cmd/tailscale, ipn/ipnlocal: add suggest exit node CLI option (#11407)

Updates tailscale/corp#17516

Signed-off-by: Claire Wang <claire@tailscale.com>

client/tailscale/apitype/apitype.go

@@ -49,3 +49,11 @@ type ReloadConfigResponse struct {
 	Reloaded bool   // whether the config was reloaded
 	Err      string // any error message
 }
+
+// ExitNodeSuggestionResponse is the response to a LocalAPI suggest-exit-node GET request.
+// It returns the StableNodeID, name, and location of a suggested exit node for the client making the request.
+type ExitNodeSuggestionResponse struct {
+	ID       tailcfg.StableNodeID
+	Name     string
+	Location tailcfg.LocationView `json:",omitempty"`
+}

client/tailscale/localclient.go

@@ -1514,3 +1514,12 @@ func (w *IPNBusWatcher) Next() (ipn.Notify, error) {
 	}
 	return n, nil
 }
+
+// SuggestExitNode requests an exit node suggestion and returns the exit node's details.
+func (lc *LocalClient) SuggestExitNode(ctx context.Context) (apitype.ExitNodeSuggestionResponse, error) {
+	body, err := lc.get200(ctx, "/localapi/v0/suggest-exit-node")
+	if err != nil {
+		return apitype.ExitNodeSuggestionResponse{}, err
+	}
+	return decodeJSON[apitype.ExitNodeSuggestionResponse](body)
+}

cmd/tailscale/cli/exitnode.go

@@ -40,6 +40,12 @@ func exitNodeCmd() *ffcli.Command {
 					fs.StringVar(&exitNodeArgs.filter, "filter", "", "filter exit nodes by country")
 					return fs
 				})(),
+			},
+			{
+				Name:       "suggest",
+				ShortUsage: "tailscale exit-node suggest",
+				ShortHelp:  "Suggests the best available exit node",
+				Exec:       runExitNodeSuggest,
 			}},
 			(func() []*ffcli.Command {
 				if !envknob.UseWIPCode() {
@@ -134,11 +140,37 @@ func runExitNodeList(ctx context.Context, args []string) error {
 	}
 	fmt.Fprintln(w)
 	fmt.Fprintln(w)
-	fmt.Fprintln(w, "# To use an exit node, use `tailscale set --exit-node=` followed by the hostname or IP")
-
+	fmt.Fprintln(w, "# To use an exit node, use `tailscale set --exit-node=` followed by the hostname or IP.")
+	if hasAnyExitNodeSuggestions(peers) {
+		fmt.Fprintln(w, "# To have Tailscale suggest an exit node, use `tailscale exit-node suggest`.")
+	}
 	return nil
 }
 
+// runExitNodeSuggest returns a suggested exit node ID to connect to and shows the chosen exit node tailcfg.StableNodeID.
+// If there are no derp based exit nodes to choose from or there is a failure in finding a suggestion, the command will return an error indicating so.
+func runExitNodeSuggest(ctx context.Context, args []string) error {
+	res, err := localClient.SuggestExitNode(ctx)
+	if err != nil {
+		return fmt.Errorf("suggest exit node: %w", err)
+	}
+	if res.ID == "" {
+		fmt.Println("No exit node suggestion is available.")
+		return nil
+	}
+	fmt.Printf("Suggested exit node: %v\nTo accept this suggestion, use `tailscale set --exit-node=%v`.\n", res.Name, res.ID)
+	return nil
+}
+
+func hasAnyExitNodeSuggestions(peers []*ipnstate.PeerStatus) bool {
+	for _, peer := range peers {
+		if peer.HasCap(tailcfg.NodeAttrSuggestExitNode) {
+			return true
+		}
+	}
+	return false
+}
+
 // peerStatus returns a string representing the current state of
 // a peer. If there is no notable state, a - is returned.
 func peerStatus(peer *ipnstate.PeerStatus) string {

