/*
 * This is a module which is used for rejecting packets.
 */

/* (C) 1999-2001 Paul `Rusty' Russell
 * (C) 2002-2004 Netfilter Core Team <coreteam@netfilter.org>
 *
 * 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/module.h>
#include <linux/skbuff.h>
#include <linux/slab.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <linux/icmp.h>
#include <net/icmp.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/route.h>
#include <net/dst.h>
#include <linux/netfilter/x_tables.h>
#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv4/ipt_REJECT.h>
#ifdef CONFIG_BRIDGE_NETFILTER
#include <linux/netfilter_bridge.h>
#endif

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Netfilter Core Team <coreteam@netfilter.org>");
MODULE_DESCRIPTION("Xtables: packet \"rejection\" target for IPv4");

/* Send RST reply */
static void send_reset(struct sk_buff *oldskb, int hook)
{
        struct sk_buff *nskb;
        const struct iphdr *oiph;
        struct iphdr *niph;
        const struct tcphdr *oth;
        struct tcphdr _otcph, *tcph;

        /* IP header checks: fragment. */
        if (ip_hdr(oldskb)->frag_off & htons(IP_OFFSET))
                return;

        oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
                                 sizeof(_otcph), &_otcph);
        if (oth == NULL)
                return;

        /* No RST for RST. */
        if (oth->rst)
                return;

        if (skb_rtable(oldskb)->rt_flags & (RTCF_BROADCAST | RTCF_MULTICAST))
                return;

        /* Check checksum */
        if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
                return;
        oiph = ip_hdr(oldskb);

        nskb = alloc_skb(sizeof(struct iphdr) + sizeof(struct tcphdr) +
                         LL_MAX_HEADER, GFP_ATOMIC);
        if (!nskb)
                return;

        skb_reserve(nskb, LL_MAX_HEADER);

        skb_reset_network_header(nskb);
        niph = (struct iphdr *)skb_put(nskb, sizeof(struct iphdr));
        niph->version   = 4;
        niph->ihl       = sizeof(struct iphdr) / 4;
        niph->tos       = 0;
        niph->id        = 0;
        niph->frag_off  = htons(IP_DF);
        niph->protocol  = IPPROTO_TCP;
        niph->check     = 0;
        niph->saddr     = oiph->daddr;
        niph->daddr     = oiph->saddr;

        skb_reset_transport_header(nskb);
        tcph = (struct tcphdr *)skb_put(nskb, sizeof(struct tcphdr));
        memset(tcph, 0, sizeof(*tcph));
        tcph->source    = oth->dest;
        tcph->dest      = oth->source;
        tcph->doff      = sizeof(struct tcphdr) / 4;

        if (oth->ack)
                tcph->seq = oth->ack_seq;
        else {
                tcph->ack_seq = htonl(ntohl(oth->seq) + oth->syn + oth->fin +
                                      oldskb->len - ip_hdrlen(oldskb) -
                                      (oth->doff << 2));
                tcph->ack = 1;
        }

        tcph->rst       = 1;
        tcph->check = ~tcp_v4_check(sizeof(struct tcphdr), niph->saddr,
                                    niph->daddr, 0);
        nskb->ip_summed = CHECKSUM_PARTIAL;
        nskb->csum_start = (unsigned char *)tcph - nskb->head;
        nskb->csum_offset = offsetof(struct tcphdr, check);

        /* ip_route_me_harder expects skb->dst to be set */
        skb_dst_set_noref(nskb, skb_dst(oldskb));

        nskb->protocol = htons(ETH_P_IP);
        if (ip_route_me_harder(nskb, RTN_UNSPEC))
                goto free_nskb;

        niph->ttl       = ip4_dst_hoplimit(skb_dst(nskb));

        /* "Never happens" */
        if (nskb->len > dst_mtu(skb_dst(nskb)))
                goto free_nskb;

        nf_ct_attach(nskb, oldskb);

