receiving multicast on a server with multiple interfaces (linux)

linux networking routing multicast
Gaetano Mendola picture Gaetano Mendola · Mar 30, 2011 · Viewed 18.1k times · Source

To receive a multicast on my not default NIC (dvb) I do the following:

  • open a socket (AF_INET, SOCK_DGRAM)
  • join the multicast address with IP_ADD_MEMBERSHIP on the dvb interface
  • bind the multicast address (note that a common error is to bind "0.0.0.0" and then receive on that socket even multicast you are not interested in) and the port

at this point the only way to receive the needed multicast packets is to add in the routing table a rule to reach the network where the sender is (another network) trough the dvb, as if the dvb needs to reply to the multicast sender; let say a sort of source sender multicast mode. Anyone knows what is going on? The problem is annoying to me because in principle I don't know the ip of sender.

Answer

caf picture caf · Mar 30, 2011

You appear to be being stung by rp_filter reverse-path filtering. This drops packets if they arrive on an interface that doesn't have a route for the source address.

You can disable it on a per-interface basis with the sysctl /proc/sys/net/ipv4/conf/<if>/rp_filter.

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