p2p/nat: new package for port mapping stuff

I have verified that UPnP and NAT-PMP work against an older version of
the MiniUPnP daemon running on pfSense. This code is kind of hard to
test automatically.

p2p/nat/natpmp.go

@@ -0,0 +1,115 @@
+package nat
+
+import (
+	"fmt"
+	"net"
+	"strings"
+	"time"
+
+	"github.com/jackpal/go-nat-pmp"
+)
+
+// natPMPClient adapts the NAT-PMP protocol implementation so it conforms to
+// the common interface.
+type pmp struct {
+	gw net.IP
+	c  *natpmp.Client
+}
+
+func (n *pmp) String() string {
+	return fmt.Sprintf("NAT-PMP(%v)", n.gw)
+}
+
+func (n *pmp) ExternalIP() (net.IP, error) {
+	response, err := n.c.GetExternalAddress()
+	if err != nil {
+		return nil, err
+	}
+	return response.ExternalIPAddress[:], nil
+}
+
+func (n *pmp) AddMapping(protocol string, extport, intport int, name string, lifetime time.Duration) error {
+	if lifetime <= 0 {
+		return fmt.Errorf("lifetime must not be <= 0")
+	}
+	// Note order of port arguments is switched between our
+	// AddMapping and the client's AddPortMapping.
+	_, err := n.c.AddPortMapping(strings.ToLower(protocol), intport, extport, int(lifetime/time.Second))
+	return err
+}
+
+func (n *pmp) DeleteMapping(protocol string, extport, intport int) (err error) {
+	// To destroy a mapping, send an add-port with an internalPort of
+	// the internal port to destroy, an external port of zero and a
+	// time of zero.
+	_, err = n.c.AddPortMapping(strings.ToLower(protocol), intport, 0, 0)
+	return err
+}
+
+func discoverPMP() Interface {
+	// run external address lookups on all potential gateways
+	gws := potentialGateways()
+	found := make(chan *pmp, len(gws))
+	for i := range gws {
+		gw := gws[i]
+		go func() {
+			c := natpmp.NewClient(gw)
+			if _, err := c.GetExternalAddress(); err != nil {
+				found <- nil
+			} else {
+				found <- &pmp{gw, c}
+			}
+		}()
+	}
+	// return the one that responds first.
+	// discovery needs to be quick, so we stop caring about
+	// any responses after a very short timeout.
+	timeout := time.NewTimer(1 * time.Second)
+	defer timeout.Stop()
+	for _ = range gws {
+		select {
+		case c := <-found:
+			if c != nil {
+				return c
+			}
+		case <-timeout.C:
+			return nil
+		}
+	}
+	return nil
+}
+
+var (
+	// LAN IP ranges
+	_, lan10, _  = net.ParseCIDR("10.0.0.0/8")
+	_, lan176, _ = net.ParseCIDR("172.16.0.0/12")
+	_, lan192, _ = net.ParseCIDR("192.168.0.0/16")
+)
+
+// TODO: improve this. We currently assume that (on most networks)
+// the router is X.X.X.1 in a local LAN range.
+func potentialGateways() (gws []net.IP) {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return nil
+	}
+	for _, iface := range ifaces {
+		ifaddrs, err := iface.Addrs()
+		if err != nil {
+			return gws
+		}
+		for _, addr := range ifaddrs {
+			switch x := addr.(type) {
+			case *net.IPNet:
+				if lan10.Contains(x.IP) || lan176.Contains(x.IP) || lan192.Contains(x.IP) {
+					ip := x.IP.Mask(x.Mask).To4()
+					if ip != nil {
+						ip[3] = ip[3] | 0x01
+						gws = append(gws, ip)
+					}
+				}
+			}
+		}
+	}
+	return gws
+}