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The C and C++ Include Header Files

cat -n /usr/include/linux/netfilter/xt_l2tp.h

     1	/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
     2	#ifndef _LINUX_NETFILTER_XT_L2TP_H
     3	#define _LINUX_NETFILTER_XT_L2TP_H
     4	
     5	#include <linux/types.h>
     6	
     7	enum xt_l2tp_type {
     8		XT_L2TP_TYPE_CONTROL,
     9		XT_L2TP_TYPE_DATA,
    10	};
    11	
    12	/* L2TP matching stuff */
    13	struct xt_l2tp_info {
    14		__u32 tid;			/* tunnel id */
    15		__u32 sid;			/* session id */
    16		__u8 version;			/* L2TP protocol version */
    17		__u8 type;			/* L2TP packet type */
    18		__u8 flags;			/* which fields to match */
    19	};
    20	
    21	enum {
    22		XT_L2TP_TID	= (1 << 0),	/* match L2TP tunnel id */
    23		XT_L2TP_SID	= (1 << 1),	/* match L2TP session id */
    24		XT_L2TP_VERSION	= (1 << 2),	/* match L2TP protocol version */
    25		XT_L2TP_TYPE	= (1 << 3),	/* match L2TP packet type */
    26	};
    27	
    28	#endif /* _LINUX_NETFILTER_XT_L2TP_H */