raw sockets in c - bogus header length - c

Question

#include<stdlib.h>
#include<netinet/ip.h>
#include<netinet/tcp.h>
#include<sys/socket.h>
#include<arpa/inet.h>
#include<netinet/in.h>
#include<stdio.h>
#include<unistd.h>
#include<string.h>
#include<sys/types.h>
#define P 5000 /* lets flood the port */
unsigned short /* this function generates header checksums */
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 (void)
{
char ipp[10] = "1.2.3.4";
int s = socket (PF_INET, SOCK_RAW, IPPROTO_TCP); /* open raw socket */
char datagram[4096]; /* this buffer will contain ip header, tcp header,
and payload. we'll point an ip header structure
at its beginning, and a tcp header structure after
that to write the header values into it */
struct ip *iph = (struct ip *) datagram;
struct tcphdr *tcph = (struct tcphdr *) datagram + sizeof (struct ip);
struct sockaddr_in sin;
/* the sockaddr_in containing the dest. address is used
in sendto() to determine the datagrams path */
sin.sin_family = AF_INET;
sin.sin_port = htons (P);/* you byte-order >1byte header values to network
byte order (not needed on big endian machines) */
sin.sin_addr.s_addr = inet_addr ("192.168.1.12");
memset (datagram, 0, 4096); /* zero out the buffer */
/* we'll now fill in the ip/tcp header values, see above for explanations */
iph->ip_hl = 5;
iph->ip_v = 4;
iph->ip_tos = 0;
iph->ip_len = sizeof (struct ip) + sizeof (struct tcphdr); /* no payload */
iph->ip_id = htonl (54321); /* the value doesn't matter here */
iph->ip_off = 0;
iph->ip_ttl = 255;
iph->ip_p = 6;
iph->ip_sum = 0; /* set it to 0 before computing the actual checksum later */
iph->ip_src.s_addr = inet_addr (ipp);/* SYN's can be blindly spoofed */
iph->ip_dst.s_addr = sin.sin_addr.s_addr;
tcph->source = htons (1234); /* arbitrary port */
tcph->dest = htons (P);
tcph->seq = random ();/* in a SYN packet, the sequence is a random */
tcph->ack_seq = 0;/* number, and the ack sequence is 0 in the 1st packet */
tcph->doff = 0; /* first and only tcp segment */
tcph->syn = 1;
tcph->window = htonl (65535); /* maximum allowed window size */
tcph->check = 0;/* if you set a checksum to zero, your kernel's IP stack
should fill in the correct checksum during transmission */
tcph->urg_ptr = 0;
iph->ip_sum = csum ((unsigned short *) datagram, iph->ip_len >> 1);
/* finally, it is very advisable to do a IP_HDRINCL call, to make sure
that the kernel knows the header is included in the data, and doesn't
insert its own header into the packet before our data */
{ /* lets do it the ugly way.. */
int one = 1;
const int *val = &one;
if (setsockopt (s, IPPROTO_IP, IP_HDRINCL, val, sizeof (one)) < 0)
printf ("Warning: Cannot set HDRINCL!\n");
}
int i1=49;
char i2 = 49;
int cc = 0;
int r1;
while (cc<99)
{
ipp[6] = i1;
ipp[7] = i2;
if(i1 == 57)
{
break;
}
if(i2 == 57 )
{ i2 = 48;
i1++;
}
i2++;
cc++;
iph->ip_src.s_addr = inet_addr (ipp);
if (sendto (s, /* our socket */
datagram, /* the buffer containing headers and data */
iph->ip_len, /* total length of our datagram */
0, /* routing flags, normally always 0 */
(struct sockaddr *) &sin, /* socket addr, just like in */
sizeof (sin)) < 0) /* a normal send() */
printf ("error\n");
else
printf ("sent \n");
}
return 0;
}
the code creates a raw socket with proper ip address and the source addrewss is 192.168.1.12...port 5000 ... the problem is that when i try to capture the packets sent using wireshark, it says something like bogus tcp headee length...should be atleast 20 ... how can i correct this error ...
P.S. you can refer link for the code explanation if you find the code lengthy ...

Answer 1

After further reading: change this line
struct tcphdr *tcph = (struct tcphdr *) datagram + sizeof (struct ip);
to this
struct tcphdr *tcph = (struct tcphdr *) (datagram + sizeof (struct ip));
In the first case tcph would pointer far beyond iph.
char *datagramshould be unsigned char *too.