cmd/tailscaled/depaware.txt

@@ -297,7 +297,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/flowtrack                                  from tailscale.com/net/packet+
      💣 tailscale.com/net/interfaces                                 from tailscale.com/cmd/tailscaled+
         tailscale.com/net/netaddr                                    from tailscale.com/ipn+
-        tailscale.com/net/netcheck                                   from tailscale.com/wgengine/magicsock
+        tailscale.com/net/netcheck                                   from tailscale.com/wgengine/magicsock+
         tailscale.com/net/neterror                                   from tailscale.com/net/dns/resolver+
         tailscale.com/net/netkernelconf                              from tailscale.com/ipn/ipnlocal
         tailscale.com/net/netknob                                    from tailscale.com/logpolicy+

ipn/ipnlocal/local.go

@@ -14,6 +14,7 @@
 	"log"
 	"maps"
 	"math"
+	"math/rand"
 	"net"
 	"net/http"
 	"net/netip"
@@ -59,6 +60,7 @@
 	"tailscale.com/net/dnscache"
 	"tailscale.com/net/dnsfallback"
 	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netcheck"
 	"tailscale.com/net/netkernelconf"
 	"tailscale.com/net/netmon"
 	"tailscale.com/net/netns"
@@ -6295,3 +6297,217 @@ func mayDeref[T any](p *T) (v T) {
 	}
 	return *p
 }
+
+var ErrNoPreferredDERP = errors.New("no preferred DERP, try again later")
+
+// SuggestExitNode 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 (b *LocalBackend) SuggestExitNode() (response apitype.ExitNodeSuggestionResponse, err error) {
+	b.mu.Lock()
+	lastReport := b.MagicConn().GetLastNetcheckReport(b.ctx)
+	netMap := b.netMap
+	b.mu.Unlock()
+	seed := time.Now().UnixNano()
+	r := rand.New(rand.NewSource(seed))
+	return suggestExitNode(lastReport, netMap, r)
+}
+
+func suggestExitNode(report *netcheck.Report, netMap *netmap.NetworkMap, r *rand.Rand) (res apitype.ExitNodeSuggestionResponse, err error) {
+	if report.PreferredDERP == 0 {
+		return res, ErrNoPreferredDERP
+	}
+	candidates := make([]tailcfg.NodeView, 0, len(netMap.Peers))
+	for _, peer := range netMap.Peers {
+		if peer.CapMap().Has(tailcfg.NodeAttrSuggestExitNode) && tsaddr.ContainsExitRoutes(peer.AllowedIPs()) {
+			candidates = append(candidates, peer)
+		}
+	}
+	if len(candidates) == 0 {
+		return res, nil
+	}
+	if len(candidates) == 1 {
+		peer := candidates[0]
+		if hi := peer.Hostinfo(); hi.Valid() {
+			if loc := hi.Location(); loc != nil {
+				res.Location = loc.View()
+			}
+		}
+		res.ID = peer.StableID()
+		res.Name = peer.Name()
+		return res, nil
+	}
+
+	candidatesByRegion := make(map[int][]tailcfg.NodeView, len(netMap.DERPMap.Regions))
+	var preferredDERP *tailcfg.DERPRegion = netMap.DERPMap.Regions[report.PreferredDERP]
+	var minDistance float64 = math.MaxFloat64
+	type nodeDistance struct {
+		nv       tailcfg.NodeView
+		distance float64 // in meters, approximately
+	}
+	distances := make([]nodeDistance, 0, len(candidates))
+	for _, c := range candidates {
+		if !c.Valid() {
+			continue
+		}
+		if c.DERP() != "" {
+			ipp, err := netip.ParseAddrPort(c.DERP())
+			if err != nil {
+				continue
+			}
+			if ipp.Addr() != tailcfg.DerpMagicIPAddr {
+				continue
+			}
+			regionID := int(ipp.Port())
+			candidatesByRegion[regionID] = append(candidatesByRegion[regionID], c)
+			continue
+		}
+		if len(candidatesByRegion) > 0 {
+			// Since a candidate exists that does have a DERP home, skip this candidate. We never select
+			// a candidate without a DERP home if there is a candidate available with a DERP home.
+			continue
+		}
+		// This candidate does not have a DERP home.
+		// Use geographic distance from our DERP home to estimate how good this candidate is.
+		hi := c.Hostinfo()
+		if !hi.Valid() {
+			continue
+		}
+		loc := hi.Location()
+		if loc == nil {
+			continue
+		}
+		distance := longLatDistance(preferredDERP.Latitude, preferredDERP.Longitude, loc.Latitude, loc.Longitude)
+		if distance < minDistance {
+			minDistance = distance
+		}
+		distances = append(distances, nodeDistance{nv: c, distance: distance})
+	}
+	// First, try to select an exit node that has the closest DERP home, based on lastReport's DERP latency.
