Connection testing:
ping: ICMP echo, tests basic connectivitytelnet host port: Tests if port is opennc -zv host port: Netcat, similar to telnetcurl -v: Verbose HTTP request
Traffic analysis:
tcpdump: Capture packets on interfacetcpdump -i eth0 port 80: Filter by portwireshark: GUI packet analyzer
Connection info:
netstat -tulpn: Show listening ports and processesss -tulpn: Modern replacement for netstatlsof -i :8080: What process is using port ?
DNS:
dig example.com: Query DNSdig +trace example.com: Show full resolution path