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Old 09-20-2005, 07:13 PM   #1
LQ Newbie
Registered: Sep 2005
Posts: 9

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netfilter hook function error

my hook function code, called at 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 *))
  print_string("Packet reached IN_HOOK.");

  struct iphdr *my_ipheader;
  //u32 this_address;

  if (out)
	  print_string("out is: ");

	  // compare the out device with the list of rules
	  // TODO: replace by matches_rule()
	  if (strcmp(out->name, "eth1") == 0) 

	      print_string("packet matches rule");

	      if(skb == NULL)
		  return -1;
	      my_ipheader = skb->nh->iph;
	      print_string("printk-ing saddr...");
	      //this_address = *my_ipheader->saddr;
	      //printk("the ip is : %d.%d.%d.%d\n",NIPQUAD(*my_ipheader->saddr));

	  print_string("out is null");

  return NF_QUEUE;           /* Drop ALL packets */
line 150 is: my_ipheader = skb->nh->iph;

on compile, I get this error:

[root@whatever spider]# make -C /usr/src/kernels/`uname -r`-i686 SUBDIRS=$PWD modules
make: Entering directory `/usr/src/kernels/2.6.12-1.1447_FC4-i686'
CC [M] /home/jc/code/spider/spider.o
/home/jc/code/spider/spider.c: In function ‘in_hook’:
/home/jc/code/spider/spider.c:126: warning: ISO C90 forbids mixed declarations and code
/home/jc/code/spider/spider.c:134: warning: passing argument 1 of ‘print_string’ discards qualifiers from pointer target type
/home/jc/code/spider/spider.c:150: error: request for member ‘nh’ in something not a structure or union

I've seen that statement being made more than a number of times, so what am I doing wrong??

I need to change the saddr (source address) in that if to another one (basicly, NATing it). How will I commit that change to the skb and then send it back into the stack?

thanx in advance...

Old 09-23-2005, 04:24 PM   #2
Registered: Feb 2002
Location: Grenoble
Distribution: Debian
Posts: 9,571

Rep: Reputation: 180Reputation: 180
I guess you want to change content of the struct of type sk_buff, right? You have a pointer to pointer, so you should write:
my_ipheader = (*skb)->nh->iph;
Of course, if I understand what you want to do correctly.


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