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FR: Support for general purpose multicast #11134

Description

@mrnoisytiger

What are you trying to do?

I have a small handful of applications that rely on general purpose UDP multicast to be available between devices on a network. These include ROON and some miscellaneous key servers. While I know there are existing FRs for mDNS and Bonjour across the tailnet, I've yet to find one for multicast.

How should we solve this?

Given that TS is fundamentally a point-to-point connection, it ultimately makes less sense to broadcast multicast packets across the entire tailnet. Rather, a proxy system could be in place, where you can specify to what machines/tags multicast packets should be proxied if received at a certain node.

For example, I have 3 nodes in the tailnet, and node1 is on a LAN that receives multicast packets. I want node2 to receive the multicast, but have no need for node3 to receive. It would be nice to assign node1 and node2 to a multicast "domain" in a sense, where node1 will proxy the packets through the tailnet to node2 but not to node3.

What is the impact of not solving this?

Apps that rely on UDP multicast ultimately fail to traverse the tailnet. This pushes those that rely on such apps to "layer 2" virtual ethernet solutions like ZeroTier instead of Tailscale. Multicast is unfortunately rather common in the legacy enterprise application space and the lack of multicast hinders the ability to fully migrate legacy networking over to TS.

Anything else?

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    connectivityIssues with general connectivity with TailscalefrFeature request

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