fake IFF_NO_PI on macOS (thanks @eliezedeck ) (#21)

README.md

@@ -147,8 +147,8 @@ $ ping 10.1.0.20
 You'd see the ICMP packets printed out:
 
 ```
-2016/10/23 20:21:53 Interface Name: utun2
-2016/10/23 20:22:40 Packet Received: 00 00 00 02 45 00 00 54 4a 2e 00 00 40 01 1c 5c 0a 01 00 0a 0a 01 00 14 08 00 31 51 f0 f9 00 00 58 0d 7e 80 00 03 14 21 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37
+2017/03/20 21:17:30 Interface Name: utun2
+2017/03/20 21:17:40 Packet Received: 45 00 00 54 e9 1d 00 00 40 01 7d 6c 0a 01 00 0a 0a 01 00 14 08 00 ee 04 21 15 00 00 58 d0 a9 64 00 08 fb a5 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 14 15 16 17 18 19 1a 1b 1c 1d 1e 1f 20 21 22 23 24 25 26 27 28 29 2a 2b 2c 2d 2e 2f 30 31 32 33 34 35 36 37
 ```
 
 ### TAP on Windows:

syscalls_darwin.go

@@ -5,7 +5,9 @@ package water
 import (
 	"errors"
 	"fmt"
+	"io"
 	"os"
+	"sync"
 	"syscall"
 	"unsafe"
 )
@@ -114,10 +116,66 @@ func newTUN(string) (ifce *Interface, err error) {
 	return &Interface{
 		isTAP: false,
 		name:  string(ifName.name[:ifNameSize-1 /* -1 is for \0 */]),
-		ReadWriteCloser: os.NewFile(uintptr(fd), string(ifName.name[:])),
+		ReadWriteCloser: &tunReadCloser{
+			f: os.NewFile(uintptr(fd), string(ifName.name[:])),
+		},
 	}, nil
 }
 
+// tunReadCloser is a hack to work around the first 4 bytes "packet
+// information" because there doesn't seem to be an IFF_NO_PI for darwin.
+type tunReadCloser struct {
+	f io.ReadWriteCloser
+
+	rMu  sync.Mutex
+	rBuf []byte
+
+	wMu  sync.Mutex
+	wBuf []byte
+}
+
+var _ io.ReadWriteCloser = (*tunReadCloser)(nil)
+
+func (t *tunReadCloser) Read(to []byte) (int, error) {
+	t.rMu.Lock()
+	defer t.rMu.Unlock()
+
+	if cap(t.rBuf) < len(to)+4 {
+		t.rBuf = make([]byte, len(to)+4)
+	}
+	t.rBuf = t.rBuf[:len(to)+4]
+
+	n, err := t.f.Read(t.rBuf)
+	copy(to, t.rBuf[4:])
+	return n - 4, err
+}
+
+func (t *tunReadCloser) Write(from []byte) (int, error) {
+	t.wMu.Lock()
+	defer t.wMu.Unlock()
+
+	if cap(t.wBuf) < len(from)+4 {
+		t.wBuf = make([]byte, len(from)+4)
+	}
+	t.wBuf = t.wBuf[:len(from)+4]
+
+	t.wBuf[3] = 2 // Family: IP (2)
+	copy(t.wBuf[4:], from)
+
+	n, err := t.f.Write(t.wBuf)
+	return n - 4, err
+}
+
+func (t *tunReadCloser) Close() error {
+	// lock to make sure no read/write is in process.
+	t.rMu.Lock()
+	defer t.rMu.Unlock()
+	t.wMu.Lock()
+	defer t.wMu.Unlock()
+
+	return t.f.Close()
+}
+
 func newTAP(ifName string) (ifce *Interface, err error) {
 	return nil, errors.New("tap interface not implemented on this platform")
 }