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macOS: RTSP multicast sessions start, but no multicast RTP is emitted #5574
Description
Which version are you using?
1.16.3
Which operating system are you using?
macOS amd64 standard
Describe the issue
Describe the bug
On macOS, MediaMTX negotiates RTSP UDP-multicast successfully, but no RTP multicast packets are actually sent.
TCP RTSP works normally. Multicast networking on the same Mac also works with a standalone Python UDP multicast sender, so this does not appear to be a general macOS multicast issue.
Environment
- MediaMTX v1.16.3
- Installed via Homebrew
- macOS 15.x on Apple Silicon (arm64)
Minimal config
logLevel: debug
rtspAddress: :8555
rtspTransports: [udp, multicast, tcp]
multicastIPRange: 239.255.0.0/16
multicastRTPPort: 8002
multicastRTCPPort: 8003
paths:
test:Reproduction
Publish a test stream:
ffmpeg -re -f lavfi -i testsrc=size=640x360:rate=10 -c:v libx264 -tune zerolatency -f rtsp -rtsp_transport tcp rtsp://127.0.0.1:8555/testRead it with multicast from another host:
ffplay -rtsp_transport udp_multicast rtsp://192.168.1.90:8555/testDebug log
Transport: RTP/AVP/UDP;multicast
Transport: RTP/AVP;multicast;destination=239.255.0.1;port=8002-8003;ttl=127;ssrc=...
[session ...] is reading from path 'test', with UDP-multicast, 1 track (H264)
Observed behavior
The RTSP session is set up successfully and MediaMTX reports UDP-multicast, but no packets are emitted on the multicast address.
sudo tcpdump -ni en0 'udp and dst 239.255.0.1 and (port 8002 or port 8003)'No packets are captured.
As a control test, sending multicast UDP directly from the same Mac to 239.255.0.1:8002 works and is received by another host on the LAN.
Expected behavior
After successful multicast negotiation, MediaMTX should emit RTP/RTCP packets to the configured multicast destination and ports.
Describe how to replicate the issue
- start the MediaMTX
- publish with ...
- read with ...
MediaMTX configuration
MediaMTX logs
No response
Packet dump
No response
Activity
alexchernyakov commentedon Mar 15, 2026
Additional findings from local instrumentation:
This does not appear to be a general macOS multicast networking problem. A standalone Python multicast sender on the same host/interface works and packets are received by another LAN host.
I patched
gortspliblocally to log multicast send errors and continue on partial success. With that change, RTSP multicast started working.The logged failures show the non-Linux multicast sender is attempting to send on multiple multicast-capable interfaces, including unrelated ones such as
gif0,bridge0,utun*, and additionalen*interfaces. Many of these fail with:After changing the sender to ignore per-interface failures and keep sending on interfaces that succeed, ffplay -rtsp_transport udp_multicast ... started receiving data.
So the issue seems to be in the non-Linux multicast sender path: failures on unrelated multicast-capable interfaces can break the whole sender, even when the correct interface works.
make multicast errors on single interfaces non-fatal
make multicast errors on single interfaces non-fatal
aler9 commentedon Aug 2, 2026
Thanks for reporting the bug, this is fixed by bluenviron/gortsplib#1115
make multicast errors on single interfaces non-fatal
make multicast errors on single interfaces non-fatal (#1115)