Network Debug Guide

# Basic ping ping google.com # Limit count ping -c 4 google.com # Specify interval (0.2s) ping -i 0.2 -c 10 8.8.8.8 # Set packet size (test MTU) ping -s 1472 -c 3 10.0.0.1 # Ping with timestamp ping -D google.com

# Linux/macOS traceroute google.com # Use ICMP (not UDP) traceroute -I google.com # Use TCP port 80 traceroute -T -p 80 google.com # Windows tracert google.com # mtr (better traceroute) mtr --report --report-cycles 10 google.com

# Basic nslookup nslookup example.com # Query specific record type nslookup -type=MX example.com nslookup -type=TXT example.com nslookup -type=NS example.com # dig (more detailed) dig example.com dig example.com MX dig example.com NS +short # Reverse DNS dig -x 8.8.8.8 # Query specific DNS server dig @8.8.8.8 example.com # Trace full resolution dig +trace example.com

# Basic request with headers curl -v https://example.com # Show only response headers curl -I https://example.com # Time breakdown curl -w "@curl-format.txt" -o /dev/null -s https://example.com # curl-format.txt: # time_namelookup: %{time_namelookup}\n # time_connect: %{time_connect}\n # time_appconnect: %{time_appconnect}\n # time_pretransfer: %{time_pretransfer}\n # time_redirect: %{time_redirect}\n # time_starttransfer: %{time_starttransfer}\n # time_total: %{time_total}\n # POST JSON curl -X POST https://api.example.com/data \ -H "Content-Type: application/json" \ -d '{"key":"value"}'

# List listening ports netstat -tlnp # Linux (needs sudo for PID) ss -tlnp # modern replacement # All connections netstat -an ss -an # Filter by port ss -tnp sport = :80 # Show established connections ss -tn state established # Port usage summary ss -s

# Capture on interface eth0 tcpdump -i eth0 # Filter by host tcpdump -i eth0 host 10.0.0.1 # Filter by port tcpdump -i eth0 port 80 # Capture HTTP traffic (GET/POST) tcpdump -i eth0 -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' # Save to file tcpdump -i eth0 -w capture.pcap # Read saved capture tcpdump -r capture.pcap