To port bind an interactive transient PBS session to the head node, you must establish a secure SSH tunnel that forwards local traffic through the login node to the compute node where your job is running. This two-hop forwarding technique allows you to access services like Jupyter notebooks or web interfaces directly from your local machine without exposing them to the public internet. The process requires configuring SSH with specific flags to maintain the connection and handle host key verification across multiple hops.
Understanding the Network Topology
In High-Performance Computing (HPC) environments, direct connections from your local workstation to compute nodes are typically blocked by firewalls for security reasons. All traffic must route through the head node, also known as the login or gateway node. When you submit a PBS job, it runs on a compute node that has no public IP address. To interact with this node, you need to create a chain of tunnels: one from your local machine to the head node, and another from the head node to the compute node. This setup ensures that your data remains encrypted and that only authorized users can access the resources.
Step-by-Step Port Forwarding Configuration
The first step is to identify the compute node assigned to your PBS job. You can usually find this information in the job output file or by using commands like
qstat -f. Once you have the hostname of the compute node, you can set up the SSH tunnel. Use the following command structure in your local terminal:
ssh -L 8888:localhost:8888 -J [email protected] [email protected] -N -f
This command uses several critical flags. The
-L 8888:localhost:8888 option binds local port 8888 to port 8888 on the remote destination. The
-J flag specifies the jump host, which is the head node. This simplifies the multi-hop process by telling SSH to automatically connect to the head node first before attempting to reach the compute node. The
-N flag indicates that no remote command will be executed, which is ideal for keeping the tunnel open without starting a shell session. The
-f flag sends SSH to the background after authentication, allowing you to continue using your terminal.
If you encounter issues with host key verification during the multi-hop process, you may need to adjust your SSH configuration. For detailed guidance on handling complex SSH scenarios, see our guide on
X11 forwarding after multi hopping in SSH. This resource explains how to manage authentication states when traversing multiple network segments.
Maintaining Persistent Connections
Interactive sessions on HPC clusters are often transient, meaning they may disconnect due to inactivity or system maintenance. To prevent your SSH tunnel from dropping, configure the keep-alive settings in your
~/.ssh/config file. Add the following lines to ensure the connection remains active:
Host *
ServerAliveInterval 60
ServerAliveCountMax 3
These settings send a null packet to the server every 60 seconds. If three consecutive packets fail to receive a response, SSH will terminate the connection. This proactive approach helps maintain stability during long-running jobs. Additionally, consider using tools like
tmux or
screen on the compute node to keep your application running even if the SSH connection is temporarily interrupted.
For more advanced security practices, such as setting up passwordless access for MPI jobs, refer to
Enabling Passwordless SSH Access for Running MPI jobs on Two Nodes. While this article focuses on MPI, the principles of secure key management apply to general SSH tunneling as well.
Troubleshooting Common Issues
One common error is "Connection refused" when trying to access the forwarded port. This usually means the service on the compute node is not listening on the specified port or is bound to a different interface. Ensure that the application inside your PBS job is configured to listen on
0.0.0.0 or
localhost rather than a specific external IP. Another issue is firewall restrictions on the head node. Some HPC centers block certain ports for security reasons. If your chosen port is blocked, try using a higher port number, such as 9000 or above, which are less likely to be restricted.
If you experience slow performance, check your network latency between your local machine and the head node. Compressing the SSH stream can help by adding the
-C flag to your command. However, this may increase CPU usage on both ends. Always monitor the resource usage of your PBS job to ensure that the overhead of SSH tunneling does not impact the scientific computation you are performing.
Safety and Best Practices
Never expose sensitive services directly to the internet. Using SSH tunneling ensures that only your local machine can access the compute node's services. Avoid sharing your SSH keys or passwords with unauthorized personnel. Regularly update your SSH client to patch any security vulnerabilities. If you suspect a security breach, immediately revoke access and change your credentials. For insights on securing outbound connections, review
Fix SSH outbound undesired/unknown connections to understand how to monitor and control outgoing traffic from your cluster.
Frequently Asked Questions
Why can't I connect directly to the compute node?
Direct connections are blocked by design to protect the internal network infrastructure. All traffic must pass through the head node, which acts as a secure gateway. This architecture prevents external attackers from scanning or accessing compute resources directly.
What happens if my SSH connection drops?
If the connection drops, the PBS job continues running on the compute node, but you lose access to its outputs. You will need to re-establish the SSH tunnel using the same steps. Consider using
tmux to keep your session alive independently of the SSH link.
Can I forward multiple ports at once?
Yes, you can specify multiple
-L options in a single SSH command. Each option maps a local port to a remote port on the compute node. This is useful for applications that require multiple services, such as a database and a web interface.
Is it safe to use dynamic port forwarding?
Dynamic port forwarding (
-D) creates a SOCKS proxy, which can be less secure for HPC environments. It is generally safer to use explicit local port forwarding (
-L) because it restricts access to specific ports and services. This minimizes the attack surface and provides better control over traffic.
How do I find the correct port for my application?
Check the documentation for your specific application to determine which port it listens on. Many scientific tools allow you to specify a custom port via command-line arguments. Ensure that the port is not already in use on the compute node to avoid conflicts.