text: #include <stdio.h>
  #include <xtables.h>
  #include <linux/netfilter/xt_helper.h>
  
  enum {
  	O_HELPER = 0,
  };
  
  static void helper_help(void)
  {
  	printf(
  "helper match options:\n"
  "[!] --helper string        Match helper identified by string\n");
  }
  
  static const struct xt_option_entry helper_opts[] = {
  	{.name = "helper", .id = O_HELPER, .type = XTTYPE_STRING,
  	 .flags = XTOPT_MAND | XTOPT_INVERT | XTOPT_PUT,
  	 XTOPT_POINTER(struct xt_helper_info, name)},
  	XTOPT_TABLEEND,
  };
  
  static void helper_parse(struct xt_option_call *cb)
  {
  	struct xt_helper_info *info = cb->data;
  
  	xtables_option_parse(cb);
  	if (cb->invert)
  		info->invert = 1;
  }
  
  static void
  helper_print(const void *ip, const struct xt_entry_match *match, int numeric)
  {
  	const struct xt_helper_info *info = (const void *)match->data;
  
  	printf(" helper match %s\"%s\"", info->invert ? "! " : "", info->name);
  }
  
  static void helper_save(const void *ip, const struct xt_entry_match *match)
  {
  	const struct xt_helper_info *info = (const void *)match->data;
  
  	printf("%s --helper", info->invert ? " !" : "");
  	xtables_save_string(info->name);
  }
  
  static struct xtables_match helper_match = {
  	.family		= NFPROTO_UNSPEC,
  	.name		= "helper",
  	.version	= XTABLES_VERSION,
  	.size		= XT_ALIGN(sizeof(struct xt_helper_info)),
  	.help		= helper_help,
  	.print		= helper_print,
  	.save		= helper_save,
  	.x6_parse	= helper_parse,
  	.x6_options	= helper_opts,
  };
  
  void _init(void)
  {
  	xtables_register_match(&helper_match);
  }
  