Add note

content/en/docs/collector/configuration.md

@@ -38,8 +38,23 @@ which provide capabilities that can be added to the Collector, such as
 diagnostic tools. Extensions don't require direct access to telemetry data and
 are enabled through the [service](#service) section.
 
-The following is an example of Collector configuration with a receiver, a
-processor, an exporter, and three extensions:
+<a id="endpoint-0.0.0.0-warning"></a> The following is an example of
+Collector configuration with a receiver, a processor, an exporter, and three
+extensions.
+
+{{% alert title="Important" color="warning" %}}
+
+While it is generally preferable to bind endpoints to `localhost` when all
+clients are local, our example configurations use the "unspecified" address
+`0.0.0.0` as a convenience. The Collector currently defaults to `0.0.0.0`, but
+the default will be changed to `localhost` in the near future. For details
+concerning either of these choices as endpoint configuration value, see
+[Safeguards against denial of service attacks].
+
+[Safeguards against denial of service attacks]:
+  https://github.com/open-telemetry/opentelemetry-collector/blob/main/docs/security-best-practices.md#safeguards-against-denial-of-service-attacks
+
+{{% /alert %}}
 
 ```yaml
 receivers: