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, struct xt_action_param *par)
  29 {
  30         const struct xt_tcpmss_match_info *info = par->matchinfo;
  31         const struct tcphdr *th;
  32         struct tcphdr _tcph;
  33         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  34         const u_int8_t *op;
  35         u8 _opt[15 * 4 - sizeof(_tcph)];
  36         unsigned int i, optlen;
  37
  38         /* If we don't have the whole header, drop packet. */
  39         th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
  40         if (th == NULL)
  41                 goto dropit;
  42
  43         /* Malformed. */
  44         if (th->doff*4 < sizeof(*th))
  45                 goto dropit;
  46
  47         optlen = th->doff*4 - sizeof(*th);
  48         if (!optlen)
  49                 goto out;
  50
  51         /* Truncated options. */
  52         op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
  53         if (op == NULL)
  54                 goto dropit;
  55
  56         for (i = 0; i < optlen; ) {
  57                 if (op[i] == TCPOPT_MSS
  58                     && (optlen - i) >= TCPOLEN_MSS
  59                     && op[i+1] == TCPOLEN_MSS) {
  60                         u_int16_t mssval;
  61
  62                         mssval = (op[i+2] << 8) | op[i+3];
  63
  64                         return (mssval >= info->mss_min &&
  65                                 mssval <= info->mss_max) ^ info->invert;
  66                 }
  67                 if (op[i] < 2)
  68                         i++;
  69                 else
  70                         i += op[i+1] ? : 1;
  71         }
  72 out:
  73         return info->invert;
  74
  75 dropit:
  76         par->hotdrop = true;
  77         return false;
  78 }
  79
  80 static struct xt_match tcpmss_mt_reg[] __read_mostly = {
  81         {
  82                 .name           = "tcpmss",
  83                 .family         = NFPROTO_IPV4,
  84                 .match          = tcpmss_mt,
  85                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  86                 .proto          = IPPROTO_TCP,
  87                 .me             = THIS_MODULE,
  88         },
  89         {
  90                 .name           = "tcpmss",
  91                 .family         = NFPROTO_IPV6,
  92                 .match          = tcpmss_mt,
  93                 .matchsize      = sizeof(struct xt_tcpmss_match_info),
  94                 .proto          = IPPROTO_TCP,
  95                 .me             = THIS_MODULE,
  96         },
  97 };
  98
  99 static int __init tcpmss_mt_init(void)
 100 {
 101         return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 102 }
 103
 104 static void __exit tcpmss_mt_exit(void)
 105 {
 106         xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
 107 }
 108
 109 module_init(tcpmss_mt_init);
 110 module_exit(tcpmss_mt_exit);