added README.md

README.md

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+# water
+`water` is a native Go library for `TUN`/`TAP` interfaces. It's designed to be simple and scalable. `water` wraps almost only syscalls and uses only Go standard types, so it plays well with standard packages like `io`, `bufio`, etc.
+
+`water/waterutil` has some useful functions to interpret MAC farme headers and IP packet headers.
+
+## 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
+
+```go
+package main
+
+import (
+	"github.com/songgao/water"
+	"github.com/songgao/water/waterutil"
+	"fmt"
+)
+
+const BUFFERSIZE = 1522
+
+func main() {
+	ifce, err := water.NewTAP("")
+	fmt.Printf("%v, %v\n\n", err, ifce)
+	buffer := make([]byte, BUFFERSIZE)
+	for {
+		_, err = ifce.Read(buffer)
+		if err != nil {
+			break
+		}
+		ethertype := waterutil.MACEthertype(buffer)
+		if ethertype == waterutil.IPv4 {
+			packet := waterutil.MACPayload(buffer)
+			if waterutil.IsIPv4(packet) {
+				fmt.Printf("Source:      %v [%v]\n", waterutil.MACSource(buffer), waterutil.IPv4Source(packet))
+				fmt.Printf("Destination: %v [%v]\n", waterutil.MACDestination(buffer), waterutil.IPv4Destination(packet))
+				fmt.Printf("Protocol:    %v\n\n", waterutil.IPv4Protocol(packet))
+			}
+		}
+	}
+}
+```
+
+This piece of code creates a `TAP` interface, and prints some header information for every IPv4 packet. After pull up the `main.go`, you'll need to bring up the interface and assign IP address. All of these need root permission.
+
+```bash
+sudo go run main.go
+```
+
+```bash
+sudo ip link set dev tap0 up
+sudo ip addr add 10.0.0.1/24 dev tap0
+```
+
+Now, try sending some ICMP broadcast message:
+```bash
+ping -b 10.0.0.255
+```
+
+You'll see the `main.go` print something like:
+```
+<nil>, &{true 0xf84003f058 tap0}
+
+Source:      42:35:da:af:2b:00 [10.10.10.1]
+Destination: ff:ff:ff:ff:ff:ff [10.10.10.255]
+Protocol:    1
+
+Source:      42:35:da:af:2b:00 [10.10.10.1]
+Destination: ff:ff:ff:ff:ff:ff [10.10.10.255]
+Protocol:    1
+
+Source:      42:35:da:af:2b:00 [10.10.10.1]
+Destination: ff:ff:ff:ff:ff:ff [10.10.10.255]
+Protocol:    1
+```