#include <stdint.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <poll.h>
#include <dirent.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/time.h>
#include <net/if.h>

struct ethernet_packet {
	uint8_t destination[6];
	uint8_t source[6];
	uint16_t type;
	uint8_t payload[];
} __attribute__((packed)) __attribute__((aligned(2)));

struct ipv4_packet {
	uint8_t  version_ihl;
	uint8_t  dscp_ecn;
	uint16_t length;
	uint16_t ident;
	uint16_t flags_fragment;
	uint8_t  ttl;
	uint8_t  protocol;
	uint16_t checksum;
	uint32_t source;
	uint32_t destination;
	uint8_t  payload[];
} __attribute__ ((packed)) __attribute__((aligned(2)));

struct udp_packet {
	uint16_t source_port;
	uint16_t destination_port;
	uint16_t length;
	uint16_t checksum;
	uint8_t  payload[];
} __attribute__ ((packed)) __attribute__((aligned(2)));

struct dhcp_packet {
	uint8_t op;
	uint8_t htype;
	uint8_t hlen;
	uint8_t hops;

	uint32_t xid;

	uint16_t secs;
	uint16_t flags;

	uint32_t ciaddr;
	uint32_t yiaddr;
	uint32_t siaddr;
	uint32_t giaddr;

	uint8_t  chaddr[16];

	uint8_t sname[64];
	uint8_t file[128];

	uint32_t magic;

	uint8_t  options[];
} __attribute__ ((packed)) __attribute__((aligned(2)));

struct dns_packet {
	uint16_t qid;
	uint16_t flags;
	uint16_t questions;
	uint16_t answers;
	uint16_t authorities;
	uint16_t additional;
	uint8_t data[];
} __attribute__ ((packed)) __attribute__((aligned(2)));

struct tcp_header {
	uint16_t source_port;
	uint16_t destination_port;

	uint32_t seq_number;
	uint32_t ack_number;

	uint16_t flags;
	uint16_t window_size;
	uint16_t checksum;
	uint16_t urgent;

	uint8_t  payload[];
} __attribute__((packed)) __attribute__((aligned(2)));

struct tcp_check_header {
	uint32_t source;
	uint32_t destination;
	uint8_t  zeros;
	uint8_t  protocol;
	uint16_t tcp_len;
	uint8_t  tcp_header[];
};

#define SOCK_STREAM 1
#define SOCK_DGRAM 2

// Note: Data offset is in upper 4 bits of flags field. Shift and subtract 5 since that is the min TCP size.
//       If the value is more than 5, multiply by 4 because this field is specified in number of words
#define TCP_OPTIONS_LENGTH(tcp) (((((tcp)->flags) >> 12) - 5) * 4)
#define TCP_HEADER_LENGTH(tcp) ((((tcp)->flags) >> 12) * 4)
#define TCP_HEADER_LENGTH_FLIPPED(tcp) (((htons((tcp)->flags)) >> 12) * 4)

#define htonl(l)  ( (((l) & 0xFF) << 24) | (((l) & 0xFF00) << 8) | (((l) & 0xFF0000) >> 8) | (((l) & 0xFF000000) >> 24))
#define htons(s)  ( (((s) & 0xFF) << 8) | (((s) & 0xFF00) >> 8) )
#define ntohl(l)  htonl((l))
#define ntohs(s)  htons((s))

#define BROADCAST_MAC {0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF}
#define IPV4_PROT_UDP 17
#define IPV4_PROT_TCP 6
#define DHCP_MAGIC 0x63825363

#define TCP_FLAGS_FIN (1 << 0)
#define TCP_FLAGS_SYN (1 << 1)
#define TCP_FLAGS_RES (1 << 2)
#define TCP_FLAGS_PSH (1 << 3)
#define TCP_FLAGS_ACK (1 << 4)
#define TCP_FLAGS_URG (1 << 5)
#define TCP_FLAGS_ECE (1 << 6)
#define TCP_FLAGS_CWR (1 << 7)
#define TCP_FLAGS_NS  (1 << 8)
#define DATA_OFFSET_5 (0x5 << 12)

#define ETHERNET_TYPE_IPV4 0x0800
#define ETHERNET_TYPE_ARP  0x0806

struct payload {
	struct ethernet_packet eth_header;
	struct ipv4_packet     ip_header;
	struct udp_packet      udp_header;
	struct dhcp_packet     dhcp_header;
	uint8_t payload[32];
};

static void ip_ntoa(const uint32_t src_addr, char * out) {
	snprintf(out, 16, "%d.%d.%d.%d",
		(src_addr & 0xFF000000) >> 24,
		(src_addr & 0xFF0000) >> 16,
		(src_addr & 0xFF00) >> 8,
		(src_addr & 0xFF));
}

uint16_t calculate_ipv4_checksum(struct ipv4_packet * p) {
	uint32_t sum = 0;
	uint16_t * s = (uint16_t *)p;

