Linux bridge does not propagate multicast ntp messages

H.Arslan

3/18/24, 11:05 PM

My hardware, based on imx8, has two ethernet interfaces. These are connected together as a bridge, all messages not for this he are forwarded. Everything works as expected except multicast messages: ntp server sends time as a multicast message (ff05::101). When I use tcpdump (tcpdump -i br0), I see the NTP messages arriving, but the local NTP client does not seem to receive them. When I delete the bridge and configure eth0, then NTP client reacts to the message.

Ntp multicast messages arrive the bridge at eth0 and they are also routed to eth1 correctly.

I already experimented with multicast snooping, multicast_router and multicast_querier, did not change anything. Any suggestions, what might cause the problem?

254

0 + 5

linux

ntp

multicast

bridge

A.B

3/18/24, 11:14 PM

Is the client actually listening? You're supposed to see `ff05::101` in `ip -6 maddr show dev br0`'s output. If not it's a client configuration problem.

H.Arslan

3/19/24, 11:08 PM

This command indeed does not show this address. It might seem to be a client problem, which brings me to the point: the client does not know anything about physical network, but only the configuration to listen to the ff05::101. This apparently works fine, when I delete br0 and configure eth0 instead, If this would be a problem with the client, then it shouldn't work also when connected with eth0. It seems to be something with bridge configuration. The bridge forwards the message from eth0 to eth1, but but somehow the message is not propagated to local application.

Paul Gear

3/20/24, 12:39 AM

I have seen (and experienced) some reports of IPv6 multicast issues, which I summarised here: https://www.libertysys.com.au/2022/12/pain-points-mld-snooping-on-linux-bridges/ If that ends up fixing the issue for you, let's write up an answer together...

H.Arslan

3/20/24, 8:03 AM

Thanks @PaulGear, I already experimented with multicast_snooping, it did not change anything. Btw, I am running kernel 5.10

Paul Gear

3/21/24, 8:37 PM

For what it's worth, NTP over broadcast/multicast is generally considered not the best idea, and it might be removed in NTPv5.

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