DNS Failover

ELI5 - The Vibe Check

DNS failover is having a health checker constantly poke your primary server, and the moment it stops answering, quietly swapping the DNS answer to point everyone at a backup instead. Since DNS is the very first step in reaching any service, this is one of the crudest but most universal ways to reroute traffic during an outage, no fancy load balancer required, just a health check and a DNS record with a short TTL.

Real Talk

DNS failover automatically updates DNS responses to redirect traffic from a failed or unhealthy endpoint to a healthy backup, based on continuous health checks performed by the DNS provider (Route 53, Cloudflare, NS1). It requires low TTLs on the relevant records to limit how long clients cache the stale answer, and because DNS caching happens at multiple layers outside the provider's control, failover is never instantaneous, some clients keep hitting the dead endpoint until their local cache expires.

When You'll Hear This

"DNS failover kicked in within 30 seconds once health checks failed three times in a row." / "We dropped the TTL to 60 seconds specifically to make DNS failover faster during incidents."