	/* TODO: Checksums for options? */
	for (int i = 0; i < 10; ++i) {
		sum += ntohs(s[i]);
	}

	if (sum > 0xFFFF) {
		sum = (sum >> 16) + (sum & 0xFFFF);
	}

	return ~(sum & 0xFFFF) & 0xFFFF;
}

uint8_t mac_addr[6];
uint32_t xid = 0x1337;

void fill(struct payload *it, size_t payload_size) {

	it->eth_header.source[0] = mac_addr[0];
	it->eth_header.source[1] = mac_addr[1];
	it->eth_header.source[2] = mac_addr[2];
	it->eth_header.source[3] = mac_addr[3];
	it->eth_header.source[4] = mac_addr[4];
	it->eth_header.source[5] = mac_addr[5];

	it->eth_header.destination[0] = 0xFF;
	it->eth_header.destination[1] = 0xFF;
	it->eth_header.destination[2] = 0xFF;
	it->eth_header.destination[3] = 0xFF;
	it->eth_header.destination[4] = 0xFF;
	it->eth_header.destination[5] = 0xFF;

	it->eth_header.type = htons(0x0800);

	it->ip_header.version_ihl = ((0x4 << 4) | (0x5 << 0));
	it->ip_header.dscp_ecn    = 0;
	it->ip_header.length      = htons(sizeof(struct ipv4_packet) + sizeof(struct udp_packet) + sizeof(struct dhcp_packet) + payload_size);
	it->ip_header.ident       = htons(1);
	it->ip_header.flags_fragment = 0;
	it->ip_header.ttl         = 0x40;
	it->ip_header.protocol    = IPV4_PROT_UDP;
	it->ip_header.checksum    = 0;
	it->ip_header.source      = htonl(0);
	it->ip_header.destination = htonl(0xFFFFFFFF);

	it->ip_header.checksum = htons(calculate_ipv4_checksum(&it->ip_header));

	it->udp_header.source_port = htons(68);
	it->udp_header.destination_port = htons(67);
	it->udp_header.length = htons(sizeof(struct udp_packet) + sizeof(struct dhcp_packet) + payload_size);
	it->udp_header.checksum = 0; /* uh */

	it->dhcp_header.op = 1;
	it->dhcp_header.htype = 1;
	it->dhcp_header.hlen = 6;
	it->dhcp_header.hops = 0;
	it->dhcp_header.xid = htonl(xid); /* transaction id... */
	it->dhcp_header.secs = 0;
	it->dhcp_header.flags = 0;

	it->dhcp_header.ciaddr = 0;
	it->dhcp_header.yiaddr = 0;
	it->dhcp_header.siaddr = 0;
	it->dhcp_header.giaddr = 0;
	it->dhcp_header.chaddr[0] = mac_addr[0];
	it->dhcp_header.chaddr[1] = mac_addr[1];
	it->dhcp_header.chaddr[2] = mac_addr[2];
	it->dhcp_header.chaddr[3] = mac_addr[3];
	it->dhcp_header.chaddr[4] = mac_addr[4];
	it->dhcp_header.chaddr[5] = mac_addr[5];

	it->dhcp_header.magic = htonl(DHCP_MAGIC);
}


static void time_diff(struct timeval *start, struct timeval *end, time_t *sec_diff, suseconds_t *usec_diff) {
	*sec_diff = end->tv_sec - start->tv_sec;
	*usec_diff = end->tv_usec - start->tv_usec;
	if (end->tv_usec < start->tv_usec) {
		*sec_diff -= 1;
		*usec_diff = (1000000 + end->tv_usec) - start->tv_usec;
	}
}

extern char * _argv_0;

static int configure_interface(const char * if_name) {
	/* Open a raw socket. */
	int sock = socket(AF_RAW, SOCK_RAW, 0);
	if (!sock) {
		perror(_argv_0);
		return 1;
	}

	/* Bind to this interface */
	if (setsockopt(sock, SOL_SOCKET, SO_BINDTODEVICE, if_name, strlen(if_name)+1)) {
		perror(_argv_0);
		return 1;
	}

	/* Request the mac address */
	char if_path[100];
	snprintf(if_path, 100, "/dev/net/%s", if_name);
	int netdev = open(if_path, O_RDWR);

	if (netdev < 0) {
		perror(_argv_0);
		return 1;
	}

	if (ioctl(netdev, SIOCGIFHWADDR, &mac_addr)) {
		fprintf(stderr, "%s: %s: could not get mac address\n", _argv_0, if_name);
		return 1;
	}

	xid = rand();

