ADDITIONAL SYSTEM INFORMATION :
Tested on two systems, identical behaviour:

#1 Microsoft Windows 10 Enterprise, 10.0.19045 Build 19045
output of uname -o -i -s -r -v in WSL:
Linux 5.15.153.1-microsoft-standard-WSL2 #1 SMP Fri Mar 29 23:14:13 UTC 2024 x86_64 GNU/Linux
output of java --version in WSL:
openjdk 21.0.4 2024-07-16
OpenJDK Runtime Environment (build 21.0.4+7-Ubuntu-1ubuntu222.04)
OpenJDK 64-Bit Server VM (build 21.0.4+7-Ubuntu-1ubuntu222.04, mixed mode, sharing)

#2 Microsoft Windows 11 Pro, 10.0.22631 Build 22631
output of uname -o -i -s -r -v in WSL:
Linux 5.15.153.1-microsoft-standard-WSL2 #1 SMP Fri Mar 29 23:14:13 UTC 2024 x86_64 GNU/Linux
output of java --version in WSL:
openjdk 21.0.4 2024-07-16
OpenJDK Runtime Environment (build 21.0.4+7-Ubuntu-1ubuntu224.04)
OpenJDK 64-Bit Server VM (build 21.0.4+7-Ubuntu-1ubuntu224.04, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
The "localhost" is resolved to the IPv4 address instead of the IPv6 address, this is problematic in case a server started on the loopback address (resolved to the IPv6 when java.net.preferIPv6Addresses is set true) and the HttpClient does try to access it. The URL "http://localhost" will be resolved by the HttpClient to IPv4 and the call will fail.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run code below, the "localhost => " line should print an IPv6 address.
Running on W10/W11 will print the expected output.
Running on WSL2 inside W10/W11 will print the - in my mind - incorrect result.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
should return IPv6 address for InetAddress.getByName("localhost") when java.net.preferIPv6Addresses set to true
ACTUAL -
IPv4 address

---------- BEGIN SOURCE ----------
import java.net.InetAddress;

public class Test {
    public static void main(String[] args) throws Exception {
        System.setProperty("java.net.preferIPv6Addresses", "true");
            System.out.println("null => " + InetAddress.getByName(null));
            System.out.println("loopback => " + InetAddress.getLoopbackAddress());
            System.out.println("localhost => " + InetAddress.getByName("localhost"));
    }
}

---------- END SOURCE ----------

FREQUENCY : always