nat: introduce new NAT traversal package with UPnP/NAT-PMP implementations

In this commit, we introduce a new package `nat`. This package is
responsible for handling the different techniques for NAT traversals.
Specifically, we have implemented UPnP and NAT-PMP support. This will
allow users to easily advertise their nodes to the network as long as
their devices are behind a single NAT. Devices behind multiple NATs are
not supported.

nat/traversal.go

@@ -0,0 +1,58 @@
+package nat
+
+import (
+	"errors"
+	"net"
+)
+
+var (
+	// private24BitBlock contains the set of private IPv4 addresses within
+	// the 10.0.0.0/8 adddress space.
+	private24BitBlock *net.IPNet
+
+	// private20BitBlock contains the set of private IPv4 addresses within
+	// the 172.16.0.0/12 address space.
+	private20BitBlock *net.IPNet
+
+	// private16BitBlock contains the set of private IPv4 addresses within
+	// the 192.168.0.0/16 address space.
+	private16BitBlock *net.IPNet
+
+	// ErrMultipleNAT is an error returned when multiple NATs have been
+	// detected.
+	ErrMultipleNAT = errors.New("multiple NATs detected")
+)
+
+func init() {
+	_, private24BitBlock, _ = net.ParseCIDR("10.0.0.0/8")
+	_, private20BitBlock, _ = net.ParseCIDR("172.16.0.0/12")
+	_, private16BitBlock, _ = net.ParseCIDR("192.168.0.0/16")
+}
+
+// Traversal is an interface that brings together the different NAT traversal
+// techniques.
+type Traversal interface {
+	// ExternalIP returns the external IP address.
+	ExternalIP() (net.IP, error)
+
+	// AddPortMapping adds a port mapping for the given port between the
+	// private and public addresses.
+	AddPortMapping(port uint16) error
+
+	// DeletePortMapping deletes a port mapping for the given port between
+	// the private and public addresses.
+	DeletePortMapping(port uint16) error
+
+	// ForwardedPorts returns the ports currently being forwarded using NAT
+	// traversal.
+	ForwardedPorts() []uint16
+
+	// Name returns the name of the specific NAT traversal technique used.
+	Name() string
+}
+
+// isPrivateIP determines if the IP is private.
+func isPrivateIP(ip net.IP) bool {
+	return private24BitBlock.Contains(ip) ||
+		private20BitBlock.Contains(ip) || private16BitBlock.Contains(ip)
+}