ipn/ipnlocal: add traffic steering support to exit-node suggestions (#16527)

When `tailscale exit-node suggest` contacts the LocalAPI for a
suggested exit node, the client consults its netmap for peers that
contain the `suggest-exit-node` peercap. It currently uses a series of
heuristics to determine the exit node to suggest.

When the `traffic-steering` feature flag is enabled on its tailnet,
the client will defer to Control’s priority scores for a particular
peer. These scores, in `tailcfg.Hostinfo.Location.Priority`, were
historically only used for Mullvad exit nodes, but they have now been
extended to score any peer that could host a redundant resource.

Client capability version 119 is the earliest client that understands
these traffic steering scores. Control tells the client to switch to
rely on these scores by adding `tailcfg.NodeAttrTrafficSteering` to
its `AllCaps`.

Updates tailscale/corp#29966

Signed-off-by: Simon Law <sfllaw@tailscale.com>

ipn/ipnlocal/local.go

@@ -7675,13 +7675,10 @@ func allowedAutoRoute(ipp netip.Prefix) bool {
 
 var ErrNoPreferredDERP = errors.New("no preferred DERP, try again later")
 
-// suggestExitNodeLocked computes a suggestion based on the current netmap and last netcheck report. If
-// there are multiple equally good options, one is selected at random, so the result is not stable. To be
-// eligible for consideration, the peer must have NodeAttrSuggestExitNode in its CapMap.
-//
-// Currently, peers with a DERP home are preferred over those without (typically this means Mullvad).
-// Peers are selected based on having a DERP home that is the lowest latency to this device. For peers
-// without a DERP home, we look for geographic proximity to this device's DERP home.
+// suggestExitNodeLocked computes a suggestion based on the current netmap and
+// other optional factors. If there are multiple equally good options, one may
+// be selected at random, so the result is not stable. To be eligible for
+// consideration, the peer must have NodeAttrSuggestExitNode in its CapMap.
 //
 // b.mu.lock() must be held.
 func (b *LocalBackend) suggestExitNodeLocked() (response apitype.ExitNodeSuggestionResponse, err error) {
@@ -7743,7 +7740,32 @@ func fillAllowedSuggestions() set.Set[tailcfg.StableNodeID] {
 	return s
 }
 
+// suggestExitNode returns a suggestion for reasonably good exit node based on
+// the current netmap and the previous suggestion.
 func suggestExitNode(report *netcheck.Report, nb *nodeBackend, prevSuggestion tailcfg.StableNodeID, selectRegion selectRegionFunc, selectNode selectNodeFunc, allowList set.Set[tailcfg.StableNodeID]) (res apitype.ExitNodeSuggestionResponse, err error) {
+	switch {
+	case nb.SelfHasCap(tailcfg.NodeAttrTrafficSteering):
+		// The traffic-steering feature flag is enabled on this tailnet.
+		return suggestExitNodeUsingTrafficSteering(nb, prevSuggestion, allowList)
+	default:
+		return suggestExitNodeUsingDERP(report, nb, prevSuggestion, selectRegion, selectNode, allowList)
+	}
+}
+
+// suggestExitNodeUsingDERP is the classic algorithm used to suggest exit nodes,
+// before traffic steering was implemented. This handles the plain failover
+// case, in addition to the optional Regional Routing.
+//
+// It computes a suggestion based on the current netmap and last netcheck
+// report. If there are multiple equally good options, one is selected at
+// random, so the result is not stable. To be eligible for consideration, the
+// peer must have NodeAttrSuggestExitNode in its CapMap.
+//
+// Currently, peers with a DERP home are preferred over those without (typically
+// this means Mullvad). Peers are selected based on having a DERP home that is
+// the lowest latency to this device. For peers without a DERP home, we look for
+// geographic proximity to this device's DERP home.
+func suggestExitNodeUsingDERP(report *netcheck.Report, nb *nodeBackend, prevSuggestion tailcfg.StableNodeID, selectRegion selectRegionFunc, selectNode selectNodeFunc, allowList set.Set[tailcfg.StableNodeID]) (res apitype.ExitNodeSuggestionResponse, err error) {
 	netMap := nb.NetMap()
 	if report == nil || report.PreferredDERP == 0 || netMap == nil || netMap.DERPMap == nil {
 		return res, ErrNoPreferredDERP
@@ -7864,6 +7886,104 @@ func suggestExitNode(report *netcheck.Report, nb *nodeBackend, prevSuggestion ta
 	return res, nil
 }
 
