Q: Two EC2 instances in the exact same VPC and subnet cannot ping each other, but they can both reach the internet. What is the most likely cause?
If they can reach the internet, the routing table and internet gateways are correct. The issue is security at the instance or subnet level.
#Networking #Networking #L2 #VPC #DNS #Security
🎙️ Candidate Opening & Architectural Context
""When an interviewer asks how I diagnose network connectivity, I follow an outside-in OSI model approach. The interviewer is testing: Security Groups vs Network ACLs, default behaviors.. I structure my answer around systematic triage first, root cause analysis second, and permanent remediation third.""
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🛠️ Production Runbook & Step-by-Step Resolution
1️⃣
Initial Diagnostics & Root Cause Analysis
If they can reach the internet, the routing table and internet gateways are correct. The issue is security at the instance or subnet level.
- Local OS firewall (
iptablesorfirewalld) blocking ICMP. - Network ACLs (NACLs) are usually stateless and evaluated before SGs, but if they were blocking local traffic, they'd likely block the internet return traffic too, unless misconfigured with exact IP denies.
2️⃣
Remediation & Permanent Safeguards
The most likely culprit is the Security Group. By default, AWS Security Groups permit all outbound traffic but deny all inbound traffic. If they are in the same security group, they still cannot ping each other unless there is an explicit inbound rule allowing ICMP traffic from the self-referencing security group ID (or the subnet's CIDR). Other possibilities:
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: Local OS firewall (iptables or firewalld) blocking ICMP.."
⚡ 60-Second Elevator Pitch Talking Points
- Local OS firewall (iptables or firewalld) blocking ICMP.
- Network ACLs (NACLs) are usually stateless and evaluated before SGs, but if they were blocking lo...
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