#include<unistd.h>
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<net/ethernet.h>
#include<net/if.h>
#include<netinet/ip.h>
#include<netpacket/packet.h>
#include<sys/socket.h>
#include<sys/ioctl.h>
#include<arpa/inet.h>
#include<linux/if_ether.h>
#include<errno.h>

unsigned short checksum(unsigned short* buff, int _16bitword)
{
	unsigned long sum;
	for(sum=0;_16bitword>0;_16bitword--)
		sum+=htons(*(buff)++);
	sum = ((sum >> 16) + (sum & 0xFFFF));
	sum += (sum>>16);
	return (unsigned short)(~sum);
}

void die(char * msg) {
	perror(msg);
	exit(1);
}

void _recv(unsigned char * buffer, int rx, int if_idx, int tx) {
	int n;
	n = recvfrom(rx , buffer, 65536, 0 , NULL, NULL);
	if (n == -1) die("Failed to get packets\n");

	// even though this is a AF_PACKET socket,
	// since we are on a wireguard interface, there are no layer 2 headers
	// AF_INET/IPPROTO_RAW did not work, i don't know why
	struct iphdr *ip_packet = (struct iphdr *)(buffer);
	if (ntohl(ip_packet->daddr) != 0x0a6300ff) return;
	struct sockaddr_in addr = {0};
	addr.sin_family=AF_INET;
	// unicast to everyone because wireguard
	for (int32_t daddr = 0x0a630081; daddr < 0x0a6300ff; daddr++) {
		addr.sin_addr.s_addr = htonl(daddr);
		ip_packet->daddr = htonl(daddr);
		ip_packet->check = checksum((unsigned short *)ip_packet, sizeof(struct iphdr) / 2);
		int res = sendto(tx, ip_packet, ntohs(ip_packet->tot_len), 0, (struct sockaddr *) &addr, sizeof(addr));
		if (res == -1) {
			//perror("aaah shit");
			// it's ok if this fails, since we spam a lot of adresses, that don't exist
		}
	}
}

int main() {
	unsigned char *buffer = (unsigned char *)malloc(65536);
	int rx = socket (AF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
	if(rx == -1) die("Failed to create rx socket");

	struct sockaddr_ll addr = {0};
	addr.sll_family=AF_PACKET;
	addr.sll_protocol=htons(ETH_P_ALL);
	char * if_name = "wg-spielwiese";
	struct ifreq ifr;
	size_t if_name_len=strlen(if_name);
	memcpy(ifr.ifr_name,if_name,if_name_len + 1);
	if (ioctl(rx,SIOCGIFINDEX,&ifr)==-1) {
		printf("%s",strerror(errno));
		exit(2);
	}
	int if_idx=ifr.ifr_ifindex;
	addr.sll_ifindex=if_idx;
	bind(rx, (struct sockaddr *)&addr, sizeof(addr));

	int tx = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
	if(tx == -1) die("Failed to create tx socket");
	while(1) {
		_recv(buffer, rx, if_idx, tx);
	}

	return 0;
}