#include <linux/socket.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/netlink.h>
#include <net/sock.h>
#include <net/tcp_states.h>

/* Sollte in .../include/linux/netlink.h stehen */
#define NETLINK_TEST 25

#define MY_NL_COMMAND 0x11
#define MY_MESSAGE "Answer from Kernel"

static struct sock *nl_sk = NULL;
static DEFINE_MUTEX(nltest_mutex);

static void 
nltest_netlink_server(struct sk_buff *skb)
{
  struct nlmsghdr *nlh, *hdr;
  u32 pid;
  struct sk_buff *ret_skb;
  char *data_ptr;

  printk("nltest_netlink_server(%p)\n", skb);
  BUG_ON(!skb);
  nlh = nlmsg_hdr(skb);
  pid = nlh->nlmsg_pid;
  printk("pid: %d, seq: %d, data: \"%s\"\n",
         pid, nlh->nlmsg_seq,
	 (char *)nlmsg_data(nlh));

  /* Antwort verschicken  */
  ret_skb = nlmsg_new(NLMSG_GOODSIZE, GFP_KERNEL);
  if (ret_skb == NULL) {
    printk("%s: no mem\n", __FUNCTION__);
    return;
  }

  /* Header vorbereiten */
  hdr = nlmsg_put(ret_skb,0,nlh->nlmsg_seq,
    NLMSG_DONE, strlen(MY_MESSAGE),0);
  if(IS_ERR(hdr)) {
    printk("nlmsg_put failed\n");
    nlmsg_free( ret_skb );
    return;
  }

  data_ptr = nlmsg_data(hdr);
  strcpy(data_ptr, MY_MESSAGE);
  nlmsg_end( ret_skb, nlh );

  netlink_unicast(nl_sk, ret_skb, pid, MSG_DONTWAIT);
}

static int 
__init nltest_module_init(void)
{
  printk("nltest_module_init\n");
  nl_sk = netlink_kernel_create(&init_net, NETLINK_TEST,
    0, nltest_netlink_server, &nltest_mutex, THIS_MODULE );
  return 0;
}

static void 
__exit nltest_module_exit(void)
{
  printk("nltest_module_exit\n");
  if( nl_sk )
    sock_release(nl_sk->sk_socket);
}

module_init(nltest_module_init);
module_exit(nltest_module_exit);
MODULE_LICENSE("GPL");