	/* Try to frob the whatsit */
	{
		struct payload thething = {
			.payload = {53,1,1,55,2,3,6,255,0}
		};

		fill(&thething, 8);

		send(sock, &thething, sizeof(struct payload), 0);
	}

	uint32_t yiaddr;
	int stage = 1;

	struct timeval start, end;
	time_t sec_diff;
	suseconds_t usec_diff;
	gettimeofday(&start, NULL);

	static uint8_t eth_broadcast[6] = {255,255,255,255,255,255};

	do {
		char buf[8192] = {0};

		gettimeofday(&end, NULL);
		time_diff(&start,&end,&sec_diff,&usec_diff);
		if (sec_diff > 2) {
			close(netdev);
			return 1;
		}

		struct pollfd fds[1];
		fds[0].fd = sock;
		fds[0].events = POLLIN;
		int ret = poll(fds,1,200);
		if (ret == 0) {
			continue;
		}
		if (ret < 0) {
			fprintf(stderr, "poll: failed\n");
			return 1;
		}
		ssize_t rsize = recv(sock, &buf, 8192, 0);

		if (rsize <= 0) {
			fprintf(stderr, "%s: %s: bad size? %zd\n", _argv_0, if_name, rsize);
			continue;
		}

		struct payload * response = (void*)buf;

		if (memcmp(response->eth_header.destination,mac_addr,6) &&
		    memcmp(response->eth_header.destination,eth_broadcast,6)) {
			/* Not ours */
			continue;
		}

		if (ntohs(response->udp_header.destination_port) != 68) {
			/* Not DHCP */
			continue;
		}

		if (ntohl(response->dhcp_header.xid) != xid) {
			/* Not our transaction */
			continue;
		}

		if (stage == 1) {
			yiaddr = response->dhcp_header.yiaddr;
			struct payload thething = {
				.payload = {53,1,3,50,4,
					(yiaddr) & 0xFF,
					(yiaddr >> 8) & 0xFF,
					(yiaddr >> 16) & 0xFF,
					(yiaddr >> 24) & 0xFF,
					55,2,3,6,255,0}
			};
			fill(&thething, 14);
			send(sock, &thething, sizeof(struct payload), 0);
			stage = 2;
			gettimeofday(&start, NULL);
		} else if (stage == 2) {
			yiaddr = response->dhcp_header.yiaddr;
			char yiaddr_ip[16];
			ip_ntoa(ntohl(yiaddr), yiaddr_ip);
			if (!ioctl(netdev, SIOCSIFADDR, &yiaddr)) {
				printf("%s: %s: configured for %s\n", _argv_0, if_name, yiaddr_ip);
			} else {
				perror(_argv_0);
			}

			/* See if we got a gateway and subnet out of it as well, those are cool... */
			uint8_t * opt = response->dhcp_header.options;
			while (*opt && *opt != 255) {
				uint8_t opt_type = *opt++;
				uint8_t len = *opt++;
				if (opt_type == 1) {
					/* Subnet mask */
					uint32_t ip_data;
					memcpy(&ip_data, opt, 4);
					char addr[16];
					ip_ntoa(ntohl(ip_data), addr);
					printf("%s: %s: subnet mask %s\n", _argv_0, if_name, addr);
					ioctl(netdev, SIOCSIFNETMASK, &ip_data);
				} else if (opt_type == 3) {
					/* Gateway address - add this to a route table? */
					uint32_t ip_data;
					memcpy(&ip_data, opt, 4);
					char addr[16];
					ip_ntoa(ntohl(ip_data), addr);
					printf("%s: %s: gateway %s\n", _argv_0, if_name, addr);
					ioctl(netdev, SIOCSIFGATEWAY, &ip_data);
				} else if (opt_type == 6) {
					/* DNS server */
					uint32_t ip_data;
					memcpy(&ip_data, opt, 4);
					char addr[16];
					ip_ntoa(ntohl(ip_data), addr);
					printf("%s: %s: nameserver %s\n", _argv_0, if_name, addr);
					FILE * resolve = fopen("/etc/resolv.conf","w");
					if (!resolve) resolve = fopen("/var/resolv.conf","w");
					if (resolve) {
						fprintf(resolve, "nameserver %s\n", addr);
						fclose(resolve);
					} /* else, read-only file system? */
				}
				opt += len;
			}

			close(netdev);
			close(sock);
			return 0;
		}
	} while (1);

	return 1;
}

static int configure_interface_with_backoff(const char * if_name) {
	int sleep_times[] = {1,3,5,0};

	for (int *time = sleep_times; *time; time++) {
		if (!configure_interface(if_name)) return 0;
		sleep(*time);
	}

	return 1;
}

int main(int argc, char * argv[]) {
	int retval = 0;

	if (argc > 1) {
		return configure_interface(argv[1]);
	} else {
		/* Read /dev/net for interfaces */
		DIR * d = opendir("/dev/net");
		if (!d) {
			fprintf(stderr, "%s: no network?\n", _argv_0);
			return 1;
		}

		struct dirent * ent;
		while ((ent = readdir(d))) {
			if (ent->d_name[0] == '.') continue;
			if (configure_interface_with_backoff(ent->d_name)) {
				retval = 1;
			}
		}

		closedir(d);
	}

	return retval;
}