Keepalived

Keepalived is an open-source routing software for Linux systems that provides high-availability and load-balancing capabilities through the implementation of the Virtual Router Redundancy Protocol (VRRP). Its primary function is to ensure uninterrupted service availability by allowing a group of two or more servers to share a virtual IP address, where one server acts as the active master while others remain in standby. If the master fails, one of the standby servers automatically takes over the virtual IP within seconds, minimizing downtime. Beyond failover, Keepalived also features a built-in health-checking subsystem that monitors the status of backend real servers (e.g., web servers, database servers) using various probes such as TCP connection checks, HTTP/HTTPS GET requests, SSL handshake verification, SMTP, LDAP, DNS queries, and custom scripts. When a backend server fails its health check, the software dynamically removes it from the load-balancing pool, and re-adds it once it recovers. This makes Keepalived an essential tool for building resilient server clusters, such as reverse proxy farms, database load balancers, and any service that demands high availability. The software also supports advanced features like VRRP preemption, multicast or unicast VRRP advertisements, multiple VRRP instances per interface, virtual server grouping, and fine-grained failure detection thresholds. Administrators can configure Notify scripts that trigger custom actions (e.g., sending alerts, updating DNS) when VRRP state transitions occur. Keepalived integrates tightly with the Linux kernel's IP Virtual Server (IPVS) framework, enabling L4 load balancing across multiple backends, but it can also operate independently just for failover.