Recently I've set up a new Ubuntu Server 10.04 and noticed my UDP server is no longer able to see any multicast data sent to the interface, even after joining the multicast group. I've got the exact same set up on two other Ubuntu 8.04.4 LTS machines and there is no problem receiving data after joining the same multicast group. The ethernet card is a Broadcom netXtreme II BCM5709 and the driver used is: b $ ethtool -i eth1 driver: bnx2 version: 2.0.2 firmware-version: 5.0.11 NCSI 2.0.5 bus-info: 0000:01:00.1 I'm using smcroute to manage my multicast registrations. b$ smcroute -d b$ smcroute -j eth1 233.37.54.71 After joining the group ip maddr shows the newly added registration. b$ ip maddr 1: lo inet 224.0.0.1 inet6 ff02::1 2: eth0 link 33:33:ff:40:c6:ad link 01:00:5e:00:00:01 link 33:33:00:00:00:01 inet 224.0.0.1 inet6 ff02::1:ff40:c6ad inet6 ff02::1 3: eth1 link 01:00:5e:25:36:47 link 01:00:5e:25:36:3e link 01:00:5e:25:36:3d link 33:33:ff:40:c6:af link 01:00:5e:00:00:01 link 33:33:00:00:00:01 inet 233.37.54.71 <------- McastGroup. inet 224.0.0.1 inet6 ff02::1:ff40:c6af inet6 ff02::1 So far so good, I can see that I'm receiving data for this multicast group. b$ sudo tcpdump -i eth1 -s 65534 host 233.37.54.71 tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth1, link-type EN10MB (Ethernet), capture size 65534 bytes 09:30:09.924337 IP 192.164.1.120.58848 > 233.37.54.71.15572: UDP, length 212 09:30:09.947547 IP 192.164.1.120.58848 > 233.37.54.71.15572: UDP, length 212 09:30:10.108378 IP 192.164.1.120.58866 > 233.37.54.71.15574: UDP, length 268 09:30:10.196841 IP 192.164.1.120.58848 > 233.37.54.71.15572: UDP, length 212 ... I can also confirm that the interface is receiving mcast packets. b $ ethtool -S eth1 | grep mcast_pack rx_mcast_packets: 103998 tx_mcast_packets: 33 Now here's the problem. When I try to capture the traffic using a simple ruby UDP server I receive zero data! Here's a simple server that reads data send on port 15572 and prints the first two characters. This works on the two 8.04.4 Ubuntu Servers, but not the 10.04 server. require 'socket' s = UDPSocket.new s.bind("", 15572) 5.times do text, sender = s.recvfrom(2) puts text end If I send a UDP packet crafted in ruby to localhost, the server receives it and prints out the first two characters. So I know that the server above is working correctly. irb(main):001:0> require 'socket' => true irb(main):002:0> s = UDPSocket.new => #<UDPSocket:0x7f3ccd6615f0> irb(main):003:0> s.send("I2 XXX", 0, 'localhost', 15572) When I check the protocol statistics I see that InMcastPkts is not increasing. While on the other 8.04 servers, on the same network, received a few thousands packets in 10 seconds. b $ netstat -sgu ; sleep 10 ; netstat -sgu IcmpMsg: InType3: 11 OutType3: 11 Udp: 446 packets received 4 packets to unknown port received. 0 packet receive errors 461 packets sent UdpLite: IpExt: InMcastPkts: 4654 <--------- Same as below OutMcastPkts: 3426 InBcastPkts: 9854 InOctets: -1691733021 OutOctets: 51187936 InMcastOctets: 145207 OutMcastOctets: 109680 InBcastOctets: 1246341 IcmpMsg: InType3: 11 OutType3: 11 Udp: 446 packets received 4 packets to unknown port received. 0 packet receive errors 461 packets sent UdpLite: IpExt: InMcastPkts: 4656 <-------------- Same as above OutMcastPkts: 3427 InBcastPkts: 9854 InOctets: -1690886265 OutOctets: 51188788 InMcastOctets: 145267 OutMcastOctets: 109712 InBcastOctets: 1246341 If I try forcing the interface into promisc mode nothing changes. At this point I'm stuck. I've confirmed the kernel config has multicast enabled. Perhaps there are other config options I should be checking? b $ grep CONFIG_IP_MULTICAST /boot/config-2.6.32-23-server CONFIG_IP_MULTICAST=y Any thoughts on where to go from here?