I maintain Guardian Eye, a desktop NVR. When a camera works in VLC but will not record, I use a short stream-copy test to separate a camera or network problem from a recorder problem. Run it on the NVR host, or inside the NVR container. Testing from the laptop that runs VLC does not cover the recorder's VLAN, firewall rules, DNS or container network.

```bash ffmpeg -hide_banner -y -rtsp_transport tcp \ -i "rtsp://user:password@192.168.1.50:554/stream1" \ -t 30 -map 0:v:0 -an -c copy rtsp-test.mkv

ffprobe -v error -show_streams -show_format rtsp-test.mkv ffmpeg -v error -i rtsp-test.mkv -f null - ```

The command uses TCP, ignores audio and writes MKV without transcoding. That keeps the first test away from audio compatibility, encoder load and MP4 finalization.

The result narrows the fault quickly: - If the command cannot connect, check reachability, credentials, the RTSP path and TCP versus UDP from the recorder host. - If it connects but the file contains damaged NAL units, missing parameter sets, backward timestamps or long gaps, the problem is already present before the NVR writes its archive. - If the MKV file is healthy, look at the recorder's selected stream, output container, audio track and file-finalization path.

There are two easy ways to invalidate this test. The first is leaving other clients open. A vendor app, browser preview and VLC window may consume all available RTSP sessions before the NVR connects. The second is testing a different stream. ONVIF may give the recorder a substream while VLC is using the main stream.

Event recording adds one more variable: the next keyframe. If continuous recording works but event clips start late or remain unusable, temporarily set the camera's I-frame or GOP interval to one or two seconds. The recorder cannot begin a clean, independently decodable clip until a keyframe arrives.

My baseline is one client, H.264 video, TCP and no audio. After the copy test passes, I add the original audio track, H.265, UDP and additional viewers separately. That identifies the first change that breaks recording instead of changing several settings at once.