UDP multicast client does not see UDP multicast traffic generated by tcpreplay
I have two programs:
- server ... it generates UDP traffic on a chosen multicast
- listener ... it prints UDP traffic on a chosen multicast (it subscribes to a multicast and prints whatever it receives).
When I run the server on one machine and listeners on some (other) machine(s), the listener sees UDP traffic and prints it correctly. So these programs should be in a good shape.
However, when I try to capture the traffic, on whatever machine, with tcpdump:
sudo tcpdump -i eth0 'dst 233.65.120.153' -w 0.pcap
and when I later try to replay it, on whatever machine, with tcpreplay:
sudo tcpreplay -i eth0 0.pcap
none of the listeners sees those captured packets:
09:38:40.975604 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
0x0000: 4500 0020 0000 4000 0111 6527 ac1b 06b0 [email protected]'....
0x0010: e941 7899 d103 fdc8 000c 579c 6162 6364 .Ax.......W.abcd
0x0020: 0000 0000 0000 0000 0000 0000 0000 ..............
09:38:41.975709 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
0x0000: 4500 0020 0000 4000 0111 6527 ac1b 06b0 [email protected]'....
0x0010: e941 7899 d103 fdc8 000c 579c 6162 6364 .Ax.......W.abcd
0x0020: 0000 0000 0000 0000 0000 0000 0000 ..............
09:38:42.975810 IP (tos 0x0, ttl 1, id 0, offset 0, flags [DF], proto UDP (17), length 32)
172.27.6.176.53507 > 233.65.120.153.64968: [udp sum ok] UDP, length 4
0x0000: 4500 0020 0000 4000 0111 6527 ac1b 06b0 [email protected]'....
0x0010: e941 7899 d103 fdc8 000c 579c 6162 6364 .Ax.......W.abcd
0x0020: 0000 0000 0000 0000 0000 0000 0000 ..............
Note that even though none of the listeners sees UDP multicast traffic, I am still able to see it, on whatever machine, with tcpdump:
sudo tcpdump -i eth0 'dst 233.65.120.153' -X
My question: What should I do (differently) if I want to tcpreplay the UDP multicast traffic I am creating so that I can see it on application level (e.g. my listener program), not only by tcpdump?
$ cat sender.c
#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 PORT 64968
#define 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
GROUP ... the IP-address of the multicast group
to which we want to multicast
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(GROUP);
addr.sin_port=htons(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);
}
}
$ cat listener.c
#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 PORT 64968
#define 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;
/* 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(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(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");
}
}
Answer
We had the same problem. With tcpdump we saw the data; however, the multicast client/listener was not picking up the data. Then we realized that the Reverse Path Filter (rp_filter) was rejecting the packets.
After disabling the rp-filter, the client/listener application started picking up the packets. Use the below command to disable rp_filter:
echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter
In the above, replace 'eth0' with the interface receiving the multicast if other than eth0