I was trying to use the multicast-iface property of udpsink/udpsrc elements on 2 machines with 2 network cards.
Setting this property on udpsrc worked fine but not for udpsink whereas this property is set when joining the multicast in both elements.
Here are my pipelines:

gst-launch-1.0 --gst-debug=*udp*:5 audiotestsrc ! audioconvert ! rtpL16pay ! udpsink host=239.5.5.5 port=5555 multicast-iface=eth1

gst-launch-1.0 --gst-debug=*udp*:5 udpsrc uri=udp://239.5.5.5:5555 multicast-iface=eth1 ! application/x-rtp, media=audio, clock-rate=44100, encoding-name=L16, channels=1, payload=96 ! rtpL16depay ! audioconvert ! alsasink

and my routing table:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
172.16.86.0     0.0.0.0         255.255.255.0   U     0      0        0 eth0
172.16.86.0     0.0.0.0         255.255.255.0   U     0      0        0 eth1
0.0.0.0         172.16.86.1     0.0.0.0         UG    0      0        0 eth0

udpsrc will correctly get the stream from eth1 whereas udpsink will send the stream on eth0. Of course, if I change the default gateway on eth1 or add a static route for my stream, the stream will be sent on eth1 but not using multicast-iface for udpsink.
I also tried to set the option like this:

if (sink->multi_iface)
      setsockopt (g_socket_get_fd (sink->used_socket), SOL_SOCKET, SO_BINDTODEVICE, sink->multi_iface, strlen (sink->multi_iface) * sizeof (gchar));

after creating the socket in gst_multiudpsink_start and in this case it was using the good interface.

Any idea, why it does not work like it udpsrc?

Cheers,

Paul HENRYS