Receiving multicast data on specific interface

Question

tcmpdump can view all the multicast traffic to specific group and port on eth2, but my Python program cannot. The Python program, running on Ubuntu 12.04: ``` sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM) sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) # Multicast port is 52122 sock.bind(('', 52122)) # Interface eth2 IP is 1.2.3.4, multicast group is 6.7.8.9 mreq = socket.inet_aton('6.7.8.9')+socket.inet_aton('1.2.3.4') sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq) while True: print '
waiting to receive message' data, address = sock.recvfrom(1024) print data ``` When I use another program to send a multicast packet to eth2, it works and prints the packet. But it fails to see all the current multicast traffic. If I run tcpdump on eth2 on the same port and group as the above program: ``` sudo tcpdump -i eth2 host 6.7.8.9 and port 52122 ``` it sees both the packets I send from another program AND all the current multicast traffic. It's output lo…

Accepted Answer

Finally! Found this question on ServerFault that addresses the same thing. Basically the kernel was not forwarding on / was filtering out the packets because it thought the sourced address was spoofed.

Changed the settings in /etc/sysctl.conf to match:

```
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.all.rp_filter = 0
net.ipv4.ip_forward = 1
```

Rebooted and everything works.

Receiving multicast data on specific interface

Solution

Problem