New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/loop/README.md

@@ -0,0 +1,40 @@
+# loop
+
+## Name
+
+*loop* - detect forwarding loops and halt the server.
+
+## Description
+
+The *loop* plugin will send a random query to ourselves and will then keep track of how many times
+we see it. If we see it more than twice, we assume CoreDNS is looping and we halt the process.
+
+The plugin will try to send the query for up to 30 seconds. This is done to give CoreDNS enough time
+to start up. Once a query has been successfully sent *loop* disables itself to prevent a query of
+death.
+
+The query send is `<random number>.<random number>.zone` with type set to HINFO.
+
+## Syntax
+
+~~~ txt
+loop
+~~~
+
+## Examples
+
+Start a server on the default port and load the *loop* and *forward* plugins. The *forward* plugin
+forwards to it self.
+
+~~~ txt
+. {
+    loop
+    forward . 127.0.0.1
+}
+~~~
+
+After CoreDNS has started it stops the process while logging:
+
+~~~ txt
+plugin/loop: Seen "HINFO IN 5577006791947779410.8674665223082153551." more than twice, loop detected
+~~~