocol (IKEv2, SSTP, or OpenVPN).
• Ensure the correct certificate is being selected during authentication.

4. Network and Firewall Rules

• Check if any firewall or antivirus software is blocking the VPN connection. Ensure that UDP ports 500, 4500, and TCP 443 are open.
• If behind a corporate network, check if there is a proxy or network policy blocking VPN traffic.

5. VPN Logs and Debugging

• If using Windows, check the logs in Event Viewer > Applications and Services Logs > Microsoft > Windows > RasClient for more details.
• Also check from powershell command, Get-WinEvent -LogName Application -MaxEvents 50 | Where-Object { $_.Message -match "RAS" }
• Restart the IKE and IPsec services
• Disconnect and try reconnecting after a minute.

If the issue persists, try re-importing the certificate and re-downloading the VPN configuration package from Azure. Let me know if you need further troubleshooting!

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