C: UDP packets source Ip faking - can't receive packet - c

Question

After hours on google I still cant figure it out.
What I am trying to do is:
I want to send UDP packets with a fake source-Ip with a C programm (see code below).
What is not working:
When I send a packet from server-1 to server-2 and trying to receive it, it will only appear in 'iftop', when the sourceip of the packet is not faked. When I put a fake ip in the UDP header it still sais me that the packet is sent (I see that on 'iftop' too on server-1), but i won't receive anything on server-2.
So for example when I send it with the correct sender IP, I'll receive the packet and see it on 'iftop', but when i take for example '1.2.3.4' as source IP i cant receive it (but 'iftop' on server-1 still says it has been send).
I read a lot of stuff and everybody says that it is no problem to fake the source IP of a UDP packet, so I'm wondering what I am doing wrong. I tried it in python too and I didn't receive anything too.
/*
Raw UDP sockets
*/
#include<stdio.h> //for printf
#include<string.h> //memset
#include<sys/socket.h> //for socket ofcourse
#include<stdlib.h> //for exit(0);
#include<errno.h> //For errno - the error number
#include<netinet/udp.h> //Provides declarations for udp header
#include<netinet/ip.h> //Provides declarations for ip header
/*
96 bit (12 bytes) pseudo header needed for udp header checksum calculation
*/
struct pseudo_header
{
u_int32_t source_address;
u_int32_t dest_address;
u_int8_t placeholder;
u_int8_t protocol;
u_int16_t udp_length;
};
/*
Generic checksum calculation function
*/
unsigned short csum(unsigned short *ptr,int nbytes)
{
register long sum;
unsigned short oddbyte;
register 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 + (sum>>16);
answer=(short)~sum;
return(answer);
}
int main (void)
{
//Create a raw socket of type IPPROTO
int s = socket (AF_INET, SOCK_RAW, IPPROTO_RAW);
if(s == -1)
{
//socket creation failed, may be because of non-root privileges
perror("Failed to create raw socket");
exit(1);
}
//Datagram to represent the packet
char datagram[4096] , source_ip[32] , *data , *pseudogram;
//zero out the packet buffer
memset (datagram, 0, 4096);
//IP header
struct iphdr *iph = (struct iphdr *) datagram;
//UDP header
struct udphdr *udph = (struct udphdr *) (datagram + sizeof (struct ip));
struct sockaddr_in sin;
struct pseudo_header psh;
//Data part
data = datagram + sizeof(struct iphdr) + sizeof(struct udphdr);
strcpy(data , "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
//some address resolution
strcpy(source_ip , "***.***.***.***"); // <- This is the (fake-)source IP
sin.sin_family = AF_INET;
sin.sin_port = htons(80);
sin.sin_addr.s_addr = inet_addr ("***.***.***.***"); // <- Receiver IP
//Fill in the IP Header
iph->ihl = 5;
iph->version = 4;
iph->tos = 0;
iph->tot_len = sizeof (struct iphdr) + sizeof (struct udphdr) + strlen(data);
iph->id = htonl (54321); //Id of this packet
iph->frag_off = 0;
iph->ttl = 255;
iph->protocol = IPPROTO_UDP;
iph->check = 0; //Set to 0 before calculating checksum
iph->saddr = inet_addr ( source_ip ); //Spoof the source ip address
iph->daddr = sin.sin_addr.s_addr;
//Ip checksum
iph->check = csum ((unsigned short *) datagram, iph->tot_len);
//UDP header
udph->source = htons (45242);
udph->dest = htons (12345);
udph->len = htons(8 + strlen(data)); //tcp header size
udph->check = 0; //leave checksum 0 now, filled later by pseudo header
//Now the UDP checksum using the pseudo header
psh.source_address = inet_addr( source_ip );
psh.dest_address = sin.sin_addr.s_addr;
psh.placeholder = 0;
psh.protocol = IPPROTO_UDP;
psh.udp_length = htons(sizeof(struct udphdr) + strlen(data) );
int psize = sizeof(struct pseudo_header) + sizeof(struct udphdr) + strlen(data);
pseudogram = malloc(psize);
memcpy(pseudogram , (char*) &psh , sizeof (struct pseudo_header));
memcpy(pseudogram + sizeof(struct pseudo_header) , udph , sizeof(struct udphdr) + strlen(data));
udph->check = csum( (unsigned short*) pseudogram , psize);
//loop if you want to flood :)
//while (1)
{
//Send the packet
if (sendto (s, datagram, iph->tot_len , 0, (struct sockaddr *) &sin, sizeof (sin)) < 0)
{
perror("sendto failed");
}
//Data send successfully
else
{
printf ("Packet Send. Length : %d \n" , iph->tot_len);
}
}
return 0;
}
Thanks for your time.

Answer 1

You put a spoofed source ip - the server will receive your packets but will send the reply (or will try to send it) to the spoofed source (1.2.3.4) which, I guess , has no proper back route. Therefore , they get lost.