What is TTL in DNS?

TTL (Time To Live) in DNS is the lifetime of a DNS record in cache, measured in seconds.

In simpler terms: TTL shows how long a DNS answer can be stored and used without asking the DNS server again.

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#Why TTL is needed

DNS queries can be made by:

• the browser
• the operating system
• the provider's DNS servers

To avoid asking the DNS server every time, caching is used. TTL says:

"This answer can be considered valid for N seconds."

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#How TTL works (a simple example)

Suppose a record has:

This means:

• the DNS answer is cached for 300 seconds (5 minutes)
• during this time the IP is taken from cache
• new queries do not go to the authoritative DNS

After the TTL expires:

• the record is removed from cache
• on the next request, DNS is queried again

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#Where TTL applies

TTL is taken into account:

• in the browser
• in the OS
• in recursive DNS servers
• in proxies and CDNs

TTL is not the same as the lifetime of the record on the server, only the caching time.

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#Why TTL matters

#High TTL

• fewer DNS queries
• faster performance
• slow propagation of changes

#Low TTL

• changes take effect quickly
• more DNS queries
• higher load

TTL is a trade-off between speed and flexibility.

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#A common real-world scenario

Before migrating a site:

• TTL is lowered (for example, to 60 seconds)
• the IP is changed
• after the migration, TTL is raised back

This allows:

• minimizing downtime
• switching users over faster

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#Real-life example

TTL is like the validity period of a certificate:

• while the period has not expired, it is accepted
• once it expires, a new one is needed

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#Short interview answer

Remember this formulation:

TTL in DNS is the time, in seconds, during which a DNS record can be stored in cache and used without querying the DNS server again.