diff --git a/.travis.yml b/.travis.yml
index 45eabe8..95a78f1 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,5 @@
 language: go
-go:
-  - "1.12.1"
-  - "1.10.8"
+
 go_import_path: github.com/songgao/water
 install: go get -u golang.org/x/lint/golint
 script: make ci
@@ -10,4 +8,11 @@ matrix:
   include:
     - os: linux
       dist: xenial
+      go: 1.12.1
+    - os: linux
+      dist: xenial
+      go: 1.10.8
+    - os: osx
+      go: 1.12.1
     - os: osx
+      go: 1.10.8
diff --git a/README.md b/README.md
index 00969a5..0c797b5 100644
--- a/README.md
+++ b/README.md
@@ -1,16 +1,9 @@
-# water
+[![Build Status](https://travis-ci.org/songgao/water.svg?branch=master)](https://travis-ci.org/songgao/water)
+[![GoDoc](https://godoc.org/github.com/songgao/water?status.svg)](https://godoc.org/github.com/songgao/water)
 
-`water` is a native Go library for [TUN/TAP](http://en.wikipedia.org/wiki/TUN/TAP) interfaces.
-
-`water` is designed to be simple and efficient. It
-
-* wraps almost only syscalls and uses only Go standard types;
-* exposes standard interfaces; plays well with standard packages like `io`, `bufio`, etc..
-* does not handle memory management (allocating/destructing slice). It's up to user to decide whether/how to reuse buffers.
-
-~~`water/waterutil` has some useful functions to interpret MAC frame headers and IP packet headers. It also contains some constants such as protocol numbers and ethernet frame types.~~
-
-See https://github.com/songgao/packets for functions for parsing various packets.
+`water` is a pure Go package for working with
+[TUN/TAP](http://en.wikipedia.org/wiki/TUN/TAP) interfaces. It works well with
+Go's standard library and has no external dependency.
 
 ## Supported Platforms
 
@@ -19,15 +12,18 @@ See https://github.com/songgao/packets for functions for parsing various packets
 * macOS (point-to-point TUN only)
 
 ## Installation
+
 ```
 go get -u github.com/songgao/water
-go get -u github.com/songgao/water/waterutil
 ```
 
 ## Documentation
-[http://godoc.org/github.com/songgao/water](http://godoc.org/github.com/songgao/water)
 
-## Example
+* [Linux](http://godoc.org/github.com/songgao/water?GOOS=linux)
+* [macOS](http://godoc.org/github.com/songgao/water?GOOS=darwin)
+* [Windows](http://godoc.org/github.com/songgao/water?GOOS=windows)
+
+## Getting Started
 
