NETWORK TROUBLESHOOTING AND DEBUGGING FRAMEWORK (FAANG-LEVEL)

NETWORK TROUBLESHOOTING AND DEBUGGING FRAMEWORK (FAANG-LEVEL)

By At June 14, 2025 0



 




Here is a FAANG-level Network Troubleshooting & Debugging Framework, with a deep dive, real-world examples, and interview Q&A to help you master this area like a top-tier engineer.

🔧 NETWORK TROUBLESHOOTING & DEBUGGING FRAMEWORK (FAANG-LEVEL)

🌐 1. Layered Troubleshooting (OSI Model-Based)

OSI Layer

Component

Troubleshooting Focus

Tools

L1 - Physical

Cables, Ports, NICs

Cable damage, unplugged, bad SFPs

ethtool, ip -s link, visual inspection

L2 - Data Link

MAC addresses, Switches

ARP issues, MAC learning, loops

arp, brctl, tcpdump, switch logs

L3 - Network

IP, Routers

IP conflicts, unreachable hosts

ping, traceroute, ip route, netstat, tcpdump

L4 - Transport

TCP/UDP

Port issues, retransmissions

ss, netstat, nmap, Wireshark

L5-7 - App

SSL, DNS, HTTP

DNS failures, SSL handshake, app bugs

curl, dig, openssl, app logs

🧰 2. The FAANG-Level 5-Step Network Debugging Framework

Step

Description

Example

1. Define the Problem

User complaint, system alert, logs

“Service A can’t reach Service B”

2. Isolate the Layer

Use OSI to localize the issue

If ping fails → L3; if TLS fails → L5-6

3. Reproduce & Measure

Reproduce the issue, trace path

curl -v, tcpdump, latency breakdown

4. Analyze & Compare

Compare working vs failing flows

iptables, netstat, compare routing tables

5. Fix & Monitor

Apply change, monitor after fix

Restart service, update DNS, fix route

⚙️ 3. Real-World Use Case Scenarios

Scenario 1: Microservice A → B Call Fails

  • Symptom: 504 Gateway Timeout

  • Tools: curl, ping, ss -tulnp, iptables, tcpdump

  • Flow:

    • ✅ DNS resolves

    • ✅ Ping to B OK

    • ❌ TCP port 8080 connection fails

    • → iptables -L shows port blocked → FIXED

Scenario 2: Sudden Latency Spike

  • Metric: P99 latency jumps from 120ms to 3s

  • Tools: traceroute, tcptraceroute, Wireshark, iftop

  • Steps:

    • ✅ DNS + TCP handshake OK

    • ❌ App takes 2.8s to respond

    • Wireshark shows multiple TCP retransmissions

    • → Congestion or packet loss → Contact Network Ops

Scenario 3: NodePod in K8s Can’t Reach Internet

  • Symptom: curl google.com fails inside pod

  • Tools: kubectl exec, nslookup, ip route, iptables, ip a

  • Steps:

    • ❌ DNS fails

    • → /etc/resolv.conf has wrong nameserver

    • Fix CoreDNS config → Restart pod → FIXED

🧠 FAANG-LEVEL INTERVIEW QUESTIONS & ANSWERS

🔍 Q1: How do you debug a microservice that is not reachable?

Answer:
 Use layered debugging:

  1. Check DNS resolution → nslookup, dig

  2. Check TCP reachability → telnet, curl, ss -lnt

  3. Analyze network routes → ip route, traceroute

  4. Check firewalls or security groups → iptables, nft, cloud console

  5. Application-level logs for errors or crashes

🔍 Q2: What if you see TCP retransmissions in Wireshark?

Answer:
 TCP retransmissions usually mean:

  • Packet loss on the network path

  • MTU mismatch (try ping -M do -s 1472)

  • Buffer overflow due to congestion
     Fix:

  • Use QoS policies

  • Identify faulty NIC, link, or congested switch

🔍 Q3: A web app is slow for users in one region. How would you debug?

Answer:

  • Check regional CDN edge health

  • Use traceroute/MTR from that region

  • Measure RTT, jitter

  • Look at service logs and dashboards (Prometheus, Grafana)

  • Cross-compare with healthy regions

🔍 Q4: DNS is resolving slowly. How would you debug it?

Answer:

  1. Use dig +trace to identify delays

  2. Compare lookup time across multiple resolvers

  3. Check if local DNS cache is stale

  4. Investigate upstream DNS server performance

🔍 Q5: Explain a situation where MTU caused network failure.

Answer:

  • In VPN or overlay networks (e.g., VxLAN), if packets exceed MTU, they may drop silently.

  • Symptom: Large HTTP POST requests hang or timeout.

  • Solution: Use ping -M do -s 1472 to test path MTU.

  • Fix by setting lower MTU on NICs or tunnels.

🧪 KEY TOOLS CHEAT SHEET

Tool

Use

ping

Basic reachability

traceroute, mtr

Path analysis

tcpdump, Wireshark

Packet-level inspection

dig, nslookup

DNS resolution

ss, netstat

Socket monitoring

iftop, nethogs

Bandwidth usage

iptables, nft

Firewall rules

curl -v, telnet

App-level debugging

ethtool, ip -s

NIC/PHY diagnostics

📌 BONUS: VISUAL DEBUGGING FLOWCHART

          [User Reports Issue]
                     ↓
        ┌──────── Is it a DNS issue? ─────────┐
        │             Yes                     │
        ↓                                     ↓
   [dig/nslookup]                      [Ping the IP]
        ↓                                     ↓
   [DNS Broken]                    ┌──── No response? ─────┐
   Fix /etc/resolv.conf            │         Yes          │
                                   ↓                      ↓
                             [Traceroute]        [Check Firewall]
                                   ↓                      ↓
                             [Packet Loss?]        [Blocked Port?]
                                   ↓                      ↓
                             [Fix Routing]         [Open Ports / Rules]








RECOMMENDED YOUTUBE VIDEO TUTORIALS — HANDS-ON

🔹 1. tcpdump & Wireshark (Packet Capture & Analysis)

🔹 2. traceroute, mtr, ping (Network Path & Latency Tools)

🔹 3. dig, nslookup (DNS Debugging)

🔹 4. ss, netstat (Socket Debugging & Port Monitoring)

🔹 5. iptables (Firewall & Port Blocking)

🔹 6. iftop, nethogs (Real-Time Bandwidth Monitoring)

🔹 7. ethtool, ip, iproute2 (NIC, Interface, Routing)

🔹 8. curl, telnet, openssl (Application-Layer Debugging)

🧪 BONUS: COMPLETE NETWORK TROUBLESHOOTING COURSE (All-in-One)




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