net/netfilter/xt_TCPOPTSTRIP.c

   1 /*
   2  * A module for stripping a specific TCP option from TCP packets.
   3  *
   4  * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
   5  * Copyright © CC Computer Consultants GmbH, 2007
   6  * Contact: Jan Engelhardt <jengelh@computergmbh.de>
   7  *
   8  * This program is free software; you can redistribute it and/or modify
   9  * it under the terms of the GNU General Public License version 2 as
  10  * published by the Free Software Foundation.
  11  */
  12
  13 #include <linux/module.h>
  14 #include <linux/skbuff.h>
  15 #include <linux/ip.h>
  16 #include <linux/ipv6.h>
  17 #include <linux/tcp.h>
  18 #include <net/ipv6.h>
  19 #include <net/tcp.h>
  20 #include <linux/netfilter/x_tables.h>
  21 #include <linux/netfilter/xt_TCPOPTSTRIP.h>
  22
  23 static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
  24 {
  25         /* Beware zero-length options: make finite progress */
  26         if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  27                 return 1;
  28         else
  29                 return opt[offset+1];
  30 }
  31
  32 static unsigned int
  33 tcpoptstrip_mangle_packet(struct sk_buff *skb,
  34                           const struct xt_tcpoptstrip_target_info *info,
  35                           unsigned int tcphoff, unsigned int minlen)
  36 {
  37         unsigned int optl, i, j;
  38         struct tcphdr *tcph;
  39         u_int16_t n, o;
  40         u_int8_t *opt;
  41
  42         if (!skb_make_writable(skb, skb->len))
  43                 return NF_DROP;
  44
  45         tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  46         opt  = (u_int8_t *)tcph;
  47
  48         /*
  49          * Walk through all TCP options - if we find some option to remove,
  50          * set all octets to %TCPOPT_NOP and adjust checksum.
  51          */
  52         for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  53                 optl = optlen(opt, i);
  54
  55                 if (i + optl > tcp_hdrlen(skb))
  56                         break;
  57
  58                 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  59                         continue;
  60
  61                 for (j = 0; j < optl; ++j) {
  62                         o = opt[i+j];
  63                         n = TCPOPT_NOP;
  64                         if ((i + j) % 2 == 0) {
  65                                 o <<= 8;
  66                                 n <<= 8;
  67                         }
  68                         inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  69                                                  htons(n), 0);
  70                 }
  71                 memset(opt + i, TCPOPT_NOP, optl);
  72         }
  73
  74         return XT_CONTINUE;
  75 }
  76
  77 static unsigned int
  78 tcpoptstrip_tg4(struct sk_buff *skb, const struct net_device *in,
  79                 const struct net_device *out, unsigned int hooknum,
  80                 const struct xt_target *target, const void *targinfo)
  81 {
  82         return tcpoptstrip_mangle_packet(skb, targinfo, ip_hdrlen(skb),
  83                sizeof(struct iphdr) + sizeof(struct tcphdr));
  84 }
  85
  86 #if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE)
  87 static unsigned int
  88 tcpoptstrip_tg6(struct sk_buff *skb, const struct net_device *in,
  89                 const struct net_device *out, unsigned int hooknum,
  90                 const struct xt_target *target, const void *targinfo)
  91 {
  92         struct ipv6hdr *ipv6h = ipv6_hdr(skb);
  93         int tcphoff;
  94         u_int8_t nexthdr;
  95
  96         nexthdr = ipv6h->nexthdr;
  97         tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
  98         if (tcphoff < 0)
  99                 return NF_DROP;
 100
 101         return tcpoptstrip_mangle_packet(skb, targinfo, tcphoff,
 102                sizeof(*ipv6h) + sizeof(struct tcphdr));
 103 }
 104 #endif
 105
 106 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 107         {
 108                 .name       = "TCPOPTSTRIP",
 109                 .family     = AF_INET,
 110                 .table      = "mangle",
 111                 .proto      = IPPROTO_TCP,
 112                 .target     = tcpoptstrip_tg4,
 113                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 114                 .me         = THIS_MODULE,
 115         },
 116 #if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE)
 117         {
 118                 .name       = "TCPOPTSTRIP",
 119                 .family     = AF_INET6,
 120                 .table      = "mangle",
 121                 .proto      = IPPROTO_TCP,
 122                 .target     = tcpoptstrip_tg6,
 123                 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 124                 .me         = THIS_MODULE,
 125         },
 126 #endif
 127 };
 128
 129 static int __init tcpoptstrip_tg_init(void)
 130 {
 131         return xt_register_targets(tcpoptstrip_tg_reg,
 132                                    ARRAY_SIZE(tcpoptstrip_tg_reg));
 133 }
 134
 135 static void __exit tcpoptstrip_tg_exit(void)
 136 {
 137         xt_unregister_targets(tcpoptstrip_tg_reg,
 138                               ARRAY_SIZE(tcpoptstrip_tg_reg));
 139 }
 140
 141 module_init(tcpoptstrip_tg_init);
 142 module_exit(tcpoptstrip_tg_exit);
 143 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@computergmbh.de>");
 144 MODULE_DESCRIPTION("Xtables: TCP option stripping");
 145 MODULE_LICENSE("GPL");
 146 MODULE_ALIAS("ipt_TCPOPTSTRIP");
 147 MODULE_ALIAS("ip6t_TCPOPTSTRIP");