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