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

syscalls_darwin.go

@@ -5,7 +5,9 @@ package water
 import (
 	"errors"
 	"fmt"
+	"io"
 	"os"
+	"sync"
 	"syscall"
 	"unsafe"
 )
@@ -112,12 +114,68 @@ 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[:])),
+		isTAP: false,
+		name:  string(ifName.name[:ifNameSize-1 /* -1 is for \0 */]),
+		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")
 }