Create UDP client socket without socket listening

Question

I'm using the standard way, shown in many examples, to create a UDP socket in C++, keep it alive and keep sending stuff over it.

I only call sendto. I never call recvfrom. In this code fragment I only use sendto once and then sleep for 5 seconds:

C code:

static int do_the_rest( int sock )
{
    struct sockaddr_in server_addr;

    bzero(&server_addr,sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr=inet_addr("192.168.32.32");
    server_addr.sin_port=htons(32000);

    char sendline[100];
    bzero(sendline,sizeof(sendline));

    const struct sockaddr * addr = (const struct sockaddr*)&server_addr;

    sendto(sock,sendline,sizeof(sendline),0,addr,sizeof(server_addr));

    sleep( 5 );

    return 0;
}

int main(int argc, char**argv)
{
    int sock;

    sock=socket(AF_INET,SOCK_DGRAM,0);
    if( sock < 0 ) {
        perror( "socket" );
        return 1;
    }

    int ret = do_the_rest( sock );
    close( sock );

    return ret;
}

Now, if I run "netstat -na", I can identify that the system seems to listen on the local port of the socket (I remove the lines in my program that print the local port, for clarity):

netstat -na:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
...
udp        0    304 0.0.0.0:53735               0.0.0.0:*

When I try something similar in Java, I also seem to get some listening, although it looks a bit different (perhaps IPv6?):

Java code:

import java.io.*;
import java.net.*;

class Udp {
    public static void main(String[] args) throws Throwable {
        DatagramSocket sock = new DatagramSocket(null);

        try {
            InetAddress ipAddress = InetAddress.getByName("192.168.32.32");

            byte[] sendData = new byte[50000];

            DatagramPacket sendPacket = new DatagramPacket(
                sendData, sendData.length, ipAddress, 32000);

            sock.send(sendPacket);

            Thread.sleep(5000);

        } finally {

            sock.close();
        }
    }
}

netstat -na:

Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
...
udp        0      0 :::37053                    :::*

I understand this is done in order to support a possible recvfrom (receive in Java) that may follow. However, is there a way to tell the socket not to listen to incoming packets at all?

Thanks

Answer 1

Now, if I run "netstat -na", I can identify that the system seems to listen on the local port of the socket

UDP sockets have a kernel buffer for incoming messages. This buffer is maintained by the kernel regardless whether you call recv/recvfrom/recvmsg from user-space code.