Screen Sharing Troubles on OS X: "channel 3: open failed: administratively prohibited"
Facing a "channel 3: open failed: administratively prohibited" error while trying to use Screen Sharing over an SSH tunnel on your OS X machine? This frustrating issue can arise when trying to remotely access another Mac, often after setting up an SSH tunnel. Let's delve into the root cause of this problem and explore effective solutions.
The Scenario:
Imagine you're working on a project with a colleague and need to remotely access their Mac for collaboration. You set up an SSH tunnel, but when you attempt Screen Sharing through the tunnel, you encounter the infamous error: "channel 3: open failed: administratively prohibited".
Code Example (Optional):
# Setting up an SSH tunnel
ssh -N -L 5901:remote-mac-ip:5901 user@remote-mac-ip
The Issue:
This error is a security measure built into OS X. When you connect over an SSH tunnel, Screen Sharing relies on a specific port (usually 5901) to establish the connection. The "administratively prohibited" message signals that this port is blocked by a security setting, preventing the connection from being established.
Understanding the Cause:
- Firewall Restrictions: Your firewall (either on the remote Mac or your local system) could be blocking the necessary ports for Screen Sharing.
- VNC Server Configuration: The VNC server (the application enabling screen sharing) on the remote Mac might have restrictions preventing connections through SSH tunnels.
- SSH Tunnel Configuration: There might be issues with the SSH tunnel setup itself, preventing the Screen Sharing connection from passing through properly.
Troubleshooting Solutions:
-
Check Firewall Settings:
- On the remote Mac: Make sure the firewall allows connections on the Screen Sharing port (usually 5901). Check the "Firewall" settings in "System Preferences".
- On your local machine: Ensure your firewall is not blocking outgoing connections to the remote Mac.
-
Enable Screen Sharing:
- On the remote Mac: Go to "System Preferences" -> "Sharing" and make sure "Screen Sharing" is enabled. You may need to add your user to the list of allowed users for Screen Sharing.
-
VNC Server Configuration:
- If you're using a third-party VNC server, check its configuration settings to allow connections through SSH tunnels.
- The default VNC server in OS X usually works seamlessly with SSH tunnels, but there could be settings preventing this behavior.
-
SSH Tunnel Setup:
- Make sure the SSH tunnel is properly configured and active. Double-check the command you used to establish the tunnel and ensure the ports are correctly specified.
- You can also use the
-Doption with SSH to create a SOCKS proxy, which can sometimes resolve the issue.
Additional Tips:
- Disable Security Software: Temporarily disable any security software on your local or remote machines that might interfere with the connection.
- Restart Services: Restart the VNC server and SSH services on both machines after making changes to their configurations.
Debugging Techniques:
- Network Tracing: Use network monitoring tools like Wireshark to analyze the traffic between your machines and identify any blocked packets.
- Log Files: Check the logs of your firewall, SSH client, and VNC server for error messages or clues.
Remember: These solutions might require advanced technical knowledge and a thorough understanding of your network configuration. If you're unsure about any steps, consult a knowledgeable user or a professional IT support team.
Conclusion:
While seemingly complex, the "channel 3: open failed: administratively prohibited" error usually boils down to security restrictions on your machines. By carefully reviewing firewall settings, VNC server configurations, and SSH tunnel setups, you can troubleshoot this issue and successfully establish a Screen Sharing connection over your SSH tunnel.
