what is your kernel version?
maybe you should use NF_INET_PRE_ROUTING instead.

2012-05-22



c265n46



????Chir0n
?????2012-05-08 12:14
???Why I can't compile a simple netfilter hook module?
????"kernelnewbies"[off-list ref]
???

I'm using this Makefile:

obj-m += hello.o

all:
       make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
       make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean


The hello.c is this:

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netfilter.h>
#include <linux/netfilter_ipv4.h>

static struct nf_hook_ops nfho;         //struct holding set of hook function options

//function to be called by hook
unsigned int hook_func(unsigned int hooknum, struct sk_buff **skb, const struct net_device *in, const struct net_device *out, int (*okfn)(struct sk_buff *))
{
 printk(KERN_INFO "packet dropped\n");                                             //log to var/log/messages
 return NF_DROP;                                                                   //drops the packet
}

//Called when module loaded using 'insmod'
int init_module()
{
 nfho.hook = hook_func;                       //function to call when conditions below met
 nfho.hooknum = NF_IP_PRE_ROUTING;            //called right after packet recieved, first hook in Netfilter
 nfho.pf = PF_INET;                           //IPV4 packets
 nfho.priority = NF_IP_PRI_FIRST;             //set to highest priority over all other hook functions
 nf_register_hook(&nfho);                     //register hook

 return 0;                                    //return 0 for success
}

//Called when module unloaded using 'rmmod'
void cleanup_module()
{
 nf_unregister_hook(&nfho);                     //cleanup ? unregister hook
}


Here are the error message:

$ make
make -C /lib/modules/3.3.2-6.fc16.x86_64/build M=/home/fabio/Desktop/modules modules
make[1]: Entering directory `/usr/src/kernels/3.3.2-6.fc16.x86_64'
 CC [M]  /home/fabio/Desktop/modules/hello.o
/home/fabio/Desktop/modules/hello.c: In function ?init_module?:
/home/fabio/Desktop/modules/hello.c:18:13: warning: assignment from incompatible pointer type [enabled by default]
/home/fabio/Desktop/modules/hello.c:19:18: error: ?NF_IP_PRE_ROUTING? undeclared (first use in this function)
/home/fabio/Desktop/modules/hello.c:19:18: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [/home/fabio/Desktop/modules/hello.o] Error 1
make[1]: *** [_module_/home/fabio/Desktop/modules] Error 2
make[1]: Leaving directory `/usr/src/kernels/3.3.2-6.fc16.x86_64'
make: *** [all] Error 2
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