Troubleshooting SSH connection termination after `sshd_config` changes

Steps to verify and resolve the issue:

1. Check if SSH server (sshd) is installed

   dpkg -l | grep openssh-server

   If not installed:

   sudo apt-get install openssh-server

2. Check if the SSH service is active
   Identify the SSH service and verify its status:

   sudo systemctl status ssh.service

   If not running:

   sudo systemctl start ssh.service
   sudo systemctl enable ssh.service

   If the service name differs:

   systemctl list-units --type=service | grep ssh

3. Validate Changes to sshd_config
   Test for syntax errors in the configuration:

   sudo sshd -t

   Correct any errors found in /etc/ssh/sshd_config.

4. Prevent system suspend by disabling suspend mode.
   Temporarily disable suspend mode:

   sudo systemctl mask sleep.target suspend.target hibernate.target hybrid-sleep.target

   To make this change persistent, edit /etc/systemd/logind.conf:

   sudo nano /etc/systemd/logind.conf

   Add or modify the following lines:

   HandleLidSwitch=ignore
   HandleSuspendKey=ignore
   HandleLidSwitchDocked=ignore
   IdleAction=ignore

   !!! IMPORTANT
   `Do not mixed up configuration if you did any manipulations with preventing machine be sleeping when ‘lid’ is closed.’

   Reload the configuration:

   sudosystemctlrestartsystemd−logindz

5. Confirm SSH Usage

   Check for active SSH sessions:

   who∣grepssh

   Verify the SSH port is open:

   sudonetstat−tuln∣grep:22

   If using a custom port, update firewall rules:

   sudoufwallow<customport>/tcpsudoufwreload

6. Custom scrtipt for preventing suspend mode from /usr/lib/systemd/system-sleep/

   1. Create script fielssh_keep_awake.sh

      sudo nano /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

   2. Add there script for getting ip of connected via SSH remote machine

      #!/bin/bash
      # add some sleep delay to manage execution of script. 
      sleep 1
      echo "Checking for active SSH sessions..." 
      
      # Run the 'w' command and store the output
      w_output=$(w -h)
      echo "w command output: $w_output"
      
      # Extract the IP addresses of users logged in via SSH
      ip=$(echo "$w_output" | grep -oP '(\d{1,3}\.){3}\d{1,3}')
      
      echo "Detected IP: $ip"
      
      if [ -z "$ip" ]; then
         echo "No active SSH session detected."
         exit 0
      else
         echo "User is still logged in from $ip"
         exit 1
      fi

   3. Check the script location and permissions:

      sudo ls -l /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

      It should show something like:

      -rwxr-xr-x 1 root root 123 Dec 19 14:00 /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

   4. Grant access to the file

      sudo chmod +x /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

   5. For checking active SSH session on remote Linux machine execute

      w -h 

      Console should show exisintg session for all users.

   6. Run and check script execution via

      sudo /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

      Expected Output:

      • The output should print the raw result of w -h and then show the detected IP address.
      • If there is an active SSH session, it should print “User is still logged in from “.
      • If no session is found, it should print “No active SSH session detected.”.

7. Custom scrtipt for preventing suspend mode from /etc/pm/sleep.d/

   1. Create script fielssh_keep_awake.sh

      sudo nano /usr/lib/systemd/system-sleep/ssh_keep_awake.sh

   2. Add there script for getting ip of connected via SSH remote machine

      #!/bin/bash
      sleep 1
      echo "Checking for active SSH sessions..." 
      
      # Run the 'w' command and store the output
      w_output=$(w -h)
      echo "w command output: $w_output"
      
      # Extract the IP addresses of users logged in via SSH
      ip=$(echo "$w_output" | grep -oP '(\d{1,3}\.){3}\d{1,3}')
      
      echo "Detected IP: $ip"
      
      if [ -z "$ip" ]; then
         echo "No active SSH session detected."
         exit 0
      else
         echo "User is still logged in from $ip"
         exit 1
      fi

   3. Check the script location and permissions:

      sudo ls -l /etc/pm/sleep.d/ssh_keep_awake.sh

      It should show something like:

      -rwxr-xr-x 1 root root 123 Dec 19 14:00 /etc/pm/sleep.d/ssh_keep_awake.sh

   4. Grant access to the file

      sudo chmod +x /etc/pm/sleep.d/ssh_keep_awake.sh

   5. For checking active SSH session on remote Linux machine execute

      w -h 

      Console should show exisintg session for all users.

   6. Run and check script execution via

      sudo /etc/pm/sleep.d/ssh_keep_awake.sh

      Expected Output:

      • The output should print the raw result of w -h and then show the detected IP address.
      • If there is an active SSH session, it should print “User is still logged in from “.
      • If no session is found, it should print “No active SSH session detected.”.