Code-Memo

Network troubleshooting (ping, traceroute, netstat, ss)

Fast checklist (host)

ip -br addr
ip route
resolvectl status 2>/dev/null || cat /etc/resolv.conf

ping (reachability / ICMP)

ping -c 4 1.1.1.1
ping -c 4 example.com

If IP works but DNS fails → likely DNS issue (or blocked DNS).

traceroute (path)

traceroute 1.1.1.1
traceroute -n 1.1.1.1

Alternative:

mtr -rw 1.1.1.1   # if installed

ss / netstat (ports)

ss -tulpn
ss -tan
netstat -tulpn    # legacy

Capture packets (quick)

sudo tcpdump -i eth0 -nn host 1.1.1.1
sudo tcpdump -i eth0 -nn port 53

Common failure patterns