+var ErrNoNetMap = errors.New("no network map, try again later")
+
+// suggestExitNodeUsingTrafficSteering uses traffic steering priority scores to
+// pick one of the best exit nodes. These priorities are provided by Control in
+// the node’s [tailcfg.Location]. To be eligible for consideration, the node
+// must have NodeAttrSuggestExitNode in its CapMap.
+func suggestExitNodeUsingTrafficSteering(nb *nodeBackend, prev tailcfg.StableNodeID, allowed set.Set[tailcfg.StableNodeID]) (apitype.ExitNodeSuggestionResponse, error) {
+	nm := nb.NetMap()
+	if nm == nil {
+		return apitype.ExitNodeSuggestionResponse{}, ErrNoNetMap
+	}
+
+	if !nb.SelfHasCap(tailcfg.NodeAttrTrafficSteering) {
+		panic("missing traffic-steering capability")
+	}
+
+	peers := nm.Peers
+	nodes := make([]tailcfg.NodeView, 0, len(peers))
+
+	for _, p := range peers {
+		if !p.Valid() {
+			continue
+		}
+		if allowed != nil && !allowed.Contains(p.StableID()) {
+			continue
+		}
+		if !p.CapMap().Contains(tailcfg.NodeAttrSuggestExitNode) {
+			continue
+		}
+		if !tsaddr.ContainsExitRoutes(p.AllowedIPs()) {
+			continue
+		}
+		if p.StableID() == prev {
+			// Prevent flapping: since prev is a valid suggestion,
+			// force prev to be the only valid pick.
+			nodes = []tailcfg.NodeView{p}
+			break
+		}
+		nodes = append(nodes, p)
+	}
+
+	var pick tailcfg.NodeView
+
+	scores := make(map[tailcfg.NodeID]int, len(nodes))
+	score := func(n tailcfg.NodeView) int {
+		id := n.ID()
+		s, ok := scores[id]
+		if !ok {
+			s = 0 // score of zero means incomparable
+			if hi := n.Hostinfo(); hi.Valid() {
+				if loc := hi.Location(); loc.Valid() {
+					s = loc.Priority()
+				}
+			}
+			scores[id] = s
+		}
+		return s
+	}
+
+	if len(nodes) > 0 {
+		// Find the highest scoring exit nodes.
+		slices.SortFunc(nodes, func(a, b tailcfg.NodeView) int {
+			return cmp.Compare(score(b), score(a)) // reverse sort
+		})
+
+		// Find the top exit nodes, which all have the same score.
+		topI := len(nodes)
+		ts := score(nodes[0])
+		for i, n := range nodes[1:] {
+			if score(n) < ts {
+				// n is the first node with a lower score.
+				// Make nodes[:topI] to slice the top exit nodes.
+				topI = i + 1
+				break
+			}
+		}
+
+		// TODO(sfllaw): add a temperature knob so that this client has
+		// a chance of picking the next best option.
+		randSeed := uint64(nm.SelfNode.ID())
+		pick = nodes[rands.IntN(randSeed, topI)]
+	}
+
+	if !pick.Valid() {
+		return apitype.ExitNodeSuggestionResponse{}, nil
+	}
+	res := apitype.ExitNodeSuggestionResponse{
+		ID:   pick.StableID(),
+		Name: pick.Name(),
+	}
+	if hi := pick.Hostinfo(); hi.Valid() {
+		if loc := hi.Location(); loc.Valid() {
+			res.Location = loc
+		}
+	}
+	return res, nil
+}
+
 // pickWeighted chooses the node with highest priority given a list of mullvad nodes.
 func pickWeighted(candidates []tailcfg.NodeView) []tailcfg.NodeView {
 	maxWeight := 0

ipn/ipnlocal/local_test.go

@@ -4229,6 +4229,23 @@ func withLocation(loc tailcfg.LocationView) peerOptFunc {
 	}
 }
 
