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 #define TH_SYN 0x02
  22
  23 MODULE_LICENSE("GPL");
  24 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  25 MODULE_DESCRIPTION("iptables TCP MSS match module");
  26 MODULE_ALIAS("ipt_tcpmss");
  27
  28 /* Returns 1 if the mss option is set and matched by the range, 0 otherwise */
  29 static inline int
  30 mssoption_match(u_int16_t min, u_int16_t max,
  31                 const struct sk_buff *skb,
  32                 unsigned int protoff,
  33                 int invert,
  34                 int *hotdrop)
  35 {
  36         struct tcphdr _tcph, *th;
  37         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  38         u8 _opt[15 * 4 - sizeof(_tcph)], *op;
  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 >= min && mssval <= max) ^ invert;
  68                 }
  69                 if (op[i] < 2) i++;
  70                 else i += op[i+1]?:1;
  71         }
  72 out:
  73         return invert;
  74
  75  dropit:
  76         *hotdrop = 1;
  77         return 0;
  78 }
  79
  80 static int
  81 match(const struct sk_buff *skb,
  82       const struct net_device *in,
  83       const struct net_device *out,
  84       const void *matchinfo,
  85       int offset,
  86       unsigned int protoff,
  87       int *hotdrop)
  88 {
  89         const struct xt_tcpmss_match_info *info = matchinfo;
  90
  91         return mssoption_match(info->mss_min, info->mss_max, skb, protoff,
  92                                info->invert, hotdrop);
  93 }
  94
  95 static int
  96 checkentry(const char *tablename,
  97            const void *ipinfo,
  98            void *matchinfo,
  99            unsigned int matchsize,
 100            unsigned int hook_mask)
 101 {
 102         const struct ipt_ip *ip = ipinfo;
 103         if (matchsize != XT_ALIGN(sizeof(struct xt_tcpmss_match_info)))
 104                 return 0;
 105
 106         /* Must specify -p tcp */
 107         if (ip->proto != IPPROTO_TCP || (ip->invflags & IPT_INV_PROTO)) {
 108                 printk("tcpmss: Only works on TCP packets\n");
 109                 return 0;
 110         }
 111
 112         return 1;
 113 }
 114
 115 static int
 116 checkentry6(const char *tablename,
 117            const void *ipinfo,
 118            void *matchinfo,
 119            unsigned int matchsize,
 120            unsigned int hook_mask)
 121 {
 122         const struct ip6t_ip6 *ip = ipinfo;
 123
 124         if (matchsize != XT_ALIGN(sizeof(struct xt_tcpmss_match_info)))
 125                 return 0;
 126
 127         /* Must specify -p tcp */
 128         if (ip->proto != IPPROTO_TCP || (ip->invflags & XT_INV_PROTO)) {
 129                 printk("tcpmss: Only works on TCP packets\n");
 130                 return 0;
 131         }
 132
 133         return 1;
 134 }
 135
 136 static struct xt_match tcpmss_match = {
 137         .name           = "tcpmss",
 138         .match          = &match,
 139         .checkentry     = &checkentry,
 140         .me             = THIS_MODULE,
 141 };
 142
 143 static struct xt_match tcpmss6_match = {
 144         .name           = "tcpmss",
 145         .match          = &match,
 146         .checkentry     = &checkentry6,
 147         .me             = THIS_MODULE,
 148 };
 149
 150
 151 static int __init init(void)
 152 {
 153         int ret;
 154         ret = xt_register_match(AF_INET, &tcpmss_match);
 155         if (ret)
 156                 return ret;
 157
 158         ret = xt_register_match(AF_INET6, &tcpmss6_match);
 159         if (ret)
 160                 xt_unregister_match(AF_INET, &tcpmss_match);
 161
 162         return ret;
 163 }
 164
 165 static void __exit fini(void)
 166 {
 167         xt_unregister_match(AF_INET6, &tcpmss6_match);
 168         xt_unregister_match(AF_INET, &tcpmss_match);
 169 }
 170
 171 module_init(init);
 172 module_exit(fini);