[Gllug] DNS - identifying primary nameserver

Jim Cheetham jim.gllug at gonzul.net
Wed Apr 10 13:55:15 UTC 2002

On Wed, Apr 10, 2002 at 02:41:47PM +0100, Dave Turner wrote:
> On Wed, Apr 10, 2002 at 02:29:15PM +0100, Jim Cheetham wrote:
> > And if you're thinking of making changes to a zone, drop the TTL value
> > to something less than a day, to help the changes propogate into the
> > wider world ... 5 minutes isn't a bad number.
> I've seen this advice a few times and just to make it clear in my mind
> I'm going to have to ask a dumb question.
> If I set the TTL to 5 minutes, how long before making the changes that
> I want to propogate should I do this?  Presumably if my TTL is currently
> 3 days I should do this 3 days before making any zone changes so that
> I can be sure all machines are working of a record with a TTL of 5 minutes.
> Is that correct?

Absolutely spot on.
The TTL value of the domain immediately before an interesting change
represents the worst-case TTL that someone else will cache.

So, at time 't', I query for your domain, and receive TTL of 3 days.
At time t+1, you change your domain TTL to 5 minutes.
Any of my requests (for data that I already have) within the next 3 days
will not make it as far as your system.
At time t+3 days, my queries will come through to you again.

I think a TTL of 1 day is reasonable for most non-changing domains,
however if you're running DNS as a service for customers you probably
should have a much lower DNS, say 3 hours, to enable you to be able to
honestly claim that a change request received in the morning will be
achieved by the end of the day.

If you're expecting to be making changes soon, drop your value way down
in advance.

The question of what makes a good TTL value for different situations is
related to how many people query your domain, and how many resources you
have for the domain server. CPU and bandwidth are not as important as
memory, but it's not a heavy-weight application.


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