DNS TTL (time to live)
Every DNS record has a TTL, a number of seconds that says how long resolvers may keep the answer before asking again. It's why DNS changes aren't instant, and choosing it well is how you make them fast when it matters.
What the TTL does
When a resolver (your internet provider's, or a public one such as 1.1.1.1) looks up a record, it remembers the answer for the record's TTL. Everyone using that resolver gets the cached answer until it expires. So after you change a record, some people see the old value for up to the old TTL. There's no way to clear other people's caches; the TTL is the only control you have.
"DNS propagation" is this caching. Nothing is pushed around the internet: each resolver simply picks up the new value when its cached copy expires.
Common values
| TTL | Seconds | Good for |
|---|---|---|
| 1 minute | 60 | Failover setups that must switch quickly. More queries, slightly slower for visitors. |
| 5 minutes | 300 | Records you're about to change, or that change often. Cloudflare's Auto. |
| 1 hour | 3600 | The sensible default for most records. |
| 1 day | 86400 | Stable records such as NS, and anything that never changes. |
Planning a change
- A day (or one old TTL) before: lower the TTL of the records you'll change to 300.
- Wait for the old TTL to pass, so every cache holds the short one.
- Make the change. It reaches everyone within about 5 minutes.
- Once you're happy, raise the TTL again.
TTLs you don't control
- Nameserver changes: the NS records at the registry (such as .com) have their own TTL, often one or two days.
- Negative answers: "this name doesn't exist" is cached for the SOA record's minimum value. Look up a name before creating it and you may wait that long to see it.
- Some resolvers ignore very short TTLs and keep answers for at least 30 seconds to a few minutes, and cap long ones at a day or a week.
- Browsers and operating systems keep their own short caches on top. Flush DNS cache has the steps to clear yours.
Questions
What is a good DNS TTL?
3600 seconds (one hour) for most records. Use 300 for records you're about to change, and 86400 for things that never change.
How long does DNS propagation take?
Up to the TTL of the old record, because resolvers keep their cached copy until it expires. Lower the TTL in advance to make a change fast.
Does a low TTL slow my website down?
Barely. Visitors' resolvers have to ask again more often, which adds a few milliseconds now and then. It does mean more queries to your DNS host.
Can I force everyone to see my change now?
No. You can clear a specific public resolver's cache (Google and Cloudflare have pages for that) and your own computer's, but every other resolver waits for the TTL. Flush DNS cache links to both and has the commands for every device.
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