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document dns cache default values #1216

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9 changes: 7 additions & 2 deletions docs/index.asciidoc
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Expand Up @@ -303,12 +303,17 @@ single request.
==== DNS Caching

This plugin uses the JVM to lookup DNS entries and is subject to the value of
https://docs.oracle.com/javase/7/docs/technotes/guides/net/properties.html[networkaddress.cache.ttl],
a global setting for the JVM.
https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/net/doc-files/net-properties.html#address-cache-heading[Address Cache settings]
such as `networkaddress.cache.ttl` and `networkaddress.cache.negative.ttl`, global settings for the JVM.

As an example, to set your DNS TTL to 1 second you would set
the `LS_JAVA_OPTS` environment variable to `-Dnetworkaddress.cache.ttl=1`.

The default value for `networkaddress.cache.ttl` depends on the JVM implementation,
which is 30 seconds for the JDK bundled with Logstash.
The `networkaddress.cache.negative.ttl` setting, that controls how long Java caches
the result of failed DNS lookups, defaults to 10 seconds.

Keep in mind that a connection with keepalive enabled will
not reevaluate its DNS value while the keepalive is in effect.

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