DNSfailover

DNS failover is a technique used to maintain service availability by automatically directing client requests away from unhealthy endpoints to healthy ones through DNS record changes. It relies on monitoring checks and DNS records to minimize downtime without requiring client-side intervention.

In operation, a DNS provider or monitoring system periodically checks the health of endpoints such as web servers or APIs. If a failure is detected, the system updates DNS records (for example A, AAAA, or CNAME records) to point to a functioning endpoint and reduces the Time to Live (TTL) to speed up subsequent updates. Clients and recursive resolvers that cache the old address will eventually receive the new IP after TTL expiration. Because DNS changes propagate through caches, DNS failover is not instantaneous and is influenced by DNS caching behavior and propagation delays.

DNS failover is commonly used for web services with multi-region deployments or disaster recovery plans and is often combined with additional redundancy such as mirrored endpoints, load balancers, or anycast networks. Best practices include maintaining multiple healthy endpoints, using short TTL values during failover windows, and rigorous health checks that reflect actual usability. It is important to recognize limitations: DNS-based failover cannot guarantee sub-second recovery, may be complicated by cache inconsistencies, and can lead to split-brain scenarios if not carefully managed. It is generally most effective when used as part of a broader strategy that includes application-level failover and reliable infrastructure.