+func withLocationPriority(pri int) peerOptFunc {
+	return func(n *tailcfg.Node) {
+		var hi *tailcfg.Hostinfo
+		if n.Hostinfo.Valid() {
+			hi = n.Hostinfo.AsStruct()
+		} else {
+			hi = new(tailcfg.Hostinfo)
+		}
+		if hi.Location == nil {
+			hi.Location = new(tailcfg.Location)
+		}
+		hi.Location.Priority = pri
+
+		n.Hostinfo = hi.View()
+	}
+}
+
 func withExitRoutes() peerOptFunc {
 	return func(n *tailcfg.Node) {
 		n.AllowedIPs = append(n.AllowedIPs, tsaddr.ExitRoutes()...)
@@ -4895,6 +4912,406 @@ func TestSuggestExitNodeLongLatDistance(t *testing.T) {
 	}
 }
 
+func TestSuggestExitNodeTrafficSteering(t *testing.T) {
+	city := &tailcfg.Location{
+		Country:     "Canada",
+		CountryCode: "CA",
+		City:        "Montreal",
+		CityCode:    "MTR",
+		Latitude:    45.5053,
+		Longitude:   -73.5525,
+	}
+	noLatLng := &tailcfg.Location{
+		Country:     "Canada",
+		CountryCode: "CA",
+		City:        "Montreal",
+		CityCode:    "MTR",
+	}
+
+	selfNode := tailcfg.Node{
+		ID: 0, // randomness is seeded off NetMap.SelfNode.ID
+		Addresses: []netip.Prefix{
+			netip.MustParsePrefix("100.64.1.1/32"),
+			netip.MustParsePrefix("fe70::1/128"),
+		},
+		CapMap: tailcfg.NodeCapMap{
+			tailcfg.NodeAttrTrafficSteering: []tailcfg.RawMessage{},
+		},
+	}
+
+	for _, tt := range []struct {
+		name string
+
+		netMap      *netmap.NetworkMap
+		lastExit    tailcfg.StableNodeID
+		allowPolicy []tailcfg.StableNodeID
+
+		wantID   tailcfg.StableNodeID
+		wantName string
+		wantLoc  *tailcfg.Location
+		wantPri  int
+
+		wantErr error
+	}{
+		{
+			name:    "no-netmap",
+			netMap:  nil,
+			wantErr: ErrNoNetMap,
+		},
+		{
+			name: "no-nodes",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers:    []tailcfg.NodeView{},
+			},
+			wantID: "",
+		},
+		{
+			name: "no-exit-nodes",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1),
+				},
+			},
+			wantID: "",
+		},
+		{
+			name: "exit-node-without-suggestion",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes()),
+				},
+			},
+			wantID: "",
+		},
+		{
+			name: "suggested-exit-node-without-routes",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withSuggest()),
+				},
+			},
+			wantID: "",
+		},
+		{
+			name: "suggested-exit-node",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+		},
+		{
+			name: "many-suggested-exit-nodes",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			wantID:   "stable3",
+			wantName: "peer3",
+		},
+		{
+			name: "suggested-exit-node-was-last-suggested",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			lastExit: "stable2", // overrides many-suggested-exit-nodes
+			wantID:   "stable2",
+			wantName: "peer2",
+		},
+		{
+			name: "suggested-exit-node-was-never-suggested",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			lastExit: "stable10",
+			wantID:   "stable3", // matches many-suggested-exit-nodes
+			wantName: "peer3",
+		},
+		{
+			name: "exit-nodes-with-and-without-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(1)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantPri:  1,
+		},
+		{
+			name: "exit-nodes-without-and-with-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(1)),
+				},
+			},
+			wantID:   "stable2",
+			wantName: "peer2",
+			wantPri:  1,
+		},
+		{
+			name: "exit-nodes-with-negative-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-1)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-2)),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-3)),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-4)),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantPri:  -1,
+		},
+		{
+			name: "exit-nodes-no-priority-beats-negative-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-1)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(-2)),
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest()),
+				},
+			},
+			wantID:   "stable3",
