DNS Cache TTL Calculator

DNS Cache TTL & Resolver Calculator

The TTL (Time To Live) field on a DNS resource record tells every downstream resolver how long it may hold onto the answer before going back to the authoritative server. The base protocol comes from RFC 1035 (1987), and the value is counted in seconds. Picking a number means weighing two sides against each other. A low TTL (60–300s) lets changes take effect fast, which is what you want for CDN failover, blue/green deploys, or dynamic GSLB, but it sends far more queries to the authoritative server. A high TTL (86 400s = 24h, or more) cuts that query load and speeds up resolution for records that rarely move, like MX and NS.

Negative caching (RFC 2308) follows its own rule, set by the MINIMUM field of the zone's SOA record. Keep it short (300–3600s) so a record you just created shows up quickly instead of being remembered as missing. Round-robin DNS spreads load by handing back several A/AAAA records in rotating order. Public anycast resolvers, such as Cloudflare 1.1.1.1 and Google 8.8.8.8, send each query to the nearest PoP and shave a lot off tail latency in the process.

Applications

Reach for this calculator when you need to estimate how much query traffic hits your authoritative servers, plan the TTL-lowering window before a migration (drop to 60s a day ahead, then raise it again once the cutover settles), size the memory a resolver cache will eat, or think through how long a negative cache lingers in typo and wildcard cases. It comes in handy whether you run BIND, Unbound, PowerDNS, Route 53, Cloudflare DNS, or any GeoDNS / weighted-routing setup.

FAQ

Why don't my DNS changes propagate instantly even after the TTL expires? Plenty of ISPs and corporate resolvers shrug off short TTLs and clamp them to a floor of their own, often 300s and sometimes 3600s, to ease the load upstream. Run dig +trace against an authoritative server if you want to see the real value the zone is publishing.

What's a sensible default TTL? Static records are fine at 3600–86400s. If a record might move around, say CDN edges or A records sitting behind load balancers, 300s is safer. Active failover usually wants 60s, though count on a good chunk of clients clinging to longer values anyway.

Should I worry about negative caching SOA MINIMUM? If you spin up new subdomains often, yes. Set MINIMUM to 24h and a single mistyped query can stick around as NXDOMAIN for a whole day. Most people land somewhere in the 900–3600s range as a compromise.