DNS TTLs revisited

[David Botham]

bind-users-bounce at isc.org wrote on 02/05/2004 04:55:19 PM:
> At 11:51 AM 2/5/2004, David Botham wrote:
> 
> 
> >bind-users-bounce at isc.org wrote on 02/05/2004 02:25:15 PM:
> > > My company is the holder of the domain netflame.cc
> > >
> > > While the TTL on the domain has been set to 24 hours, we have set an
> > > individual TTL on one particular machine (ssl-hints.netflame.cc) to 
30 
> > seconds.
> > >
> > > My question is this:
> > >
> > > Does this play well across all DNS? I haven't done this until 9.x, 
so I
> > > don't know about version 8 or, for that matter, version 4. Is there 
a
> > > version out there where they only take the domain value (1 day) 
rather 
> > than
> > > the individual value (30 sec)?
> >
> >Reality is that, regardless of how you configure your zone db file, all 
RR
> >always carry an explicit TTL.
> 
> So, then, I assume that reversing my logic would be a good thing? That 
is, 
> set the default to 30 seconds and then explicitly set each and every 
(but 
> non-applicable to this situation) IN A record to 86400 seconds, so in 
the 
> event that someone isn't seeing the individual TTL values, they are 
picking 
> the right default value (30 seconds).


No.  What I was trying to say is that when you configure a zone database 
file, you use a $TTL statement to set the default TTL for every RR in the 
zone where one is not explicitly stated.

When the name server loads the zone, it inserts each RR into memory with 
an explicit TTL on it (if you were able to look into memory, you would see 
this).

Another place to see this effect is to look at the backup file of a slave 
zone.  You will notice that regardless of how you typed the information 
into the master zone database file (by using the single $TTL directive), 
each and every RR in the backup copy of zone on the slave has an explicit 
TTL.


In the end, if you want a TTL of 30 seconds set for just one RR, then, 
explicitly state one for that RR and use the $TTL directive for the rest. 

You may be interested in reading RFC2308, particulary section 4.

http://www.ietf.org/rfc/rfc2308.txt?number=2308



Hope this helps.


Dave...


> 
> Gads, this bothers me to do this :( I will, but I don't like it.
> 
> 
> ===
> Bob Gahl Bicycle (Ryan Vanguard) Mobile ||     @
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> mountain has no tigers, the monkey will also declare himself
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>