+			wantName: "peer3",
+		},
+		{
+			name: "exit-nodes-same-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(1)),
+					makePeer(2,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(2)), // top
+					makePeer(3,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(1)),
+					makePeer(4,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(2)), // top
+					makePeer(5,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(2)), // top
+					makePeer(6,
+						withExitRoutes(),
+						withSuggest()),
+					makePeer(7,
+						withExitRoutes(),
+						withSuggest(),
+						withLocationPriority(2)), // top
+				},
+			},
+			wantID:   "stable5",
+			wantName: "peer5",
+			wantPri:  2,
+		},
+		{
+			name: "suggested-exit-node-with-city",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocation(city.View())),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantLoc:  city,
+		},
+		{
+			name: "suggested-exit-node-with-city-and-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocation(city.View()),
+						withLocationPriority(1)),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantLoc:  city,
+			wantPri:  1,
+		},
+		{
+			name: "suggested-exit-node-without-latlng",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocation(noLatLng.View())),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantLoc:  noLatLng,
+		},
+		{
+			name: "suggested-exit-node-without-latlng-with-priority",
+			netMap: &netmap.NetworkMap{
+				SelfNode: selfNode.View(),
+				Peers: []tailcfg.NodeView{
+					makePeer(1,
+						withExitRoutes(),
+						withSuggest(),
+						withLocation(noLatLng.View()),
+						withLocationPriority(1)),
+				},
+			},
+			wantID:   "stable1",
+			wantName: "peer1",
+			wantLoc:  noLatLng,
+			wantPri:  1,
+		},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			var allowList set.Set[tailcfg.StableNodeID]
+			if tt.allowPolicy != nil {
+				allowList = set.SetOf(tt.allowPolicy)
+			}
+
+			// HACK: NetMap.AllCaps is populated by Control:
+			if tt.netMap != nil {
+				caps := maps.Keys(tt.netMap.SelfNode.CapMap().AsMap())
+				tt.netMap.AllCaps = set.SetOf(slices.Collect(caps))
+			}
+
+			nb := newNodeBackend(t.Context(), eventbus.New())
+			defer nb.shutdown(errShutdown)
+			nb.SetNetMap(tt.netMap)
+
+			got, err := suggestExitNodeUsingTrafficSteering(nb, tt.lastExit, allowList)
+			if tt.wantErr == nil && err != nil {
+				t.Fatalf("err=%v, want nil", err)
+			}
+			if tt.wantErr != nil && !errors.Is(err, tt.wantErr) {
+				t.Fatalf("err=%v, want %v", err, tt.wantErr)
+			}
+
+			if got.Name != tt.wantName {
+				t.Errorf("name=%q, want %q", got.Name, tt.wantName)
+			}
+
+			if got.ID != tt.wantID {
+				t.Errorf("ID=%q, want %q", got.ID, tt.wantID)
+			}
+
+			wantLoc := tt.wantLoc
+			if tt.wantPri != 0 {
+				if wantLoc == nil {
+					wantLoc = new(tailcfg.Location)
+				}
+				wantLoc.Priority = tt.wantPri
+			}
+			if diff := cmp.Diff(got.Location.AsStruct(), wantLoc); diff != "" {
+				t.Errorf("location mismatch (+want -got)\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestMinLatencyDERPregion(t *testing.T) {
 	tests := []struct {
 		name       string

tailcfg/tailcfg.go

@@ -163,7 +163,8 @@ type CapabilityVersion int
 //   - 116: 2025-05-05: Client serves MagicDNS "AAAA" if NodeAttrMagicDNSPeerAAAA set on self node
 //   - 117: 2025-05-28: Client understands DisplayMessages (structured health messages), but not necessarily PrimaryAction.
 //   - 118: 2025-07-01: Client sends Hostinfo.StateEncrypted to report whether the state file is encrypted at rest (#15830)
-const CurrentCapabilityVersion CapabilityVersion = 118
+//   - 119: 2025-07-10: Client uses Hostinfo.Location.Priority to prioritize one route over another.
+const CurrentCapabilityVersion CapabilityVersion = 119
 
 // ID is an integer ID for a user, node, or login allocated by the
 // control plane.