Q: Our application uses a third-party API. The API vendor claims they are receiving requests, but our app throws timeout errors. How do you prove whether the traffic is leaving our server successfully?
To definitively prove if the traffic is leaving the server and if we are receiving a response, I would use tcpdump.
#Linux #Linux / SRE — Scenario-Based Interview Questions #L2 #SRE #Systems #Troubleshooting
🎙️ Candidate Opening & Architectural Context
""Never reboot a server blindly; always capture top process telemetry, lsof descriptors, and thread dumps first. The interviewer is testing: Packet capture, `tcpdump` usage.. 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
To definitively prove if the traffic is leaving the server and if we are receiving a response, I would use tcpdump.
- If I see
SYNleaving, but noSYN-ACKreturning, the traffic is being dropped aggressively (firewall, routing issue on the path, or vendor blocking us). - If the handshake completes, but SSL/TLS negotiation stalls, or we send an HTTP request and get no
PSHdata back, the vendor's application is hanging, proving the fault is on their end. - If no
SYNpackets appear in tcpdump at all, the issue is internal to the host (e.g., the app is locked up, or localiptablesrules drop outbound traffic before it hits the wire).
2️⃣
Remediation & Permanent Safeguards
I'd run: tcpdump -nni eth0 host I'd look at the TCP handshake.
💡 The Senior SRE Gold Nugget (Key Architectural Takeaway)
"Pro-Tip: If I see SYN leaving, but no SYN-ACK returning, the traffic is being dropped aggressively (firewall, routing issue on the path, or."
⚡ 60-Second Elevator Pitch Talking Points
- If I see SYN leaving, but no SYN-ACK returning, the traffic is being dropped aggressively (firewa...
- If the handshake completes, but SSL/TLS negotiation stalls, or we send an HTTP request and get no...
- If no SYN packets appear in tcpdump at all, the issue is internal to the host (e.g., the app is l...
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