TTL question

Jeff Lasman blists at
Tue Jan 27 18:36:31 UTC 2004

On Monday 26 January 2004 06:30 pm, joe wrote:

>  Which value determines the time a zones data will sit in a remote
> servers cache. Is it the minimum SOA value or the TTL value.

It's the per record TTL value (which, if you're looking at the actual 
zonefiles on your own server, may not be in each record line, but 
rather a default $TTL value at the top of the zonefile).

The TTL value in the SOA is the cache time for negative replies.

> From
> what I have read, the minimum value doesn't seem to get used for
> this. If that is the case if I need to make changes to a zone and I
> adjust the TTL period to a lower value temporarily (say 2 hours) is
> it safe to say that my zone changes on remote DNS's would be updated
> accordingly. Obviously I would need to adjust the TTL so that based
> on its current value the "desired" value would take effect.

It's as safe as it can be.  Some ISPs are known to ignore TTL and use 
their own algorithm for when to request updated information from your 
nameserver, but you really have to ignore them because (a) you don't 
know who they are, and (b) you can't do anything about them anyway.

> Also for the following example I performed a "dig + trace
>". How are the TTL's values (which I believe are the
> second column values from the left) established ? I can understand
> that the zone would have a value of 86400 as setup by their
> own DNS, but wouldn't that value propagate to the upper nameservers?
> Why is the value different and how would subsequent changes be
> updated to upper name servers ?

What you're seeing, for example:

> ..                       55628   IN      NS

is the current TTL, in other words, how long it will be before the name 
server you're queried will consider the record to be correct before it 
looks again.  For example, the actual TTL may be 56000 seconds (it's 
probably much higher) and it may have been 362 seconds since your name 
server queried the rootserver for it's NS records.

Jeff Lasman,, P. O. Box 52672, Riverside, CA  92517 US
Professional Internet Services & Support / Consulting / Colocation
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