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This path is currently only used by DERP servers that have also enabled `verify-clients` to ensure that only authorized clients within a Tailnet are allowed to use said DERP server. The previous naive linear scan in NodeByKey would almost certainly lead to bad outcomes with a large enough netmap, so address an existing todo by building a map of node key -> node ID. Updates #19042 Signed-off-by: Amal Bansode <amal@tailscale.com>