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If there are multiple <listen> elements, then the bus listens on multiple addresses. The bus will pass its address to started services or other interested parties with the last address given in <listen> first. That is, apps will try to connect to the last <listen> address first.

tcp sockets can accept IPv4 addresses, IPv6 addresses or hostnames. If a hostname resolves to multiple addresses, the server will bind to all of them. The family=ipv4 or family=ipv6 options an be used to force it to bind to a subset of addresses
Example: <listen>tcp:host=localhost,port=0,family=ipv4</listen>

R-Bus*

RBus is a 3 layered RPC communication bus.

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* Note: R-Bus implementation is in progress.