Scenario : Connection refused
App can't connect to database on port .
. telnet db-host 5432 to test connectivity
. Check if database is running: ss -tulpn | grep 5432
. Check firewall rules, security groups
. Verify DNS: dig db-host
Scenario : Intermittent timeouts
. Check if client or server side
. Use curl -v --trace-time to find delay
. Check for packet loss, network saturation
Scenario : DNS not resolving
. Check /etc/resolv.conf for nameserver
. Try dig @8.8.8.8 hostname with known DNS
. Verify network path to DNS server