+	// If there are no latency values, it returns an arbitrary region
+	if len(candidatesByRegion) > 0 {
+		minRegion := minLatencyDERPRegion(xmaps.Keys(candidatesByRegion), report)
+		if minRegion == 0 {
+			minRegion = randomRegion(xmaps.Keys(candidatesByRegion), r)
+		}
+		regionCandidates, ok := candidatesByRegion[minRegion]
+		if !ok {
+			return res, errors.New("no candidates in expected region: this is a bug")
+		}
+		chosen := randomNode(regionCandidates, r)
+		res.ID = chosen.StableID()
+		res.Name = chosen.Name()
+		if hi := chosen.Hostinfo(); hi.Valid() {
+			if loc := hi.Location(); loc != nil {
+				res.Location = loc.View()
+			}
+		}
+		return res, nil
+	}
+	// None of the candidates have a DERP home, so proceed to select based on geographical distance from our preferred DERP region.
+
+	// allowanceMeters is the extra distance that will be permitted when considering peers. By this point, there
+	// are multiple approximations taking place (DERP location standing in for this device's location, the peer's
+	// location may only be city granularity, the distance algorithm assumes a spherical planet, etc.) so it is
+	// reasonable to consider peers that are similar distances. Those peers are good enough to be within
+	// measurement error. 100km corresponds to approximately 1ms of additional round trip light
+	// propagation delay in a fiber optic cable and seems like a reasonable heuristic. It may be adjusted in
+	// future.
+	const allowanceMeters = 100000
+	pickFrom := make([]tailcfg.NodeView, 0, len(distances))
+	for _, candidate := range distances {
+		if candidate.nv.Valid() && candidate.distance <= minDistance+allowanceMeters {
+			pickFrom = append(pickFrom, candidate.nv)
+		}
+	}
+	chosen := pickWeighted(pickFrom)
+	if !chosen.Valid() {
+		return res, errors.New("chosen candidate invalid: this is a bug")
+	}
+	res.ID = chosen.StableID()
+	res.Name = chosen.Name()
+	if hi := chosen.Hostinfo(); hi.Valid() {
+		if loc := hi.Location(); loc != nil {
+			res.Location = loc.View()
+		}
+	}
+	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
+	var best tailcfg.NodeView
+	for _, c := range candidates {
+		hi := c.Hostinfo()
+		if !hi.Valid() {
+			continue
+		}
+		loc := hi.Location()
+		if loc == nil || loc.Priority <= maxWeight {
+			continue
+		}
+		maxWeight = loc.Priority
+		best = c
+	}
+	return best
+}
+
+// randomNode chooses a node randomly given a list of nodes and a *rand.Rand.
+func randomNode(nodes []tailcfg.NodeView, r *rand.Rand) tailcfg.NodeView {
+	return nodes[r.Intn(len(nodes))]
+}
+
+// randomRegion chooses a region randomly given a list of ints and a *rand.Rand
+func randomRegion(regions []int, r *rand.Rand) int {
+	if testenv.InTest() {
+		regions = slices.Clone(regions)
+		slices.Sort(regions)
+	}
+	return regions[r.Intn(len(regions))]
+}
+
+// minLatencyDERPRegion returns the region with the lowest latency value given the last netcheck report.
+// If there are no latency values, it returns 0.
+func minLatencyDERPRegion(regions []int, report *netcheck.Report) int {
+	min := slices.MinFunc(regions, func(i, j int) int {
+		const largeDuration time.Duration = math.MaxInt64
+		iLatency, ok := report.RegionLatency[i]
+		if !ok {
+			iLatency = largeDuration
+		}
+		jLatency, ok := report.RegionLatency[j]
+		if !ok {
+			jLatency = largeDuration
+		}
+		if c := cmp.Compare(iLatency, jLatency); c != 0 {
+			return c
+		}
+		return cmp.Compare(i, j)
+	})
+	latency, ok := report.RegionLatency[min]
+	if !ok || latency == 0 {
+		return 0
+	} else {
+		return min
+	}
+}
+
+// longLatDistance returns an estimated distance given the geographic coordinates of two locations, in degrees.
+// The coordinates are separated into four separate float64 values.
+// Value is returned in meters.
+func longLatDistance(fromLat, fromLong, toLat, toLong float64) float64 {
+	const toRadians = math.Pi / 180
+	diffLat := (fromLat - toLat) * toRadians
+	diffLong := (fromLong - toLong) * toRadians
+	lat1 := fromLat * toRadians
+	lat2 := toLat * toRadians
+	a := math.Pow(math.Sin(diffLat/2), 2) + math.Cos(lat1)*math.Cos(lat2)*math.Pow(math.Sin(diffLong/2), 2)
+	const earthRadiusMeters = 6371000
+	c := 2 * math.Atan2(math.Sqrt(a), math.Sqrt(1-a))
+	return earthRadiusMeters * c
+}