 ### TAP on Linux:
 
@@ -234,9 +230,3 @@ If you are going to use multiple TAP devices on the Windows, there is a way to s
 		},
 	})
 ```
-
-## TODO
-* tuntaposx for TAP on Darwin
-
-## Alternatives
-`tuntap`: [https://code.google.com/p/tuntap/](https://code.google.com/p/tuntap/)
diff --git a/doc.go b/doc.go
index 287412b..8992a24 100644
--- a/doc.go
+++ b/doc.go
@@ -1,4 +1,11 @@
-// Package water is a simple TUN/TAP interface library that efficiently works
-// with standard packages like io, bufio, etc.. Use waterutil with it to work
-// with TUN/TAP packets/frames.
+// Package water encapsulates system calls for working with TUN/TAP interfaces,
+// in pure Go.
+//
+// Platform specific docs:
+//
+// Linux: https://godoc.org/github.com/songgao/water?GOOS=lnux
+//
+// macOS: https://godoc.org/github.com/songgao/water?GOOS=darwin
+//
+// Windows: https://godoc.org/github.com/songgao/water?GOOS=windows
 package water
diff --git a/if.go b/if.go
index 4023a1a..b7def99 100644
--- a/if.go
+++ b/if.go
@@ -5,23 +5,36 @@ import (
 	"io"
 )
 
-// Interface is a TUN/TAP interface.
-//
-// MultiQueue(Linux kernel > 3.8): With MultiQueue enabled, user should hold multiple
-// interfaces to send/receive packet in parallel.
-// Kernel document about MultiQueue: https://www.kernel.org/doc/Documentation/networking/tuntap.txt
-type Interface struct {
-	isTAP bool
+// Interface represents a TUN/TAP interface.
+type Interface interface {
+	// Use Read() and Write() methods to read from and write into this TUN/TAP
+	// interface. In TAP interface, each call corresponds to an Ethernet frame.
+	// In TUN mode, each call corresponds to an IP packet.
 	io.ReadWriteCloser
-	name string
+
+	// IsTUN returns true if ifce is a TUN interface.
+	IsTUN() bool
+	// IsTAP returns true if ifce is a TAP interface.
+	IsTAP() bool
+	// DeviceType returns the interface's device type.
+	Type() DeviceType
+	// Name returns the name of the interface.
+	Name() string
+	// Sys returns the underlying system interface for the interface. This is
+	// useful if caller needs to perform system calls directly on the tun/tap
+	// device.
+	//
+	// On Unix systems, this returns a file descriptor of uintptr type. On
+	// Windows, this returns a syscall.Handle.
+	Sys() interface{}
 }
 
 // DeviceType is the type for specifying device types.
 type DeviceType int
 
-// TUN and TAP device types.
+// Constants for TUN and TAP interfaces.
 const (
-	_ = iota
+	_ DeviceType = iota
 	TUN
 	TAP
 )
@@ -49,7 +62,7 @@ func defaultConfig() Config {
 var zeroConfig Config
 
 // New creates a new TUN/TAP interface using config.
-func New(config Config) (ifce *Interface, err error) {
+func New(config Config) (ifce Interface, err error) {
 	if zeroConfig == config {
 		config = defaultConfig()
 	}
@@ -63,18 +76,3 @@ func New(config Config) (ifce *Interface, err error) {
 		return nil, errors.New("unknown device type")
 	}
 }
-
-// IsTUN returns true if ifce is a TUN interface.
-func (ifce *Interface) IsTUN() bool {
-	return !ifce.isTAP
-}
-
-// IsTAP returns true if ifce is a TAP interface.
-func (ifce *Interface) IsTAP() bool {
-	return ifce.isTAP
-}
-
-// Name returns the interface name of ifce, e.g. tun0, tap1, tun0, etc..
-func (ifce *Interface) Name() string {
-	return ifce.name
-}
diff --git a/if_linux.go b/if_linux.go
index 227e8a8..e6463ea 100644
--- a/if_linux.go
+++ b/if_linux.go
@@ -10,7 +10,7 @@ import (
 // ifName cannot be specified on windows, you will need ifce.Name() to use some cmds.
 //
 // Deprecated: This function may be removed in the future. Please use New() instead.
-func NewTAP(ifName string) (ifce *Interface, err error) {
+func NewTAP(ifName string) (ifce Interface, err error) {
 	fmt.Println("Deprecated: NewTAP(..) may be removed in the future. Please use New() instead.")
 	config := Config{DeviceType: TAP}
 	config.Name = ifName
@@ -22,7 +22,7 @@ func NewTAP(ifName string) (ifce *Interface, err error) {
 // ifName cannot be specified on windows, you will need ifce.Name() to use some cmds.
 //
 // Deprecated: This function will be removed in the future. Please use New() instead.
-func NewTUN(ifName string) (ifce *Interface, err error) {
