Thread: Controlling IPv4/IPv6 Socket Behavior in Java NIO (Similar to IPV6_ONLY Flag in BSD)

Hello Everyone,

I am working on a cross-platform project using Java NIO channels, and I’m trying to replicate the behavior of the IPV6_ONLY flag (similar to what is available in BSD) to control how IPv4 and IPv6 sockets behave when binding to the same port.

The goal is to have two options:

Separate sockets for IPv4 and IPv6 – I want to control whether I can open separate sockets for IPv4 and IPv6 (e.g., one DatagramChannel for IPv4 and another for IPv6, each bound to the same port). A dual-mode IPv6 socket – I want to use a single IPv6 socket that can accept both IPv4 and IPv6 packets, mimicking the behavior of the IPV6_ONLY flag in BSD, which restricts an IPv6 socket to only accept IPv6 packets. Here’s a simplified version of the code I’m using:

// Opening IPv4 DatagramChannel
DatagramChannel ipv4Channel = DatagramChannel.open(StandardProtocolFamily.INET);
ipv4Channel.bind(new InetSocketAddress(port));

// Opening IPv6 DatagramChannel
DatagramChannel ipv6Channel = DatagramChannel.open(StandardProtocolFamily.INET6) ;
ipv6Channel.bind(new InetSocketAddress(port));
The Limitation I Encountered:
The issue I’m facing is that while DatagramChannel allows opening separate channels for IPv4 and IPv6, there’s a limitation where you cannot open a second socket with a different IP family (IPv4 vs IPv6) on the same port. This occurs if one socket is already opened for a specific IP family (either IPv4 or IPv6). I receive the error "port already in use" when attempting to bind the second socket with a different IP family to the same port.

What I Need:
I want the ability to:

Open separate sockets for IPv4 and IPv6 on the same port (for both DatagramChannel and possibly ServerSocketChannel or SocketChannel).
Configure an IPv6 socket to accept both IPv4 and IPv6 packets (like BSD’s IPV6_ONLY flag), and toggle its behavior between accepting only IPv6 packets or dual-mode (IPv4/IPv6).
Is there a way in Java NIO to:

Open separate IPv4 and IPv6 sockets on the same port (without encountering the "port already in use" error)?
Configure an IPv6 socket to accept both IPv4 and IPv6 packets, or implement similar behavior to BSD's IPV6_ONLY flag?
Since I’m writing cross-platform code, I am looking for a solution that works across different operating systems while providing the same level of control over socket behavior.

Any insights or guidance would be greatly appreciated.

Thank you!

Some other theards from past on same topics by me:

Configuring Java NIO Channels for IPv4 and IPv6 Separately Java NIO DatagramChannel: Unable to Open Multiple Sockets on Same Port with IPv4 and IPv6

--- Update ---

Stack overflow question links: https://stackoverflow.com/questions/...pv6-separately
https://stackoverflow.com/questions/...port-with-ipv4