packet_id.h

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/*
 *  OpenVPN -- An application to securely tunnel IP networks
 *             over a single TCP/UDP port, with support for SSL/TLS-based
 *             session authentication and key exchange,
 *             packet encryption, packet authentication, and
 *             packet compression.
 *
 *  Copyright (C) 2002-2025 OpenVPN Inc <sales@openvpn.net>
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License version 2
 *  as published by the Free Software Foundation.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, see <https://www.gnu.org/licenses/>.
 */
 
/*
 * These routines are designed to catch replay attacks,
 * where a man-in-the-middle captures packets and then
 * attempts to replay them back later.
 */
 
#ifndef PACKET_ID_H
#define PACKET_ID_H
 
#include "circ_list.h"
#include "buffer.h"
#include "error.h"
#include "otime.h"
 
/*
 * These are the types that members of a struct packet_id_net are converted
 * to for network transmission and for saving to a persistent file.
 *
 * Note: data epoch data uses a 64 bit packet ID
 * compromised of 16 bit epoch and 48 bit per-epoch packet counter.
 * These are ephemeral and are never saved to a file.
 */
typedef uint32_t packet_id_type;
#define PACKET_ID_MAX       UINT32_MAX
#define PACKET_ID_EPOCH_MAX 0x0000ffffffffffffull
#define PACKET_ID_MASK      0x0000ffffffffffffull
typedef uint32_t net_time_t;
 
/*
 * In TLS mode, when a packet ID gets to this level,
 * start thinking about triggering a new
 * SSL/TLS handshake.
 */
#define PACKET_ID_WRAP_TRIGGER 0xFF000000
 
/* convert a packet_id_type from host to network order */
#define htonpid(x) htonl(x)
 
/* convert a packet_id_type from network to host order */
#define ntohpid(x) ntohl(x)
 
/* convert a time_t in host order to a net_time_t in network order */
#define htontime(x) htonl((net_time_t)x)
 
/* convert a net_time_t in network order to a time_t in host order */
#define ntohtime(x) ((time_t)ntohl(x))
 
 
/*
 * Printf formats for special types
 */
#define packet_id_format "%" PRIu64
typedef uint64_t packet_id_print_type;
 
/*
 * Maximum allowed backtrack in
 * sequence number due to packets arriving
 * out of order.
 */
#define MIN_SEQ_BACKTRACK     0
#define MAX_SEQ_BACKTRACK     65536
#define DEFAULT_SEQ_BACKTRACK 64
 
/*
 * Maximum allowed backtrack in
 * seconds due to packets arriving
 * out of order.
 */
#define MIN_TIME_BACKTRACK     0
#define MAX_TIME_BACKTRACK     600
#define DEFAULT_TIME_BACKTRACK 15
 
/*
 * Do a reap pass through the sequence number
 * array once every n seconds in order to
 * expire sequence numbers which can no longer
 * be accepted because they would violate
 * TIME_BACKTRACK.
 */
#define SEQ_REAP_INTERVAL 5
 
CIRC_LIST(seq_list, time_t);
 
/*
 * This is the data structure we keep on the receiving side,
 * to check that no packet-id (i.e. sequence number + optional timestamp)
 * is accepted more than once.
 */
struct packet_id_rec
{
    time_t last_reap;            /* last call of packet_id_reap */
    time_t time;                 /* highest time stamp received */
    uint64_t id;                 /* highest sequence number received */
    uint64_t seq_backtrack;      /* set from --replay-window */
    int time_backtrack;          /* set from --replay-window */
    uint64_t max_backtrack_stat; /* maximum backtrack seen so far */
    bool initialized;            /* true if packet_id_init was called */
    struct seq_list *seq_list;   /* packet-id "memory" */
    const char *name;
    int unit;
};
 
/*
 * file to facilitate cross-session persistence
 * of time/id
 */
struct packet_id_persist
{
    const char *filename;
    int fd;
    time_t time;       /* time stamp */
    packet_id_type id; /* sequence number */
    time_t time_last_written;
    packet_id_type id_last_written;
};
 
struct packet_id_persist_file_image
{
    time_t time;       /* time stamp */
    packet_id_type id; /* sequence number */
};
 
/*
 * Keep a record of our current packet-id state
 * on the sending side.
 */
struct packet_id_send
{
    uint64_t id;
    time_t time;
};
 