ipn/ipnlocal/local_test.go

@@ -8,6 +8,8 @@
 	"encoding/json"
 	"errors"
 	"fmt"
+	"math"
+	"math/rand"
 	"net"
 	"net/http"
 	"net/netip"
@@ -29,6 +31,7 @@
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/store/mem"
 	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netcheck"
 	"tailscale.com/net/tsaddr"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tsd"
@@ -2634,3 +2637,734 @@ func (b *LocalBackend) SetPrefsForTest(newp *ipn.Prefs) {
 	defer unlock()
 	b.setPrefsLockedOnEntry(newp, unlock)
 }
+
+func TestSuggestExitNode(t *testing.T) {
+	tests := []struct {
+		name         string
+		lastReport   netcheck.Report
+		netMap       netmap.NetworkMap
+		wantID       tailcfg.StableNodeID
+		wantName     string
+		wantLocation tailcfg.LocationView
+		wantError    error
+	}{
+		{
+			name: "2 exit nodes in same region",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 10 * time.Millisecond,
+					2: 20 * time.Millisecond,
+					3: 30 * time.Millisecond,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						Name:     "2",
+						StableID: "2",
+						DERP:     "127.3.3.40:1",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						Name:     "3",
+						StableID: "3",
+						DERP:     "127.3.3.40:1",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantName: "3",
+			wantID:   tailcfg.StableNodeID("3"),
+		},
+		{
+			name: "2 derp based exit nodes, different regions, no latency measurements",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						Name:     "2",
+						DERP:     "127.3.3.40:2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						Name:     "3",
+						DERP:     "127.3.3.40:3",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantName: "3",
+			wantID:   tailcfg.StableNodeID("3"),
+		},
+		{
+			name: "2 derp based exit nodes, different regions, same latency",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 10,
+					2: 10,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						Name:     "2",
+						DERP:     "127.3.3.40:1",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						Name:     "3",
+						DERP:     "127.3.3.40:2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantName: "2",
+			wantID:   tailcfg.StableNodeID("2"),
+		},
+		{
+			name: "mullvad nodes, no derp based exit nodes",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {
+							Latitude:  40.73061,
+							Longitude: -73.935242,
+						},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "Dallas",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  32.89748,
+								Longitude: -97.040443,
+								Priority:  100,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "San Jose",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  37.3382082,
+								Longitude: -121.8863286,
+								Priority:  20,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantID: tailcfg.StableNodeID("2"),
+			wantLocation: (&tailcfg.Location{
+				Latitude:  32.89748,
+				Longitude: -97.040443,
+				Priority:  100,
+			}).View(),
+			wantName: "Dallas",
+		},
+		{
+			name: "mullvad nodes close to each other, different priorities",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {
+							Latitude:  40.73061,
+							Longitude: -73.935242,
+						},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "Dallas",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  32.89748,
+								Longitude: -97.040443,
+								Priority:  10,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "Fort Worth",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  37.768799,
+								Longitude: -97.309341,
+								Priority:  50,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantID: tailcfg.StableNodeID("3"),
+			wantLocation: (&tailcfg.Location{
+				Latitude:  37.768799,
+				Longitude: -97.309341,
+				Priority:  50,
+			}).View(),
+			wantName: "Fort Worth",
+		},
+		{
+			name: "mullvad nodes, no preferred derp region exit nodes",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {
