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