Thread: creating packets

hello!

my packetgenerator, isnt working properly.
when i use tcpdump to look at the packets i am sending they dont equal the ones i was specifying in the source code. this isnt working.

Code:

#include <string.h>
#include <stdio.h>
#include <netinet/tcp.h>
#include <netinet/ip.h>
#include <netinet/udp.h>
#include <netdb.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <linux/slab.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>


char datagram[4096];

struct iphdr *iphead = (struct iphdr *)datagram;
struct tcphdr *tcphead = (struct tcphdr *)datagram + sizeof(iphead); 


void help(void);

unsigned short csum(unsigned short *buf, int nwords)
{
    unsigned long sum;
    for(sum=0;nwords>0;nwords--)
	sum += *buf++;

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

int main (int argc, char **argv)
{
    char opt;
    unsigned int count, i;
    int sockfd;
    struct sockaddr_in dest;    
    int sin_size;
    char buffer[2048];
    int destport, sourceport;
    char *sourceip, *destip;
    const int on = 1;
        
    srand(time(NULL));
    
    if(getuid() != 0){
	fprintf(stderr, "you must be r00t to build packets\n");
	exit(1);
    }
        
    while((opt = getopt(argc, argv, "S:D:P:p:c:")) != EOF){
	switch(opt){
	    case 'S': { sourceip = optarg; break; }
	    case 'D': { destip = optarg; break; }
	    case 'P': { sourceport = atoi(optarg); break; }
	    case 'p': { destport = atoi(optarg); break; }
	    case 'c': { count = atoi(optarg); break; }
	    default:  { help(); break; }
	}
    }    

    if((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_TCP)) == -1){
	perror("socket");
	exit(1);
    }
    
    sin_size = sizeof(struct sockaddr);

    dest.sin_family = AF_INET;
    dest.sin_port = htons(destport);
    dest.sin_addr.s_addr = inet_addr(destip);

    memset(&(dest.sin_zero), '\0', 8);    
    memset(datagram, 0, 4096);
    memset(iphead, 0, sizeof(iphead));
    memset(tcphead, 0, sizeof(tcphead));
    
    
    iphead->ihl = 5;
    iphead->version = 4;
    iphead->tos = 0;
    iphead->tot_len = sizeof(struct iphdr) + sizeof(struct tcphdr);
    iphead->id = htons(getpid());
    iphead->frag_off = 0;
    iphead->ttl = 255;
    iphead->protocol = 6;
    iphead->check = csum((unsigned short *)datagram, iphead->tot_len >> 1);
    iphead->saddr = INADDR_ANY;
    iphead->daddr = inet_addr(destip);

    tcphead->source = htons(sourceport);
    tcphead->dest = htons(destport);
    tcphead->seq = htons(random());
    tcphead->ack_seq = 0;
    tcphead->fin = 0;
    tcphead->syn = 1;
    tcphead->rst = 0;
    tcphead->psh = 0;
    tcphead->ack = 0;
    tcphead->urg = 0;
    tcphead->window = htons(65535); 
    tcphead->check = 0;
    tcphead->urg_ptr = 0;


    if(setsockopt(sockfd, IPPROTO_IP, IP_HDRINCL, &on, sizeof(on)) < 0){
	perror("setsockopt");
	exit(1);
    }

    if( (sendto(sockfd, datagram, iphead->tot_len, 0, (struct sockaddr *)&dest, sizeof(struct sockaddr))) == -1){
    	perror("sendto");
	exit(1);
    }
    
    if( (recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr *)&dest, &sin_size)) == -1){
	perror("recvfrom");
	exit(1);
    }

    close(sockfd);
    return 0;
}

void help()
{
    printf("usage: ./userpacket -S <sourceip> -D <destip> -P <sourceport> -p <destport> -c <# of packets>\n\n");
}

i hope that someone of you can help me.
maybe you have a clue whats wrong with that code.

i compiled and executed that code on an x86 arch with slackware.

thanks

yes.

i updated my code and got it finally working.
now i can send udp packets.
still not working are tcp and icmp packets.
because the tcp and icmp header checksums are wrong.

is there a pseudoheader needed for both or only for tcp packets?

thanks