LXR linux/net/netfilter/xt

   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 *
   7 * This program is free software; you can redistribute it and/or modify
   8 * it under the terms of the GNU General Public License version 2 as
   9 * published by the Free Software Foundation.
  10 */
  11
  12#include <linux/module.h>
  13#include <linux/skbuff.h>
  14#include <linux/ip.h>
  15#include <linux/ipv6.h>
  16#include <linux/tcp.h>
  17#include <net/ipv6.h>
  18#include <net/tcp.h>
  19#include <linux/netfilter/x_tables.h>
  20#include <linux/netfilter/xt_TCPOPTSTRIP.h>
  21
  22static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
  23{
  24        /* Beware zero-length options: make finite progress */
  25        if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
  26                return 1;
  27        else
  28                return opt[offset+1];
  29}
  30
  31static unsigned int
  32tcpoptstrip_mangle_packet(struct sk_buff *skb,
  33                          const struct xt_action_param *par,
  34                          unsigned int tcphoff, unsigned int minlen)
  35{
  36        const struct xt_tcpoptstrip_target_info *info = par->targinfo;
  37        unsigned int optl, i, j;
  38        struct tcphdr *tcph;
  39        u_int16_t n, o;
  40        u_int8_t *opt;
  41        int len;
  42
  43        /* This is a fragment, no TCP header is available */
  44        if (par->fragoff != 0)
  45                return XT_CONTINUE;
  46
  47        if (!skb_make_writable(skb, skb->len))
  48                return NF_DROP;
  49
  50        len = skb->len - tcphoff;
  51        if (len < (int)sizeof(struct tcphdr))
  52                return NF_DROP;
  53
  54        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  55        if (tcph->doff * 4 > len)
  56                return NF_DROP;
  57
  58        opt  = (u_int8_t *)tcph;
  59
  60        /*
  61         * Walk through all TCP options - if we find some option to remove,
  62         * set all octets to %TCPOPT_NOP and adjust checksum.
  63         */
  64        for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  65                optl = optlen(opt, i);
  66
  67                if (i + optl > tcp_hdrlen(skb))
  68                        break;
  69
  70                if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  71                        continue;
  72
  73                for (j = 0; j < optl; ++j) {
  74                        o = opt[i+j];
  75                        n = TCPOPT_NOP;
  76                        if ((i + j) % 2 == 0) {
  77                                o <<= 8;
  78                                n <<= 8;
  79                        }
  80                        inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  81                                                 htons(n), 0);
  82                }
  83                memset(opt + i, TCPOPT_NOP, optl);
  84        }
  85
  86        return XT_CONTINUE;
  87}
  88
  89static unsigned int
  90tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  91{
  92        return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
  93               sizeof(struct iphdr) + sizeof(struct tcphdr));
  94}
  95
  96#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  97static unsigned int
  98tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  99{
 100        struct ipv6hdr *ipv6h = ipv6_hdr(skb);
 101        int tcphoff;
 102        u_int8_t nexthdr;
 103        __be16 frag_off;
 104
 105        nexthdr = ipv6h->nexthdr;
 106        tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
 107        if (tcphoff < 0)
 108                return NF_DROP;
 109
 110        return tcpoptstrip_mangle_packet(skb, par, tcphoff,
 111               sizeof(*ipv6h) + sizeof(struct tcphdr));
 112}
 113#endif
 114
 115static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 116        {
 117                .name       = "TCPOPTSTRIP",
 118                .family     = NFPROTO_IPV4,
 119                .table      = "mangle",
 120                .proto      = IPPROTO_TCP,
 121                .target     = tcpoptstrip_tg4,
 122                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 123                .me         = THIS_MODULE,
 124        },
 125#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 126        {
 127                .name       = "TCPOPTSTRIP",
 128                .family     = NFPROTO_IPV6,
 129                .table      = "mangle",
 130                .proto      = IPPROTO_TCP,
 131                .target     = tcpoptstrip_tg6,
 132                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 133                .me         = THIS_MODULE,
 134        },
 135#endif
 136};
 137
 138static int __init tcpoptstrip_tg_init(void)
 139{
 140        return xt_register_targets(tcpoptstrip_tg_reg,
 141                                   ARRAY_SIZE(tcpoptstrip_tg_reg));
 142}
 143
 144static void __exit tcpoptstrip_tg_exit(void)
 145{
 146        xt_unregister_targets(tcpoptstrip_tg_reg,
 147                              ARRAY_SIZE(tcpoptstrip_tg_reg));
 148}
 149
 150module_init(tcpoptstrip_tg_init);
 151module_exit(tcpoptstrip_tg_exit);
 152MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 153MODULE_DESCRIPTION("Xtables: TCP option stripping");
 154MODULE_LICENSE("GPL");
 155MODULE_ALIAS("ipt_TCPOPTSTRIP");
 156MODULE_ALIAS("ip6t_TCPOPTSTRIP");
 157