Glossary
DNS
TTL
How long a resolver may cache a record, in seconds. It bounds how fast a change takes effect and, more importantly, how fast a mistake can be undone.
Defined in RFC 1035 §3.2.1.
Where this appears
The lessons that use this term, and what each is for.
SPF PractitionerChanging a record safelyMake an SPF change you can verify and undo within the TTL.DKIM PractitionerThe overlap windowSize a rotation’s overlap from TTL and delivery latency rather than from habit.DMARC PractitionerThe rollbackDefine in advance what would make you revert, and make reverting take minutes.Advanced Email TrustChanging one safelyChange MX hosts or mode under an active policy without making your domain unreachable.Email Security PractitionerAn outage you causedRecover from a policy change that stopped legitimate mail, and find out what you missed.Domain Security FundamentalsCaching, and why "propagation" is the wrong wordExplain what you are actually waiting for after a DNS change, and compute how long.Domain Security FundamentalsTTL as a planning constraintPlan a DNS cutover so the rollback is as fast as the change.DNS Security PractitionerHow a secondary stays currentDescribe the transfer mechanism a secondary uses and how it is authenticated.DNS Security PractitionerRetiring a service safelyDefine a decommission order that cannot leave a record pointing at claimable infrastructure.Domain Abuse & ImpersonationWhat passive DNS addsSay what a record’s history tells you that its current value cannot.Brand ProtectionWatching your own estateDetect an unauthorised change to your own records before anybody outside does.