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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("iptables TCP MSS match module");
  24 MODULE_ALIAS("ipt_tcpmss");
  25 MODULE_ALIAS("ip6t_tcpmss");
  26
  27 static bool
  28 match(const struct sk_buff *skb,
  29       const struct net_device *in,
  30       const struct net_device *out,
  31       const struct xt_match *match,
  32       const void *matchinfo,
  33       int offset,
  34       unsigned int protoff,
  35       bool *hotdrop)
  36 {
  37         const struct xt_tcpmss_match_info *info = matchinfo;
  38         struct tcphdr _tcph, *th;
  39         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  40         u8 _opt[15 * 4 - sizeof(_tcph)], *op;
  41         unsigned int i, optlen;
  42
  43         /* If we don't have the whole header, drop packet. */
  44         th = skb_header_pointer(skb, protoff, sizeof(_tcph), &_tcph);
  45         if (th == NULL)
  46                 goto dropit;
  47
  48         /* Malformed. */
  49         if (th->doff*4 < sizeof(*th))
  50                 goto dropit;
  51
  52         optlen = th->doff*4 - sizeof(*th);
  53         if (!optlen)
  54                 goto out;
  55
  56         /* Truncated options. */
  57         op = skb_header_pointer(skb, protoff + sizeof(*th), optlen, _opt);
  58         if (op == NULL)
  59                 goto dropit;
  60
  61         for (i = 0; i < optlen; ) {
  62                 if (op[i] == TCPOPT_MSS
  63                     && (optlen - i) >= TCPOLEN_MSS
  64                     && op[i+1] == TCPOLEN_MSS) {
  65                         u_int16_t mssval;
  66
  67                         mssval = (op[i+2] << 8) | op[i+3];
  68
  69                         return (mssval >= info->mss_min &&
  70                                 mssval <= info->mss_max) ^ info->invert;
  71                 }
  72                 if (op[i] < 2)
  73                         i++;
  74                 else
  75                         i += op[i+1] ? : 1;
  76         }
  77 out:
  78         return info->invert;
  79
  80 dropit:
  81         *hotdrop = true;
  82         return false;
  83 }
  84
  85 static struct xt_match xt_tcpmss_match[] __read_mostly = {
  86         {
  87                 .name           = "tcpmss",
  88                 .family         = AF_INET,
  89                 .match          = match,
  90                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  91                 .proto          = IPPROTO_TCP,
  92                 .me             = THIS_MODULE,
  93         },
  94         {
  95                 .name           = "tcpmss",
  96                 .family         = AF_INET6,
  97                 .match          = match,
  98                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  99                 .proto          = IPPROTO_TCP,
 100                 .me             = THIS_MODULE,
 101         },
 102 };
 103
 104 static int __init xt_tcpmss_init(void)
 105 {
 106         return xt_register_matches(xt_tcpmss_match,
 107                                    ARRAY_SIZE(xt_tcpmss_match));
 108 }
 109
 110 static void __exit xt_tcpmss_fini(void)
 111 {
 112         xt_unregister_matches(xt_tcpmss_match, ARRAY_SIZE(xt_tcpmss_match));
 113 }
 114
 115 module_init(xt_tcpmss_init);
 116 module_exit(xt_tcpmss_fini);