/*
   Copyright 2010 Gabriel Serme

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
*/
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/ip.h>
#include <netinet/tcp.h>
#include <netinet/udp.h>
#include <arpa/inet.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

#define LEN 512

typedef unsigned short u16;
typedef unsigned long u32;

//2nd
unsigned short csum (unsigned short *buf, int nwords);
uint16_t udp_checksum(const struct iphdr  *ip,
        const struct udphdr *udp,
        const uint16_t *buf);



int main(int argc, char * argv[])
{

#  if __BYTE_ORDER == __LITTLE_ENDIAN //reverse per 8 bits
    printf("little endian\n");
#else
    printf("big endian\n");
#endif

    if(argc != 5)
    {
        printf("- Usage %s <IP source> <port source> <IP dest> <port dest>\n", argv[0]);
        exit(1);
    }else{
        printf ("Args : \n"
                "\tip source : %s:%s\n"
                "\tip dest : %s:%s\n", 
                argv[1], argv[2], argv[3], argv[4]);
    }
    int PSOURCE = atoi (argv[2]);
    int PDEST = atoi(argv[4]);
    char * SOURCE = argv[1];
    char * DEST = argv[3];
    char * ANSWER = argv[5];
    int s;
    struct sockaddr_in daddr, saddr, answerip;
    char packet[LEN];
    /* point the iphdr to the beginning of the packet */
    struct iphdr *ip = (struct iphdr *)packet;  
    struct udphdr *udp = (struct udphdr *)((void *) ip + sizeof(struct iphdr));
    struct dnshdr *dns = (struct dnshdr *)((void *) udp + sizeof(struct udphdr));

    if ((s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
        perror("error:");
        exit(EXIT_FAILURE);
    }

    daddr.sin_family = AF_INET;
    saddr.sin_family = AF_INET;
    daddr.sin_port = htons(PDEST); 
    saddr.sin_port = htons(PSOURCE); 
    inet_pton(AF_INET, DEST, (struct in_addr *)&daddr.sin_addr.s_addr);
    inet_pton(AF_INET, SOURCE, (struct in_addr *)&saddr.sin_addr.s_addr);

    memset(daddr.sin_zero, 0, sizeof(daddr.sin_zero));
    memset(saddr.sin_zero, 0, sizeof(saddr.sin_zero));
    memset(udp, 0, sizeof(struct udphdr));
    memset(ip, 0, sizeof(struct iphdr));

    ip->ihl = 5; //header length
    ip->version = 4;
    ip->tos = 0x0;
    ip->id = 0;
    ip->frag_off = htons(0x4000);		/* DF */
    ip->ttl = 64;			/* default value */
    ip->protocol = 17;	//IPPROTO_RAW;	/* protocol at L4 */
    ip->check = 0;			/* not needed in iphdr */
    ip->saddr = saddr.sin_addr.s_addr;
    ip->daddr = daddr.sin_addr.s_addr;

    udp->source = htons(PSOURCE);
    udp->dest = htons (PDEST);

    int sizedata = 100;
    memset(((void *) udp) + sizeof(struct udphdr), 'A', sizedata);

    int sizeudpdata = sizeof(struct udphdr) + sizedata;
    ip->tot_len = htons(sizeudpdata + sizeof(struct iphdr));	/* 16 byte value */
    udp->len = htons(sizeudpdata);

    udp->check = udp_checksum(
            ip,
            udp,
            udp);
    printf ("Checksum : 0x%x\n", udp->check);
    printf ("Sizes :  \n\t[+] iphdr %d"
            "\n\t[+] udphdr %d\n",
            sizeof(struct iphdr), sizeof(struct udphdr));
    printf ("Total size : %d\n", sizeudpdata);

    int optval = 1;

    if (setsockopt(s, IPPROTO_IP, IP_HDRINCL, &optval, sizeof(int))
            < 0)
        perror ("IP HDRINCL");

    while(1) {
        int sizepacket = sizeof(struct iphdr) + sizeudpdata;
        if (sendto(s, (char *)packet, sizepacket, 0, 
                    (struct sockaddr *)&daddr, (socklen_t)sizeof(daddr)) < 0)
            perror("packet send error:");
        else
            printf("Sent packet\n");
        sleep(5);
    }
    exit(EXIT_SUCCESS);
}

//http://www.linuxquestions.org/questions/linux-networking-3/udp-checksum-algorithm-845618/
//modified by Gabriel Serme
struct pseudo_hdr {
    u_int32_t source;
    u_int32_t dest;
    u_int8_t zero; //reserved, check http://www.rhyshaden.com/udp.htm
    u_int8_t protocol;
    u_int16_t udp_length;
};

uint16_t udp_checksum(const struct iphdr  *ip,
        const struct udphdr *udp,
        const uint16_t *buf)
{
    //take in account padding if necessary
    int calculated_length = ntohs(udp->len)%2 == 0 ? ntohs(udp->len) : ntohs(udp->len) + 1;

    struct pseudo_hdr ps_hdr = {0};
    bzero (&ps_hdr, sizeof(struct pseudo_hdr));
    uint8_t data[sizeof(struct pseudo_hdr) + calculated_length];
    bzero (data, sizeof(struct pseudo_hdr) + calculated_length );

    ps_hdr.source = ip->saddr;
    ps_hdr.dest = ip->daddr;
    ps_hdr.protocol = IPPROTO_UDP; //17
    ps_hdr.udp_length = udp->len;

    memcpy(data, &ps_hdr, sizeof(struct pseudo_hdr));
    memcpy(data + sizeof(struct pseudo_hdr), buf, ntohs(udp->len) ); //the remaining bytes are set to 0

    return csum((uint16_t *)data, sizeof(data)/2);
}

/* Not my code */
unsigned short csum (unsigned short *buf, int nwords)
{
    unsigned long sum;

    for (sum = 0; nwords > 0; nwords--)
        sum += *buf++;

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);
    return ~sum;
}