SSH closed ! [IMPORTANT]

I am glad you resolved the issue with the AWS Systems Manager - Sessions Manager. However, to comprehensively diagnose and monitor the status of your EC2 instance and SSH access, here are several recommended steps and configurations to consider:

1. Review AWS CloudTrail Logs

• Start by examining your AWS CloudTrail logs, which record API activities across AWS resources. This can help identify any recent changes or actions, such as modifications to security groups or IAM permissions, that could impact instance connectivity. Look for any API calls related to EC2 or VPC that might explain recent network behavior changes affecting SSH access.

2. Check System and Login Logs

• System Logs: Check /var/log/syslog or /var/log/messages on the instance for any signs of network or system issues. These logs may reveal resource usage spikes or networking configurations that affect SSH.
• Login Logs: Examine /var/log/secure to review recent SSH connection attempts or rejections, which can highlight potential access issues or unauthorized attempts to access the instance.

3. Use AWS Systems Manager (SSM) Run Command

• If Systems Manager (SSM) is configured with the appropriate IAM role, it allows you to run commands remotely on the instance without needing direct SSH access. You can use the Run Command to check the SSH daemon status (sudo systemctl status sshd), verify firewall configurations, and troubleshoot networking settings. This minimizes downtime and enables direct, secure access to the instance’s internal settings.

4. Enable VPC Flow Logs

• To diagnose potential networking issues on port 22, consider enabling VPC Flow Logs. These logs capture information about the IP traffic to and from the instance and can show whether SSH connection attempts are being accepted, rejected, or dropped at the network interface level. Flow logs can reveal connectivity issues related to security group or network ACL configurations.

5. Stream Logs to Amazon CloudWatch Logs

• Operating System Logs: Configure the instance to stream critical logs—such as system, secure, and authentication logs—to Amazon CloudWatch Logs. This provides real-time access to diagnostic information and enables filtering for specific error types, helping to monitor access events, process health, and network connectivity issues.
• Enable automatic log grouping and filtering, allowing you to set alerts for significant or repeated failures and track SSH-related events in real-time.

6. Stream Custom Metrics to Amazon CloudWatch

• Resource Monitoring: To proactively manage the instance’s performance, set up CloudWatch custom metrics for CPU, memory, and network usage. This data helps monitor resource utilization trends and provides visibility into performance bottlenecks.
• Key Process Monitoring: Monitor essential services such as the SSH daemon (sshd), Docker containers, and any application-specific processes by configuring custom metrics in CloudWatch. This setup allows you to create alerts for critical events, like when a key service unexpectedly stops.

For more detailed guidance on troubleshooting EC2 instances, you can refer to the official AWS EC2 troubleshooting documentation. Additionally, for a detailed overview of how to detect and remediate process issues on EC2 instances using CloudWatch and AWS Systems Manager, check out this AWS blog post.

Be vigilant about the costs associated when using the recommendations 4, 5, and 6.