How to Check and Configure SSH Timeout Settings on Your Server

SSH (Secure Shell) is a crucial tool for remotely managing and administering servers. To ensure security and efficiency, it’s essential to understand and configure SSH timeout settings. These settings help manage inactive sessions and prevent unauthorized access due to idle connections. In this tutorial, we’ll walk you through how to check and configure SSH timeout and session expiry settings on your server.

Table of Contents

1. Introduction to SSH Timeout Settings
2. Understanding SSH Timeout Parameters

• ClientAliveInterval
• ClientAliveCountMax
• LoginGraceTime

1. Checking Current SSH Timeout Settings
2. Configuring SSH Timeout Settings
3. Restarting SSH to Apply Changes
4. Conclusion

1. Introduction to SSH Timeout Settings

SSH timeout settings are vital for maintaining server security and managing resources efficiently. By setting appropriate timeout values, you can ensure that idle sessions are automatically closed, reducing the risk of unauthorized access. These settings also help conserve server resources by freeing up connections that are no longer active.

2. Understanding SSH Timeout Parameters

To configure SSH timeout settings, you need to be familiar with the following key parameters in the SSH configuration file (sshd_config):

ClientAliveInterval

• Definition: This parameter specifies the time interval (in seconds) between sending keepalive messages to the client. If the server doesn't receive a response, it will eventually close the connection.
• Example: ClientAliveInterval 300 (Sends a keepalive message every 300 seconds, or 5 minutes).

ClientAliveCountMax

• Definition: This parameter defines the number of unanswered keepalive messages before the server closes the connection. It works in conjunction with ClientAliveInterval.
• Example: ClientAliveCountMax 3 (The server will allow three unanswered keepalive messages before terminating the session).

LoginGraceTime

• Definition: This parameter sets the time allowed for a user to authenticate to the server after establishing a connection. If the user doesn’t authenticate within this time, the server disconnects.
• Example: LoginGraceTime 120 (Allows 120 seconds for a user to log in).

3. Checking Current SSH Timeout Settings

To check the current SSH timeout settings on your server, follow these steps:

1. Open a Terminal: Connect to your server via SSH or access the terminal directly.
2. View the SSH Configuration File:

Use a text editor like nano or vim, or simply use cat to view the SSH configuration file:

   sudo cat /etc/ssh/sshd_config

1. Check for Timeout Parameters:

Search for the timeout parameters in the file:

   grep -E "ClientAliveInterval|ClientAliveCountMax|LoginGraceTime" /etc/ssh/sshd_config

This command will output the relevant lines from the sshd_config file, showing you the current settings.

4. Configuring SSH Timeout Settings

If you need to configure or modify the SSH timeout settings, follow these steps:

1. Open the SSH Configuration File:

   sudo nano /etc/ssh/sshd_config

1. Edit the Timeout Parameters:

Add or modify the following lines to set your desired timeout values:

   ClientAliveInterval 300
   ClientAliveCountMax 3
   LoginGraceTime 120

• Total Session Timeout Calculation: The total timeout before the SSH session expires due to inactivity is calculated as ClientAliveInterval * ClientAliveCountMax. For example, with the settings above, the session will timeout after 900 seconds (15 minutes) of inactivity.

1. Save and Exit:

• If you’re using nano, press CTRL + X, then Y, and Enter to save and exit.
• If using vim, press ESC, then type :wq and hit Enter.

5. Restarting SSH to Apply Changes

After modifying the SSH configuration, you need to restart the SSH service to apply the changes:

sudo systemctl restart sshd

This command restarts the SSH service, ensuring that the new configuration is applied to all new SSH sessions.

6. Conclusion

Configuring SSH timeout settings is a simple but effective way to enhance the security and efficiency of your server. By setting appropriate timeouts, you can automatically close inactive sessions, reducing the risk of unauthorized access and conserving server resources. Regularly review and adjust these settings based on your server’s usage patterns to maintain optimal performance and security.

Now that you know how to check and configure SSH timeout settings, you can keep your server secure and running efficiently. Remember to always test your changes in a controlled environment before applying them to a production server.