Network interface check when setting rtspTransport to multicast.

[yu-fukusako]

Question

We are considering using MediaMTX to stream video from a camera.
The camera connects via RTSP and then streams video using multicast.

Therefore, we have configured MediaMTX to use rtspTransport: multicast as follows:

  camera/path:
    source: rtsp://user:password@10.1.2.3:554/stream
    sourceOnDemand: yes
    rtspTransport: multicast

However, we are encountering the following error in MediaMTX:

found no interface that is multicast-capable and can communicate with IP 10.1.2.3

After checking the source code of gortsplib, it appears that the error occurs at the following part of the code.
https://github.com/bluenviron/gortsplib/blob/main/pkg/multicast/multicast.go#L42
https://github.com/bluenviron/gortsplib/blob/v4.12.3/client.go#L1568

Looking into the surrounding code, it seems that the system is checking whether the camera's IP (SourceIP) exists on the MediaMTX server side nw-interface.(IP 10.10.2.3)
Since the camera is on a different network from the server running MediaMTX, this check will always fail.

Is there any way to ignore this check?
Alternatively, shouldn't this check be performed against the DestinationIP (the multicast address) returned during the SETUP process, rather than the SourceIP?

[yu-fukusako]

I reviewed the code a bit more.
In the case of multicast, shouldn't the address specified in readIP be the local address rather than the remote address?
https://github.com/bluenviron/gortsplib/blob/v4.12.3/client.go#L1563

It looks like the process after this includes joining a multicast group on the interface specified by readIP. However, I believe it is necessary to join the multicast group using a local interface rather than a remote interface.
https://github.com/bluenviron/gortsplib/blob/v4.12.3/client_udp_listener.go#L96