Add very tentative UPnP NAT traversal

Relates to #230

client.go

@@ -303,6 +303,7 @@ func NewClient(cfg *Config) (cl *Client, err error) {
 	if err != nil {
 		return
 	}
+	go cl.forwardPort()
 	if cl.tcpListener != nil {
 		go cl.acceptConnections(cl.tcpListener, false)
 	}

client_test.go

@@ -33,10 +33,11 @@ import (
 
 func TestingConfig() *Config {
 	return &Config{
-		ListenAddr:      "localhost:0",
-		NoDHT:           true,
-		DataDir:         tempDir(),
-		DisableTrackers: true,
+		ListenAddr:              "localhost:0",
+		NoDHT:                   true,
+		DataDir:                 tempDir(),
+		DisableTrackers:         true,
+		NoDefaultPortForwarding: true,
 		// Debug:           true,
 	}
 }

config.go

@@ -33,7 +33,8 @@ type Config struct {
 	// The address to listen for new uTP and TCP bittorrent protocol
 	// connections. DHT shares a UDP socket with uTP unless configured
 	// otherwise.
-	ListenAddr string `long:"listen-addr" value-name:"HOST:PORT"`
+	ListenAddr              string `long:"listen-addr" value-name:"HOST:PORT"`
+	NoDefaultPortForwarding bool
 	// Don't announce to trackers. This only leaves DHT to discover peers.
 	DisableTrackers bool `long:"disable-trackers"`
 	DisablePEX      bool `long:"disable-pex"`

portfwd.go

@@ -0,0 +1,41 @@
+package torrent
+
+import (
+	"log"
+	"time"
+
+	flog "github.com/anacrolix/log"
+
+	"github.com/syncthing/syncthing/lib/nat"
+	"github.com/syncthing/syncthing/lib/upnp"
+)
+
+func addPortMapping(d nat.Device, proto nat.Protocol, internalPort int, debug bool) {
+	externalPort, err := d.AddPortMapping(proto, internalPort, internalPort, "anacrolix/torrent", 0)
+	if err != nil {
+		log.Printf("error adding %s port mapping: %s", proto, err)
+	} else if externalPort != internalPort {
+		log.Printf("external port %d does not match internal port %d in port mapping", externalPort, internalPort)
+	} else if debug {
+		log.Printf("forwarded external %s port %d", proto, externalPort)
+	}
+}
+
+func (cl *Client) forwardPort() {
+	cl.mu.Lock()
+	defer cl.mu.Unlock()
+	if cl.config.NoDefaultPortForwarding {
+		return
+	}
+	cl.mu.Unlock()
+	ds := upnp.Discover(0, 2*time.Second)
+	cl.mu.Lock()
+	flog.Default.Emit(flog.Fmsg("discovered %d upnp devices", len(ds)))
+	port := cl.incomingPeerPort()
+	cl.mu.Unlock()
+	for _, d := range ds {
+		go addPortMapping(d, nat.TCP, port, cl.config.Debug)
+		go addPortMapping(d, nat.UDP, port, cl.config.Debug)
+	}
+	cl.mu.Lock()
+}