Slashdot: Providers Ignoring DNS TTL?

Sat Apr 23 09:22:03 UTC 2005

On Fri, 22 Apr 2005, Dean Anderson wrote:

> On Thu, 21 Apr 2005, Stephen J. Wilcox wrote:
> 
> > On Wed, 20 Apr 2005, Dean Anderson wrote:
> > 
> > > On Wed, 20 Apr 2005 sthaug at nethelp.no 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.

Ok gotcha, and you point seems valid except aiui the previous post was 
concerning providers who are actually overriding the TTL eg your zone has a 5m 
ttl, the provider caches it but sets TTL to 10 days.

i think this thread forked quite early :)

Steve