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root/net/netfilter/xt_TCPOPTSTRIP.c

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DEFINITIONS

This source file includes following definitions.
  1. optlen
  2. tcpoptstrip_mangle_packet
  3. tcpoptstrip_tg4
  4. tcpoptstrip_tg6
  5. tcpoptstrip_tg_init
  6. tcpoptstrip_tg_exit

/*
 * A module for stripping a specific TCP option from TCP packets.
 *
 * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
 * Copyright © CC Computer Consultants GmbH, 2007
 * Contact: Jan Engelhardt <jengelh@computergmbh.de>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <linux/tcp.h>
#include <net/ipv6.h>
#include <net/tcp.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter/xt_TCPOPTSTRIP.h>

static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
{
        /* Beware zero-length options: make finite progress */
        if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
                return 1;
        else
                return opt[offset+1];
}

static unsigned int
tcpoptstrip_mangle_packet(struct sk_buff *skb,
                          const struct xt_tcpoptstrip_target_info *info,
                          unsigned int tcphoff, unsigned int minlen)
{
        unsigned int optl, i, j;
        struct tcphdr *tcph;
        u_int16_t n, o;
        u_int8_t *opt;

        if (!skb_make_writable(skb, skb->len))
                return NF_DROP;

        tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
        opt  = (u_int8_t *)tcph;

        /*
         * Walk through all TCP options - if we find some option to remove,
         * set all octets to %TCPOPT_NOP and adjust checksum.
         */
        for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
                optl = optlen(opt, i);

                if (i + optl > tcp_hdrlen(skb))
                        break;

                if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
                        continue;

                for (j = 0; j < optl; ++j) {
                        o = opt[i+j];
                        n = TCPOPT_NOP;
                        if ((i + j) % 2 == 0) {
                                o <<= 8;
                                n <<= 8;
                        }
                        inet_proto_csum_replace2(&tcph->check, skb, htons(o),
                                                 htons(n), 0);
                }
                memset(opt + i, TCPOPT_NOP, optl);
        }

        return XT_CONTINUE;
}

static unsigned int
tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_target_param *par)
{
        return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
               sizeof(struct iphdr) + sizeof(struct tcphdr));
}

#if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE)
static unsigned int
tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_target_param *par)
{
        struct ipv6hdr *ipv6h = ipv6_hdr(skb);
        int tcphoff;
        u_int8_t nexthdr;

        nexthdr = ipv6h->nexthdr;
        tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr);
        if (tcphoff < 0)
                return NF_DROP;

        return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
               sizeof(*ipv6h) + sizeof(struct tcphdr));
}
#endif

static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
        {
                .name       = "TCPOPTSTRIP",
                .family     = NFPROTO_IPV4,
                .table      = "mangle",
                .proto      = IPPROTO_TCP,
                .target     = tcpoptstrip_tg4,
                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
                .me         = THIS_MODULE,
        },
#if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE)
        {
                .name       = "TCPOPTSTRIP",
                .family     = NFPROTO_IPV6,
                .table      = "mangle",
                .proto      = IPPROTO_TCP,
                .target     = tcpoptstrip_tg6,
                .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
                .me         = THIS_MODULE,
        },
#endif
};

static int __init tcpoptstrip_tg_init(void)
{
        return xt_register_targets(tcpoptstrip_tg_reg,
                                   ARRAY_SIZE(tcpoptstrip_tg_reg));
}

static void __exit tcpoptstrip_tg_exit(void)
{
        xt_unregister_targets(tcpoptstrip_tg_reg,
                              ARRAY_SIZE(tcpoptstrip_tg_reg));
}

module_init(tcpoptstrip_tg_init);
module_exit(tcpoptstrip_tg_exit);
MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@computergmbh.de>");
MODULE_DESCRIPTION("Xtables: TCP option stripping");
MODULE_LICENSE("GPL");
MODULE_ALIAS("ipt_TCPOPTSTRIP");
MODULE_ALIAS("ip6t_TCPOPTSTRIP");

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