>>If I drop our TTL value for our mail server to two hours how long does it >>take to propagate thru the dns servers? > It takes: >>Currently the TTL value is set for 12 hours ... > 12 hours.
plus Refresh interval if the slaves don't support NOTIFY.
slaves don't respect TTL (used for caching only), because they are authoritative for the zone, so don't cache the records.
Assume a 4 hour Refresh interval
- slave makes a SOA query and finds it has the latest serial number - you change your master - 3.99+ hours later someone queries the slave and receives and caches the old IP with a 12 hour TTL - 11.99+ hours later that IP is still in the cache
Theoretically correct, but BIND and W2K DNs both send notify to all NS records by default, so in practice, you're splitting frog hairs worrying about slave latency.
Any DNS admin that is has turned of NOTIFY on public zones needs to turn it on.
As I said, the key value to manage is the TTL on the A record to be changed.
Len
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