+							Latitude:  40.73061,
+							Longitude: -73.935242,
+						},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "Dallas",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  32.89748,
+								Longitude: -97.040443,
+								Priority:  20,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "San Jose",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  37.3382082,
+								Longitude: -121.8863286,
+								Priority:  30,
+							},
+						}).View(),
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+					(&tailcfg.Node{
+						ID:       3,
+						StableID: "3",
+						Name:     "3",
+						DERP:     "127.3.3.40:2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						CapMap: (tailcfg.NodeCapMap)(map[tailcfg.NodeCapability][]tailcfg.RawMessage{
+							tailcfg.NodeAttrSuggestExitNode: {},
+						}),
+					}).View(),
+				},
+			},
+			wantID:   tailcfg.StableNodeID("3"),
+			wantName: "3",
+		},
+		{
+			name: "no mullvad nodes; no derp nodes",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+			},
+		},
+		{
+			name: "no preferred derp region",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: -1,
+					3: 0,
+				},
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+			},
+			wantError: ErrNoPreferredDERP,
+		},
+		{
+			name: "derp exit node and mullvad exit node both with no suggest exit node attribute",
+			lastReport: netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 0,
+					2: 0,
+					3: 0,
+				},
+				PreferredDERP: 1,
+			},
+			netMap: netmap.NetworkMap{
+				SelfNode: (&tailcfg.Node{
+					Addresses: []netip.Prefix{
+						netip.MustParsePrefix("100.64.1.1/32"),
+						netip.MustParsePrefix("fe70::1/128"),
+					},
+				}).View(),
+				DERPMap: &tailcfg.DERPMap{
+					Regions: map[int]*tailcfg.DERPRegion{
+						1: {},
+						2: {},
+						3: {},
+					},
+				},
+				Peers: []tailcfg.NodeView{
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						Name:     "2",
+						DERP:     "127.3.3.40:1",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+					}).View(),
+					(&tailcfg.Node{
+						ID:       2,
+						StableID: "2",
+						AllowedIPs: []netip.Prefix{
+							netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+						},
+						Name: "Dallas",
+						Hostinfo: (&tailcfg.Hostinfo{
+							Location: &tailcfg.Location{
+								Latitude:  32.89748,
+								Longitude: -97.040443,
+								Priority:  30,
+							},
+						}).View(),
+					}).View(),
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			r := rand.New(rand.NewSource(100))
+			got, err := suggestExitNode(&tt.lastReport, &tt.netMap, r)
+			if got.Name != tt.wantName {
+				t.Errorf("name=%v, want %v", got.Name, tt.wantName)
+			}
+			if got.ID != tt.wantID {
+				t.Errorf("ID=%v, want %v", got.ID, tt.wantID)
+			}
+			if tt.wantError == nil && err != nil {
+				t.Errorf("err=%v, want no error", err)
+			}
+			if tt.wantError != nil && !errors.Is(err, tt.wantError) {
+				t.Errorf("err=%v, want %v", err, tt.wantError)
+			}
+			if !reflect.DeepEqual(got.Location, tt.wantLocation) {
+				t.Errorf("location=%v, want %v", got.Location, tt.wantLocation)
+			}
+		})
+	}
+}
+
+func TestSuggestExitNodePickWeighted(t *testing.T) {
+	tests := []struct {
+		name       string
+		candidates []tailcfg.NodeView
+		wantValue  tailcfg.NodeView
+		wantValid  bool
+	}{
+		{
+			name: ">1 candidates",
+			candidates: []tailcfg.NodeView{
+				(&tailcfg.Node{
+					ID:       2,
+					StableID: "2",
+					AllowedIPs: []netip.Prefix{
+						netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+					},
+					Hostinfo: (&tailcfg.Hostinfo{
+						Location: &tailcfg.Location{
+							Priority: 20,
+						},
+					}).View(),
+				}).View(),
+				(&tailcfg.Node{
+					ID:       3,
+					StableID: "3",
+					AllowedIPs: []netip.Prefix{
+						netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+					},
+					Hostinfo: (&tailcfg.Hostinfo{
+						Location: &tailcfg.Location{
+							Priority: 10,
