--On Monday, October 05, 2009 04:53:51 PM -0700 "Buhrmaster, Gary"
<[email protected]> wrote:
Ultimately, there's some interesting things that can be done with load
balancing if TTLs are obeyed, thus the question.
Note that in practice a number of ISP's will "adjust" the TTL
on their caching DNS servers, regardless of what the original
TTL is(*).
True. And in fact, they may do so in both directions, not only discarding
data whose TTL has not expired (which is perfectly legal) but also caching
data well beyond its TTL. Such behavior is noncompliant, but some
providers don't care; this is one reason I prefer not to use a caching
resolver not under my control. Nonetheless, part of the point here is that
behavior such as this _breaks_ DNS-based load-balancing, sometimes quite
badly.
And when there are multiple levels of DNS caching,
the practical TTL may end up being longer.
That shouldn't happen, but yes, it sometimes does.
Also, TTL's of zero are not always safe to use, because some resolvers will
throw them away too quickly.
-- Jeff
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