HowTo solve: setsockoption IP_DROP_MEMBERSHIP fails with 'Cannot assign requested address'

Problem

With our IP-TV probes, there may occur an error message like "Multicast leave failed: Cannot assign requested address".

Reason is: leaving a multicast group with 'setsockopt' with 'optname' 'IP_DROP_MEMBERSHIP' failed. The system reports "Cannot assign requested address".

Solution

The root cause is usually a missing multicast route in the system routing table. It may be fixed quickly by setting a mutlicast default routing entry:

# netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
10.19.2.0       0.0.0.0         255.255.255.0   U         0 0          0 eth0
10.19.2.0       0.0.0.0         255.255.255.0   U         0 0          0 eth2
0.0.0.0         10.19.2.252     0.0.0.0         UG        0 0          0 eth2
0.0.0.0         10.19.2.252     0.0.0.0         UG        0 0          0 eth0

The issue is, there is no clear routing rule for multicast for single interface. To solve the issue:

# route add -net 224.0.0.0 netmask 224.0.0.0 eth0