/* (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.
 */

#include <linux/module.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/route.h>
#include <net/dst.h>
#include <net/netfilter/ipv4/nf_reject.h>
#include <linux/netfilter_ipv4.h>
#include <linux/netfilter_bridge.h>

const struct tcphdr *nf_reject_ip_tcphdr_get(struct sk_buff *oldskb,
                                             struct tcphdr *_oth, int hook)
{
        const struct tcphdr *oth;

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

        if (ip_hdr(oldskb)->protocol != IPPROTO_TCP)
                return NULL;

        oth = skb_header_pointer(oldskb, ip_hdrlen(oldskb),
                                 sizeof(struct tcphdr), _oth);
        if (oth == NULL)
                return NULL;

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

        /* Check checksum */
        if (nf_ip_checksum(oldskb, hook, ip_hdrlen(oldskb), IPPROTO_TCP))
                return NULL;

        return oth;
}
EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_get);

struct iphdr *nf_reject_iphdr_put(struct sk_buff *nskb,
                                  const struct sk_buff *oldskb,
                                  __u8 protocol, int ttl)
{
        struct iphdr *niph, *oiph = ip_hdr(oldskb);

        skb_reset_network_header(nskb);
        niph = 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  = protocol;
        niph->check     = 0;
        niph->saddr     = oiph->daddr;
        niph->daddr     = oiph->saddr;
        niph->ttl       = ttl;

        nskb->protocol = htons(ETH_P_IP);

        return niph;
}
EXPORT_SYMBOL_GPL(nf_reject_iphdr_put);

void nf_reject_ip_tcphdr_put(struct sk_buff *nskb, const struct sk_buff *oldskb,
                          const struct tcphdr *oth)
{
        struct iphdr *niph = ip_hdr(nskb);
        struct tcphdr *tcph;

        skb_reset_transport_header(nskb);
        tcph = skb_put_zero(nskb, sizeof(struct tcphdr));
        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);
}
EXPORT_SYMBOL_GPL(nf_reject_ip_tcphdr_put);

/* Send RST reply */
void nf_send_reset(struct net *net, struct sk_buff *oldskb, int hook)
{
        struct sk_buff *nskb;
        struct iphdr *niph;
        const struct tcphdr *oth;
        struct tcphdr _oth;

        oth = nf_reject_ip_tcphdr_get(oldskb, &_oth, hook);
        if (!oth)
                return;

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

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

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

        nskb->mark = IP4_REPLY_MARK(net, oldskb->mark);

        skb_reserve(nskb, LL_MAX_HEADER);
        niph = nf_reject_iphdr_put(nskb, oldskb, IPPROTO_TCP,
                                   ip4_dst_hoplimit(skb_dst(nskb)));
        nf_reject_ip_tcphdr_put(nskb, oldskb, oth);

        if (ip_route_me_harder(net, nskb, RTN_UNSPEC))
                goto free_nskb;

        niph = ip_hdr(nskb);

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

        nf_ct_attach(nskb, oldskb);

#if IS_ENABLED(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 = nf_bridge_get_physindev(oldskb);
                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(net, nskb->sk, nskb);

        return;

 free_nskb:
        kfree_skb(nskb);
}
EXPORT_SYMBOL_GPL(nf_send_reset);

void nf_send_unreach(struct sk_buff *skb_in, int code, int hook)
{
        struct iphdr *iph = ip_hdr(skb_in);
        u8 proto;

        if (iph->frag_off & htons(IP_OFFSET))
                return;

        if (skb_csum_unnecessary(skb_in)) {
                icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
                return;
        }

        if (iph->protocol == IPPROTO_TCP || iph->protocol == IPPROTO_UDP)
                proto = iph->protocol;
        else
                proto = 0;

        if (nf_ip_checksum(skb_in, hook, ip_hdrlen(skb_in), proto) == 0)
                icmp_send(skb_in, ICMP_DEST_UNREACH, code, 0);
}
EXPORT_SYMBOL_GPL(nf_send_unreach);

MODULE_LICENSE("GPL");