feat(runner): add reachable dest addr check

internal/runner/errors.go

@@ -0,0 +1,5 @@
+package runner
+
+import "errors"
+
+var errDestUnreachable = errors.New("destination address unreachable")

internal/runner/runner.go

@@ -15,6 +15,11 @@ import (
 )
 
 func New(opt *common.Options) error {
+	reachable := isReachable(opt.Destination, 5*time.Second)
+	if !reachable {
+		return errDestUnreachable
+	}
+
 	tun, err := tunnel.NewTunnel(opt.Port, opt.Destination, opt.Config.Path, opt.Config.Format)
 	if err != nil {
 		return err

internal/runner/utils.go

@@ -0,0 +1,17 @@
+package runner
+
+import (
+	"net"
+	"time"
+)
+
+func isReachable(dest string, timeout time.Duration) bool {
+	conn, err := net.DialTimeout("tcp", dest, timeout)
+	if err != nil {
+		return false
+	}
+
+	defer conn.Close()
+
+	return true
+}