Connectivity to the OCI compute instance has failed unexpectedly. Until yesterday, access was seamless; however, as of today, all login attempts are unsuccessful, and the server remains unreachable. This incident is time-sensitive, as the monthly health check report must be generated and delivered to clients within the next 30 minutes.

An incident ticket has already been logged. Immediate assistance is required to restore access to the instance and ensure timely completion of the health check report.

Follow these steps sequentially to isolate and resolve the root cause.

1. Identify Scope & Timeline

• Validate when the issue began.

• Confirm if this is a sudden outage versus a deployment misconfiguration.

2. Gather Network Details

• Source IP (requestor’s public IP).

• Destination IP (instance IP).

• Clarify whether connection is via Public IP or Private IP (VPN/Bastion).

3. Check Instance & Region Status

• Verify instance state in OCI Console → must be RUNNING.

• Review OCI Region Health Dashboard for outages.

4. Basic Network Connectivity (ICMP)

• Run: ping

• ⚠️ If blocked, proceed to port checks.

5. Port Accessibility (Layer 4)

• Run: telnet

• Validate SSH (22), HTTPS (443), or application-specific ports.

6. Security Lists (SL) & NSGs

• Review ingress/egress rules for VNIC/Subnet.

• Ensure explicit ingress for Source IP on required port.

7. Route Tables

• Confirm subnet route table includes correct entries (e.g., 0.0.0.0/0 → Internet Gateway).

8. OS Firewall

• Linux: systemctl status firewalld

• Check allowed ports: firewall-cmd --list-all

• Windows: Inspect Advanced Firewall rules.

• Provide Proof of Sanity:

Show successful internal ping/telnet results.

Confirm OCI Region Health is green.

• Explain Outbound vs. Inbound:

OCI rules allow inbound traffic.

Corporate firewall may block outbound requests.

Analogy: “OCI lets you in, but your laptop firewall may prevent you from reaching out.”