Multicast not working on linux in golang

php editor Xinyi, what I want to discuss with you today is the problem that multicast in golang does not work on Linux. Multicast is a method of network communication that transmits data between a single sender and multiple receivers. However, in golang, we may encounter situations where multicast does not work on Linux operating systems. This article will explain why this problem occurs and provide possible solutions. let's start!

Question content

This code sends and receives multicast packets.

This code works on windows 10 but not on linux: why?

Packaged content (ip 230.0.0.1, destination port 9001) was sent, but the multicast was not received by the application

Packet (ip 230.0.0.2, destination port 9002).

what is the problem?

To test my application I used a linux vm: maybe, that's the reason?

package main

import (
    "net"
    "os"
    "strconv"
    "time"

    "github.com/rs/zerolog"
    "golang.org/x/net/ipv4"
)

const device1_tx_multicastAddr = "230.0.0.1"
const device1_tx_udp_port = 9001
const device2_tx_multicastAddr = "230.0.0.2"
const device2_tx_udp_port = 9002
const packetTxDelayMs = 1000

// const ethName = "Ethernet" // Windows
const ethName = "eth0" // Linux

const modeDevice2 = false // Device 1
//const modeDevice2 = true // Device 2

var logConsole zerolog.Logger

func main() {
    logConsole = zerolog.New(os.Stderr).With().Timestamp().
        Str("module", "main").
        Logger().Output(zerolog.ConsoleWriter{Out: os.Stderr}).
        Level(zerolog.InfoLevel)

    // **********************************
    // Initialize Tx
    localInterface := getInterfaceByName(ethName)
    logConsole.Info().Str("func", "main").Msg("localInterface: " + ethName)

    tx_multicastAddr := device1_tx_multicastAddr
    rx_multicastAddr := device2_tx_multicastAddr
    tx_udp_port := device1_tx_udp_port
    rx_udp_port := device2_tx_udp_port

    if modeDevice2 {
        tx_multicastAddr = device2_tx_multicastAddr
        rx_multicastAddr = device1_tx_multicastAddr
        tx_udp_port = device2_tx_udp_port
        rx_udp_port = device1_tx_udp_port
    }

    logConsole.Info().Str("func", "main").Msg("Open Tx UDP port " + tx_multicastAddr + ":" + strconv.Itoa(tx_udp_port) + "...")
    remoteDeviceUdpAddr, err := net.ResolveUDPAddr("udp4", tx_multicastAddr+":"+strconv.Itoa(tx_udp_port))
    if err != nil {
        panic(err)
    }

    localDeviceUdpAddr, err2 := net.ResolveUDPAddr("udp4", localInterface.String()+":"+strconv.Itoa(rx_udp_port))
    if err2 != nil {
        panic(err2)
    }

    logConsole.Info().Str("func", "main").Msg("Listen UDP: " + localDeviceUdpAddr.String() + "...")
    localDevice, err2 := net.ListenUDP("udp4", localDeviceUdpAddr)
    if err2 != nil {
        panic(err2)
    }

    // **********************************
    // Initialize Rx
    udpReceiver := ipv4.NewPacketConn(localDevice)
    ief, errInterface := net.InterfaceByName(ethName)
    if errInterface != nil {
        localDevice.Close()
        panic(errInterface)
    }
    logConsole.Info().Str("func", "main").Msg("Join Multicast: " + rx_multicastAddr + "...")
    err = udpReceiver.JoinGroup(ief, &net.UDPAddr{IP: net.ParseIP(rx_multicastAddr)})

    if err != nil {
        localDevice.Close()
        panic(err)
    }

    // **********************************
    // Run Rx/Tx tasks
    go sendData(localDevice, remoteDeviceUdpAddr, packetTxDelayMs)
    receivedData(udpReceiver)
}

