rtsp-simple-server/docs/2-features/11-forward.md

Forward

To forward incoming streams to another server, use FFmpeg inside the runOnReady parameter:

pathDefaults:
  runOnReady: >
    ffmpeg -i rtsp://localhost:$RTSP_PORT/$MTX_PATH
    -c copy
    -f rtsp rtsp://other-server:8554/another-path
  runOnReadyRestart: yes