Make Interface.Inside an interface type (#252)

This commit updates the Interface.Inside type to be a new interface
type instead of a *Tun. This will allow for an inside interface
that does not use a tun device, such as a single-binary client that
can run without elevated privileges.

tun_ios.go

@@ -30,13 +30,13 @@ func newTunFromFd(deviceFd int, cidr *net.IPNet, defaultMTU int, routes []route,
 	file := os.NewFile(uintptr(deviceFd), "/dev/tun")
 	ifce = &Tun{
 		Cidr:            cidr,
+		Device:          "iOS",
 		ReadWriteCloser: &tunReadCloser{f: file},
 	}
 	return
 }
 
 func (c *Tun) Activate() error {
-	c.Device = "iOS"
 	return nil
 }
 
@@ -103,3 +103,11 @@ func (t *tunReadCloser) Write(from []byte) (int, error) {
 func (t *tunReadCloser) Close() error {
 	return t.f.Close()
 }
+
+func (c *Tun) CidrNet() *net.IPNet {
+	return c.Cidr
+}
+
+func (c *Tun) DeviceName() string {
+	return c.Device
+}