use the next available port for wireguard (#2024)

check if WgPort is available, if not find the next free port

client/internal/connect.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"net"
 	"runtime"
 	"runtime/debug"
 	"strings"
@@ -330,6 +331,15 @@ func createEngineConfig(key wgtypes.Key, config *Config, peerConfig *mgmProto.Pe
 		engineConf.PreSharedKey = &preSharedKey
 	}
 
+	port, err := freePort(config.WgPort)
+	if err != nil {
+		return nil, err
+	}
+	if port != config.WgPort {
+		log.Infof("using %d as wireguard port: %d is in use", port, config.WgPort)
+	}
+	engineConf.WgPort = port
+
 	return engineConf, nil
 }
 
@@ -379,3 +389,20 @@ func statusRecorderToSignalConnStateNotifier(statusRecorder *peer.Status) signal
 	notifier, _ := sri.(signal.ConnStateNotifier)
 	return notifier
 }
+
+func freePort(start int) (int, error) {
+	addr := net.UDPAddr{}
+	if start == 0 {
+		start = iface.DefaultWgPort
+	}
+	for x := start; x <= 65535; x++ {
+		addr.Port = x
+		conn, err := net.ListenUDP("udp", &addr)
+		if err != nil {
+			continue
+		}
+		conn.Close()
+		return x, nil
+	}
+	return 0, errors.New("no free ports")
+}

client/internal/connect_test.go

@@ -0,0 +1,57 @@
+package internal
+
+import (
+	"net"
+	"testing"
+)
+
+func Test_freePort(t *testing.T) {
+	tests := []struct {
+		name    string
+		port    int
+		want    int
+		wantErr bool
+	}{
+		{
+			name:    "available",
+			port:    51820,
+			want:    51820,
+			wantErr: false,
+		},
+		{
+			name:    "notavailable",
+			port:    51830,
+			want:    51831,
+			wantErr: false,
+		},
+		{
+			name:    "noports",
+			port:    65535,
+			want:    0,
+			wantErr: true,
+		},
+	}
+	for _, tt := range tests {
+
+		c1, err := net.ListenUDP("udp", &net.UDPAddr{Port: 51830})
+		if err != nil {
+			t.Errorf("freePort error = %v", err)
+		}
+		c2, err := net.ListenUDP("udp", &net.UDPAddr{Port: 65535})
+		if err != nil {
+			t.Errorf("freePort error = %v", err)
+		}
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := freePort(tt.port)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("freePort() error = %v, wantErr %v", err, tt.wantErr)
+				return
+			}
+			if got != tt.want {
+				t.Errorf("freePort() = %v, want %v", got, tt.want)
+			}
+		})
+		c1.Close()
+		c2.Close()
+	}
+}