Motivation:
According to the Oracle documentation:
> java.net.preferIPv4Stack (default: false)
>
> If IPv6 is available on the operating system, the underlying native
> socket will be an IPv6 socket. This allows Java applications to connect
> to, and accept connections from, both IPv4 and IPv6 hosts.
>
> If an application has a preference to only use IPv4 sockets, then this
> property can be set to true. The implication is that the application
> will not be able to communicate with IPv6 hosts.
which means, if DnsNameResolver returns an IPv6 address, a user (or
Netty) will not be able to connect to it.
Modifications:
- Move the code that retrieves java.net.prefer* properties from
DnsNameResolver to NetUtil
- Add NetUtil.isIpV6AddressesPreferred()
- Revise the API documentation of NetUtil.isIpV*Preferred()
- Set the default resolveAddressTypes to IPv4 only when
NetUtil.isIpv4StackPreferred() returns true
Result:
- Fixes#5657
9fef4ba1bf