1. 根目录:
2. include
3. net
4. gre.h

#ifndef __LINUX_GRE_H
#define __LINUX_GRE_H

#include <linux/skbuff.h>
#include <net/ip_tunnels.h>

#define GREPROTO_CISCO		0
#define GREPROTO_PPTP		1
#define GREPROTO_MAX		2

struct gre_protocol {
	int  (*handler)(struct sk_buff *skb);
	void (*err_handler)(struct sk_buff *skb, u32 info);
};

struct gre_base_hdr {
	__be16 flags;
	__be16 protocol;
};
#define GRE_HEADER_SECTION 4

int gre_add_protocol(const struct gre_protocol *proto, u8 version);
int gre_del_protocol(const struct gre_protocol *proto, u8 version);

static inline __be16 gre_flags_to_tnl_flags(__be16 flags)
{
	__be16 tflags = 0;

	if (flags & GRE_CSUM)
		tflags |= TUNNEL_CSUM;
	if (flags & GRE_ROUTING)
		tflags |= TUNNEL_ROUTING;
	if (flags & GRE_KEY)
		tflags |= TUNNEL_KEY;
	if (flags & GRE_SEQ)
		tflags |= TUNNEL_SEQ;
	if (flags & GRE_STRICT)
		tflags |= TUNNEL_STRICT;
	if (flags & GRE_REC)
		tflags |= TUNNEL_REC;
	if (flags & GRE_VERSION)
		tflags |= TUNNEL_VERSION;

	return tflags;
}

static inline __be16 tnl_flags_to_gre_flags(__be16 tflags)
{
	__be16 flags = 0;

	if (tflags & TUNNEL_CSUM)
		flags |= GRE_CSUM;
	if (tflags & TUNNEL_ROUTING)
		flags |= GRE_ROUTING;
	if (tflags & TUNNEL_KEY)
		flags |= GRE_KEY;
	if (tflags & TUNNEL_SEQ)
		flags |= GRE_SEQ;
	if (tflags & TUNNEL_STRICT)
		flags |= GRE_STRICT;
	if (tflags & TUNNEL_REC)
		flags |= GRE_REC;
	if (tflags & TUNNEL_VERSION)
		flags |= GRE_VERSION;

	return flags;
}

#endif