arrange services and protocols

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+mDNS Repeater
+-------------
+
+Starting with VyOS 1.2 a :abbr:`mDNS (Multicast DNS)` repeater functionality is
+provided. Additional information can be obtained from
+https://en.wikipedia.org/wiki/Multicast_DNS.
+
+Multicast DNS uses the 224.0.0.251 address, which is "administratively scoped"
+and does not leave the subnet. It retransmits mDNS packets from one interface
+to other interfaces. This enables support for e.g. Apple Airplay devices across
+multiple VLANs.
+
+Since the mDNS protocol sends the AA records in the packet itself, the repeater
+does not need to forge the source address. Instead, the source address is of
+the interface that repeats the packet.
+
+Configuration
+=============
+
+.. cfgcmd:: set service mdns repeater interface <interface>
+
+   To enable mDNS repeater you need to configure at least two interfaces. To
+   re-broadcast all incoming mDNS packets from any interface configured here to
+   any other interface configured under this section.
+
+.. cfgcmd:: set service mdns repeater disable
+
+   mDNS repeater can be temporarily disabled without deleting the service using
+
+.. note:: You can not run this in a VRRP setup, if multiple mDNS repeaters
+   are launched in a subnet you will experience the mDNS packet storm death!
+
+Example
+=======
+
+To listen on both `eth0` and `eth1` mDNS packets and also repeat packets
+received on `eth0` to `eth1` (and vice-versa) use the following commands:
+
+.. code-block:: none
+
+  set service mdns repeater interface 'eth0'
+  set service mdns repeater interface 'eth1'
+
+.. _`Multicast DNS`: https://en.wikipedia.org/wiki/Multicast_DNS