Fix up the tun for android (#249)

tun_android.go

@@ -0,0 +1,69 @@
+package nebula
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+type Tun struct {
+	io.ReadWriteCloser
+	fd           int
+	Device       string
+	Cidr         *net.IPNet
+	MaxMTU       int
+	DefaultMTU   int
+	TXQueueLen   int
+	Routes       []route
+	UnsafeRoutes []route
+}
+
+func newTunFromFd(deviceFd int, cidr *net.IPNet, defaultMTU int, routes []route, unsafeRoutes []route, txQueueLen int) (ifce *Tun, err error) {
+	file := os.NewFile(uintptr(deviceFd), "/dev/net/tun")
+
+	ifce = &Tun{
+		ReadWriteCloser: file,
+		fd:              int(file.Fd()),
+		Device:          "tun0",
+		Cidr:            cidr,
+		DefaultMTU:      defaultMTU,
+		TXQueueLen:      txQueueLen,
+		Routes:          routes,
+		UnsafeRoutes:    unsafeRoutes,
+	}
+	return
+}
+
+func newTun(deviceName string, cidr *net.IPNet, defaultMTU int, routes []route, unsafeRoutes []route, txQueueLen int) (ifce *Tun, err error) {
+	return nil, fmt.Errorf("newTun not supported in Android")
+}
+
+func (c *Tun) WriteRaw(b []byte) error {
+	var nn int
+	for {
+		max := len(b)
+		n, err := unix.Write(c.fd, b[nn:max])
+		if n > 0 {
+			nn += n
+		}
+		if nn == len(b) {
+			return err
+		}
+
+		if err != nil {
+			return err
+		}
+
+		if n == 0 {
+			return io.ErrUnexpectedEOF
+		}
+	}
+}
+
+func (c Tun) Activate() error {
+	c.Device = "android"
+	return nil
+}

tun_linux.go

@@ -1,3 +1,5 @@
+// +build !android
+
 package nebula
 
 import (