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   1 /* Kernel module to match TCP MSS values. */
   2
   3 /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   4  * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License version 2 as
   8  * published by the Free Software Foundation.
   9  */
  10
  11 #include <linux/module.h>
  12 #include <linux/skbuff.h>
  13 #include <net/tcp.h>
  14
  15 #include <linux/netfilter/xt_tcpmss.h>
  16 #include <linux/netfilter/x_tables.h>
  17
  18 #include <linux/netfilter_ipv4/ip_tables.h>
  19 #include <linux/netfilter_ipv6/ip6_tables.h>
  20
  21 MODULE_LICENSE("GPL");
  22 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  23 MODULE_DESCRIPTION("Xtables: TCP MSS match");
  24 MODULE_ALIAS("ipt_tcpmss");
  25 MODULE_ALIAS("ip6t_tcpmss");
  26
  27 static bool
  28 tcpmss_mt(const struct sk_buff *skb, const struct net_device *in,
  29           const struct net_device *out, const struct xt_match *match,
  30           const void *matchinfo, int offset, unsigned int protoff,
  31           bool *hotdrop)
  32 {
  33         const struct xt_tcpmss_match_info *info = matchinfo;
  34         struct tcphdr _tcph, *th;
  35         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  36         u8 _opt[15 * 4 - sizeof(_tcph)], *op;
  37         unsigned int i, optlen;
  38
  39         /* If we don't have the whole header, drop packet. */
  40         th = skb_header_pointer(skb, protoff, sizeof(_tcph), &_tcph);
  41         if (th == NULL)
  42                 goto dropit;
  43
  44         /* Malformed. */
  45         if (th->doff*4 < sizeof(*th))
  46                 goto dropit;
  47
  48         optlen = th->doff*4 - sizeof(*th);
  49         if (!optlen)
  50                 goto out;
  51
  52         /* Truncated options. */
  53         op = skb_header_pointer(skb, protoff + sizeof(*th), optlen, _opt);
  54         if (op == NULL)
  55                 goto dropit;
  56
  57         for (i = 0; i < optlen; ) {
  58                 if (op[i] == TCPOPT_MSS
  59                     && (optlen - i) >= TCPOLEN_MSS
  60                     && op[i+1] == TCPOLEN_MSS) {
  61                         u_int16_t mssval;
  62
  63                         mssval = (op[i+2] << 8) | op[i+3];
  64
  65                         return (mssval >= info->mss_min &&
  66                                 mssval <= info->mss_max) ^ info->invert;
  67                 }
  68                 if (op[i] < 2)
  69                         i++;
  70                 else
  71                         i += op[i+1] ? : 1;
  72         }
  73 out:
  74         return info->invert;
  75
  76 dropit:
  77         *hotdrop = true;
  78         return false;
  79 }
  80
  81 static struct xt_match tcpmss_mt_reg[] __read_mostly = {
  82         {
  83                 .name           = "tcpmss",
  84                 .family         = AF_INET,
  85                 .match          = tcpmss_mt,
  86                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  87                 .proto          = IPPROTO_TCP,
  88                 .me             = THIS_MODULE,
  89         },
  90         {
  91                 .name           = "tcpmss",
  92                 .family         = AF_INET6,
  93                 .match          = tcpmss_mt,
  94                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  95                 .proto          = IPPROTO_TCP,
  96                 .me             = THIS_MODULE,
  97         },
  98 };
  99
 100 static int __init tcpmss_mt_init(void)
 101 {
 102         return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 103 }
 104
 105 static void __exit tcpmss_mt_exit(void)
 106 {
 107         xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 108 }
 109
 110 module_init(tcpmss_mt_init);
 111 module_exit(tcpmss_mt_exit);