// *************************************************
func sendData(localDevice *net.UDPConn, remoteDeviceUdpAddr *net.UDPAddr, packetDelay uint) {
    data := []byte("1234567890")

    for {
        //logConsole.Info().Str("func", "sendData").Msg("Send...")
        _, err := localDevice.WriteTo(data, remoteDeviceUdpAddr)
        if err != nil {
            panic(err)
        }
        time.Sleep(time.Duration(packetDelay) * time.Millisecond)
    }
}
func receivedData(receiver *ipv4.PacketConn) {
    buf := make([]byte, 1500)
    for {
        n, _, _, err := receiver.ReadFrom(buf)
        if err == nil {
            logConsole.Info().Str("func", "receivedData").Msg("Receive Data: " + string(buf[0:n]))
        }
    }
}

// *************************************************
func getInterfaceByName(name string) net.IP {
    ief, err := net.InterfaceByName(name)
    if err != nil {
        panic(err)
    }
    addrs, err := ief.Addrs()
    if err != nil {
        panic(err)
    }

    var ipAddr net.IP
    for _, addr := range addrs {
        ipAddr = addr.(*net.IPNet).IP.To4()
        if ipAddr != nil {
            break
        }
    }
    if ipAddr == nil {
        panic("ipAddr is nil")
    }
    return ipAddr
}

Workaround

Modifying the application to listen on one of the following ip addresses will make it run on linux and macos:

• The ip address of the multicast group (rx_multicastaddr in the question)
• Wildcard address (0.0.0.0).

But it's not clear if it will work when it listens on a nic's ip address (e.g. 192.168.0.5). Based on my testing and the description in the question, it works on windows, but not on linux or macos. I haven't been able to find an authoritative source describing this behavior.

Below is a simplified demonstration of the accept flag.

On device 1, run it using the following command (replace the interface name with the name of your device):

go run . -listen 230.0.0.1:9001 -join 230.0.0.1:9001 -send 230.0.0.2:9002 -ifname eth0

On Device 2, run it using the following command (replace the interface name with the interface name of your device):

go run . -listen 0.0.0.0:9002 -join 230.0.0.2:9002 -send 230.0.0.1:9001 -ifname ethernet

package main

import (
    "flag"
    "log"
    "net"
    "time"

    "golang.org/x/net/ipv4"
)

var (
    listen string
    join   string
    send   string
    ifname string
)

func main() {
    flag.StringVar(&listen, "listen", "230.0.0.1:9001", "")
    flag.StringVar(&join, "join", "230.0.0.1:9001", "the multicast group address to receive data from")
    flag.StringVar(&send, "send", "230.0.0.2:9002", "the multicast group address to send data to")
    flag.StringVar(&ifname, "ifname", "eth0", "the name of the interface")
    flag.Parse()

    itf, err := net.InterfaceByName(ifname)
    if err != nil {
        panic(err)
    }

    groupAddr, err := net.ResolveUDPAddr("udp4", join)
    if err != nil {
        panic(err)
    }

    c, err := net.ListenPacket("udp4", listen)
    if err != nil {
        panic(err)
    }
    defer c.Close()

    p := ipv4.NewPacketConn(c)
    if err := p.JoinGroup(itf, &net.UDPAddr{IP: groupAddr.IP}); err != nil {
        panic(err)
    }
    log.Printf("join multicast group %s, waiting...", join)

    go sendData(c, send)
    receivedData(p)
}

func sendData(c net.PacketConn, target string) {
    data := []byte(ifname)

    addr, err := net.ResolveUDPAddr("udp4", target)
    if err != nil {
        panic(err)
    }

    for {
        _, err := c.WriteTo(data, addr)
        if err != nil {
            panic(err)
        }
        time.Sleep(time.Second)
    }
}

func receivedData(receiver *ipv4.PacketConn) {
    buf := make([]byte, 1500)
    for {
        n, _, _, err := receiver.ReadFrom(buf)
        if err == nil {
            log.Printf("Receive Data from: %s\n", buf[0:n])
        }
    }
}

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