Add atping: a tcp ping tool

cmd/atping/README

@@ -0,0 +1,8 @@
+atping: automated tcping
+Usage of atping:
+  -6	require IPv6
+  -p port
+    	port to connect to instead of 80
+  -t timeout
+    	connect deadline (default 3s)
+  -v	verbose mode: print server and protocol when connecting

cmd/atping/main.go

@@ -0,0 +1,75 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"net"
+	"os"
+	"time"
+)
+
+const (
+	defaultServer = "google.com"
+	defaultPort   = "80"
+)
+
+var verbose bool
+
+func connect(addr string, dport string, six bool, timeout time.Duration) error {
+	_, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		addr = net.JoinHostPort(addr, dport)
+	}
+
+	proto := "tcp"
+	if six {
+		proto += "6"
+	}
+
+	if verbose {
+		fmt.Printf("connecting to %s/%s... ", addr, proto)
+		os.Stdout.Sync()
+	}
+
+	conn, err := net.DialTimeout(proto, addr, timeout)
+	if err != nil {
+		if verbose {
+			fmt.Println("failed.")
+		}
+		return err
+	}
+
+	if verbose {
+		fmt.Println("OK")
+	}
+	conn.Close()
+	return nil
+}
+
+func main() {
+	var (
+		port    string
+		timeout time.Duration
+		six     bool
+	)
+
+	flag.BoolVar(&six, "6", false, "require IPv6")
+	flag.StringVar(&port, "p", defaultPort, "`port` to connect to instead of "+defaultPort)
+	flag.DurationVar(&timeout, "t", 3*time.Second, "`timeout`")
+	flag.BoolVar(&verbose, "v", false, "verbose mode: print server and protocol when connecting")
+	flag.Parse()
+
+	var servers []string
+	if flag.NArg() == 0 {
+		servers = []string{defaultServer}
+	} else {
+		servers = flag.Args()
+	}
+
+	for _, server := range servers {
+		err := connect(server, port, six, timeout)
+		if err != nil {
+			os.Exit(1)
+		}
+	}
+}