Network Diagnostics Pocket Reference

Command-line tools for diagnosing network failures, from DNS to the firewall. A timeout is not the same as NXDOMAIN. A local resolver returning the wrong answer is not the same as a broken authoritative record. This pocket reference walks the full diagnostic path: DNS and name resolution, reachability and routing, connection and port testing, TLS and certificate inspection, performance and packet behaviour, and firewall filtering. Each chapter opens with the question it answers and the tools — dig, drill, resolvectl, mtr, ss, openssl, tcpdump, nft — you reach for to answer it.

Table of Contents

1. 1 DNS & Name Resolution
2. 2 Reachability & Routing
3. 3 Connection & Port Testing
4. 4 TLS & Certificate Inspection
5. 5 Performance & Packet Behaviour
6. 6 Firewall & Filtering
7. A Appendix