Update

vendor/tailscale.com/net/routecheck/routes.go

@@ -0,0 +1,56 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package routecheck
+
+import (
+	"net/netip"
+
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/views"
+	"tailscale.com/util/mak"
+)
+
+// RoutersByPrefix represents a map of nodes grouped by the subnet that they route.
+// Nodes that route for /0 prefixes are exit nodes, their subnet is the Internet.
+// The result omits any prefix that is one of a node’s local addresses.
+//
+// Note: Fallback routes are not supported by design. If a subnet prefix
+// contained within another more general prefix has no reachable routers,
+// traffic is still sent to one of those unreachable routers.
+// Routers for the general prefix aren’t candidates. See tailscale/tailscale#18550.
+type RoutersByPrefix map[netip.Prefix][]tailcfg.NodeView
+
+// RoutersByPrefix returns a map of nodes grouped by the subnet that they route.
+// See [RoutersByPrefix] for more detail.
+func (c *Client) RoutersByPrefix() RoutersByPrefix {
+	return GroupRoutersByPrefix(c.nb.NodeBackend().Peers())
+}
+
+// GroupRoutersByPrefix returns a map of nodes grouped by the subnet that they route.
+// See [RoutersByPrefix] for more detail.
+func GroupRoutersByPrefix(nodes []tailcfg.NodeView) RoutersByPrefix {
+	var routers RoutersByPrefix
+	for _, n := range nodes {
+		for _, pfx := range routes(n) {
+			mak.Set(&routers, pfx, append(routers[pfx], n))
+		}
+	}
+	return routers
+}
+
+// Routes returns a slice of subnets that the given node will route.
+// If the node is an exit node, the result will contain at least one /0 prefix.
+// If the node is a subnet router, the result will contain a smaller prefix.
+// The result omits any prefix that is one of the node’s local addresses.
+func routes(n tailcfg.NodeView) []netip.Prefix {
+	var routes []netip.Prefix
+	for _, pfx := range n.AllowedIPs().All() {
+		// Routers never forward their own local addresses.
+		if views.SliceContains(n.Addresses(), pfx) {
+			continue
+		}
+		routes = append(routes, pfx)
+	}
+	return routes
+}