VerneMQ uses the Erlang distribution mechanism for most inter-node communication. VerneMQ identifies other machines in the cluster using Erlang identifiers (e.g. [email protected]). Erlang resolves these node identifiers to a TCP port on a given machine via the Erlang Port Mapper daemon (epmd) running on each cluster node.
By default, epmd binds to TCP port 4369 and listens on the wildcard interface. For inter-node communication, Erlang uses an unpredictable port by default; it binds to port 0, which means the first available port.
For ease of firewall configuration, VerneMQ can be configured to instruct the Erlang interpreter to use a limited range of ports. For example, to restrict the range of ports that Erlang will use for inter-Erlang node communication to 6000-7999, add the following lines to vernemq.conf on each VerneMQ node:
erlang.distribution.port_range.minimum = 6000
erlang.distribution.port_range.maximum = 7999
The settings above are only used for distributing subscription updates and maintenance messages. For distributing the 'real' MQTT messages the proper vmq listener must be configured in the vernemq.conf.
listener.vmq.clustering = 0.0.0.0:44053
It isn't necessary to configure the same port on every machine, as the nodes will probe each other for this information.
This section, "VerneMQ Inter-node Communication", is a derivative of Security and Firewalls by Riak, used under Creative Commons Attribution 3.0 Unported License.