/* Kernel module to match TCP MSS values. */
  
  /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>

   * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>

   *
   * This program is free software; you can redistribute it and/or modify
   * it under the terms of the GNU General Public License version 2 as
   * published by the Free Software Foundation.
   */
  
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <net/tcp.h>

  #include <linux/netfilter/xt_tcpmss.h>
  #include <linux/netfilter/x_tables.h>

  #include <linux/netfilter_ipv4/ip_tables.h>

  #include <linux/netfilter_ipv6/ip6_tables.h>

  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");

  MODULE_DESCRIPTION("Xtables: TCP MSS match");

  MODULE_ALIAS("ipt_tcpmss");

  MODULE_ALIAS("ip6t_tcpmss");

  static bool

  tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)

  {

  	const struct xt_tcpmss_match_info *info = par->matchinfo;

  	const struct tcphdr *th;
  	struct tcphdr _tcph;

  	/* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */

  	const u_int8_t *op;
  	u8 _opt[15 * 4 - sizeof(_tcph)];

  	unsigned int i, optlen;
  
  	/* If we don't have the whole header, drop packet. */

  	th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);

  	if (th == NULL)
  		goto dropit;
  
  	/* Malformed. */
  	if (th->doff*4 < sizeof(*th))
  		goto dropit;
  
  	optlen = th->doff*4 - sizeof(*th);
  	if (!optlen)
  		goto out;
  
  	/* Truncated options. */

  	op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);

  	if (op == NULL)
  		goto dropit;
  
  	for (i = 0; i < optlen; ) {
  		if (op[i] == TCPOPT_MSS
  		    && (optlen - i) >= TCPOLEN_MSS
  		    && op[i+1] == TCPOLEN_MSS) {
  			u_int16_t mssval;
  
  			mssval = (op[i+2] << 8) | op[i+3];

  			return (mssval >= info->mss_min &&

  				mssval <= info->mss_max) ^ info->invert;

  		}

  		if (op[i] < 2)
  			i++;
  		else
  			i += op[i+1] ? : 1;

  	}
  out:

  	return info->invert;

  dropit:

  	par->hotdrop = true;

  	return false;

  }

  static struct xt_match tcpmss_mt_reg[] __read_mostly = {

  	{
  		.name		= "tcpmss",

  		.family		= NFPROTO_IPV4,

  		.match		= tcpmss_mt,

  		.matchsize	= sizeof(struct xt_tcpmss_match_info),
  		.proto		= IPPROTO_TCP,
  		.me		= THIS_MODULE,
  	},
  	{
  		.name		= "tcpmss",

  		.family		= NFPROTO_IPV6,

  		.match		= tcpmss_mt,

  		.matchsize	= sizeof(struct xt_tcpmss_match_info),
  		.proto		= IPPROTO_TCP,
  		.me		= THIS_MODULE,
  	},

  };

  static int __init tcpmss_mt_init(void)

  {

  	return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));

  }

  static void __exit tcpmss_mt_exit(void)

  {

  	xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));

  }

  module_init(tcpmss_mt_init);
  module_exit(tcpmss_mt_exit);