Code-Memo

Observability & Network Debugging

Ping, traceroute, nslookup tools

• ping: checks reachability/latency (ICMP echo); may be blocked/rate-limited.
• traceroute/tracert: shows hop-by-hop path (TTL exceeded); useful for routing issues.
• nslookup/dig: tests DNS resolution and records from specific resolvers.

netstat / ss basics

• Show listening ports, established connections, and socket states.
• Windows often uses netstat; Linux uses ss for faster/more detail.
• Useful for verifying “is the service actually listening?” and “are connections stuck?”

curl for debugging HTTP

• Inspect status codes, headers, redirects, and TLS details (-v).
• Test specific routes, auth headers, and timeouts; reproduce issues without browsers.

Packet capture basics

• Tools like Wireshark/tcpdump capture packets for deep inspection (retransmits, resets, handshake failures).
• Use filters to focus; beware sensitive data in captures.

Common failure patterns

• DNS: NXDOMAIN, wrong record, stale TTL.
• Routing: asymmetric paths, missing routes, blackholes.
• Filtering: security groups/firewalls/NACLs blocking ports or return traffic.
• MTU: fragmentation/PMTUD issues causing hangs.
• TLS: cert mismatch/expiry, protocol/cipher mismatch, SNI issues.