Re: Recalculate checksums in netfilter queue

[Justin Yaple <yaplej@gmail.com>]

> The RFC for IP contains C code that you can literally copy and paste for
> this task.
>
> -- Morgon

Thanks.  I finally have both my tcp checksum, and ip checksum
functions working.  I found where I was doing some calculations in
reverse order than the example given in the RFC.  Would there be any
reason not to add checksum functions to libnetfilter_queue?

unsigned short tcp_sum_calc(unsigned short len_tcp, unsigned short
*src_addr, unsigned short *dest_addr, unsigned short *buff)
{
	unsigned short prot_tcp = 6;
	long sum = 0;
	int i = 0;
	
	/* Check if the tcp length is even or odd.  Add padding if odd. */
	if((len_tcp % 2) == 1){
		buff[len_tcp] = 0;  // Empty space in the ip buffer should be 0 anyway.
      	len_tcp += 1; // increase length to make even.
	}
	
	/* add the pseudo header */	
	sum += ntohs(src_addr[0]);
	sum += ntohs(src_addr[1]);
	sum += ntohs(dest_addr[0]);
	sum += ntohs(dest_addr[1]);
	sum += len_tcp; // already in host format.
	sum += prot_tcp; // already in host format.

	/*
	 * calculate the checksum for the tcp header and payload
	 * len_tcp represents number of 8-bit bytes,
	 * we are working with 16-bit words so divide len_tcp by 2.
	 */
	for(i=0;i<(len_tcp/2);i++){
		sum += ntohs(buff[i]);
	}

	// keep only the last 16 bits of the 32 bit calculated sum and add the carries
	while (sum >> 16){
		sum = (sum & 0xFFFF) + (sum >> 16);
	}
	
	// Take the bitwise complement of sum
	sum = ~sum;

	return htons(((unsigned short) sum));
}

unsigned short ip_sum_calc(unsigned short len_ip_header, unsigned short *buff){
	long sum = 0;
	int i = 0;
	
	for (i=0;i<len_ip_header/2;i++){
		sum += ntohs(buff[i]);
	}
	
	while (sum >> 16){
		sum = (sum & 0xFFFF) + (sum >> 16);
	}
	
	sum = ~sum;
	
	return htons(((unsigned short) sum));
}