How to Prevent SSH Session Disconnects and Timeouts

Last updated: 2024-12-06
Solution verified: 2024-12-06

Environment

  • SSH client software:

    • OpenSSH (Linux/macOS)

    • PuTTY (Windows)

    • MobaXterm (Windows)

  • ITSO HPC clusters

Issue

  • SSH sessions disconnect during long operations due to network inactivity timeouts (e.g., 15-30 minutes).

Resolution

Configure SSH Keep-Alive Settings

For OpenSSH (Linux/macOS)

Add these settings to your SSH config file:

# Edit ~/.ssh/config
Host server_name           # e.g., ust-hpc4
    HostName hostname      # e.g., hpc4.ust.hk
    User username
    ServerAliveInterval 60
    ServerAliveCountMax 3

Then connect using:

ssh server_name

For PuTTY (Windows)

  1. Open PuTTY configuration

  2. Navigate to Connection

  3. Set “Seconds between keepalives” to 60

  4. Save the session settings

For MobaXterm (Windows)

  1. Open Session Settings

  2. Go to SSH tab

  3. Enable “SSH keepalive”

  4. Set interval to 60 seconds

Use Terminal Multiplexers

GNU Screen:

# Start new session
screen

# Reconnect to existing session
screen -r

Tmux:

# Start new session
tmux

# Reconnect to existing session
tmux attach

Best Practices for Long Operations

  • Use batch job submission systems instead of interactive sessions

  • Always run important interactive session in terminal multiplexer sessions

  • For background processes, use nohup:

nohup long_running_command &

Root Cause

ITSO HPC clusters drop inactive SSH connections after a certain period to free up resources. This is a security measure to prevent unauthorized access to idle sessions.

Diagnosis

Check if you’re experiencing timeouts by:

  • Monitoring connection duration

  • Testing with different keepalive intervals

  • Checking system/client logs for disconnect messages

References

HPC Support Team
ITSO, HKUST
Email: cchelp@ust.hk
Web: https://itso.hkust.edu.hk/
Article Info
Issued: 2024-12-06
Issued by: chtaihei@ust.hk