Understanding TTL Values in DNS

What Is DNS TTL?

TTL (Time to Live) is a value in seconds that tells DNS resolvers how long to cache a record before querying the authoritative server again. TTL directly affects how quickly DNS changes propagate and how much query load your nameservers receive.

TTL in Zone File Records

; TTL is the second field (in seconds)
example.com.    3600     IN    A       203.0.113.50    ; 1 hour
example.com.    86400    IN    NS      ns1.example.com. ; 24 hours
example.com.    300      IN    TXT     "v=spf1..."     ; 5 minutes

; Default TTL for the zone (applies to records without explicit TTL)
$TTL 3600

How TTL Affects DNS Propagation

When you change a DNS record, the old value remains cached by resolvers worldwide for the duration of the previously set TTL. This is why DNS changes are not instant:

• TTL = 86400 (24 hours): Changes can take up to 24 hours to fully propagate
• TTL = 3600 (1 hour): Changes propagate within 1 hour
• TTL = 300 (5 minutes): Changes propagate within 5 minutes
• TTL = 60 (1 minute): Near-instant propagation, but higher load on nameservers

Recommended TTL Values

Record Type	Recommended TTL	Reason
NS	86400 (24h)	Nameservers change rarely
MX	3600 (1h)	Mail routing needs reasonable stability
A / AAAA	3600 (1h)	Good balance of caching and flexibility
CNAME	3600 (1h)	Same as A records
TXT (SPF/DKIM/DMARC)	3600 (1h)	Changes infrequently
CAA	3600 (1h)	Changes infrequently
SRV	3600 (1h)	Service endpoints are relatively stable
SOA Minimum	300-3600	Controls negative caching (NXDOMAIN)

TTL Strategy for Migrations

When planning a DNS migration (e.g., moving to a new server):

1. 48 hours before: Lower TTL to 300 seconds
2. Wait for the old TTL to expire (so all caches refresh with the new low TTL)
3. Make the change: Update the record to the new IP
4. After the change: Within 5 minutes, most resolvers will pick up the new value
5. After confirmation: Raise TTL back to 3600 or higher

The Trade-Off

Low TTL:

• Faster propagation of changes
• Better for failover scenarios
• Higher query volume to nameservers
• Slightly higher latency for first-time lookups

High TTL:

• Lower query volume and nameserver load
• Faster user experience (more cache hits)
• Slower propagation of changes
• Harder to recover from misconfiguration

SOA Record and Negative TTL

The SOA (Start of Authority) record's Minimum TTL field (last number) controls negative caching — how long resolvers cache the fact that a record does not exist (NXDOMAIN). If a visitor queries a non-existent subdomain, resolvers cache the negative response for this duration.

Common Mistakes

• Setting TTL too high before a migration: Lowering from 86400 means you must wait 24 hours for caches to expire before your low TTL takes effect
• Very low TTL in production: A TTL of 60 seconds means every DNS query hits your nameserver, increasing latency and cost
• Forgetting to raise TTL after migration: Leaving TTL at 300 permanently wastes resources