text: #ifndef __LINUX_NETFILTER_H
  #define __LINUX_NETFILTER_H
  
  #ifdef __KERNEL__
  #include <linux/init.h>
  #include <linux/types.h>
  #include <linux/skbuff.h>
  #include <linux/net.h>
  #include <linux/if.h>
  #include <linux/wait.h>
  #include <linux/list.h>
  #endif
  
  /* Responses from hook functions. */
  #define NF_DROP 0
  #define NF_ACCEPT 1
  #define NF_STOLEN 2
  #define NF_QUEUE 3
  #define NF_REPEAT 4
  #define NF_MAX_VERDICT NF_REPEAT
  
  /* Generic cache responses from hook functions.
     <= 0x2000 is used for protocol-flags. */
  #define NFC_UNKNOWN 0x4000
  #define NFC_ALTERED 0x8000
  
  #ifdef __KERNEL__
  #include <linux/config.h>
  #ifdef CONFIG_NETFILTER
  
  extern void netfilter_init(void);
  
  /* Largest hook number + 1 */
  #define NF_MAX_HOOKS 8
  
  struct sk_buff;
  struct net_device;
  
  typedef unsigned int nf_hookfn(unsigned int hooknum,
  			       struct sk_buff **skb,
  			       const struct net_device *in,
  			       const struct net_device *out,
  			       int (*okfn)(struct sk_buff *));
  
  struct nf_hook_ops
  {
  	struct list_head list;
  
  	/* User fills in from here down. */
  	nf_hookfn *hook;
  	struct module *owner;
  	int pf;
  	int hooknum;
  	/* Hooks are ordered in ascending priority. */
  	int priority;
  };
  
  struct nf_sockopt_ops
  {
  	struct list_head list;
  
  	int pf;
  
  	/* Non-inclusive ranges: use 0/0/NULL to never get called. */
  	int set_optmin;
  	int set_optmax;
  	int (*set)(struct sock *sk, int optval, void *user, unsigned int len);
  
  	int get_optmin;
  	int get_optmax;
  	int (*get)(struct sock *sk, int optval, void *user, int *len);
  
  	/* Number of users inside set() or get(). */
  	unsigned int use;
  	struct task_struct *cleanup_task;
  };
  
  /* Each queued (to userspace) skbuff has one of these. */
  struct nf_info
  {
  	/* The ops struct which sent us to userspace. */
  	struct nf_hook_ops *elem;
  	
  	/* If we're sent to userspace, this keeps housekeeping info */
  	int pf;
  	unsigned int hook;
  	struct net_device *indev, *outdev;
  	int (*okfn)(struct sk_buff *);
  };
                                                                                  
  /* Function to register/unregister hook points. */
  int nf_register_hook(struct nf_hook_ops *reg);
  void nf_unregister_hook(struct nf_hook_ops *reg);
  
  /* Functions to register get/setsockopt ranges (non-inclusive).  You
     need to check permissions yourself! */
  int nf_register_sockopt(struct nf_sockopt_ops *reg);
  void nf_unregister_sockopt(struct nf_sockopt_ops *reg);
  
  extern struct list_head nf_hooks[NPROTO][NF_MAX_HOOKS];
  
  /* Activate hook; either okfn or kfree_skb called, unless a hook
     returns NF_STOLEN (in which case, it's up to the hook to deal with
     the consequences).
  
     Returns -ERRNO if packet dropped.  Zero means queued, stolen or
     accepted.
  */
  
  /* RR:
     > I don't want nf_hook to return anything because people might forget
     > about async and trust the return value to mean "packet was ok".
  
     AK:
     Just document it clearly, then you can expect some sense from kernel
     coders :)
  */
  
  /* This is gross, but inline doesn't cut it for avoiding the function
     call in fast path: gcc doesn't inline (needs value tracking?). --RR */
  #ifdef CONFIG_NETFILTER_DEBUG
  #define NF_HOOK(pf, hook, skb, indev, outdev, okfn)			\
   nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), INT_MIN)
  #define NF_HOOK_THRESH nf_hook_slow
  #else
  #define NF_HOOK(pf, hook, skb, indev, outdev, okfn)			\
  (list_empty(&nf_hooks[(pf)][(hook)])					\
   ? (okfn)(skb)								\
   : nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), INT_MIN))
  #define NF_HOOK_THRESH(pf, hook, skb, indev, outdev, okfn, thresh)	\
  (list_empty(&nf_hooks[(pf)][(hook)])					\
   ? (okfn)(skb)								\
   : nf_hook_slow((pf), (hook), (skb), (indev), (outdev), (okfn), (thresh)))
  #endif
  
  int nf_hook_slow(int pf, unsigned int hook, struct sk_buff *skb,
  		 struct net_device *indev, struct net_device *outdev,
  		 int (*okfn)(struct sk_buff *), int thresh);
  
  /* Call setsockopt() */
  int nf_setsockopt(struct sock *sk, int pf, int optval, char *opt, 
  		  int len);
  int nf_getsockopt(struct sock *sk, int pf, int optval, char *opt,
  		  int *len);
  
  /* Packet queuing */
  typedef int (*nf_queue_outfn_t)(struct sk_buff *skb, 
                                  struct nf_info *info, void *data);
  extern int nf_register_queue_handler(int pf, 
                                       nf_queue_outfn_t outfn, void *data);
  extern int nf_unregister_queue_handler(int pf);
  extern void nf_reinject(struct sk_buff *skb,
  			struct nf_info *info,
  			unsigned int verdict);
  
  extern void (*ip_ct_attach)(struct sk_buff *, struct nf_ct_info *);
  
  #ifdef CONFIG_NETFILTER_DEBUG
  extern void nf_dump_skb(int pf, struct sk_buff *skb);
  #endif
  
  /* FIXME: Before cache is ever used, this must be implemented for real. */
  extern void nf_invalidate_cache(int pf);
  
  #else /* !CONFIG_NETFILTER */
  #define NF_HOOK(pf, hook, skb, indev, outdev, okfn) (okfn)(skb)
  #endif /*CONFIG_NETFILTER*/
  
  #endif /*__KERNEL__*/
  #endif /*__LINUX_NETFILTER_H*/
  