#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>
#include <sys/socket.h>
#include <resolv.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/ip_icmp.h>
#include <arpa/inet.h>
#include <string.h>
#include <strings.h>
#include <pthread.h>

#define PACKETSIZE  64
struct packet{
    struct icmphdr hdr;
    char msg[PACKETSIZE-sizeof(struct icmphdr)];
};


struct protoent *proto=NULL;
unsigned int usecs_before;
unsigned int usecs_after;

unsigned short checksum(void *b, int len)
{   unsigned short *buf = b;
    unsigned int sum=0;
    unsigned short result;

    for ( sum = 0; len > 1; len -= 2 )
        sum += *buf++;
    if ( len == 1 )
        sum += *(unsigned char*)buf;
    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    result = ~sum;
    return result;
}

void display(void *buf, int bytes){
    int i;
    struct in_addr saddr;
    struct in_addr daddr;
    struct iphdr *ip = buf;
    struct icmphdr *icmp = buf+ip->ihl*4;


    bzero(&saddr, sizeof(struct in_addr));
    bzero(&daddr, sizeof(struct in_addr));
    saddr.s_addr = ip->saddr;
    daddr.s_addr = ip->daddr;

    printf("----------------\n");
    for ( i = 0; i < bytes; i++ ){
        if ( !(i & 15) ) printf("\n%02X:  ", i);
        printf("%02X ", ((unsigned char*)buf)[i]);
    }
    printf("\n");
    printf("IPv%d: hdr-size=%d pkt-size=%d protocol=%d TTL=%d src=%s ",
        ip->version, ip->ihl*4, ntohs(ip->tot_len), ip->protocol,
        ip->ttl, inet_ntoa(saddr));
    printf("dst=%s\n", inet_ntoa(daddr));
    printf("ICMP: type[%d/%d] checksum[%d] id[%d] seq[%d]\n",
        icmp->type, icmp->code, ntohs(icmp->checksum),
        icmp->un.echo.id, icmp->un.echo.sequence);
}

void ping_reply(void *buf, int bytes){
    int i, sd, ttl, pckt_size;
    struct packet pckt;
    struct sockaddr_in r_addr;
    struct sockaddr_in s_addr;
    struct in_addr saddr;

    uint32_t *timestamp;

    pckt_size = bytes - sizeof(struct iphdr);
    struct iphdr *ip = buf;
    memcpy(&pckt, buf+ip->ihl*4, pckt_size);
    bzero(&saddr, sizeof(struct in_addr));
    saddr.s_addr = ip->saddr;
    ttl = ip->ttl / 2;

    r_addr.sin_family = AF_INET;
    r_addr.sin_port = 0;
    r_addr.sin_addr = saddr;

    sd = socket(PF_INET, SOCK_RAW, proto->p_proto);
    if ( sd < 0 ){
        perror("socket");
        return;
    }
    if ( setsockopt(sd, SOL_IP, IP_TTL, &ttl, sizeof(ttl)) != 0)
        perror("Set TTL option");
    if ( fcntl(sd, F_SETFL, O_NONBLOCK) != 0 )
        perror("Request nonblocking I/O");

    pckt.hdr.type = ICMP_ECHOREPLY;
    {
        socklen_t len=sizeof(s_addr);
        if (recvfrom(sd, &pckt, sizeof(pckt), 0, (struct sockaddr*)&s_addr, &len) > 0 )
            printf("recieved\n");

        pckt.hdr.checksum = 0;
        uint32_t *timestamp = (uint32_t *)pckt.msg;
        printf("%i\n",*timestamp);
        (*timestamp)++;
        printf("%i\n",*timestamp);
        pckt.hdr.checksum = checksum(&pckt, pckt_size);
        if ( sendto(sd, &pckt, pckt_size, 0, (struct sockaddr*)&r_addr, sizeof(r_addr)) <= 0 )
            perror("send");
    }

}

void listener(void){
    int sd;
    struct sockaddr_in addr;
    unsigned char buf[1024];

    sd = socket(PF_INET, SOCK_RAW, proto->p_proto);
    if ( sd < 0 ){
        perror("socket");
        exit(0);
    }
    for (;;){
        int bytes; socklen_t len=sizeof(addr);

        bzero(buf, sizeof(buf));
        bytes = recvfrom(sd, buf, sizeof(buf), 0, (struct sockaddr*)&addr, &len);
        if (bytes>0){
            display(buf, bytes);
            ping_reply(buf, bytes);
        }
        else
            perror("recv");
    }
    exit(0);
}

int main(int argc, char const *argv[]){
    proto = getprotobyname("ICMP");
    listener();
    return 0;
}