docs: Add mdns plugin (#200)

content/explugins/mdns.md

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+title = "mdns"
+description = "*mdns* - serves '.local' mDNS info over normal DNS."
+weight = 10
+tags = [  "plugin" , "mdns" ]
+categories = [ "plugin", "external" ]
+date = "2020-06-17T11:07:00+12:00"
+repo = "https://github.com/openshift/coredns-mdns"
+home = "https://github.com/openshift/coredns-mdns/blob/master/README.md"
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+
+## Description
+
+This plugin reads mDNS records from the local network and responds to queries based on those records.
+
+Useful for providing mDNS records to non-mDNS-aware applications by making them accessible through a standard DNS server.
+
+## Syntax
+
+~~~ txt
+mdns example.com [minimum SRV records] [filter text] [bind address]
+~~~
+
+## Examples
+
+As a prerequisite to using this plugin, there must be systems on the local
+network broadcasting mDNS records. Note that the .local domain will be
+replaced with the configured domain. For example, `test.local` would become
+`test.example.com` using the configuration below.
+
+Specify the domain for the records.
+
+~~~ corefile
+example.com {
+	mdns example.com
+}
+~~~
+
+And test with `dig`:
+
+~~~ txt
+dig @localhost baremetal-test-extra-1.example.com
+
+;; ANSWER SECTION:
+baremetal-test-extra-1.example.com. 60 IN A   12.0.0.24
+baremetal-test-extra-1.example.com. 60 IN AAAA fe80::f816:3eff:fe49:19b3
+~~~
+
+If `minimum SRV records` is specified in the configuration, the plugin will wait
+until it has at least that many SRV records before responding with any of them.
+`minimum SRV records` defaults to `3`.
+
+~~~ corefile
+example.com {
+    mdns example.com 2
+}
+~~~
+
+This would mean that at least two SRV records of a given type would need to be
+present for any SRV records to be returned. If only one record is found, any
+requests for that type of SRV record would receive no results.
+
+If `filter text` is specified in the configuration, the plugin will ignore any
+mDNS records that do not include the specified text in the service name. This
+allows the plugin to be used in environments where there may be mDNS services
+advertised that are not intended for use with it. When `filter text` is not
+set, all records will be processed.
+
+~~~ corefile
+example.com {
+    mdns example.com 3 my-id
+}
+~~~
+
+This configuration would ignore any mDNS records that do not contain the
+string "my-id" in their service name.
+
+If `bind address` is specified in the configuration, the plugin will only send
+mDNS traffic to the associated interface. This prevents sending multicast
+packets on interfaces where that may not be desirable. To use `bind address`
+without setting a filter, set `filter text` to "".
+
+~~~ corefile
+example.com {
+    mdns example.com 3 "" 192.168.1.1
+}
+~~~
+
+This configuration will only send multicast packets to the interface assigned
+the `192.168.1.1` address. The interface lookup is dynamic each time an mDNS
+query is sent, so if the address moves to a different interface the plugin
+will automatically switch to the new one.