Hello,
I have two programs:
- sender ... which generates some UDP multicast traffic
- listener ... which listens to a given UDP multicast group
(in this case 233.65.120.153:64968)
When I compile these two programs, all seems to be working fine.
All "listeners" on the local network are getting data from the "sender".
So I believe that routers/switches are configured properly and the
transmitted packets have correct Ethernet/IP/UDP headers.
Now I am trying to capture the traffic with "tcpdump":
sudo tcpdump -i eth0 -X 'dst 233.65.120.153' -w 0.pcap
(attached).
However, when I try to replay this pcap-file:
$ sudo tcpreplay -i eth0 0.pcap
sending out eth0
processing file: 0.pcap
Actual: 4 packets (240 bytes) sent in 3.00 seconds
Rated: 80.0 bps, 0.00 Mbps, 1.33 pps
Statistics for network device: eth0
Attempted packets: 4
Successful packets: 4
Failed packets: 0
Retried packets (ENOBUFS): 0
Retried packets (EAGAIN):
None of the listeners on a given multicast sees that traffic.
I am sure there is a pretty good reason for that but I cannot figure
this out. :-/
I have check that:
- last 3 bytes at of the destination MAC address
correspond to last three bytes of the destination IP address.
- destination IP is OK
- destination port is OK
If anybody has some good advice what I did wrong, any help would be most
welcome. Thanks in advance.
/* Compatible listeners:
* - ~/work/net/udp-multicast/2/listener ... works on the local machine as well as on the remote one
* - sudo tcpdump -i eth0 -X dst 225.0.0.37 ... works on the local machine
* works on a remote machine if some of its processes subscribed to a given multicast
* (tcpdump does not subscribe itself)
* To subscribe the remote machine to our multicast, we can either run the aforementioned listener,
* or execute this command:
* - iperf -s -u -B 225.0.0.37
*/
#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>
#include <stdlib.h>
#define HELLO_PORT 64968
#define HELLO_GROUP "233.65.120.153"
main(int argc, char *argv[])
{
struct sockaddr_in addr;
int fd, cnt;
struct ip_mreq mreq;
char *message="abcd";
/* Create what looks like an ordinary UDP socket:
AF_INET ... IPv4
SOCK_DGRAM ... UDP
0 ... required constant
*/
if ((fd=socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror("socket");
exit(1);
}
/* Set up destination address:
AF_INET ... IPv4
HELLO_GROUP ... the IP-address of the multicast group
to which we want to multicast
HELLO_PORT ... the UDP port that on which we want to multicast
*/
memset(&addr, 0, sizeof(addr));
addr.sin_family=AF_INET;
addr.sin_addr.s_addr=inet_addr(HELLO_GROUP);
addr.sin_port=htons(HELLO_PORT);
/* now just sendto() our destination! */
while (1) {
if (sendto(fd, message, strlen(message), 0, (struct sockaddr *) &addr, sizeof(addr)) < 0) {
perror("sendto");
exit(1);
}
sleep(1);
}
}
/*
* Compatible senders:
* - ~/work/net/udp-multicast/2/sender
* - ~/doc/ocaml/net/udp-multicast/1/sender
*/
/* sudo tcpdump -X -vv -i eth0 'dst 225.0.0.37' */
#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>
#include <stdlib.h>
#define HELLO_PORT 64968
#define HELLO_GROUP "233.65.120.153"
#define MSGBUFSIZE 1000000
char msgbuf[MSGBUFSIZE];
main(int argc, char *argv[])
{
struct sockaddr_in addr;
int fd, nbytes,addrlen;
struct ip_mreq mreq;
u_int yes=1; /*** MODIFICATION TO ORIGINAL */
/* Create what looks like an ordinary UDP socket:
AF_INET ... IPv4
SOCK_DGRAM ... UDP
0 ... required constant
*/
if ((fd=socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
perror("socket");
exit(1);
}
/* Allow multiple sockets to use the same PORT number:
SOL_SOCKET ... manipulate properties of the socket API itself
SO_REUSEADDR ... Allow reuse of local addresses for bind
*/
if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) < 0) {
perror("Reusing ADDR failed");
exit(1);
}
/* set up destination address */
memset(&addr,0,sizeof(addr));
addr.sin_family=AF_INET;
addr.sin_addr.s_addr=htonl(INADDR_ANY); /* N.B.: differs from sender */
addr.sin_port=htons(HELLO_PORT);
/* bind to receive address */
if (bind(fd,(struct sockaddr *) &addr,sizeof(addr)) < 0) {
perror("bind");
exit(1);
}
/* use setsockopt() to request that the kernel join a multicast group */
mreq.imr_multiaddr.s_addr=inet_addr(HELLO_GROUP);
mreq.imr_interface.s_addr=htonl(INADDR_ANY);
if (setsockopt(fd,IPPROTO_IP,IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq)) < 0) {
perror("setsockopt");
exit(1);
}
/* now just enter a read-print loop */
while (1) {
addrlen=sizeof(addr);
memset(msgbuf, 0, MSGBUFSIZE);
if ((nbytes=recvfrom(fd, msgbuf, MSGBUFSIZE,0,
(struct sockaddr *) &addr, &addrlen)) < 0) {
perror("recvfrom");
exit(1);
}
printf("Incoming message size = %d\n", nbytes);
int i;
for (i=0; i < nbytes; i++)
printf("%02x ", ((unsigned char) msgbuf[i]));
printf("\n");
}
}
Attachment:
0.pcap
Description: application/vnd.tcpdump.pcap