Observability & Network Debugging
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.