+func NewTUN(ifName string) (ifce Interface, err error) {
 	fmt.Println("Deprecated: NewTUN(..) may be removed in the future. Please use New() instead.")
 	config := Config{DeviceType: TUN}
 	config.Name = ifName
diff --git a/if_nix.go b/if_nix.go
new file mode 100644
index 0000000..f6bdbde
--- /dev/null
+++ b/if_nix.go
@@ -0,0 +1,32 @@
+// +build linux darwin
+
+package water
+
+import "io"
+
+type ifce struct {
+	deviceType DeviceType
+	fd         uintptr
+	name       string
+	io.ReadWriteCloser
+}
+
+func (i *ifce) IsTUN() bool {
+	return i.deviceType == TUN
+}
+
+func (i *ifce) IsTAP() bool {
+	return i.deviceType == TAP
+}
+
+func (i *ifce) Type() DeviceType {
+	return i.deviceType
+}
+
+func (i *ifce) Name() string {
+	return i.name
+}
+
+func (i *ifce) Sys() interface{} {
+	return i.fd
+}
diff --git a/ipv4_darwin_test.go b/ipv4_darwin_test.go
index 2a5d646..685b6d5 100644
--- a/ipv4_darwin_test.go
+++ b/ipv4_darwin_test.go
@@ -22,7 +22,7 @@ func setupIfce(t *testing.T, self net.IP, remote net.IP, dev string) {
 	}
 }
 
-func teardownIfce(t *testing.T, ifce *Interface) {
+func teardownIfce(t *testing.T, ifce Interface) {
 	if err := ifce.Close(); err != nil {
 		t.Fatal(err)
 	}
diff --git a/ipv4_linux_test.go b/ipv4_linux_test.go
index 6544809..dd804fc 100644
--- a/ipv4_linux_test.go
+++ b/ipv4_linux_test.go
@@ -25,7 +25,7 @@ func setupIfce(t *testing.T, ipNet net.IPNet, dev string) {
 	}
 }
 
-func teardownIfce(t *testing.T, ifce *Interface) {
+func teardownIfce(t *testing.T, ifce Interface) {
 	if err := ifce.Close(); err != nil {
 		t.Fatal(err)
 	}
diff --git a/ipv4_test.go b/ipv4_test.go
index 7a8bf1e..3ecfd6b 100644
--- a/ipv4_test.go
+++ b/ipv4_test.go
@@ -10,7 +10,7 @@ import (
 
 const BUFFERSIZE = 1522
 
-func startRead(t *testing.T, ifce *Interface) (dataChan <-chan []byte, errChan <-chan error) {
+func startRead(t *testing.T, ifce Interface) (dataChan <-chan []byte, errChan <-chan error) {
 	dataCh := make(chan []byte)
 	errCh := make(chan error)
 	go func() {
diff --git a/ipv4_windows_test.go b/ipv4_windows_test.go
index e45f174..da0e3c5 100644
--- a/ipv4_windows_test.go
+++ b/ipv4_windows_test.go
@@ -26,7 +26,7 @@ func setupIfce(t *testing.T, ipNet net.IPNet, dev string) {
 	}
 }
 
-func teardownIfce(t *testing.T, ifce *Interface) {
+func teardownIfce(t *testing.T, ifce Interface) {
 	if err := ifce.Close(); err != nil {
 		t.Fatal(err)
 	}
diff --git a/params_linux.go b/params_linux.go
index 34e9898..e1b24b0 100644
--- a/params_linux.go
+++ b/params_linux.go
@@ -37,6 +37,11 @@ type PlatformSpecificParams struct {
 	// interface.  From version 3.8, Linux supports multiqueue tuntap which can
 	// uses multiple file descriptors (queues) to parallelize packets sending
 	// or receiving.
+	//
+	// MultiQueue (Linux kernel > 3.8): With MultiQueue enabled, user should
+	// hold multiple interfaces to send/receive packet in parallel.  Kernel
+	// document about MultiQueue:
+	// https://www.kernel.org/doc/Documentation/networking/tuntap.txt
 	MultiQueue bool
 }
 
diff --git a/syscalls_darwin.go b/syscalls_darwin.go
index b344034..6343627 100644
--- a/syscalls_darwin.go
+++ b/syscalls_darwin.go
@@ -59,11 +59,12 @@ type sockaddrCtl struct {
 
 var sockaddrCtlSize uintptr = 32
 
-func openDev(config Config) (ifce *Interface, err error) {
+func openDev(config Config) (Interface, error) {
 	if config.DeviceType != TUN {
 		return nil, errors.New("only tun is implemented on this platform")
 	}
 	var fd int
+	var err error
 	// Supposed to be socket(PF_SYSTEM, SOCK_DGRAM, SYSPROTO_CONTROL), but ...
 	//
 	// In sys/socket.h:
@@ -118,9 +119,10 @@ func openDev(config Config) (ifce *Interface, err error) {
 		return nil, fmt.Errorf("setting non-blocking error")
 	}
 
-	return &Interface{
-		isTAP: false,
-		name:  string(ifName.name[:ifNameSize-1 /* -1 is for \0 */]),
+	return &ifce{
+		deviceType: config.DeviceType,
+		fd:         uintptr(fd),
+		name:       string(ifName.name[:ifNameSize-1 /* -1 is for \0 */]),
 		ReadWriteCloser: &tunReadCloser{
 			f: os.NewFile(uintptr(fd), string(ifName.name[:])),
 		},
