The Myth of “Instant” DNS
When you edit an A record, the change appears in your authoritative zone immediately, but end-users may wait minutes—or hours—because intermediate resolvers cache the old answer. This delay is called propagation.
TTL Controls Everything
Time-To-Live is the countdown timer (in seconds) placed in every DNS response. A TTL of 3600 means “feel free to cache this for one hour.” Lower TTL = faster updates but more queries against your authoritative server.
Layered Caches
- Browser (Chrome: 60 s – 30 min)
- Operating system stub (Windows: 5 min default)
- Router DNS proxy (varies)
- ISP recursive (OpenDNS, Comcast: honor TTL up to 24 h)
Reducing Propagation Time
Before any major migration, lower TTL to 300 seconds at least 24 hours earlier. After you confirm traffic has moved, raise back to 3600-86400 to reduce query load and improve resolver performance.
Global Testing Tools
Use WhatsMyDNS.net, DNSChecker.org, or command line dig @8.8.8.8 yourdomain.com from multiple vantage points. If 80% of locations show new IP, you can begin decommissioning old server.
Negative Caching (NXDOMAIN)
If you query a non-existent subdomain, the NXDOMAIN response is cached for the SOA MINIMUM value (often 3 hours). Create new subdomains early or temporarily wildcard to avoid “not found” stickiness.
CDN & Anycast Impact
Anycast DNS providers update edges within 30 seconds, but cached answers at ISPs still obey original TTL. Combining low TTL with anycast gives the fastest practical propagation.
Clearing Local Cache
Windows: ipconfig /flushdns. macOS: sudo dscacheutil -flushcache && sudo killall -HUP mDNSResponder. Chrome: visit chrome://net-internals/#dns and click Clear. Mobile: toggle airplane mode.
Common Misconceptions
- “DNS changes take 24-48 h” – only if TTL was high
- “Pinging proves propagation” – ICMP may use different route than TCP/443
- “Whois shows new NS so DNS is updated” – whois ≠ DNS resolution
Takeaway
Control TTL proactively, test globally, and communicate timelines with stakeholders. Proper planning turns “up to 48 hours” into “under 15 minutes” for real-world users.