Linux blocking udp socket won't receive

Question

Hello,

I am openning a udp blocking socket on an ubuntu machine, it goes successful (no errors returned).

When sending data to that machine and port from another machine the receive doesnt breaches and in a wireshark sniffer I see and Icmp error "port unreachable".

I though it might be an iptables problem and opened the port for accept.

Any other suggestions how to debug this?

Thanks.

Timmy

Answer 1

You should show a minimal test case.

Do you see your process in

sudo netstat -4lp

? What local address is its socket bound to (as reported by netstat)?

Answer 2

I usually use netcat to figure out if the problem comes from the network/firewall or from my own code

try running a test server with netcat : eg.

nc -l -u -p 9999 

will open and listen an udp socket, port 9999.

Now you can try to send a packet from the same or from another computer using

nc -u <ipaddress> 9999

Then type something and see if it reaches the first computer.

There are a lot of other cool stuffs in netcat, have a look on the manual.

Answer 3

Are you using bind() to correctly bind the socket to the local port?

Did you remember to pass the local port number through htons() to convert it to network byte order?

What value did bind() return?

Answer 4

Try this simple server and see if it works for you:

#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define BUFSZ 4096
#define PORTNUM 1099
char buffer[BUFSZ];

int main( int argc, char* argv[] )
{
    int fd;
    struct sockaddr_in servaddr, cliaddr;
    socklen_t clilen = sizeof( cliaddr );
    ssize_t nread;

    if (( fd = socket( AF_INET, SOCK_DGRAM, 0 )) == -1 )
        err( 1, "socket" );

    bzero( &cliaddr, sizeof( cliaddr ));
    bzero( &servaddr, sizeof( servaddr ));

    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = htonl( INADDR_ANY );
    servaddr.sin_port = htons( PORTNUM );

    if ( bind( fd, ( struct sockaddr* )&servaddr, sizeof( servaddr )) == -1 )
        err( 1, "bind" );

    printf( "bound to %s:%d\n", inet_ntoa( servaddr.sin_addr ),
        ntohs( servaddr.sin_port ));

    while (( nread = recvfrom( fd, buffer, BUFSZ, 0,
        ( struct sockaddr* )&cliaddr, &clilen )) != -1 )
    {
        printf( "received %lu bytes from %s:%d\n", nread,
            inet_ntoa( cliaddr.sin_addr ),
            ntohs( cliaddr.sin_port ));
    }

    return 1;
}

See if all the required steps are there in your code.