Why is my server not receiving multicast traffic?
Out of the box linux installations oftentimes enable so-called "reverse path" filtering. This essentially means that if the source ip of the udp packet is not routable through the interface in which the packet arrived, then it will be dropped.
To turn off (disable) this filtering, and allow multicast traffic through, set the rp_filter to 0:
echo "0" > /proc/sys/net/ipv4/conf/all/rp_filter
Lastly, edit /etc/sysctl.conf and change: ipv4.conf.all.rp_filter = 0
More information available: http://www.slashroot.in/linux-kernel-rpfilter-settings-reverse-path-filtering and http://centoshowtos.org/network-and-security/rp_filter/