Content Delivery Networks

andrew2 at andrew2 at
Fri Aug 10 15:14:40 UTC 2007

Rodney Joffe wrote:
> On Aug 9, 2007, at 10:55 PM, Paul Reubens wrote:
>> How do you engineer around enterprise and ISP recursors that don't
>> honor TTL, instead caching DNS records for a week or more?
> In my "little" bit of research and experience over the last 10 years
> in this field, I have often pursued this "urban myth". It remains
> largely just that.
> The most common  supposed violator of this was AOL. I found myself in
> a position at one stage to get to the "root" of this, and was rather
> impressed to find that it was indeed a myth.
> We've just finished a small research project where we looked at
> approximately 16 million recursive servers. The only ones violating
> this were some CPE devices that ran local recursive services, and
> they were generally along the lines of returning the appropriate TTL
> the first time they were queried, and if the TTL was zero, they
> returned a higher TTL (10000 seconds) to subsequent queries for a
> short period (5 minutes). It may have been a code bug, or a designed
> behavior given that these were CPE devices.

Very interesting.  We've all heard and probably all passed along that little
bromide at one time or another.  Is it possible that at one time it was true
(even possibly for AOL) but with the rise of CDNs, policies of not honoring
TTL's have fallen by the wayside?


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