How to change the source address of a tcp packet

How to change the source address of a tcp packet

[JC]

netfilter hook function called at NF_IP_PRE_ROUTING:

unsigned int in_hook(unsigned int hooknum,
struct sk_buff **skb,
const struct net_device *in,
const struct net_device *out,
int (*okfn)(struct sk_buff *))
{
  struct iphdr *my_ipheader;
  struct tcphdr *my_tcpheader;
  struct udphdr *my_udpheader;

  unsigned char *my_ip1;
  unsigned char *my_ip2;
  unsigned char *my_ip3;
  unsigned char *my_ip4;
  my_ip1 = "SOMETHING_IN_HEX";  //
  my_ip2 = "SOMETHING_IN_HEX";  //
  my_ip3 = "SOMETHING_IN_HEX";  //
  my_ip4 = "SOMETHING_IN_HEX";  //

  if (out)
    {
      if(out->name)
	{
	  // We do NOT want to handle a "weird" null packet
	  if(skb == NULL)
	    {
		  print_string("NULL skb!!!");
		  return NF_ACCEPT;
	    }
	
	    my_ipheader = (struct iphdr*)((*skb)->nh.iph);
	    if (my_ipheader->protocol == IPPROTO_TCP)
	      {
		  print_string("This is a tcp packet");
		  if (my_ipheader->saddr == *(unsigned int *)my_ip3)
		    {
		       print_string("with source IP xxx.xxx.xxx.xxx");
		      ((struct iphdr*)((*skb)->nh.iph))->saddr = my_ip2;
		      ((struct sk_buff*)(*skb))->proto_csum_blank = 1;
		      return NF_ACCEPT;
		    }
	      }
	}
      else
	{
	  print_string("out is null");
	}
    }

  print_string("packet is being accepted!");
  return NF_ACCEPT;
}

Apparently, these two steps are not sufficient to change the source
address of an skb:
   ((struct iphdr*)((*skb)->nh.iph))->saddr = my_ip2;
   ((struct sk_buff*)(*skb))->proto_csum_blank = 1;

All packets that are treated by these are actually dropped.
What *IS* required so that the source address of the skb is changed to
another that is on another interface of the machine?

J