Flannel: UDP Broadcast possible between docker containers across hosts?

[Zia Syed]

Hi,

I'm running Flannel on my CoreOS cluster. Network 10.200.0.0.  I need to run a gyre (https://github.com/zeromq/gyre) based application in containers. Gyre needs to be able to UDP broadcast across the hosts. Which means I need broadcast address for my containers to be 10.200.255.255. Inside the container and on docker0 of host, I see them having Broadcast address 0.0.0.0. 

Is this broadcast supposed to work on Flannel? Is there a way I can change the broadcast address?

For testing , run the following on two containers on a same host and one container on another host within the same Flannel network. 

go run examples/ping/ping.go -group test

Thanks,

Zia

[Eugene Yakubovich]

Hi Zia,

flannel does not support broadcast (or multicast). IP broadcast is
supposed to work only on a single subnet but realistically only on a
single link segment (i.e. L2). flannel operates at IP layer (L3)
effectively as a router and routers don't forward broadcast traffic.

Best,
Eugene

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