TTL
Time to live: the number of seconds a DNS answer may be cached by resolvers before they must ask the authoritative server again.
What it means
Every DNS record carries a TTL set by its owner. A resolver that fetches a record with TTL 3600 may serve it from cache for up to an hour. RFC 2181 caps the value at 2^31 - 1 seconds.
The trade-off: long TTLs mean fewer queries and resilience to brief outages; short TTLs mean changes take effect quickly. Before a planned change to MX, SPF or DKIM records, lower the TTL at least one old TTL ahead, make the change, then raise it again.
What people call propagation is mostly caches expiring according to TTLs, including the cached negative answer for a record that did not exist yet.
Example
example.com. 300 IN TXT "v=spf1 -all" (cached for up to 5 minutes)Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 1035: Domain Names: Implementation and Specification, section 3.2.1rfc-editor.org
- RFC 2181: Clarifications to the DNS Specification, section 8rfc-editor.org
- Cloudflare docs: Time to Live (TTL)developers.cloudflare.com
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