+						},
+					}).View(),
+				}).View(),
+			},
+			wantValue: (&tailcfg.Node{
+				ID:       2,
+				StableID: "2",
+				AllowedIPs: []netip.Prefix{
+					netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+				},
+				Hostinfo: (&tailcfg.Hostinfo{
+					Location: &tailcfg.Location{
+						Priority: 20,
+					},
+				}).View(),
+			}).View(),
+			wantValid: true,
+		},
+		{
+			name:       "<1 candidates",
+			candidates: []tailcfg.NodeView{},
+			wantValid:  false,
+		},
+		{
+			name: "1 candidate",
+			candidates: []tailcfg.NodeView{
+				(&tailcfg.Node{
+					ID:       2,
+					StableID: "2",
+					AllowedIPs: []netip.Prefix{
+						netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+					},
+					Hostinfo: (&tailcfg.Hostinfo{
+						Location: &tailcfg.Location{
+							Priority: 20,
+						},
+					}).View(),
+				}).View(),
+			},
+			wantValue: (&tailcfg.Node{
+				ID:       2,
+				StableID: "2",
+				AllowedIPs: []netip.Prefix{
+					netip.MustParsePrefix("0.0.0.0/0"), netip.MustParsePrefix("::/0"),
+				},
+				Hostinfo: (&tailcfg.Hostinfo{
+					Location: &tailcfg.Location{
+						Priority: 20,
+					},
+				}).View(),
+			}).View(),
+			wantValid: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := pickWeighted(tt.candidates)
+			if !reflect.DeepEqual(got, tt.wantValue) {
+				t.Errorf("got value %v want %v", got, tt.wantValue)
+				if tt.wantValid != got.Valid() {
+					t.Errorf("got invalid candidate expected valid")
+				}
+				if tt.wantValid {
+					if !reflect.DeepEqual(got, tt.wantValue) {
+						t.Errorf("got value %v want %v", got, tt.wantValue)
+					}
+				}
+			}
+		})
+	}
+}
+
+func TestSuggestExitNodeLongLatDistance(t *testing.T) {
+	tests := []struct {
+		name     string
+		fromLat  float64
+		fromLong float64
+		toLat    float64
+		toLong   float64
+		want     float64
+	}{
+		{
+			name:     "zero values",
+			fromLat:  0,
+			fromLong: 0,
+			toLat:    0,
+			toLong:   0,
+			want:     0,
+		},
+		{
+			name:     "valid values",
+			fromLat:  40.73061,
+			fromLong: -73.935242,
+			toLat:    37.3382082,
+			toLong:   -121.8863286,
+			want:     4117266.873301274,
+		},
+		{
+			name:     "valid values, locations in north and south of equator",
+			fromLat:  40.73061,
+			fromLong: -73.935242,
+			toLat:    -33.861481,
+			toLong:   151.205475,
+			want:     15994089.144368416,
+		},
+	}
+	// The wanted values are computed using a more precise algorithm using the WGS84 model but
+	// longLatDistance uses a spherical approximation for simplicity. To account for this, we allow for
+	// 10km of error.
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := longLatDistance(tt.fromLat, tt.fromLong, tt.toLat, tt.toLong)
+			const maxError = 10000 // 10km
+			if math.Abs(got-tt.want) > maxError {
+				t.Errorf("distance=%vm, want within %vm of %vm", got, maxError, tt.want)
+			}
+		})
+	}
+}
+
+func TestMinLatencyDERPregion(t *testing.T) {
+	tests := []struct {
+		name       string
+		regions    []int
+		report     *netcheck.Report
+		wantRegion int
+	}{
+		{
+			name:       "regions, no latency values",
+			regions:    []int{1, 2, 3},
+			wantRegion: 0,
+			report:     &netcheck.Report{},
+		},
+		{
+			name:       "regions, different latency values",
+			regions:    []int{1, 2, 3},
+			wantRegion: 2,
+			report: &netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 10 * time.Millisecond,
+					2: 5 * time.Millisecond,
+					3: 30 * time.Millisecond,
+				},
+			},
+		},
+		{
+			name:       "regions, same values",
+			regions:    []int{1, 2, 3},
+			wantRegion: 1,
+			report: &netcheck.Report{
+				RegionLatency: map[int]time.Duration{
+					1: 10 * time.Millisecond,
+					2: 10 * time.Millisecond,
+					3: 10 * time.Millisecond,
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := minLatencyDERPRegion(tt.regions, tt.report)
+			if got != tt.wantRegion {
+				t.Errorf("got region %v want region %v", got, tt.wantRegion)
+			}
+		})
+	}
+}

