linux/net/netfilter/xt_TCPOPTSTRIP.c
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   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_tcpoptstrip_target_info *info,
  34                          unsigned int tcphoff, unsigned int minlen)
  35{
  36        unsigned int optl, i, j;
  37        struct tcphdr *tcph;
  38        u_int16_t n, o;
  39        u_int8_t *opt;
  40
  41        if (!skb_make_writable(skb, skb->len))
  42                return NF_DROP;
  43
  44        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
  45        opt  = (u_int8_t *)tcph;
  46
  47        /*
  48         * Walk through all TCP options - if we find some option to remove,
  49         * set all octets to %TCPOPT_NOP and adjust checksum.
  50         */
  51        for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
  52                optl = optlen(opt, i);
  53
  54                if (i + optl > tcp_hdrlen(skb))
  55                        break;
  56
  57                if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
  58                        continue;
  59
  60                for (j = 0; j < optl; ++j) {
  61                        o = opt[i+j];
  62                        n = TCPOPT_NOP;
  63                        if ((i + j) % 2 == 0) {
  64                                o <<= 8;
  65                                n <<= 8;
  66                        }
  67                        inet_proto_csum_replace2(&tcph->check, skb, htons(o),
  68                                                 htons(n), 0);
  69                }
  70                memset(opt + i, TCPOPT_NOP, optl);
  71        }
  72
  73        return XT_CONTINUE;
  74}
  75
  76static unsigned int
  77tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
  78{
  79        return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
  80               sizeof(struct iphdr) + sizeof(struct tcphdr));
  81}
  82
  83#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
  84static unsigned int
  85tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
  86{
  87        struct ipv6hdr *ipv6h = ipv6_hdr(skb);
  88        int tcphoff;
  89        u_int8_t nexthdr;
  90        __be16 frag_off;
  91
  92        nexthdr = ipv6h->nexthdr;
  93        tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
  94        if (tcphoff < 0)
  95                return NF_DROP;
  96
  97        return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
  98               sizeof(*ipv6h) + sizeof(struct tcphdr));
  99}
 100#endif
 101
 102static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
 103        {
 104                .name       = "TCPOPTSTRIP",
 105                .family     = NFPROTO_IPV4,
 106                .table      = "mangle",
 107                .proto      = IPPROTO_TCP,
 108                .target     = tcpoptstrip_tg4,
 109                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 110                .me         = THIS_MODULE,
 111        },
 112#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
 113        {
 114                .name       = "TCPOPTSTRIP",
 115                .family     = NFPROTO_IPV6,
 116                .table      = "mangle",
 117                .proto      = IPPROTO_TCP,
 118                .target     = tcpoptstrip_tg6,
 119                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
 120                .me         = THIS_MODULE,
 121        },
 122#endif
 123};
 124
 125static int __init tcpoptstrip_tg_init(void)
 126{
 127        return xt_register_targets(tcpoptstrip_tg_reg,
 128                                   ARRAY_SIZE(tcpoptstrip_tg_reg));
 129}
 130
 131static void __exit tcpoptstrip_tg_exit(void)
 132{
 133        xt_unregister_targets(tcpoptstrip_tg_reg,
 134                              ARRAY_SIZE(tcpoptstrip_tg_reg));
 135}
 136
 137module_init(tcpoptstrip_tg_init);
 138module_exit(tcpoptstrip_tg_exit);
 139MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
 140MODULE_DESCRIPTION("Xtables: TCP option stripping");
 141MODULE_LICENSE("GPL");
 142MODULE_ALIAS("ipt_TCPOPTSTRIP");
 143MODULE_ALIAS("ip6t_TCPOPTSTRIP");
 144
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