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.
[ Full content available at:
https://github.com/apache/incubator-dubbo/issues/2423 ]
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