How to fix Received signal 15; terminating. in SSH

Determine whether the termination was intentional or unexpected by checking system logs:

# On systemd systems, check sshd logs:
sudo journalctl -u sshd --no-pager | tail -20

# Or check traditional auth logs:
sudo tail -20 /var/log/auth.log          # Debian/Ubuntu
sudo tail -20 /var/log/secure            # RHEL/CentOS/Fedora

Look for messages before the "Received signal 15" entry. Look for:
- "Stopped OpenSSH server daemon" or "restarting SSH"
- System shutdown/reboot messages
- Resource-related warnings
- Package upgrade notifications

Check whether SSH is currently running and healthy:

# Check service status
sudo systemctl status sshd

# Or for non-systemd systems:
sudo service sshd status

Expected output should show:
- active (running) or running
- Recent restart timestamp (if recently restarted)

If SSH is NOT running:

# Start SSH service
sudo systemctl start sshd

# Or for non-systemd:
sudo service sshd start

# Verify it started
sudo systemctl status sshd

If SSH won't start after signal 15, there may be a configuration issue. Validate the SSH configuration:

# Test SSH configuration syntax
sudo sshd -t

# If there are errors, display them
sudo sshd -T

If the test reports errors, fix the configuration file:

sudo nano /etc/ssh/sshd_config

Common issues:
- Typo in parameter names (Port vs PORT)
- Invalid IP addresses in Listen directives
- Malformed authentication settings
- Missing or corrupted key files

After fixing, restart:

sudo systemctl restart sshd

Check if the signal 15 was triggered by recent activity:

# Check system uptime (indicates if system rebooted)
uptime

# Check last login times
last -5

# Check auth/system logs for recent changes
sudo grep -E "(ssh|reboot|shutdown)" /var/log/messages | tail -30

# Check if packages were recently updated
sudo apt list --upgradable              # Debian/Ubuntu
sudo dnf check-update                   # Fedora/RHEL

If you see SSH package upgrades around the time of signal 15, the termination was expected as part of the package update process.

If you see repeated "Received signal 15" messages, SSH may be stuck in a restart loop. Monitor it:

# Watch SSH service status in real-time
sudo watch -n 2 'systemctl status sshd | head -10'

# Or use journalctl with follow:
sudo journalctl -u sshd -f

If sshd keeps restarting:

1. Stop the service entirely:

sudo systemctl stop sshd
sudo systemctl disable sshd

2. Fix the underlying issue (configuration error, missing keys, etc.)

3. Re-enable and start:

sudo systemctl enable sshd
sudo systemctl start sshd

If running sshd in a container (Docker, Kubernetes, etc.) and seeing repeated signal 15 messages:

Docker: Ensure proper signal handling in your Dockerfile:

FROM ubuntu:22.04
RUN apt-get update && apt-get install -y openssh-server

# Use exec form to ensure signals reach sshd
ENTRYPOINT ["/usr/sbin/sshd", "-D", "-e"]

Kubernetes: Check if the pod is being terminated by the orchestrator:

kubectl get pods <pod-name> -o yaml | grep -A5 terminationGracePeriodSeconds
kubectl describe pod <pod-name> | grep -i signal

For container managers: Ensure termination grace period is sufficient for SSH to close connections gracefully.