TL;DR checklist

The most important commands, usable in most situations. See the detailed guides for specific commands/things to check.

1. Physical/Network access layer
   • Check the cables
   • Check switch port LEDs
   • Is de box connected to the correct switch?
2. Internet layer:
   1. Check network settings:
      • ip a
      • ip r (+ ping default gw)
      • cat /etc/resolv.conf (+ dig www.google.com @a.b.c.d +short)
   2. LAN connectivity:
      • Ping between hosts, default GW
      • Check DNS availability: nslookup, dig, getent ahosts
3. Transport layer
   1. Is the service running? sudo systemctl status SERVICE.service
      • sudo systemctl start SERVICE.service
      • sudo systemctl enable SERVICE.service
   2. Does it use the correct ports/sockets? sudo ss -tlnp
   3. Does the firewall allow traffic to those ports? sudo firewall-cmd --list-all
      • sudo firewall-cmd --add-service=SERVICE --permanent
      • sudo firewall-cmd --add-port=PORT/tcp --permanent
      • sudo systemctl restart firewalld
4. Application layer
   • Check the logs: sudo journalctl -f -u SERVICE.service
   • Restart after each config file change: sudo systemctl restart SERVICE.service