ipt_ECN.c

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/* iptables module for the IPv4 and TCP ECN bits, Version 1.5
 *
 * (C) 2002 by Harald Welte <[email protected]>
 *
 * 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.
*/
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/in.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <net/ip.h>
#include <linux/tcp.h>
#include <net/checksum.h>

#include <linux/netfilter/x_tables.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv4/ipt_ECN.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Harald Welte <[email protected]>");
MODULE_DESCRIPTION("Xtables: Explicit Congestion Notification (ECN) flag modification");

/* set ECT codepoint from IP header.
 *  return false if there was an error. */
static inline bool
set_ect_ip(struct sk_buff *skb, const struct ipt_ECN_info *einfo)
{
    struct iphdr *iph = ip_hdr(skb);

    if ((iph->tos & IPT_ECN_IP_MASK) != (einfo->ip_ect & IPT_ECN_IP_MASK)) {
        __u8 oldtos;
        if (!skb_make_writable(skb, sizeof(struct iphdr)))
            return false;
        iph = ip_hdr(skb);
        oldtos = iph->tos;
        iph->tos &= ~IPT_ECN_IP_MASK;
        iph->tos |= (einfo->ip_ect & IPT_ECN_IP_MASK);
        csum_replace2(&iph->check, htons(oldtos), htons(iph->tos));
    }
    return true;
}

/* Return false if there was an error. */
static inline bool
set_ect_tcp(struct sk_buff *skb, const struct ipt_ECN_info *einfo)
{
    struct tcphdr _tcph, *tcph;
    __be16 oldval;

    /* Not enough header? */
    tcph = skb_header_pointer(skb, ip_hdrlen(skb), sizeof(_tcph), &_tcph);
    if (!tcph)
        return false;

    if ((!(einfo->operation & IPT_ECN_OP_SET_ECE) ||
         tcph->ece == einfo->proto.tcp.ece) &&
        (!(einfo->operation & IPT_ECN_OP_SET_CWR) ||
         tcph->cwr == einfo->proto.tcp.cwr))
        return true;

    if (!skb_make_writable(skb, ip_hdrlen(skb) + sizeof(*tcph)))
        return false;
    tcph = (void *)ip_hdr(skb) + ip_hdrlen(skb);

    oldval = ((__be16 *)tcph)[6];
    if (einfo->operation & IPT_ECN_OP_SET_ECE)
        tcph->ece = einfo->proto.tcp.ece;
    if (einfo->operation & IPT_ECN_OP_SET_CWR)
        tcph->cwr = einfo->proto.tcp.cwr;

    inet_proto_csum_replace2(&tcph->check, skb,
                 oldval, ((__be16 *)tcph)[6], 0);
    return true;
}

static unsigned int
ecn_tg(struct sk_buff *skb, const struct xt_action_param *par)
{
    const struct ipt_ECN_info *einfo = par->targinfo;

    if (einfo->operation & IPT_ECN_OP_SET_IP)
        if (!set_ect_ip(skb, einfo))
            return NF_DROP;

    if (einfo->operation & (IPT_ECN_OP_SET_ECE | IPT_ECN_OP_SET_CWR) &&
        ip_hdr(skb)->protocol == IPPROTO_TCP)
        if (!set_ect_tcp(skb, einfo))
            return NF_DROP;

    return XT_CONTINUE;
}

static int ecn_tg_check(const struct xt_tgchk_param *par)
{
    const struct ipt_ECN_info *einfo = par->targinfo;
    const struct ipt_entry *e = par->entryinfo;

    if (einfo->operation & IPT_ECN_OP_MASK) {
        pr_info("unsupported ECN operation %x\n", einfo->operation);
        return -EINVAL;
    }
    if (einfo->ip_ect & ~IPT_ECN_IP_MASK) {
        pr_info("new ECT codepoint %x out of mask\n", einfo->ip_ect);
        return -EINVAL;
    }
    if ((einfo->operation & (IPT_ECN_OP_SET_ECE|IPT_ECN_OP_SET_CWR)) &&
        (e->ip.proto != IPPROTO_TCP || (e->ip.invflags & XT_INV_PROTO))) {
        pr_info("cannot use TCP operations on a non-tcp rule\n");
        return -EINVAL;
    }
    return 0;
}

static struct xt_target ecn_tg_reg __read_mostly = {
    .name       = "ECN",
    .family     = NFPROTO_IPV4,
    .target     = ecn_tg,
    .targetsize = sizeof(struct ipt_ECN_info),
    .table      = "mangle",
    .checkentry = ecn_tg_check,
    .me     = THIS_MODULE,
};

static int __init ecn_tg_init(void)
{
    return xt_register_target(&ecn_tg_reg);
}

static void __exit ecn_tg_exit(void)
{
    xt_unregister_target(&ecn_tg_reg);
}

module_init(ecn_tg_init);
module_exit(ecn_tg_exit);