Broadcast UDP message to multiple sockets listening on the same port

Question

I'd like to sync different processes in unix environment using UDP. All the processes that need to be sync will be listening on the same port using this: How do SO_REUSEADDR and SO_REUSEPORT differ?

I tried on macOS with the following c# code without success:

using System.Net;
using System.Net.Sockets;

async Task Listen (){
    try
    {
        IPEndPoint localpt = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 4000);

        UdpClient udpServer = new UdpClient();
        udpServer.ExclusiveAddressUse = false;
        udpServer.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
        udpServer.Client.EnableBroadcast = true;
        udpServer.Client.Bind(localpt);

        Console.WriteLine("listening");
        while (true)
        {
            var msg = await udpServer.ReceiveAsync();
            Console.WriteLine("Datagram received on UDP socket " + localpt);
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
    }
}


void Send()
{
    byte[] SYNC_DATAGRAM = "SYNC"u8.ToArray();
    var client = new UdpClient();
    client.EnableBroadcast = true;
    client.Send(SYNC_DATAGRAM, SYNC_DATAGRAM.Length, new IPEndPoint(IPAddress.Parse("127.0.0.1"), 4000));
    
}

Listen();
Listen();

Thread.Sleep(1000);
Send();
Console.Read();

Only one socket receive the message and it seems that a loadbalancer is distributting the messages. How can all sockets receive the message?

Try sending on Machine IP address and listen on loopback address 127.0.0.1. — jdweng, Apr 07 '23 at 15:46
That doesn't sound right. You should be able to send to the IP address. There may be something wrong with your Network — jdweng, Apr 07 '23 at 18:20
The SO_ approach lets you bind multiple sockets to the same port, but you're sending a single packet to a single address/port (unicast), so only one of them will get it. You need to use multicast instead. — Stephen Cleary, Apr 08 '23 at 00:15
Looks like you also need `SO_REUSEPORT` so add `udpServer.Client.SetSocketOption(SocketOptionLevel.Socket, (SocketOptionName)15, true);` — Charlieface, Apr 09 '23 at 20:03
Also you are not awaiting the result of `Listen`, you should do `Task.Run(Listen);` or similar — Charlieface, Apr 09 '23 at 20:04

score 1 · Accepted Answer · answered Apr 16 '23 at 15:33

We were in the correct direcection. Finally I made it work just sending to 255.255.255.255 and listening on ANY endpoint 0.0.0.0.

Here is a working code in c#:

using System.Net;
using System.Net.Sockets;

async Task Listen (int id=0){
    try
    {
        IPEndPoint localpt = new IPEndPoint(IPAddress.Parse("0.0.0.0"), 4000);

        UdpClient udpServer = new UdpClient();
        udpServer.ExclusiveAddressUse = false;
        udpServer.Client.Bind(localpt);
        Console.WriteLine("listening");
        while (true)
        {
            var msg = await udpServer.ReceiveAsync();
            Console.WriteLine("Datagram received on UDP socket " + id);
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e.Message);
    }
}


void Send()
{
    byte[] SYNC_DATAGRAM = "SYNC"u8.ToArray();
    var client = new UdpClient();
    client.Client.EnableBroadcast = true;
    client.Send(SYNC_DATAGRAM, SYNC_DATAGRAM.Length, new IPEndPoint(IPAddress.Parse("255.255.255.255"), 4000));
    
}

Task.Run(() => Listen(1));
Task.Run(() => Listen(2));
Task.Run(() => Listen(3));
Task.Run(() => Listen(4));
Task.Run(() => Listen(5));

Thread.Sleep(1000);
Send();
Console.Read();

`IPAddress.Any` and `IPAddress.Broadcast` are simpler to use, they don't require `.Parse`. You can also create the `UdpClient` using the constructor and it will bind automatically `new UdpClient(new IPEndPoint(IPAddress.Any, 4000))` — Charlieface, Apr 16 '23 at 16:11
That's right about `.Parse`, however constructor is mandatory because `ExclusiveAddressUse` needs to be set before bind — Daniel Argüelles, Apr 17 '23 at 15:54

Broadcast UDP message to multiple sockets listening on the same port

1 Answers1