*******************************************************************
2.6.19                                                            *
                                                                  *
From skbuff.h                                                     *
#define CHECKSUM_NONE 0                                           *
#define CHECKSUM_PARTIAL 1                                        *
#define CHECKSUM_UNNECESSARY 2                                    *
#define CHECKSUM_COMPLETE 3                                       *
                                                                  *
/*                                                                *
 * Invalidate hardware checksum when packet is to be mangled, and *
 * complete checksum manually on outgoing path.                   *
 */                                                               *
int skb_checksum_help(struct sk_buff *skb)                        *
{                                                                 *
	unsigned int csum;                                        *
	int ret = 0, offset = skb->h.raw - skb->data;             *
                                                                  *
	if (skb->ip_summed == CHECKSUM_COMPLETE)                  *
		goto out_set_summed;                              *
                                                                  *
	if (unlikely(skb_shinfo(skb)->gso_size)) {                *
		/* Let GSO fix up the checksum. */                *
		goto out_set_summed;                              *
	}                                                         *
                                                                  *
	if (skb_cloned(skb)) {                                    *
		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);    *
		if (ret)                                          *
			goto out;                                 *
	}                                                         *
                                                                  *
	BUG_ON(offset > (int)skb->len);                           *
	csum = skb_checksum(skb, offset, skb->len-offset, 0);     *
                                                                  *
	offset = skb->tail - skb->h.raw;                          *
	BUG_ON(offset <= 0);                                      *
	BUG_ON(skb->csum + 2 > offset);                           *
                                                                  *
	*(u16*)(skb->h.raw + skb->csum) = csum_fold(csum);        *
                                                                  *
out_set_summed:                                                   *
	skb->ip_summed = CHECKSUM_NONE;                           *
out:	                                                          *
	return ret;                                               *
}                                                                 *
*******************************************************************
2.6.18								  *
								  *
From skbuff.h							  *
#define CHECKSUM_NONE 0						  *
#define CHECKSUM_HW 1						  *
#define CHECKSUM_UNNECESSARY 2//LINUX_VERSION_CODE		  *
								  *
/*								  *
 * Invalidate hardware checksum when packet is to be mangled, and *
 * complete checksum manually on outgoing path.			  *
 */								  *
int skb_checksum_help(struct sk_buff *skb, int inward)		  *
{								  *
	unsigned int csum;					  *
	int ret = 0, offset = skb->h.raw - skb->data;		  *
								  *
	if (inward)						  *
		goto out_set_summed;				  *
								  *
	if (unlikely(skb_shinfo(skb)->gso_size)) {		  *
		/* Let GSO fix up the checksum. */		  *
		goto out_set_summed;				  *
	}							  *
								  *
	if (skb_cloned(skb)) {					  *
		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);	  *
		if (ret)					  *
			goto out;				  *
	}							  *
								  *
	BUG_ON(offset > (int)skb->len);				  *
	csum = skb_checksum(skb, offset, skb->len-offset, 0);	  *
								  *
	offset = skb->tail - skb->h.raw;			  *
	BUG_ON(offset <= 0);					  *
	BUG_ON(skb->csum + 2 > offset);				  *
								  *
	*(u16*)(skb->h.raw + skb->csum) = csum_fold(csum);  	  *
								  *
out_set_summed:					 	          *
	skb->ip_summed = CHECKSUM_NONE;				  *
out:								  *
	return ret;						  *
}							          *
*******************************************************************