[client] Refactor/iface pkg (#2646)

Refactor the flat code structure

client/iface/netstack/env.go

@@ -0,0 +1,33 @@
+package netstack
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+
+	log "github.com/sirupsen/logrus"
+)
+
+// IsEnabled todo: move these function to cmd layer
+func IsEnabled() bool {
+	return os.Getenv("NB_USE_NETSTACK_MODE") == "true"
+}
+
+func ListenAddr() string {
+	sPort := os.Getenv("NB_SOCKS5_LISTENER_PORT")
+	port, err := strconv.Atoi(sPort)
+	if err != nil {
+		log.Warnf("invalid socks5 listener port, unable to convert it to int, falling back to default: %d", DefaultSocks5Port)
+		return listenAddr(DefaultSocks5Port)
+	}
+	if port < 1 || port > 65535 {
+		log.Warnf("invalid socks5 listener port, it should be in the range 1-65535, falling back to default: %d", DefaultSocks5Port)
+		return listenAddr(DefaultSocks5Port)
+	}
+
+	return listenAddr(port)
+}
+
+func listenAddr(port int) string {
+	return fmt.Sprintf("0.0.0.0:%d", port)
+}