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

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.