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Add information about Negative Response Caching 

Fixes #743
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highlyunavailable 2015-03-01 20:37:16 -08:00 committed by Highly Unavailable
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website/source/docs/guides/dns-cache.html.markdown
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@ -44,6 +44,22 @@ meaning no stale results may be served. The default for
if [`allow_stale`](/docs/agent/options.html#allow_stale) is enabled, we will use if [`allow_stale`](/docs/agent/options.html#allow_stale) is enabled, we will use
data from any Consul server that is within 5 seconds of the leader. data from any Consul server that is within 5 seconds of the leader.
## Negative Response Caching
Some DNS clients cache negative responses - that is, Consul returning a "not
found" style response because a service exists but there are no healthy
endpoints. What this means in practice is that cached negative responses may
mean that services appear "down" for longer than they are actually unavailable
when using DNS for service discovery.
One common example is that Windows will default to caching negative responses
for 15 minutes. DNS forwarders may also cache negative responses, with the same
effect. To avoid this problem, check the negative response cache defaults for
your client operating system and any DNS forwarder on the path between the
client and Consul and set the cache values appropriately. In many cases
"appropriately" simply is turning negative response caching off to get the best
recovery time when a service becomes available again.
## TTL Values ## TTL Values
TTL values can be set to allow DNS results to be cached downstream of Consul. Higher TTL values can be set to allow DNS results to be cached downstream of Consul. Higher