Being able to build or LD_PRELOAD an interceptor library would be extremely useful for emulating IPv6-only environments. A hacky-ipv6-only change that results in a JDK build that supports IPv6 only, see https://mail.openjdk.java.net/pipermail/net-dev/2019-March/012248.html. It's enough to get started, but not a credible long-term solution.