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Solved: How to Fix VPN Connection Issues Caused by Stunnel in Ubuntu

Troubleshoot and resolve OpenVPN and Stunnel tunneling problems on Ubuntu Linux to restore VPN connectivity.

Learn how to diagnose and fix OpenVPN and Stunnel tunneling issues on Ubuntu Linux that prevent VPN connections from working properly.


Having problems connecting to your VPN server through an OpenVPN and Stunnel tunnel on Ubuntu? This guide will walk you through troubleshooting steps to identify and resolve the root cause so you can restore VPN access.

Solved: How to Fix VPN Connection Issues Caused by Stunnel in Ubuntu


Some common symptoms that indicate an underlying issue with the OpenVPN Stunnel setup on Ubuntu include:

  • Inability to connect to the VPN server through the tunnel
  • Connection timeouts or failure messages when trying to connect
  • Stunnel errors in the logs referring to failed connections

These problems typically occur after initially configuring and verifying the VPN tunnel previously worked. The tunnel then stops functioning properly for unclear reasons.

Potential Causes

There are a few potential factors that could cause OpenVPN and Stunnel tunnels to stop working properly on Ubuntu:

  • Outdated Stunnel Version: Using an older version of Stunnel that has bugs or incompatibilities with OpenVPN could lead to connection issues.
  • Certificate Mismatch: If the tunnel certificates were renewed or regenerated on the server side without updating them on the client, this mismatch will prevent connections.
  • Port Conflicts: Another process or software installed on Ubuntu may start listening on the same port assigned to Stunnel, causing conflicts.
  • Firewall or Network Changes: Firewall rule modifications, either locally or remotely, could end up blocking the traffic for the tunnel unintentionally.

Troubleshooting Steps

Follow these troubleshooting tips to narrow down the cause:

1. Verify Stunnel Status

Check if the Stunnel service is actually running using: sudo systemctl status stunnel

If it’s not running, restart it: sudo systemctl start stunnel

2. Check Stunnel Logs

Examine the Stunnel logs located at /var/log/stunnel.log for any errors that indicate connection failures or certificate issues.

3. Confirm OpenVPN Configuration

Double check the OpenVPN client configuration file and ensure the connection settings like protocol, port, and certificates match the server.

4. Check for Port Conflicts

Use sudo netstat -plunt to list processes bound to each port. Look for other services using port 443 or other ports assigned to Stunnel.

5. Verify Firewall Rules

Check UFW and any network firewalls to confirm TCP port 443 (or the SSL tunnel port) is allowed from the Ubuntu client IP address.

6. Update Stunnel

If using an outdated version of Stunnel try upgrading to the latest available in the Ubuntu repos to see if it resolves incompatibilities.

7. Restoring VPN Connectivity

Once you’ve identified the specific cause of the problem, you can take appropriate steps to restore VPN connectivity:

  • Update Stunnel to the newest version
  • Copy updated certificate files from the VPN server
  • Modify firewall rules to allow the tunnel traffic
  • Change the listening port assignment if a conflict exists

After addressing the underlying issue, restart Stunnel and retest connecting to the VPN server. The tunnel should successfully connect and allow VPN access again.

Frequently Asked Questions (FAQs)

Question: Does Stunnel need to match versions on client and server?

Answer: No, Stunnel versions do not need to match between client and server for tunnels to work properly in most cases. But very outdated Stunnel versions may cause incompatibility issues.

Question: How can I renew the Stunnel certificates?

Answer: The easiest way is to regenerate new certificates on the VPN server, then copy the new client certificate and key files over to replace the old ones on the Ubuntu system.

Question: What Ubuntu versions support OpenVPN with Stunnel?

Answer: OpenVPN and Stunnel are supported on all current Ubuntu versions including 16.04 LTS, 18.04 LTS, 20.04 LTS, and newer releases.


Connecting OpenVPN through a Stunnel SSL tunnel is a useful way to get around restrictive firewalls. But misconfigurations or software changes can sometimes break the tunnel on Ubuntu. By methodically troubleshooting and checking for common issues like Stunnel errors, certificate mismatches, port conflicts, and firewall rule problems, you can identify and correct the problem to restore VPN functionality through the tunnel.

Alex Lim is a certified IT Technical Support Architect with over 15 years of experience in designing, implementing, and troubleshooting complex IT systems and networks. He has worked for leading IT companies, such as Microsoft, IBM, and Cisco, providing technical support and solutions to clients across various industries and sectors. Alex has a bachelor’s degree in computer science from the National University of Singapore and a master’s degree in information security from the Massachusetts Institute of Technology. He is also the author of several best-selling books on IT technical support, such as The IT Technical Support Handbook and Troubleshooting IT Systems and Networks. Alex lives in Bandar, Johore, Malaysia with his wife and two chilrdren. You can reach him at [email protected] or follow him on Website | Twitter | Facebook

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