The TUNNEL protocol uses an external router to send messages. The external router is known as a GossipRouter. Each node has to register with the router. All messages are sent to the router and forwarded on to their destinations. The TUNNEL approach can be used to setup communication with nodes behind firewalls. A node can establish a TCP connection to the GossipRouter through the firewall (you can use port 80). The same connection is used by the router to send messages to nodes behind the firewall as most firewalls do not permit outside hosts to initiate a TCP connection to a host inside the firewall. The TUNNEL configuration is defined in the TUNNEL element in the JGroups Config element. Here is an example..

<TUNNEL router_port="12001"
    router_host="192.168.5.1"
    down_thread="false" up_thread="false/>
                

The available attributes in the TUNNEL element are listed below.