#include<iostream>
#include<cstdlib>
#include<cstdio>
#include<string.h>
#include<sys/types.h>
#include<arpa/inet.h>
#include<netinet/in.h>
#include<netinet/ip.h>
#include<netinet/udp.h>
#include<sys/socket.h>
using namespace std;

/*UDP Pseudo Header*/

struct psd_udp
{
struct in_addr src;
struct in_addr dst;
unsigned char pad;
unsigned char proto;
unsigned short udp_len;
struct udphdr udp;
};

/*Checksum function*/

unsigned short in_cksum(unsigned short *addr, int len)
{
int nleft = len;
int sum = 0;
unsigned short *w = addr;
unsigned short answer = 0;
while (nleft > 1)
{
sum += *w++;
nleft -= 2;
}

if (nleft == 1)
{
*(unsigned char *) (&answer) = *(unsigned char *) w;
sum += answer;
}

sum = (sum >> 16) + (sum & 0xFFFF);
sum += (sum >> 16);
answer = ~sum;
return (answer);
}

/*UDP Checksum*/

unsigned short in_cksum_udp(int src, int dst, unsigned short *addr, int len)
{
struct psd_udp buf;

memset(&buf, 0, sizeof(buf));
buf.src.s_addr = src;
buf.dst.s_addr = dst;
buf.pad = 0;
buf.proto = IPPROTO_UDP;
buf.udp_len = htons(len);
memcpy(&(buf.udp), addr, len);
return in_cksum((unsigned short *)&buf, 12 + len);
}


int main()
{
int udprawsock;
u_char *packet;
const int any=1;
char data[9]="Hello!!";

struct ip ip_hdr;
struct udphdr udp_hdr;
struct sockaddr_in info4kernel;

packet=(u_char*)malloc(sizeof(ip_hdr)+sizeof(udp_hdr)+sizeof(data));


ip_hdr.ip_hl=5;
ip_hdr.ip_v=4;
ip_hdr.ip_tos=0;
ip_hdr.ip_len=htons(sizeof(ip_hdr)+sizeof(udp_hdr)+sizeof(data));
ip_hdr.ip_id=htons(1000);
ip_hdr.ip_off=0;
ip_hdr.ip_ttl=64;
ip_hdr.ip_p=IPPROTO_UDP;
ip_hdr.ip_sum=0;
ip_hdr.ip_src.s_addr=inet_addr("192.168.32.128");
ip_hdr.ip_dst.s_addr=inet_addr("192.168.32.1");
ip_hdr.ip_sum=in_cksum((unsigned short *)&ip_hdr,sizeof(ip_hdr));

memcpy(packet,&ip_hdr,sizeof(ip_hdr));

udp_hdr.source=htons(1500);
udp_hdr.dest=htons(1501);
udp_hdr.len=sizeof(udp_hdr)+sizeof(data);
udp_hdr.check=0;
udp_hdr.check=in_cksum_udp(ip_hdr.ip_src.s_addr,ip_hdr.ip_dst.s_addr,(unsigned short*)&udp_hdr,sizeof(udp_hdr));

memcpy(packet+20,&udp_hdr,sizeof(udp_hdr));
memcpy(packet+28,&data,sizeof(data));


udprawsock=socket(AF_INET,SOCK_RAW,IPPROTO_RAW);

if(udprawsock<0)
{
perror("Socket Error:");
exit(1);
}

if(setsockopt(udprawsock,IPPROTO_IP,IP_HDRINCL,&any,sizeof(any))<0)
{
perror("setsock error:");
exit(1);
}

info4kernel.sin_family=AF_INET;
info4kernel.sin_addr.s_addr=ip_hdr.ip_dst.s_addr;

if(sendto(udprawsock,packet,sizeof(packet),0,(struct sockaddr *)&info4kernel,sizeof(info4kernel))<0)
{
perror("sendto error");
exit(1);
}

return 0;
}

First: Check the link in my signature on how to use [code][/code] tags.

I compiled your program and see the same error. The manual page doesn't help much.

I went for a brief search for a sendto() example and found one here. Check that out because it is very simple.

One issue may be that you've chosen to do SOCK_RAW versus SOCK_DGRAM which is the normal socket type for UDP.

I don't think you need to fully create the packet unless you're modifying the content from a standard UDP data frame. You can see in the example that you can choose the port and then send a data stream via UDP without worrying about constructing the packet, nor setting any socket options.

I'd start with the equivalent of that example and then modify as you see fit.

Note also that I do NOT need to be root to run that example, but because (I believe) you're using SOCK_RAW, then you need to be sudo or root to run what you've coded.

Don't know if this related to your problem, but sizeof(packet) is equal to sizeof(u_char *), 8 bytes on my machine.

Thank you rtmistler for a very nice explanation. Very much appreciated.

@norobro your analysis is spot on, that's where the problem was. I gave the size of u_char * but not the size of packet which was the root cause. I changed that to sizeof(ip_hdr)+sizeof(udp_hdr)+sizeof(data) and it worked like charm.

Thank you both.