build ip packet with C,socket programming

z99 · 01-30-2011, 03:47 AM

Hi,
How can we build a packet using C?
we have a structure called sockaddr_in which is use to for IPv4,
so that we can define address,port and etc in this way:

Code:

struct sockaddr_in sock;
sock.sin_family=AF_INET;
sock.sin_port=htons(25);
and
..
..

does ip here:

Code:

struct ip ipheader

refer to any standard structure which is dealing with ip packet (like sockaddr_in which deals with ipv4)?
if yes
can we define ip headers this way [not using a pointer,is it even possible??]:

Code:

struct ip iph;
iph.v = 4;
iph.tos = 0;
ip.ttl = 255;
etc 
..
..

instead of this way:

Code:

struct ip *iph = (struct ip *) datagram;
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 */
  etc
  ..

thanks in advance

dwhitney67 · 01-30-2011, 08:48 AM

It doesn't really matter how you construct the IP header, as long as it is ultimately included in the datagram that you wish to send.

I've typically seen where the datagram is allocated on the stack, with the number of bytes being allocated equal to the sum of the sizes of the IP header, the TCP (or UDP) header, and the message.

So, something like the following:

Code:

unsigned char* datagram = malloc(sizeof(struct ip) + sizeof(struct tcphdr) + messageLength);

struct ip* ip = (struct ip*) datagram;

...  /* fill in ip header */

struct tcphdr* tcph = (struct tcphdr*)(datagram + (ip->ip_hl << 2));        /* compute offset where tcp header begins */

... /* fill in tcp header */

unsigned char* msg  = datagram + (ip->ip_hl << 2) + sizeof(struct tcphdr);  /* compute offset where message begins */

... /* copy message */

... /* compute checksum(s) */

... /* send away! */

P.S. You should consider parting with some spare change to procure this book.

z99 · 01-30-2011, 02:34 PM

thanks dwhitney67,
i read about pointers
i know

Code:

struct ip *iph

will make a pointer called iph but i can't just figure this out,

Code:

(struct ip*)datagram

what does

Code:

(struct ip*)

mean?

wje_lq · 01-30-2011, 03:53 PM

Quote:

Originally Posted by dwhitney67

You should consider parting with some spare change to procure this book.

I agree, sort of. But as that page itself hints, buy the third edition instead.

z99 · 01-30-2011, 04:23 PM

well,as my country is under prohibition,i can't buy from Amazon,
so would you please just give me a brief description?

dwhitney67 · 01-31-2011, 03:47 AM

The (struct ip*) is merely a cast, so that the datagram variable, which is declared as an unsigned char*, can be interpreted differently.