What tools and techniques would you use to debug complex network issues?

Toolset: tcpdump/Wireshark (packet capture and analysis), netstat/ss (connection state), traceroute/mtr (path analysis), dig/nslookup (DNS debugging), curl -v (HTTP debugging), openssl s_client (TLS debugging), iptables/nftables -L (firewall rules), strace (syscalls including network), eBPF tools (bcc/bpftrace for kernel-level). Methodology: isolate the layer (DNS, TCP, TLS, application), reproduce issue, capture traffic, analyze headers/timing. Compare working vs non-working cases. Distributed tracing for microservices.