Efficacy of using short timeout values for an A record

Chuck Swiger cswiger at mac.com
Tue Feb 14 19:23:56 UTC 2012


On Feb 14, 2012, at 11:11 AM, Alan Clegg wrote:
> On 2/14/2012 1:42 PM, Chuck Swiger wrote:
> 
>> ISC's BIND has (or had) a MINTTL value of 5 minutes / 300 seconds.
>> It's probably unreasonable to expect other platforms to refetch DNS
>> records faster than that.
> 
> Uh... no.  BIND has always respected TTL when caching information.

See http://www.ietf.org/rfc/rfc1035.txt

"The MINIMUM value in the SOA should be used to set a floor on the TTL of
data distributed from a zone.  This floor function should be done when
the data is copied into a response.  This will allow future dynamic
update protocols to change the SOA MINIMUM field without ambiguous
semantics."

...and lib/dns/master.c dns_soa_getminimum() and limit_ttl().  At one point,
and I might be dating myself back to the BIND-4.x days, these used to set
a minimum floor value of 300 seconds, even if the SOA or per-record TTL was
smaller.

Maybe that is no longer the case in BIND-9.x and more common use of dynamic
updates, but I repeat my observation that it's not reasonable to update DNS
at sub-minute intervals and expect all clients to honor such....

Regards,
-- 
-Chuck




More information about the bind-users mailing list