DNS propagation: why changes take time and how to check them

After changing DNS records, the new values appear immediately in some locations but not others, causing inconsistent behavior that is hard to debug.

Understand TTL and use tools to check propagation status:

# Check what a specific DNS server sees
dig @8.8.8.8 example.com A        # Google DNS
dig @1.1.1.1 example.com A        # Cloudflare DNS
dig @208.67.222.222 example.com A  # OpenDNS

# Check TTL of current record (how long caches hold the old value)
dig example.com A | grep -A1 'ANSWER SECTION'

# Use https://dnschecker.org or https://whatsmydns.net for global view

# Flush local DNS cache (macOS)
sudo dscacheutil -flushcache && sudo killall -HUP mDNSResponder

# Flush local DNS cache (Linux with systemd-resolved)
sudo systemd-resolve --flush-caches

DNS is a distributed caching system. Each resolver caches records for the duration of the TTL. Lowering TTL before a migration (24-48 hours ahead) reduces propagation time for the actual change.