Вывод udp пакетов. Непонятен код. Небольшая часть кода

#include<netinet/in.h>
#include<errno.h>
#include<netdb.h>
#include<stdio.h> //For standard things
#include<stdlib.h> //malloc
#include<string.h> //strlen
#include<netinet/ip_icmp.h> //Provides declarations for icmp header
#include<netinet/udp.h> //Provides declarations for udp header
#include<netinet/tcp.h> //Provides declarations for tcp header
#include<netinet/ip.h> //Provides declarations for ip header
#include<netinet/if_ether.h> //For ETH_P_ALL
#include<net/ethernet.h> //For ether_header
#include<sys/socket.h>
#include<unistd.h>
#include <net/if.h> // struct ifreq
#include <linux/if_packet.h>
 
void ProcessPacket(unsigned char* , int);
void print_ip_header(unsigned char* , int);
void print_udp_packet(unsigned char * , int );
 
struct sockaddr_in source_epc,dest_epc;
 
//uint32_t ipSrc;
//uint16_t portSrc;
 
int main()
{
 
//struct sockaddr_ll sll;
//struct ifreq ifr;
 
int saddr_size , data_size;
struct sockaddr saddr;
 
//int udp = 0, others = 0;
 
unsigned char *buffer = (unsigned char *) malloc(500); //Its Big!
 
int sock_raw = socket( AF_PACKET , SOCK_RAW , htons(ETH_P_ALL)) ;
if(sock_raw < 0)
{
//Print the error with proper message
perror("Socket Error");
return 1;
}
 
 
while(1)
{
saddr_size = sizeof saddr;
 
data_size = recvfrom(sock_raw , buffer , 65536 , 0 , &saddr , (socklen_t*)&saddr_size);
 
if(data_size <0 )
{
printf("Recvfrom error , failed to get packets\n");
return 1;
}
//Now process the packet
ProcessPacket(buffer , data_size);
}
close(sock_raw);
printf("Finished");
return 0;
 
}
 
void ProcessPacket(unsigned char* buffer, int size)
{
//Get the IP Header part of this packet , excluding the ethernet header
struct iphdr *iph = (struct iphdr*)(buffer + sizeof(struct ethhdr));
//++total;
if ( iph->protocol == 17)
{
printf("UDP Packet received\n");
print_ip_header(buffer, size);
print_udp_packet(buffer , size);
}
 
else
{
printf("Packet received in not UDP\n");
}
}
 
void print_ip_header(unsigned char* Buffer, int Size)
{
// print_ethernet_header(Buffer , Size);
 
unsigned short iphdrlen;
 
struct iphdr *iph = (struct iphdr *)(Buffer + sizeof(struct ethhdr) );
iphdrlen =iph->ihl*4;
 
memset(&source_epc, 0, sizeof(source_epc));
source_epc.sin_addr.s_addr = iph->saddr;
 
memset(&dest_epc, 0, sizeof(dest_epc));
dest_epc.sin_addr.s_addr = iph->daddr;
 
printf(" \n\n***********************IP HEADER*************************\n " );
printf(" |-IP Version : %d\n",(unsigned int)iph->version);
printf(" |-IP Header Length : %d DWORDS or %d Bytes\n",(unsigned int)iph->ihl,((unsigned int)(iph->ihl))*4);
printf(" |-Type Of Service : %d\n",(unsigned int)iph->tos);
printf(" |-IP Total Length : %d Bytes(Size of Packet)\n",ntohs(iph->tot_len));
printf(" |-Identification : %d\n",ntohs(iph->id));
//printf(" |-Reserved ZERO Field : %d\n",(unsigned int)iphdr->ip_reserved_zero);
//printf(" |-Dont Fragment Field : %d\n",(unsigned int)iphdr->ip_dont_fragment);
//printf(" |-More Fragment Field : %d\n",(unsigned int)iphdr->ip_more_fragment);
printf(" |-TTL : %d\n",(unsigned int)iph->ttl);
printf(" |-Protocol : %d\n",(unsigned int)iph->protocol);
printf(" |-Checksum : %d\n",ntohs(iph->check));
printf(" |-Source IP : %s\n",inet_ntoa(source_epc.sin_addr));
printf(" |-Destination IP : %s\n",inet_ntoa(dest_epc.sin_addr));
 
