// SPDX-License-Identifier: GPL-2.0-only
/*
 * TTL modification target for IP tables
 * (C) 2000,2005 by Harald Welte <laforge@netfilter.org>
 *
 * Hop Limit modification target for ip6tables
 * Maciej Soltysiak <solt@dns.toxicfilms.tv>
 */
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/ipv6.h>
#include <net/checksum.h>

#include <linux/netfilter/x_tables.h>
#include <linux/netfilter_ipv4/ipt_TTL.h>
#include <linux/netfilter_ipv6/ip6t_HL.h>

MODULE_AUTHOR("Harald Welte <laforge@netfilter.org>");
MODULE_AUTHOR("Maciej Soltysiak <solt@dns.toxicfilms.tv>");
MODULE_DESCRIPTION("Xtables: Hoplimit/TTL Limit field modification target");
MODULE_LICENSE("GPL");

static unsigned int
ttl_tg(struct sk_buff *skb, const struct xt_action_param *par)
{
        struct iphdr *iph;
        const struct ipt_TTL_info *info = par->targinfo;
        int new_ttl;

        if (skb_ensure_writable(skb, sizeof(*iph)))
                return NF_DROP;

        iph = ip_hdr(skb);

        switch (info->mode) {
        case IPT_TTL_SET:
                new_ttl = info->ttl;
                break;
        case IPT_TTL_INC:
                new_ttl = iph->ttl + info->ttl;
                if (new_ttl > 255)
                        new_ttl = 255;
                break;
        case IPT_TTL_DEC:
                new_ttl = iph->ttl - info->ttl;
                if (new_ttl < 0)
                        new_ttl = 0;
                break;
        default:
                new_ttl = iph->ttl;
                break;
        }

        if (new_ttl != iph->ttl) {
                csum_replace2(&iph->check, htons(iph->ttl << 8),
                                           htons(new_ttl << 8));
                iph->ttl = new_ttl;
        }

        return XT_CONTINUE;
}

static unsigned int
hl_tg6(struct sk_buff *skb, const struct xt_action_param *par)
{
        struct ipv6hdr *ip6h;
        const struct ip6t_HL_info *info = par->targinfo;
        int new_hl;

        if (skb_ensure_writable(skb, sizeof(*ip6h)))
                return NF_DROP;

        ip6h = ipv6_hdr(skb);

        switch (info->mode) {
        case IP6T_HL_SET:
                new_hl = info->hop_limit;
                break;
        case IP6T_HL_INC:
                new_hl = ip6h->hop_limit + info->hop_limit;
                if (new_hl > 255)
                        new_hl = 255;
                break;
        case IP6T_HL_DEC:
                new_hl = ip6h->hop_limit - info->hop_limit;
                if (new_hl < 0)
                        new_hl = 0;
                break;
        default:
                new_hl = ip6h->hop_limit;
                break;
        }

        ip6h->hop_limit = new_hl;

        return XT_CONTINUE;
}

static int ttl_tg_check(const struct xt_tgchk_param *par)
{
        const struct ipt_TTL_info *info = par->targinfo;

        if (info->mode > IPT_TTL_MAXMODE)
                return -EINVAL;
        if (info->mode != IPT_TTL_SET && info->ttl == 0)
                return -EINVAL;
        return 0;
}

static int hl_tg6_check(const struct xt_tgchk_param *par)
{
        const struct ip6t_HL_info *info = par->targinfo;

        if (info->mode > IP6T_HL_MAXMODE)
                return -EINVAL;
        if (info->mode != IP6T_HL_SET && info->hop_limit == 0)
                return -EINVAL;
        return 0;
}

static struct xt_target hl_tg_reg[] __read_mostly = {
        {
                .name       = "TTL",
                .revision   = 0,
                .family     = NFPROTO_IPV4,
                .target     = ttl_tg,
                .targetsize = sizeof(struct ipt_TTL_info),
                .table      = "mangle",
                .checkentry = ttl_tg_check,
                .me         = THIS_MODULE,
        },
        {
                .name       = "HL",
                .revision   = 0,
                .family     = NFPROTO_IPV6,
                .target     = hl_tg6,
                .targetsize = sizeof(struct ip6t_HL_info),
                .table      = "mangle",
                .checkentry = hl_tg6_check,
                .me         = THIS_MODULE,
        },
};

static int __init hl_tg_init(void)
{
        return xt_register_targets(hl_tg_reg, ARRAY_SIZE(hl_tg_reg));
}

static void __exit hl_tg_exit(void)
{
        xt_unregister_targets(hl_tg_reg, ARRAY_SIZE(hl_tg_reg));
}

module_init(hl_tg_init);
module_exit(hl_tg_exit);
MODULE_ALIAS("ipt_TTL");
MODULE_ALIAS("ip6t_HL");