More data in packet payload - network-programming

Question

I have the following code
int ParseData(unsigned char *packet, int len)
{
struct ethhdr *ethernet_header;
struct iphdr *ip_header;
struct tcphdr *tcp_header;
unsigned char *data;
int data_len;
/* Check if any data is there */
if(len > (sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct tcphdr)))
{
ip_header = (struct iphdr*)(packet + sizeof(struct ethhdr));
data = (packet + sizeof(struct ethhdr) + ip_header->ihl*4 + sizeof(struct tcphdr));
data_len = ntohs(ip_header->tot_len) - ip_header->ihl*4 - sizeof(struct tcphdr);
if(data_len)
{
printf("Data Len : %d\n", data_len);
PrintData("Data : ", data, data_len);
printf("\n\n");
return 1;
}
else
{
printf("No Data in packet\n");
return 0;
}
}
}
I am trying to print in ASCII the payload and with a simple function like this
PrintData(char *mesg, unsigned char *p, int len)
{
printf(mesg);
while(len--)
{
if(isprint(*p))
printf("%c", *p);
else
printf(".");
p++;
}
}
The code looks good, no compile problems/warning. The problem is that the first payload
character is not being print at position 0, but 12 bytes later.
I thought that all the "len" bytes are the exact data I have to print.
My data point at
data = (packet + sizeof(struct ethhdr) + ip_header->ihl*4 + sizeof(struct tcphdr));
however data[0] is not printable. What is the problem? Do I miss something? Do I have to check for the TCP options part maybe?
Thanks

Answer 1

That's right, adding the sizeof(struct tcphdr) is only going to get you past the header, not the options. To get to the actual data, you should use the 'offset' field from the TCP header. The offset is calculated from the start of the TCP header and is in 4-byte units, e.g. if the offset is 8 then the header + options length is 32.