net/ipv4/netfilter/ipt_tcpmss.c

   1 /* Kernel module to match TCP MSS values. */
   2
   3 /* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License version 2 as
   7  * published by the Free Software Foundation.
   8  */
   9
  10 #include <linux/module.h>
  11 #include <linux/skbuff.h>
  12 #include <net/tcp.h>
  13
  14 #include <linux/netfilter_ipv4/ipt_tcpmss.h>
  15 #include <linux/netfilter_ipv4/ip_tables.h>
  16
  17 #define TH_SYN 0x02
  18
  19 MODULE_LICENSE("GPL");
  20 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
  21 MODULE_DESCRIPTION("iptables TCP MSS match module");
  22
  23 /* Returns 1 if the mss option is set and matched by the range, 0 otherwise */
  24 static inline int
  25 mssoption_match(u_int16_t min, u_int16_t max,
  26                 const struct sk_buff *skb,
  27                 int invert,
  28                 int *hotdrop)
  29 {
  30         struct tcphdr _tcph, *th;
  31         /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
  32         u8 _opt[15 * 4 - sizeof(_tcph)], *op;
  33         unsigned int i, optlen;
  34
  35         /* If we don't have the whole header, drop packet. */
  36         th = skb_header_pointer(skb, skb->nh.iph->ihl * 4,
  37                                 sizeof(_tcph), &_tcph);
  38         if (th == NULL)
  39                 goto dropit;
  40
  41         /* Malformed. */
  42         if (th->doff*4 < sizeof(*th))
  43                 goto dropit;
  44
  45         optlen = th->doff*4 - sizeof(*th);
  46         if (!optlen)
  47                 goto out;
  48
  49         /* Truncated options. */
  50         op = skb_header_pointer(skb, skb->nh.iph->ihl * 4 + sizeof(*th),
  51                                 optlen, _opt);
  52         if (op == NULL)
  53                 goto dropit;
  54
  55         for (i = 0; i < optlen; ) {
  56                 if (op[i] == TCPOPT_MSS
  57                     && (optlen - i) >= TCPOLEN_MSS
  58                     && op[i+1] == TCPOLEN_MSS) {
  59                         u_int16_t mssval;
  60
  61                         mssval = (op[i+2] << 8) | op[i+3];
  62
  63                         return (mssval >= min && mssval <= max) ^ invert;
  64                 }
  65                 if (op[i] < 2) i++;
  66                 else i += op[i+1]?:1;
  67         }
  68 out:
  69         return invert;
  70
  71  dropit:
  72         *hotdrop = 1;
  73         return 0;
  74 }
  75
  76 static int
  77 match(const struct sk_buff *skb,
  78       const struct net_device *in,
  79       const struct net_device *out,
  80       const void *matchinfo,
  81       int offset,
  82       int *hotdrop)
  83 {
  84         const struct ipt_tcpmss_match_info *info = matchinfo;
  85
  86         return mssoption_match(info->mss_min, info->mss_max, skb,
  87                                info->invert, hotdrop);
  88 }
  89
  90 static int
  91 checkentry(const char *tablename,
  92            const struct ipt_ip *ip,
  93            void *matchinfo,
  94            unsigned int matchsize,
  95            unsigned int hook_mask)
  96 {
  97         if (matchsize != IPT_ALIGN(sizeof(struct ipt_tcpmss_match_info)))
  98                 return 0;
  99
 100         /* Must specify -p tcp */
 101         if (ip->proto != IPPROTO_TCP || (ip->invflags & IPT_INV_PROTO)) {
 102                 printk("tcpmss: Only works on TCP packets\n");
 103                 return 0;
 104         }
 105
 106         return 1;
 107 }
 108
 109 static struct ipt_match tcpmss_match = {
 110         .name           = "tcpmss",
 111         .match          = &match,
 112         .checkentry     = &checkentry,
 113         .me             = THIS_MODULE,
 114 };
 115
 116 static int __init init(void)
 117 {
 118         return ipt_register_match(&tcpmss_match);
 119 }
 120
 121 static void __exit fini(void)
 122 {
 123         ipt_unregister_match(&tcpmss_match);
 124 }
 125
 126 module_init(init);
 127 module_exit(fini);