How a Linux server was rescued after IP change left SSH unreachable for hours

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A routine public IP change on a cloud server running minimized Ubuntu caused complete SSH loss and zero ping response, because the OS lacked tools like cloud-init or NetworkManager to apply the update automatically. The data center had assigned the new IP at the hypervisor switch level, but the server's network card still held the old, defunct address. Emergency access was restored via an out-of-band VNC console, where temporary ip and route commands brought the network back online instantly. A permanent fix was achieved using systemd-networkd with the GatewayOnLink flag correctly placed under the Route section, alongside cleanup of legacy IP references in configuration files. The incident also revealed that absent ICMP responses do not confirm a server is down, as the data center's anti-DDoS firewall was silently dropping ping packets while TCP connections remained fully functional.

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