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The routecheck package parallels the netcheck package, where the former checks routes and routers while the latter checks networks. Like netcheck, it compiles reports for other systems to consume. Historically, the client has never known whether a peer is actually reachable. Most of the time this doesn’t matter, since the client will want to establish a WireGuard tunnel to any given destination. However, if the client needs to choose between two or more nodes, then it should only choose a node that it can reach. Suggested exit nodes are one such example, where the client filters out any nodes that aren’t connected to the control plane. Sometimes an exit node will get disconnected from the control plane: when the network between the two is unreliable or when the exit node is too busy to keep its control connection alive. In these cases, Control disables the Node.Online flag for the exit node and broadcasts this across the tailnet. Arguably, the client should never have relied on this flag, since it only makes sense in the admin console. This PR implements an initial routecheck client that can probe every node that your client knows about. You should not ping scan your visible tailnet, this method is for debugging only. Updates #17366 Updates tailscale/corp#33033 Signed-off-by: Simon Law <sfllaw@tailscale.com>