[MacOS] UDP Multicast: Can't assign requested address on interface 0 (as suggested in the documentation)

[MihaelBercic]

So far tested on:

• windows 11: works
• macos 14.4: doesn't work

The following code

use std::net::{Ipv6Addr, SocketAddrV6, UdpSocket};
use std::str::FromStr;

fn main() {
    let local = SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, 61666, 0, 0);
    let multicast = Ipv6Addr::from_str("ff02::fb").unwrap();
    let udp = UdpSocket::bind(&local).expect("Unable to bind!");
    udp.join_multicast_v6(&multicast, 0).expect("Unable to join to multicast...");

    match udp.send_to(b"Hello", "[::1]:61666") { // Trying to send a packet to multicast address...
        Ok(bytes_sent) => println!("Sent {} bytes", bytes_sent),
        Err(err) => eprintln!("Error sending data: {}", err),
    }
}

produces the following error:

Unable to join to multicast...: Os { code: 49, kind: AddrNotAvailable, message: "Can't assign requested address" }

It is expected for the socket to join the group (in documentation it also says setting interface to 0 allows for the OS to decide on the interface).

However instead of that, on macOS the above error happens. On windows however, it does not and works as expected.

Meta

rustc --version --verbose:

rustc 1.77.2 (25ef9e3d8 2024-04-09)
binary: rustc
commit-hash: 25ef9e3d85d934b27d9dada2f9dd52b1dc63bb04
commit-date: 2024-04-09
host: aarch64-apple-darwin
release: 1.77.2
LLVM version: 17.0.6

[bjorn3]

https://stackoverflow.com/questions/65534342/receiving-udp-multicast-on-macos-big-sur suggests that the firewall blocks receiving multicast packets unless "Block all incoming connections" and "stealth mode" are disabled. From a quick search more people seem to be having problems with multicast support on macOS in general. Not just with rust.

[MihaelBercic]

https://stackoverflow.com/questions/65534342/receiving-udp-multicast-on-macos-big-sur suggests that the firewall blocks receiving multicast packets unless "Block all incoming connections" and "stealth mode" are disabled. From a quick search more people seem to be having problems with multicast support on macOS in general. Not just with rust.

Hello, thank you for the reply.
I have both disabled but still unable to receive any multicast packets.

In JVM (Kotlin and Java), I was able to receive multicast packets just fine.

[bjorn3]

Is the java executable signed? If so "Automatically allow downloaded signed software to receive incoming connections" would apply to it, unlike rustc compiled executables, which you probably need to allowlist manually.

[MihaelBercic]

Is the java executable signed? If so "Automatically allow downloaded signed software to receive incoming connections" would apply to it, unlike rustc compiled executables, which you probably need to allowlist manually.

But if that'd be the case, after disabling the firewall I would be able to receive the packets right? However even if disabling the firewall, nothing changes.

[bjorn3]

Yeah, it should. Can you check if a C program is able to receive UDP multicast packets as sanity check?

[MihaelBercic]

Yeah, it should. Can you check if a C program is able to receive UDP multicast packets as sanity check?

I will give it my best. I'm not that familiar with C but once i have it written I'll reply to this issue.

[MihaelBercic]

Yeah, it should. Can you check if a C program is able to receive UDP multicast packets as sanity check?

https://gist.github.com/MihaelBercic/557a0386b45e5e912ff493725dfff1f2

This code doesn't work either, can you check if it's the code issue or is it actually not working.

[bjorn3]

I don't see anything obviously bad with it, but I'm not all that familiar with the socket api either. In any case for me on linux running that program seems to work as expected when I use iperf to send it multicast packets.

[MihaelBercic]

I don't see anything obviously bad with it, but I'm not all that familiar with the socket api either. In any case for me on linux running that program seems to work as expected when I use iperf to send it multicast packets.

Does the code also catch packets that are not sent by yourself? Such as tvs, spotify mdns queries and such?

[bjorn3]

I looked in wireshark for any multicast packets that are sent my direction by other devices at home, but the only ones I could find were not udp multicast packets and thus wouldn't get caught by this test program.