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Re: Slashdot: Providers Ignoring DNS TTL?

  • From: Dean Anderson
  • Date: Fri Apr 22 23:38:55 2005 
On Thu, 21 Apr 2005, Stephen J. Wilcox wrote:

> On Wed, 20 Apr 2005, Dean Anderson wrote:
> 
> > On Wed, 20 Apr 2005 [email protected] wrote:
> > 
> > > > I'd rather expect this sort of behavior with anycasted servers... 
> > > 
> > > Where do you see any connection between anycast and ignoring DNS TTL? Or is
> > > this just part of your usual rant against anycast DNS service?
> > 
> > The data he showed isn't necessarilly "ignoring ttl".  If there are multiple
> > anycasted caching servers behind a specific IP address, then those several
> > cache's will each have a different state.  Since, [as I
> 
> I fail to see the correlation still.. anycasted caches should all be operating 
> independently getting their DNS data from authoritative sources. 
> 
> If at any point one of them uses a TTL that it has not received from the 
> authoritative source it is ignoring the ttl, where does anycast get involved 
> with this particular problem?

The queries produce different data, but none of the data is inconsistent 
if there are different caches responding on the same address. Here is the 
original description: (slightly reformated with roman numerals)

  (I) I ran a query for a name in a zone I control that has a five minute 
TTL on 204.127.198.4. The first query came up with 5 minutes. 
  (II) I quickly made  a change to the zone. 
  (III) Thirty seconds after the initial query, I try 
again...err... and come up with the change. Hmm... Not caching at all? 
  (IV) Another 30 seconds and I get the change, with 5m TTL. 
  (V) Thirty seconds later, I get the original response with appropriately 
decremented TTL. 
  (VI) Another thirty seconds, I get the change, with 4m TTL.

Here is the detailed anycast explanation:
  (I) Cache 1 gets answer to query X? = Y
  (II) Authority changes X? to Z
  (III) Cache 2 gets answer to query X? = Z
  (IV) Cache 3 gets answer to query X? = Z
  (V) Cache 1 responds 
  (VI) Cache 3 responds

No TTLs were ignored.

		--Dean

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