add -v option

README.md

@@ -13,7 +13,7 @@ host, this is an [ancient
 technique](http://insecure.org/sploits/arp.games.html) known as [ARP
 spoofing](https://en.wikipedia.org/wiki/ARP_spoofing).
 
-## Get `arpfox`
+## Installing `arpfox`
 
 You can install arpfox to `/usr/local/bin` with the following command (requires
 admin privileges):
@@ -26,7 +26,7 @@ You can also grab the latest release from our [releases
 page](https://github.com/malfunkt/arpfox/releases) and install it on a
 different location.
 
-## Build it yourself
+### Building `arpfox` from source
 
 In order to build `arpfox` from source you'll need Go, a C compiler and
 libpcap's development files:

main.go

@@ -40,6 +40,8 @@ import (
 	"github.com/malfunkt/iprange"
 )
 
+const version = "1.0.0"
+
 func defaultInterface() string {
 	switch runtime.GOOS {
 	case "freebsd", "linux":
@@ -58,11 +60,17 @@ var (
 	flagListInterfaces = flag.Bool("l", false, `List available interfaces and exit.`)
 	flagWaitInterval   = flag.Float64("w", 2, `Wait <w> seconds between every broadcast, <w> must be a value greater than 0.1.`)
 	flagHelp           = flag.Bool("h", false, `Print usage instructions and exit.`)
+	flagVersion        = flag.Bool("v", false, `Print software version and exit.`)
 )
 
 func main() {
 	flag.Parse()
 
+	if *flagVersion {
+		fmt.Printf("%v\n", version)
+		os.Exit(0)
+	}
+
 	if *flagHelp {
 		fmt.Println("arpfox sends specially crafted ARP packets to a given host on a LAN in order")
 		fmt.Println("to poison its ARP cache table.")