#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

struct UdpHeader
{
    u_short src_port;
    u_short targ_port;
    u_short length;
    u_short checksum;
};

char message[] = "Hello there!\n";
char msgbuf[1024];

int main(int argc, char *argv[])
{
    int sock;
    unsigned int length; // размер структуры адреса
    struct sockaddr_in addr;
    struct UdpHeader header;
    struct hostent *hp; // структура хоста
    if (argc != 3) 
    { 
	printf("Usage: server port\n");
        exit(-1);
    }                    
    sock = socket(AF_INET, SOCK_RAW, IPPROTO_UDP);
    if(sock < 0)
    {
        perror("Cant open socket");
        exit(-2);
    }

    addr.sin_family = AF_INET;
    //addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
    hp = gethostbyname(argv[1]); // извлечение хоста
    if (hp==0) 
    {
	perror("Unknown host");
	exit(-3);
    }
    bcopy((char *)hp->h_addr, (char *)&addr.sin_addr, hp->h_length);
    //addr.sin_port = htons(atoi(argv[2])); // извлечение порта
    length=sizeof(struct sockaddr_in);
    
    header.targ_port = htons(atoi(argv[2]));
    header.length = htons(sizeof(header)+sizeof(message));
    header.checksum = 0;
    
    memcpy((void *)msgbuf, (void *)&header, sizeof(header));
    memcpy((void *)(msgbuf+sizeof(header)), (void *)message, sizeof(message));

    sendto(sock, msgbuf, sizeof(header)+sizeof(message), 0,
           (struct sockaddr *)&addr, sizeof(addr));

    close(sock);

    return 0;
}