ipn/localapi/localapi.go

@@ -122,6 +122,7 @@
 	"set-use-exit-node-enabled":   (*Handler).serveSetUseExitNodeEnabled,
 	"start":                       (*Handler).serveStart,
 	"status":                      (*Handler).serveStatus,
+	"suggest-exit-node":           (*Handler).serveSuggestExitNode,
 	"tka/affected-sigs":           (*Handler).serveTKAAffectedSigs,
 	"tka/cosign-recovery-aum":     (*Handler).serveTKACosignRecoveryAUM,
 	"tka/disable":                 (*Handler).serveTKADisable,
@@ -2872,3 +2873,18 @@ func (h *Handler) serveShares(w http.ResponseWriter, r *http.Request) {
 	// User-visible LocalAPI endpoints.
 	metricFilePutCalls = clientmetric.NewCounter("localapi_file_put")
 )
+
+// serveSuggestExitNode serves a POST endpoint for returning a suggested exit node.
+func (h *Handler) serveSuggestExitNode(w http.ResponseWriter, r *http.Request) {
+	if r.Method != "GET" {
+		http.Error(w, "only GET allowed", http.StatusMethodNotAllowed)
+		return
+	}
+	res, err := h.b.SuggestExitNode()
+	if err != nil {
+		writeErrorJSON(w, err)
+		return
+	}
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(res)
+}

wgengine/magicsock/magicsock.go

@@ -3063,3 +3063,17 @@ func getPeerMTUsProbedMetric(mtu tstun.WireMTU) *clientmetric.Metric {
 	mm, _ := metricRecvDiscoPeerMTUProbesByMTU.LoadOrInit(key, func() *clientmetric.Metric { return clientmetric.NewCounter(key) })
 	return mm
 }
+
+// GetLastNetcheckReport returns the last netcheck report, running a new one if a recent one does not exist.
+func (c *Conn) GetLastNetcheckReport(ctx context.Context) *netcheck.Report {
+	lastReport := c.lastNetCheckReport.Load()
+	if lastReport == nil {
+		nr, err := c.updateNetInfo(ctx)
+		if err != nil {
+			c.logf("magicsock.Conn.GetLastNetcheckReport: updateNetInfo: %v", err)
+			return nil
+		}
+		return nr
+	}
+	return lastReport
+}