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DNS TTL (Time To Live)
TTL (Time To Live) is a value associated with every DNS record that specifies how long a recursive DNS resolver may cache the record before querying the authoritative DNS server again.
TTL is measured in seconds.
Why Does TTL Exist?
Without caching, every DNS query would require contacting the authoritative DNS server.
Caching reduces:
- DNS lookup time
- Internet traffic
- Load on authoritative DNS servers
TTL tells recursive resolvers how long they may use a cached answer.
Example
Suppose the following record exists:
example.com. 3600 IN A 93.184.216.34
Here:
- Domain: `example.com`
- Record Type: `A`
- IP Address: `93.184.216.34`
- TTL: `3600 seconds`
A recursive resolver may cache this record for one hour.
How TTL Works
The first user requests:
example.com
The resolver:
- Queries the authoritative DNS server.
- Receives the answer.
- Stores it in its cache.
- Starts the TTL countdown.
Subsequent users receive the cached answer until the TTL expires.
User │ ▼ Recursive Resolver │ (Cache) TTL = 3600 s
When the TTL reaches zero, the resolver performs a new lookup.
Typical TTL Values
| TTL | Meaning | Typical Usage |
|---|---|---|
| 60 | 1 minute | Testing or frequently changing records |
| 300 | 5 minutes | Dynamic services |
| 900 | 15 minutes | Moderate changes |
| 3600 | 1 hour | Common default |
| 86400 | 24 hours | Stable records |
TTL and DNS Propagation
TTL directly affects DNS propagation.
Suppose:
Old Record: 93.184.216.34 TTL: 3600
If you change the IP address immediately after a resolver caches the old record, that resolver may continue serving the old address for up to:
3600 seconds
even though the authoritative server already contains the new record.
Lowering TTL Before DNS Changes
When planning a DNS migration:
- Lower the TTL several hours (or one day) before the change.
- Wait until the old TTL has expired.
- Change the DNS record.
- Increase the TTL again after propagation is complete.
Example:
Day 1 TTL = 86400 ↓ Lower TTL to 300 ↓ Wait 24 hours ↓ Change IP Address ↓ Most users update within 5 minutes
Does TTL Guarantee Propagation Time?
No.
TTL specifies the maximum recommended cache lifetime.
Some resolvers:
- Respect TTL exactly.
- Cache records for shorter periods.
- In rare cases, cache records longer than the published TTL due to implementation or policy.
Therefore propagation time is usually close to—but not always exactly equal to—the TTL.
Common Misconceptions
- TTL delays updates on the authoritative server.
- False. Changes appear immediately on the authoritative server.
- TTL tells clients how long to connect to an IP address.
- False. TTL only controls DNS caching.
- Lower TTL always improves performance.
- False. Lower TTL increases DNS traffic and load on authoritative servers.
Best Practices
- Use a long TTL (1–24 hours) for stable records.
- Lower the TTL before planned DNS migrations.
- Avoid unnecessarily short TTL values for production systems.
Checking TTL
DNS lookup tools display the remaining TTL returned by the queried DNS server.
Example:
example.com A Record: 93.184.216.34 TTL: 287
A remaining TTL of 287 seconds means the resolver will likely continue serving the cached record for another 287 seconds before querying the authoritative server again.
TTL and Telenegar DNS Check
Telenegar DNS Check displays the TTL returned by each DNS server.
Comparing TTL values from multiple resolvers can help identify:
- Cached responses
- DNS propagation status
- Resolver refresh timing
Related Topics
dns ttl cache propagation resolver authoritative