When sending a datagram using UDP, is it possible to determine the port it's sent from?

# ktor

[Sean Proctor]

When sending a datagram using UDP, is it possible to determine the port it's sent from?

[Sean Proctor]

I'd like for this function to return the outgoing port: https://github.com/sproctor/ONVIFCameraAndroid/blob/master/onvifcamera/src/commonMain/kotlin/com/seanproctor/onvifcamera/OnvifDevice.kt#L177

The SOAP discovery spec says that the service should respond on the port that the packet originated from.

[Aleksei Tirman [JB]]

You can use the address property of an incoming Datagram :

fun main() = runBlocking {
    val selectorManager = SelectorManager(<http://Dispatchers.IO|Dispatchers.IO>)
    val serverSocket = aSocket(selectorManager).udp().bind(InetSocketAddress("127.0.0.1", 9002))

    for (datagram in serverSocket.incoming) {
        serverSocket.outgoing.send(
            Datagram(
                BytePacketBuilder().apply { append("Hello client") }.build(),
                datagram.address
            )
        )
    }
}

[Sean Proctor]

I'm trying to write the client. There's a server listening on port 3702. I need to broadcast 3702 and the server will respond on the source port of the broadcast.

[Sean Proctor]

I've re-written the probe as a typical socket connection, but it doesn't receive any response (despite that I can see there was a response monitoring network traffic) https://github.com/sproctor/ONVIFCameraAndroid/blob/master/onvifcamera/src/commonMain/kotlin/com/seanproctor/onvifcamera/OnvifDevice.kt#L146

[Sean Proctor]

The probe gets sent to IP 239.255.255.250 (per the SOAP discovery spec) and the response is from 192.168.0.209 (the IP address of the device I'm probing) or multiple IP addresses if multiple devices respond.

[Sean Proctor]

Thanks, I figured it out. Your comment eventually led me in the correct direction. I needed to use the localAddress from the socket sending the probe. you can see the final result on github.