/*
 * Communicate packet-id over the wire.
 * A short packet-id is just a 32 bit
 * sequence number.  A long packet-id
 * includes a timestamp as well.
 *
 * An epoch packet-id is a 16 bit epoch
 * counter plus a 48 per-epoch packet-id.
 *
 *
 * Long packet-ids are used as IVs for
 * CFB/OFB ciphers and for control channel
 * messages.
 *
 * This data structure is always sent
 * over the net in network byte order,
 * by calling htonpid, ntohpid,
 * htontime, and ntohtime on the
 * data elements to change them
 * to and from standard sizes.
 *
 * In addition, time is converted to
 * a net_time_t before sending,
 * since openvpn always
 * uses a 32-bit time_t but some
 * 64 bit platforms use a
 * 64 bit time_t.
 */
 
struct packet_id_net
{
    /* converted to packet_id_type on non-epoch data ids, does not contain
     * the epoch but is a flat id */
    uint64_t id;
    time_t time; /* converted to net_time_t before transmission */
};
 
struct packet_id
{
    struct packet_id_send send;
    struct packet_id_rec rec;
};
 
void packet_id_init(struct packet_id *p, int seq_backtrack, int time_backtrack, const char *name,
                    int unit);
 
void packet_id_free(struct packet_id *p);
 
void packet_id_move_recv(struct packet_id_rec *dest, struct packet_id_rec *src);
 
/* should we accept an incoming packet id ? */
bool packet_id_test(struct packet_id_rec *p, const struct packet_id_net *pin);
 
/* change our current state to reflect an accepted packet id */
void packet_id_add(struct packet_id_rec *p, const struct packet_id_net *pin);
 
/* expire TIME_BACKTRACK sequence numbers */
void packet_id_reap(struct packet_id_rec *p);
 
/*
 * packet ID persistence
 */
 
/* initialize the packet_id_persist structure in a disabled state */
void packet_id_persist_init(struct packet_id_persist *p);
 
/* close the file descriptor if it is open, and switch to disabled state */
void packet_id_persist_close(struct packet_id_persist *p);
 
/* load persisted rec packet_id (time and id) only once from file, and set state to enabled */
void packet_id_persist_load(struct packet_id_persist *p, const char *filename);
 
/* save persisted rec packet_id (time and id) to file (only if enabled state) */
void packet_id_persist_save(struct packet_id_persist *p);
 
/* transfer packet_id_persist -> packet_id */
void packet_id_persist_load_obj(const struct packet_id_persist *p, struct packet_id *pid);
 
/* return an ascii string representing a packet_id_persist object */
const char *packet_id_persist_print(const struct packet_id_persist *p, struct gc_arena *gc);
 
/*
 * Read/write a packet ID to/from the buffer.  Short form is sequence number
 * only.  Long form is sequence number and timestamp.
 */
 
bool packet_id_read(struct packet_id_net *pin, struct buffer *buf, bool long_form);
 
bool packet_id_write(struct packet_id_send *p, struct buffer *buf, bool long_form, bool prepend);
 
/*
 * Inline functions.
 */
 
static inline bool
packet_id_initialized(const struct packet_id *pid)
{
    return pid->rec.initialized;
}
 
/* are we in enabled state? */
static inline bool
packet_id_persist_enabled(const struct packet_id_persist *p)
{
    return p->fd >= 0;
}
 
/* transfer packet_id -> packet_id_persist */
static inline void
packet_id_persist_save_obj(struct packet_id_persist *p, const struct packet_id *pid)
{
    if (packet_id_persist_enabled(p) && pid->rec.time)
    {
        p->time = pid->rec.time;
        p->id = pid->rec.id;
    }
}
 
static inline void
reset_packet_id_send(struct packet_id_send *p)
{
    p->time = 0;
    p->id = 0;
}
 
const char *packet_id_net_print(const struct packet_id_net *pin, bool print_timestamp,
                                struct gc_arena *gc);
 
static inline int
packet_id_size(bool long_form)
{
    return sizeof(packet_id_type) + (long_form ? sizeof(net_time_t) : 0);
}
 
static inline bool
packet_id_close_to_wrapping(const struct packet_id_send *p)
{
    return p->id >= PACKET_ID_WRAP_TRIGGER;
}
 
static inline void
packet_id_reap_test(struct packet_id_rec *p)
{
    if (p->last_reap + SEQ_REAP_INTERVAL <= now)
    {
        packet_id_reap(p);
    }
}
 
bool packet_id_write_epoch(struct packet_id_send *p, uint16_t epoch, struct buffer *buf);
 
uint16_t packet_id_read_epoch(struct packet_id_net *p, struct buffer *buf);
 
#endif /* PACKET_ID_H */