DNS failover is a technique where a DNS service monitors your servers and, on failure, stops returning the unhealthy address and serves a standby one instead. It is a form of failover implemented at the name-resolution layer.
Why it matters
It is one of the few failover methods that works across data centers, providers and regions without special network arrangements. Its weakness is caching. Clients and resolvers keep the old answer until the TTL expires, so recovery time is roughly the health check detection time plus the TTL. Some resolvers also hold records longer than instructed.
Example
A primary at 203.0.113.10 and a standby at 198.51.100.20, with a 60-second TTL:
www 60 IN A 203.0.113.10 ; healthy, served
www 60 IN A 198.51.100.20 ; returned only when the primary failsIf three consecutive checks at 30-second intervals fail, the provider switches the answer. Most users recover in two to three minutes, though stragglers take longer.
Common mistakes
- Setting a 24-hour TTL and expecting fast failover.
- Health-checking only that port 80 answers, not that the application works.
- Not testing the standby, which may be out of date or overloaded.
- Using the same DNS provider as a single point of failure.
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