get ECHO REPLY with data from server to client behind nat - c

Question

I have to write c program that send ICMP ECHO REQUEST from phone(it is connected by Mobile isp and it is behind NAT)to server with IP PUBLIC. i wrote a simple program that sends echo request and receives echo reply but now i want to send ECHO REQUEST from client to server and receive ECHO REPLY with some data(an IP PUBLIC and ICMP ID) from server to client. How can i make that?
Herre's my code
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <netinet/ip.h>
#include <netinet/ip_icmp.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
typedef unsigned char u8;
typedef unsigned short int u16;
struct icmp_header{
unsigned char type;
unsigned char code;
unsigned short checksum;
unsigned short id;
unsigned short seq;
};
unsigned short in_cksum(unsigned short *ptr, int nbytes);
int main(int argc, char **argv){
int c=100;
int ls;//lunghezza struct sockaddr_in serveraddr
int rf;//receive from
unsigned long daddr;
unsigned long saddr;
int payload_size = 0, sent = 0, sent_size;
saddr = inet_addr("IP PRIVATE");
daddr = inet_addr("IP PUBLIC");
//Raw socket - if you use IPPROTO_ICMP, then kernel will fill in the correct ICMP header checksum, if IPPROTO_RAW, then it wont
int sockfd = socket (AF_INET, SOCK_RAW, IPPROTO_ICMP);
if (sockfd < 0){
perror("could not create socket");
return (0);
}
int on = 1;
// We shall provide IP headers
if (setsockopt (sockfd, IPPROTO_IP, IP_HDRINCL, (const char*)&on, sizeof (on)) == -1){
perror("setsockopt");
return (0);
}
//allow socket to send datagrams to broadcast addresses
if (setsockopt (sockfd, SOL_SOCKET, SO_BROADCAST, (const char*)&on, sizeof (on)) == -1){
perror("setsockopt");
return (0);
}
//Calculate total packet size
int packet_size = sizeof (struct iphdr) + sizeof (struct icmp_header) + payload_size;
char *buffer = (char *) malloc (packet_size);
char *packet = (char *) malloc (packet_size);
if (!packet){
perror("out of memory");
close(sockfd);
return (0);
}
//ip header
struct iphdr *ip = (struct iphdr *) packet;
//struct icmphdr *icmp = (struct icmphdr *) (packet + sizeof (struct iphdr));
struct icmp_header *icmphdr = (struct icmp_header *) (packet + sizeof(struct iphdr));
//zero out the packet buffer
memset (packet, 0, packet_size);
memset (buffer, 0, packet_size);
ip->version = 4;
ip->ihl = 5;
ip->tos = 0;
ip->tot_len = htons (packet_size);
ip->id = rand ();
ip->frag_off = 0;
ip->ttl = 255;
ip->protocol = IPPROTO_ICMP;
ip->saddr = saddr;
ip->daddr = daddr;
//ip->check = in_cksum ((u16 *) ip, sizeof (struct iphdr));
//icmp->type = ICMP_ECHO significa ECHO REQUEST
//icmp->code = 0 è il codice dei ECHO REQUEST
icmphdr->type = ICMP_ECHO;
icmphdr->code = 0;
icmphdr->id = 5;
icmphdr->seq = 66;
//checksum
icmphdr->checksum = 0;
struct sockaddr_in servaddr;
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = daddr;
memset(&servaddr.sin_zero, 0, sizeof (servaddr.sin_zero));
puts("flooding...");
//while (1)
while(c>0){
memset(packet + sizeof(struct iphdr) + sizeof(struct icmp_header), rand() % 255, payload_size);
//memset(buffer + sizeof(struct iphdr) + sizeof(struct icmphdr), rand() % 255, payload_size);
//recalculate the icmp header checksum since we are filling the payload with random characters everytime
icmphdr->checksum = 0;
icmphdr->checksum = in_cksum((unsigned short *)icmphdr, sizeof(struct icmp_header) + payload_size);
if ( (sent_size = sendto(sockfd, packet, packet_size, 0, (struct sockaddr*) &servaddr, sizeof (servaddr))) < 1){
perror("send failed\n");
break;
}
++sent;
printf("%d packets sent\r", sent);
fflush(stdout);
ls = sizeof(servaddr);
//rf = recvfrom(sockfd, buffer, 42, 0, (struct sockaddr *)&servaddr, &ls);
rf = recvfrom(sockfd, buffer, packet_size, 0, (struct sockaddr *)&servaddr, &ls);
if(rf < 0){
perror("Errore recvfrom\n");
break;
}
else{
char *cp;
struct iphdr *ip_reply = (struct iphdr *)buffer;
cp = (char *)&ip_reply->saddr;
printf("Received %d byte reply from %u.%u.%u.%u:\n", ntohs(ip_reply->tot_len), cp[0]&0xff,cp[1]&0xff,cp[2]&0xff,cp[3]&0xff);
printf("ID: %d\n", ntohs(ip_reply->id));
printf("TTL: %d\n", ip_reply->ttl);
}
usleep(10000); //microseconds
c--;
}
free (buffer);
free(packet);
close(sockfd);
return (0);
}
/*
Function calculate checksum
*/
unsigned short in_cksum(unsigned short *ptr, int nbytes){
register long sum;
u_short oddbyte;
register u_short answer;
sum = 0;
while (nbytes > 1) {
sum += *ptr++;
nbytes -= 2;
}
if (nbytes == 1) {
oddbyte = 0;
*((u_char *) & oddbyte) = *(u_char *) ptr;
sum += oddbyte;
}
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;
return (answer);
}

Answer 1

ANSWER.
Ok, i solve problem. I create custom icmp header with unsigned char array as payload (pay in this example). i use this array, in server side, to store data in this way
memcpy(pay, (unsigned char *)&icmp->id, 2); //here i store incoming icmp echo request id.
Then i prepare sending buffer
struct eth_frame *eths = (struct eth_frame *) buffer;
crea_eth(eths,0x0800,mactarget);//create eth frame
struct ip_datagram *ips = (struct ip_datagram*) eths->payload;
struct icmp_packet *icmps = (struct icmp_packet*) ips->payload;
Then i build custom icmp echo reply and custom ip packet that i have to send from server to client
memcpy(icmps->payload, &pay, 10);
icmps->type = 0;
icmps->code = 0;
icmps->checksum = 0;
icmps->id = (icmp->id);//i use same receiving icmp id
icmps->seq = htons(1);
icmps->checksum = //calculate checksum
ips->ver_ihl = 0x45;
ips->tos = 0x00;
ips->totlen = htons(20 + 8 + 8);
ips->id = 0xABCD;
ips->flags_offs = 0;
ips->ttl = 255;
ips->proto = IPPROTO_ICMP;
ips->checksum = 0;
ips->src = *(unsigned int *)serverip;
ips->dst = *(unsigned int *)clientip;//fill with IP PUBLIC ADDRESS OF CLIENT!!
ips->checksum = htons(calcola_checksum((unsigned char*)ips,20));
Then send icmp packet from server to client
unsigned char *p = (unsigned char *)&addr;
for(i = 0;i < sizeof(struct sockaddr_ll);i++)
p[i] = 0;
addr.sll_family = AF_PACKET;
addr.sll_ifindex = 3;
/*send to*/
n=sendto(s, buffer, 14 + 20 + 8 + 8 , 0 , (struct sockaddr*) &addr , sizeof(addr));