suggestion for supporting docker udp

[realzero0]

This might be related with docker udp issues in Consul.
hashicorp/docker-consul#60

UDP Protocol seems using nf_conntrack table.
If a record in nf_conntrack is same source port(8301) and destination port(8301), host server assume that the connection is made.

container -> host -> other server

if a client sent a packet to other server like above, a conntrack record will be created and host server assume that udp stream is created.
And if other server sends a packet to the host, the packet will be sent to container because of host server's nf_conntrack even if there is no binding port.
In this case, we are not able to use bridge network in docker because sometimes if container ip changes, it will fail.

I wanna suggest a solution in this case.

In net_transport.go, memberlist reuses udp listener to send the payload.

memberlist/net_transport.go

Line 204 in 3f82dc1

_, err = t.udpListeners[0].WriteTo(b, udpAddr)

I think it can be better there is an option that can separate udp sender and listener.

memberlist/net_transport.go

Lines 49 to 60 in 3f82dc1

	type NetTransport struct {
	config *NetTransportConfig
	packetCh chan *Packet
	streamCh chan net.Conn
	logger *log.Logger
	wg sync.WaitGroup
	tcpListeners []*net.TCPListener
	udpListeners []*net.UDPConn
	shutdown int32
	metricLabels []metrics.Label
	}

In NetTransport struct, we can have udpSenders []*net.UDPConn.
In this case, sender might be created using random port like udpSender, err := net.ListenUDP("udp", nil)

If this kind of implementaion is done, we can deploy consul client in docker environment. And all connection can be done because the payload always have source ip and port.