====== 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:

<code>
example.com.    3600    IN    A    93.184.216.34
</code>

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:

<code>
example.com
</code>

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.

<code>
User
   │
   ▼
Recursive Resolver
   │
(Cache)

TTL = 3600 s
</code>

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:

<code>
Old Record:
93.184.216.34

TTL:
3600
</code>

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:

<code>
3600 seconds
</code>

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:

<code>
Day 1

TTL = 86400

↓

Lower TTL to 300

↓

Wait 24 hours

↓

Change IP Address

↓

Most users update within 5 minutes
</code>

-----

===== 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:

<code>
example.com

A Record:
93.184.216.34

TTL:
287
</code>

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 =====

  * [[networking:dns:dns|DNS Explained]]
  * [[networking:dns:dns-propagation|DNS Propagation]]
  * [[networking:dns:recursive-authoritative|Recursive vs Authoritative DNS]]
  * [[tools:dnscheck|DNS Check]]

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{{tag>dns ttl cache propagation resolver authoritative}}