shortishly / mdns

Multicast DNS in Erlang/OTP

Multicast DNS

mDNS is an implementation of the mDNS discovery protocol written in Erlang/OTP and enables two or more Erlang nodes to self discover and form a mesh network.

The implementation uses the method described in DNS-Based Service Discovery to register and discover services.

A service of type _erlang._tcp will be advertised by default. The registered services can be viewed using avahi-browse as follows:

avahi-browse _erlang._tcp

Or on OSX:

dns-sd -B _erlang._tcp

mDNS will automatically advertise its presence and discover other _erlang._tcp nodes. Whether a node will advertise or discover other nodes is controlled by the MDNS_CAN_ADVERTISE and MDNS_CAN_DISCOVER boolean environment variables.

mDNS will also automatically form an Erlang/OTP mesh network of nodes if the boolean variable MDNS_CAN_MESH is true, providing they all share the same MDNS_ENVIRONMENT (default: "dev") and have the same distribution cookie.

For example, starting mDNS on two separate machines (in this example dev001.local and dev002.local) reachable on the same local network:

On dev001.local:

MDNS_CAN_MESH=true make shell

and on dev002.local:

MDNS_CAN_MESH=true make shell

After a short period both machines will have automatically formed a mesh network:

([e-mail])2> nodes().
['[e-mail]']

([e-mail])2> nodes().
['[e-mail]']

The following variables are used to control the operation of mDNS:

application name	environment variable	default
can_advertise	MDNS_CAN_ADVERTISE	true
can_discover	MDNS_CAN_DISCOVER	true
can_mesh	MDNS_CAN_MESH	false
environment	MDNS_ENVIRONMENT	dev
multicast_address	MDNS_MULTICAST_ADDRESS	224.0.0.251
udp_port	MDNS_UDP_PORT	5353
domain	MDNS_DOMAIN	.local
service	MDNS_SERVICE	_erlang._tcp
ttl	MDNS_TTL	120

mDNS uses gproc's pub/sub pattern. Consumers can subscribe to mDNS advertisements via mdns:subscribe(advertisement). The map accompanying the advertisement has the following structure:

Key	Description
host	The hostname of the advertised node
node	The node name of the advertised node
port	The advertised distribution protocol port
env	The environment of this node

Only nodes that share the same environment can be automatically meshed together.