#ifdef CONFIG_BRIDGE_NETFILTER
        /* If we use ip_local_out for bridged traffic, the MAC source on
         * the RST will be ours, instead of the destination's.  This confuses
         * some routers/firewalls, and they drop the packet.  So we need to
         * build the eth header using the original destination's MAC as the
         * source, and send the RST packet directly.
         */
        if (oldskb->nf_bridge) {
                struct ethhdr *oeth = eth_hdr(oldskb);
                nskb->dev = oldskb->nf_bridge->physindev;
                niph->tot_len = htons(nskb->len);
                ip_send_check(niph);
                if (dev_hard_header(nskb, nskb->dev, ntohs(nskb->protocol),
                                    oeth->h_source, oeth->h_dest, nskb->len) < 0)
                        goto free_nskb;
                dev_queue_xmit(nskb);
        } else
#endif
                ip_local_out(nskb);

        return;

 free_nskb:
        kfree_skb(nskb);
}

static inline void send_unreach(struct sk_buff *skb_in, int code)
{
        icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
}

static unsigned int
reject_tg(struct sk_buff *skb, const struct xt_action_param *par)
{
        const struct ipt_reject_info *reject = par->targinfo;

        switch (reject->with) {
        case IPT_ICMP_NET_UNREACHABLE:
                send_unreach(skb, ICMP_NET_UNREACH);
                break;
        case IPT_ICMP_HOST_UNREACHABLE:
                send_unreach(skb, ICMP_HOST_UNREACH);
                break;
        case IPT_ICMP_PROT_UNREACHABLE:
                send_unreach(skb, ICMP_PROT_UNREACH);
                break;
        case IPT_ICMP_PORT_UNREACHABLE:
                send_unreach(skb, ICMP_PORT_UNREACH);
                break;
        case IPT_ICMP_NET_PROHIBITED:
                send_unreach(skb, ICMP_NET_ANO);
                break;
        case IPT_ICMP_HOST_PROHIBITED:
                send_unreach(skb, ICMP_HOST_ANO);
                break;
        case IPT_ICMP_ADMIN_PROHIBITED:
                send_unreach(skb, ICMP_PKT_FILTERED);
                break;
        case IPT_TCP_RESET:
                send_reset(skb, par->hooknum);
        case IPT_ICMP_ECHOREPLY:
                /* Doesn't happen. */
                break;
        }

        return NF_DROP;
}

static int reject_tg_check(const struct xt_tgchk_param *par)
{
        const struct ipt_reject_info *rejinfo = par->targinfo;
        const struct ipt_entry *e = par->entryinfo;

        if (rejinfo->with == IPT_ICMP_ECHOREPLY) {
                pr_info("ECHOREPLY no longer supported.\n");
                return -EINVAL;
        } else if (rejinfo->with == IPT_TCP_RESET) {
                /* Must specify that it's a TCP packet */
                if (e->ip.proto != IPPROTO_TCP ||
                    (e->ip.invflags & XT_INV_PROTO)) {
                        pr_info("TCP_RESET invalid for non-tcp\n");
                        return -EINVAL;
                }
        }
        return 0;
}

static struct xt_target reject_tg_reg __read_mostly = {
        .name           = "REJECT",
        .family         = NFPROTO_IPV4,
        .target         = reject_tg,
        .targetsize     = sizeof(struct ipt_reject_info),
        .table          = "filter",
        .hooks          = (1 << NF_INET_LOCAL_IN) | (1 << NF_INET_FORWARD) |
                          (1 << NF_INET_LOCAL_OUT),
        .checkentry     = reject_tg_check,
        .me             = THIS_MODULE,
};

static int __init reject_tg_init(void)
{
        return xt_register_target(&reject_tg_reg);
}

static void __exit reject_tg_exit(void)
{
        xt_unregister_target(&reject_tg_reg);
}

module_init(reject_tg_init);
module_exit(reject_tg_exit);