Multicast demo fails with message "Can't assign requested address"

[ralf0131]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Environment

• Dubbo version: master branch
• Operating System version: macOS 10.13.6
• Java version: 1.8

Step to reproduce this issue

1. start the provider demo

Expected Result

Provider start success.

Actual Result

Provider start failed.

If there is an exception, please attach the exception trace:

Caused by: java.net.SocketException: Can't assign requested address
	at java.net.PlainDatagramSocketImpl.join(Native Method)
	at java.net.AbstractPlainDatagramSocketImpl.join(AbstractPlainDatagramSocketImpl.java:178)
	at java.net.MulticastSocket.joinGroup(MulticastSocket.java:323)
	at org.apache.dubbo.registry.multicast.MulticastRegistry.<init>(MulticastRegistry.java:91)
	... 24 more

If I add vm args -Djava.net.preferIPv4Stack=true, everything works fine.

[ralf0131]

This is because there are multiple network interfaces in my macOS, both for ipv4 and ipv6:

en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
        ether e0:ac:cb:7f:75:20
        inet6 fe80::cfb:9054:967d:13be%en0 prefixlen 64 secured scopeid 0x5
        inet 192.168.31.235 netmask 0xffffff00 broadcast 192.168.31.255
        nd6 options=201<PERFORMNUD,DAD>
        media: autoselect
        status: active
awdl0: flags=8943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1484
        ether 62:32:71:46:6b:2e
        inet6 fe80::6032:71ff:fe46:6b2e%awdl0 prefixlen 64 scopeid 0x7
        nd6 options=201<PERFORMNUD,DAD>
        media: autoselect
        status: active

awdl0 is ipv6 only and en0 is both ipv4 and ipv6 available. However the JVM selects awdl0 as the default Network Interface in java.net.DefaultInterface#getDefault() method. So if we use a ipv4 address for multicasting, the error will occur.

If -Djava.net.preferIPv4Stack=true is specified, JVM will avoid choosing awdl0 as default network interface. So everything will be all right.

Enhancement

Basically we don't want to let user to specify that vm args manually, so I'd like to propose to automatically select network interface by the input multicast address.

If the multicast address is ipv4 address, automatically use ipv4 network address (by calling java.net.MulticastSocket#setInterface()), and vise versa.

[zonghaishang]

@ralf0131 When does this problem plan to fix?

[ralf0131]

No ETA yet :(
Do you have interest to take this?