clean cache

[Robert Gahl]

At 06:23 PM 4/3/00 +0000, Barry Margolin wrote:

>The problem may be that the data is cached at the other computers'
>nameservers.  You can't clean them out.  You can use the TTL to tell them
>how long they can cache things.

This brings up a question I've been trying to answer and can't seem to get 
the answer (from an implementation perspective).

I pretty much have a handle on how data originally ends up in an ISP's 
cache. But, what I don't seem to be able to understand is how the TTL value 
ties into updates. That is, once a set of data is in an ISP's cache, does 
it automatically update at the TTL time (or TTL time w/i reason), or does 
it only update based upon an ISP user request. By that I mean, if a user 
asks for a DNS lookup of some form, does this cause DNS to check the TTL 
value, and if expired, go get a new copy?

What I'm asking, I think, is what is the mechanism by which the DNS cache 
at an ISP updates itself? Is it automatic or is it based upon a user 
request event?

Thanks.
===
Bob Gahl Bicycle (Ryan Vanguard) Mobile  ||     @
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"Sahn joong moe low ful how jee yah ching wong" - "When the
mountain has no tigers, the monkey will also declare himself
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