diff --git a/syscalls_linux_go1.11.go b/syscalls_linux_go1.11.go
index 90c47a1..0443616 100644
--- a/syscalls_linux_go1.11.go
+++ b/syscalls_linux_go1.11.go
@@ -7,8 +7,9 @@ import (
 	"syscall"
 )
 
-func openDev(config Config) (ifce *Interface, err error) {
+func openDev(config Config) (Interface, error) {
 	var fdInt int
+	var err error
 	if fdInt, err = syscall.Open(
 		"/dev/net/tun", os.O_RDWR|syscall.O_NONBLOCK, 0); err != nil {
 		return nil, err
@@ -19,9 +20,10 @@ func openDev(config Config) (ifce *Interface, err error) {
 		return nil, err
 	}
 
-	return &Interface{
-		isTAP:           config.DeviceType == TAP,
-		ReadWriteCloser: os.NewFile(uintptr(fdInt), "tun"),
+	return &ifce{
+		deviceType:      config.DeviceType,
+		fd:              uintptr(fdInt),
 		name:            name,
+		ReadWriteCloser: os.NewFile(uintptr(fdInt), "tun"),
 	}, nil
 }
diff --git a/syscalls_linux_legacy.go b/syscalls_linux_legacy.go
index 6499b37..12d9bf9 100644
--- a/syscalls_linux_legacy.go
+++ b/syscalls_linux_legacy.go
@@ -6,8 +6,9 @@ import (
 	"os"
 )
 
-func openDev(config Config) (ifce *Interface, err error) {
+func openDev(config Config) (Interface, error) {
 	var file *os.File
+	var err error
 	if file, err = os.OpenFile(
 		"/dev/net/tun", os.O_RDWR, 0); err != nil {
 		return nil, err
@@ -18,9 +19,10 @@ func openDev(config Config) (ifce *Interface, err error) {
 		return nil, err
 	}
 
-	return &Interface{
-		isTAP:           config.DeviceType == TAP,
-		ReadWriteCloser: file,
+	return &ifce{
+		deviceType:      config.DeviceType,
+		fd:              file.Fd(),
 		name:            name,
+		ReadWriteCloser: file,
 	}, nil
 }
diff --git a/syscalls_windows.go b/syscalls_windows.go
index d495262..6189fe7 100644
--- a/syscalls_windows.go
+++ b/syscalls_windows.go
@@ -94,11 +94,29 @@ func newOverlapped() (*syscall.Overlapped, error) {
 }
 
 type wfile struct {
-	fd syscall.Handle
-	rl sync.Mutex
-	wl sync.Mutex
-	ro *syscall.Overlapped
-	wo *syscall.Overlapped
+	fd         syscall.Handle
+	rl         sync.Mutex
+	wl         sync.Mutex
+	ro         *syscall.Overlapped
+	wo         *syscall.Overlapped
+	name       string
+	deviceType DeviceType
+}
+
+func (f *wfile) Name() string {
+	return f.name
+}
+
+func (f *wfile) IsTAP() bool {
+	return f.deviceType == TAP
+}
+
+func (f *wfile) IsTUN() bool {
+	return f.deviceType == TUN
+}
+
+func (f *wfile) Type() DeviceType {
+	return f.deviceType
 }
 
 func (f *wfile) Close() error {
@@ -135,6 +153,10 @@ func (f *wfile) Read(b []byte) (int, error) {
 	return getOverlappedResult(f.fd, f.ro)
 }
 
+func (f *wfile) Sys() interface{} {
+	return f.fd
+}
+
 func ctl_code(device_type, function, method, access uint32) uint32 {
 	return (device_type << 16) | (access << 14) | (function << 2) | method
 }
@@ -233,7 +255,7 @@ func setTUN(fd syscall.Handle, network string) error {
 }
 
 // openDev find and open an interface.
-func openDev(config Config) (ifce *Interface, err error) {
+func openDev(config Config) (Interface, error) {
 	// find the device in registry.
 	deviceid, err := getdeviceid(config.PlatformSpecificParams.ComponentID, config.PlatformSpecificParams.InterfaceName)
 	if err != nil {
@@ -270,14 +292,18 @@ func openDev(config Config) (ifce *Interface, err error) {
 	// fd := os.NewFile(uintptr(file), path)
 	ro, err := newOverlapped()
 	if err != nil {
-		return
+		return nil, err
 	}
 	wo, err := newOverlapped()
 	if err != nil {
-		return
+		return nil, err
+	}
+	fd := &wfile{
+		fd:         file,
+		ro:         ro,
+		wo:         wo,
+		deviceType: config.DeviceType,
 	}
-	fd := &wfile{fd: file, ro: ro, wo: wo}
-	ifce = &Interface{isTAP: (config.DeviceType == TAP), ReadWriteCloser: fd}
 
 	// bring up device.
 	if err := setStatus(file, true); err != nil {
@@ -294,13 +320,13 @@ func openDev(config Config) (ifce *Interface, err error) {
 	// find the name of tap interface(u need it to set the ip or other command)
 	ifces, err := net.Interfaces()
 	if err != nil {
-		return
+		return nil, err
 	}
 
 	for _, v := range ifces {
 		if bytes.Equal(v.HardwareAddr[:6], mac[:6]) {
-			ifce.name = v.Name
-			return
+			fd.name = v.Name
+			return fd, nil
 		}
 	}