try to pick the best NAT when multiple NATs are discovered

nat.go

@@ -8,6 +8,8 @@ import (
 	"math/rand"
 	"net"
 	"time"
+
+	"github.com/jackpal/gateway"
 )
 
 var ErrNoExternalAddress = errors.New("no external address")
@@ -88,11 +90,37 @@ func DiscoverNATs(ctx context.Context) <-chan NAT {
 func DiscoverGateway() (NAT, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()
-	nat := <-DiscoverNATs(ctx)
-	if nat == nil {
+
+	var nats []NAT
+	for nat := range DiscoverNATs(ctx) {
+		nats = append(nats, nat)
+	}
+	switch len(nats) {
+	case 0:
 		return nil, ErrNoNATFound
+	case 1:
+		return nats[0], nil
+	}
+	gw, _ := gateway.DiscoverGateway()
+	bestNAT := nats[0]
+	natGw, _ := bestNAT.GetDeviceAddress()
+	bestNATIsGw := gw != nil && natGw.Equal(gw)
+	// 1. Prefer gateways discovered _last_. This is an OK heuristic for
+	// discovering the most-upstream (furthest) NAT.
+	// 2. Prefer gateways that actually match our known gateway address.
+	// Some relays like to claim to be NATs even if they aren't.
+	for _, nat := range nats[1:] {
+		natGw, _ := nat.GetDeviceAddress()
+		natIsGw := gw != nil && natGw.Equal(gw)
+
+		if bestNATIsGw && !natIsGw {
+			continue
+		}
+
+		bestNATIsGw = natIsGw
+		bestNAT = nat
 	}
-	return nat, nil
+	return bestNAT, nil
 }
 
 func randomPort() int {