}
 
void print_udp_packet(unsigned char *Buffer , int Size)
{
 
unsigned short iphdrlen;
 
struct iphdr *iph = (struct iphdr *)(Buffer + sizeof(struct ethhdr));
iphdrlen = iph->ihl*4;
 
struct udphdr *udph = (struct udphdr*)(Buffer + iphdrlen + sizeof(struct ethhdr));
 
int header_size = sizeof(struct ethhdr) + iphdrlen + sizeof udph;
 
printf("\n\n***********************UDP Packet*************************\n");
 
//print_ip_header(Buffer,Size); 
 
printf("\nUDP Header\n");
printf(" |-Source Port : %d\n" , ntohs(udph->source));
printf(" |-Destination Port : %d\n" , ntohs(udph->dest));
printf(" |-UDP Length : %d\n" , ntohs(udph->len));
printf(" |-UDP
Checksum : %d\n" , ntohs(udph->check));
 
printf("\n");
 
printf("\n###########################################################");
}

struct iphdr *iph =[B] (struct iphdr *)(Buffer + sizeof(struct ethhdr) );[/B]
[B]iphdrlen =iph->ihl*4;[/B]
 
memset(&source_epc, 0, sizeof(source_epc));
source_epc.sin_addr.s_addr = iph->saddr;
 
memset(&dest_epc, 0, sizeof(dest_epc));
dest_epc.sin_addr.s_addr = iph->daddr;

printf("\nUDP Header\n");
printf(" |-Source Port : %d\n" , [B]ntohs(udph->source)[/B]);
printf(" |-Destination Port : %d\n" ,[B] ntohs(udph->dest)[/B]);
printf(" |-UDP Length : %d\n" , [B]ntohs(udph->len)[/B]);
printf(" |-UDP
Checksum : %d\n" , [B]ntohs(udph->check)[/B]);

 
void ProcessPacket(unsigned char* buffer, int size)
{
//Get the IP Header part of this packet , excluding the ethernet header
struct iphdr *iph = (struct iphdr*)(buffer + sizeof(struct ethhdr)); //я так понимаю он прибавляет к buffer размер ethhdr в байтах и присваивает это iph, но почему он указывает что это  "excluding"(отделение) ethernet хэйдеров???
 
iphdrlen =iph->ihl*4; //что такое ihl?? и почему умножает на четыре?

memset(&source_epc, 0, sizeof(source_epc));    //заполняем нулями &source_epc(если честно тоже не понятно)
source_epc.sin_addr.s_addr = iph->saddr;              //???  берет значение из saddr 
 
memset(&dest_epc, 0, sizeof(dest_epc));
dest_epc.sin_addr.s_addr = iph->daddr;                          //?трудновто

printf("\nUDP Header\n");
printf(" |-Source Port : %d\n" , [B]ntohs(udph->source)[/B]);          //что за source??
printf(" |-Destination Port : %d\n" ,[B] ntohs(udph->dest)[/B]);      //где dest взяли?
printf(" |-UDP Length : %d\n" , [B]ntohs(udph->len)[/B]);            // len?
printf(" |-UDP
Checksum : %d\n" , [B]ntohs(udph->check)[/B]);

if ( iph->protocol == 17)         //почему 17?  И что это за значение в protocol , внутри хэйдера, то есть это протокол АЙпи

iphdrlen = iph->ihl*4;

printf(" |-IP Header Length : %d DWORDS or %d Bytes\n",(unsigned int)iph->ihl,((unsigned int)(iph->ihl))*4); //снова на 4 умножает