Accepting on an IPv6Only listener creates a DualMode socket on Mac

[antonfirsov]

using Socket listener = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
listener.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
listener.Listen(10);

using Socket client = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
client.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));

_ = client.ConnectAsync(listener.LocalEndPoint);

using Socket server = listener.Accept();

Console.WriteLine($"listener: {listener.DualMode} | client: {client.DualMode} | server:{server.DualMode}");

The code above will print listener: False | client: False | server:False on Linux and Windows, but listener: False | client: False | server:True on Mac.
Since we don't do anything special in our code, I assume the OS is creating an IPV6_V6ONLY == 0 accept socket despite the listener being IPV6_V6ONLY == 1.

We may want to alter the socket option in our PAL for consistency.

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Issue Details

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using Socket listener = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
listener.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
listener.Listen(10);

using Socket client = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
client.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));

_ = client.ConnectAsync(listener.LocalEndPoint);

using Socket server = listener.Accept();

Console.WriteLine($"listener: {listener.DualMode} | client: {client.DualMode} | server:{server.DualMode}");

The code above will print listener: False | client: False | server:False on Linux and Windows, but listener: False | client: False | server:True on Mac.
Since we don't do anything special in our code, I assume the OS is creating an IPV6_V6ONLY == 0 accept socket despite the listener being IPV6_V6ONLY == 1.

We may want to alter the socket option in our PAL for consistency.

Author:	antonfirsov
Assignees:	-
Labels:	area-System.Net.Sockets, untriaged
Milestone:	-

[antonfirsov]

What's most weird that the flag seems to be readable, but an attempt to write it leads to EINVAL.

Is it possible that the socket actually does not act as dual-mode, and getsockopt simply returns a wrong value for IPV6_V6ONLY ?

[wfurt]

AFAIK there is guarantee that socket setting would be inherited after Accept(). It seems like you cannot set the property once the socket is bound. You can see that this will throw:

using Socket client = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
client.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));
client.DualMode = false;

will throw but this will work (same on macOS & Linux)

using Socket client = new Socket(AddressFamily.InterNetworkV6, SocketType.Stream, ProtocolType.Tcp);
client.DualMode = false;
client.Bind(new IPEndPoint(IPAddress.IPv6Loopback, 0));

Since after Accept() the socket is in connected state, we may not be able to change the duality. It should not really matter as both EndPoints are already set, right?

[antonfirsov]

In that case there's no way we can workaround this, I guess.

There's only one situation where this could lead to an issue:
when ReceiveMessageFrom is being used with the (TCP) accept socket with IPv6, it will fail CheckDualModeReceiveSupport. I had to workaround my tests because of this.

Since ReceiveMessageFrom is intended to be used with UDP, this shouldn't be an issue in real life, considering there is no good fix, we may probably close the issue as wontfix.

[karelz]

Triage: Looks like we can't do anything about it, it is a weird corner case. Won't Fix.