net/netfilter/xt_tcpmss.c

   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         const struct tcphdr *th;
  35         struct tcphdr _tcph;
  36         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  37         const u_int8_t *op;
  38         u8 _opt[15 * 4 - sizeof(_tcph)];
  39         unsigned int i, optlen;
  40
  41         /* If we don't have the whole header, drop packet. */
  42         th = skb_header_pointer(skb, protoff, sizeof(_tcph), &_tcph);
  43         if (th == NULL)
  44                 goto dropit;
  45
  46         /* Malformed. */
  47         if (th->doff*4 < sizeof(*th))
  48                 goto dropit;
  49
  50         optlen = th->doff*4 - sizeof(*th);
  51         if (!optlen)
  52                 goto out;
  53
  54         /* Truncated options. */
  55         op = skb_header_pointer(skb, protoff + sizeof(*th), optlen, _opt);
  56         if (op == NULL)
  57                 goto dropit;
  58
  59         for (i = 0; i < optlen; ) {
  60                 if (op[i] == TCPOPT_MSS
  61                     && (optlen - i) >= TCPOLEN_MSS
  62                     && op[i+1] == TCPOLEN_MSS) {
  63                         u_int16_t mssval;
  64
  65                         mssval = (op[i+2] << 8) | op[i+3];
  66
  67                         return (mssval >= info->mss_min &&
  68                                 mssval <= info->mss_max) ^ info->invert;
  69                 }
  70                 if (op[i] < 2)
  71                         i++;
  72                 else
  73                         i += op[i+1] ? : 1;
  74         }
  75 out:
  76         return info->invert;
  77
  78 dropit:
  79         *hotdrop = true;
  80         return false;
  81 }
  82
  83 static struct xt_match tcpmss_mt_reg[] __read_mostly = {
  84         {
  85                 .name           = "tcpmss",
  86                 .family         = AF_INET,
  87                 .match          = tcpmss_mt,
  88                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  89                 .proto          = IPPROTO_TCP,
  90                 .me             = THIS_MODULE,
  91         },
  92         {
  93                 .name           = "tcpmss",
  94                 .family         = AF_INET6,
  95                 .match          = tcpmss_mt,
  96                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  97                 .proto          = IPPROTO_TCP,
  98                 .me             = THIS_MODULE,
  99         },
 100 };
 101
 102 static int __init tcpmss_mt_init(void)
 103 {
 104         return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 105 }
 106
 107 static void __exit tcpmss_mt_exit(void)
 108 {
 109         xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 110 }
 111
 112 module_init(tcpmss_mt_init);
 113 module_exit(tcpmss_mt_exit);