Thread: tcpdump & wireshark get multicast packets, receiver doesn't

This is both a java/c problem. I'm taking over a MC project that sends data from computer A to Computer B, but not the other way around (it physically can't). The MC comes through to Computer B, I can see this on WireShark & tcpdump, but I can't figure out how to troubleshoot exactly why my program won't read any MC packet. I took a project that I made a year ago and tried it, I have also used other people's projects online, but the only project that ever picked up a MC project at all was a java project, it grabbed 4 packets during 1 run, and 1 packet during another. When I use a sender/receiver on a regular network everything works fine. Does anyone have any ideas about what the problem could be?

Note: Yes this code is borrowed from another website, but I assure you I can do it on my own. I don't want to accidentally post proprietary info and end up getting fired.

It never goes past this line of code. I've quadruple checked the ports & ip addresses (so have a few other people). If we hook a separate computer (that makes this bidirectional) and use a different sender, everything works just fine.

Code:

    
recvfrom(fd,msgbuf,MSGBUFSIZE,0, (struct sockaddr *) &addr,&addrlen))

Here's a receiver that I'm using:

Code:

#include <sys/types.h> 
#include <sys/socket.h> 
#include <netinet/in.h> 
#include <arpa/inet.h> 
#include <time.h> #include <string.h> 
#include <stdio.h>  

//////changed from actual 
#define HELLO_PORT 12345 
#define HELLO_GROUP "225.0.0.37" 
#define MSGBUFSIZE 256  

int main(int argc, char *argv[]) 
{      
   struct sockaddr_in addr;      
   int fd, nbytes,addrlen;      
   struct ip_mreq mreq;      
   char msgbuf[MSGBUFSIZE];       
   u_int yes=1;
     
    if ((fd=socket(AF_INET,SOCK_DGRAM,0)) < 0) 
    {       
       perror("socket");       
       exit(1);      
    }
    
    if (setsockopt(fd,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof(yes)) < 0) 
    {        
       perror("Reusing ADDR failed");        
       exit(1);        
    } 
     
    memset(&addr,0,sizeof(addr));      
    addr.sin_family=AF_INET;      
    addr.sin_addr.s_addr=htonl(INADDR_ANY);      
    addr.sin_port=htons(HELLO_PORT);            
    if (bind(fd,(struct sockaddr *) &addr,sizeof(addr)) < 0) 
    {       
       perror("bind");       
       exit(1);      
    }            
    mreq.imr_multiaddr.s_addr=inet_addr(HELLO_GROUP);
     
    mreq.imr_interface.s_addr=htonl(INADDR_ANY//changed to listening interface ip address);      
    if (setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)) < 0) 
    {       
       perror("setsockopt");       
       exit(1);      
    }       
    while (1) 
    {       
       addrlen=sizeof(addr);       
       if((nbytes=recvfrom(fd,msgbuf,MSGBUFSIZE,0, (struct sockaddr *) &addr,&addrlen)) < 0) 
       {            
          perror("recvfrom");            
         exit(1);       
       }       
       puts(msgbuf);      
    } 
   return 0;
}

Likely a dumb question; but, I will ask it any way.
Are you positive the sender is using UDP instead of TCP protocol?

Tim S.

100% Sure =D
tcpdump -i [interface] -A dst [multicast group] and port [port]
tcpdump -i eth1 -A dst 235.35.3.5 and port 23535 (just an example of what I'm doing) and I'm getting all messages that are being sent from Computer A.

I have glanced at a few Google pages; and I think I might try recvmsg() instead of recvfrom().
An example was used in one way pipes; so, it might work for one way IP connects.

Tim S.

recvmsg() -- receive message from socket and store in structure

http://www.developer.nokia.com/Commu...ecvmsg_methods

Same thing! Thanks anyways. I'll keep messing around with it though.
That was a good idea! I'm thinking it might be the interface I'm trying to connect to.
It gives an error if I put in one that doesn't exist, but I'm wondering if the interface is set up in a way that causes problems.

Code:

mreq.imr_interface.s_addr=inet_addr("192.168.1.1");

Did you look at the last example in this link?

Open C Sockets: recv, recvfrom, recvmsg methods - Nokia Developer Wiki

It does not use structure ip_mreq instead it uses msghdr.

Tim S.