Efficacy of using short timeout values for an A record

[Chuck Swiger]

On Feb 14, 2012, at 2:16 PM, Mark Andrews 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.
> 
> To the best of my knowlege this is just plain wrong. 

Look at BIND-4.8.3 and check named/db_update.c around line 40:

int	min_cache_ttl = (5*60);		/* 5 minute minimum ttl */

...and then:

fixttl(dp)
register struct databuf *dp;
{
        if (dp->d_zone == 0 && !(dp->d_flags & DB_F_HINT)) {
                if (dp->d_ttl <= tt.tv_sec)
                        return;
                else if (dp->d_ttl < tt.tv_sec+min_cache_ttl)
                        dp->d_ttl = tt.tv_sec+min_cache_ttl;
                else if (dp->d_ttl > tt.tv_sec+max_cache_ttl)
                        dp->d_ttl = tt.tv_sec+max_cache_ttl;
        }
        return;
}

...or check named/ns_req.c around line 720 for the equivalent for a secondary NS:

                if (dp->d_ttl)
                        ttl = dp->d_ttl;
                else
                        ttl = zp->z_minimum;            /* really default */
#ifdef notdef /* don't decrease ttl based on time since verification */
                if (zp->z_type == Z_SECONDARY) {
                        /*
                         * Set ttl to value received from primary,
                         * less time since we verified it (but never
                         * less than a small positive value).
                         */
                        ttl -= tt.tv_sec - zp->z_lastupdate;
                        if (ttl <= 0)
                                ttl = 120;
                }
#endif

Regards,
-- 
-Chuck