occ.c

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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/>.
 */
 
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
 
#include "syshead.h"
 
#include "occ.h"
#include "forward.h"
#include "memdbg.h"
 
 
/*
 * This random string identifies an OpenVPN
 * Configuration Control packet.
 * It should be of sufficient length and randomness
 * so as not to collide with other tunnel data.
 *
 * The OCC protocol is as follows:
 *
 * occ_magic -- (16 octets)
 *
 * type [OCC_REQUEST | OCC_REPLY] (1 octet)
 * null terminated options string if OCC_REPLY (variable)
 *
 * When encryption is used, the OCC packet
 * is encapsulated within the encrypted
 * envelope.
 *
 * OCC_STRING_SIZE must be set to sizeof (occ_magic)
 */
 
const uint8_t occ_magic[] = { 0x28, 0x7f, 0x34, 0x6b, 0xd4, 0xef, 0x7a, 0x81,
                              0x2d, 0x56, 0xb8, 0xd3, 0xaf, 0xc5, 0x45, 0x9c };
 
static const struct mtu_load_test mtu_load_test_sequence[] = {
 
    { OCC_MTU_LOAD_REQUEST, -1000 },
    { OCC_MTU_LOAD, -1000 },
    { OCC_MTU_LOAD_REQUEST, -1000 },
    { OCC_MTU_LOAD, -1000 },
    { OCC_MTU_LOAD_REQUEST, -1000 },
    { OCC_MTU_LOAD, -1000 },
 
    { OCC_MTU_LOAD_REQUEST, -750 },
    { OCC_MTU_LOAD, -750 },
    { OCC_MTU_LOAD_REQUEST, -750 },
    { OCC_MTU_LOAD, -750 },
    { OCC_MTU_LOAD_REQUEST, -750 },
    { OCC_MTU_LOAD, -750 },
 
    { OCC_MTU_LOAD_REQUEST, -500 },
    { OCC_MTU_LOAD, -500 },
    { OCC_MTU_LOAD_REQUEST, -500 },
    { OCC_MTU_LOAD, -500 },
    { OCC_MTU_LOAD_REQUEST, -500 },
    { OCC_MTU_LOAD, -500 },
 
    { OCC_MTU_LOAD_REQUEST, -400 },
    { OCC_MTU_LOAD, -400 },
    { OCC_MTU_LOAD_REQUEST, -400 },
    { OCC_MTU_LOAD, -400 },
    { OCC_MTU_LOAD_REQUEST, -400 },
    { OCC_MTU_LOAD, -400 },
 
    { OCC_MTU_LOAD_REQUEST, -300 },
    { OCC_MTU_LOAD, -300 },
    { OCC_MTU_LOAD_REQUEST, -300 },
    { OCC_MTU_LOAD, -300 },
    { OCC_MTU_LOAD_REQUEST, -300 },
    { OCC_MTU_LOAD, -300 },
 
    { OCC_MTU_LOAD_REQUEST, -200 },
    { OCC_MTU_LOAD, -200 },
    { OCC_MTU_LOAD_REQUEST, -200 },
    { OCC_MTU_LOAD, -200 },
    { OCC_MTU_LOAD_REQUEST, -200 },
    { OCC_MTU_LOAD, -200 },
 
    { OCC_MTU_LOAD_REQUEST, -150 },
    { OCC_MTU_LOAD, -150 },
    { OCC_MTU_LOAD_REQUEST, -150 },
    { OCC_MTU_LOAD, -150 },
    { OCC_MTU_LOAD_REQUEST, -150 },
    { OCC_MTU_LOAD, -150 },
 
    { OCC_MTU_LOAD_REQUEST, -100 },
    { OCC_MTU_LOAD, -100 },
    { OCC_MTU_LOAD_REQUEST, -100 },
    { OCC_MTU_LOAD, -100 },
    { OCC_MTU_LOAD_REQUEST, -100 },
    { OCC_MTU_LOAD, -100 },
 
    { OCC_MTU_LOAD_REQUEST, -50 },
    { OCC_MTU_LOAD, -50 },
    { OCC_MTU_LOAD_REQUEST, -50 },
    { OCC_MTU_LOAD, -50 },
    { OCC_MTU_LOAD_REQUEST, -50 },
    { OCC_MTU_LOAD, -50 },
 
    { OCC_MTU_LOAD_REQUEST, 0 },
    { OCC_MTU_LOAD, 0 },
    { OCC_MTU_LOAD_REQUEST, 0 },
    { OCC_MTU_LOAD, 0 },
    { OCC_MTU_LOAD_REQUEST, 0 },
    { OCC_MTU_LOAD, 0 },
 
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
    { OCC_MTU_REQUEST, 0 },
 
    { -1, 0 }
};
 
void
check_send_occ_req_dowork(struct context *c)
{
    if (++c->c2.occ_n_tries >= OCC_N_TRIES)
    {
        if (c->options.ce.remote)
        {
            /*
             * No OCC_REPLY from peer after repeated attempts.
             * Give up.
             */
            msg(D_SHOW_OCC,
                "NOTE: failed to obtain options consistency info from peer -- "
                "this could occur if the remote peer is running a version of " PACKAGE_NAME
                " before 1.5-beta8 or if there is a network connectivity problem, and will not necessarily prevent " PACKAGE_NAME
                " from running (" counter_format " bytes received from peer, " counter_format
                " bytes authenticated data channel traffic) -- you can disable the options consistency "
                "check with --disable-occ.",
                c->c2.link_read_bytes, c->c2.link_read_bytes_auth);
        }
        event_timeout_clear(&c->c2.occ_interval);
    }
    else
    {
        c->c2.occ_op = OCC_REQUEST;
 
        /*
         * If we don't hear back from peer, send another
         * OCC_REQUEST in OCC_INTERVAL_SECONDS.
         */
        event_timeout_reset(&c->c2.occ_interval);
    }
}
 
void
check_send_occ_load_test_dowork(struct context *c)
{
    if (connection_established(c))
    {
        const struct mtu_load_test *entry;
 
        if (!c->c2.occ_mtu_load_n_tries)
        {
            msg(M_INFO,
                "NOTE: Beginning empirical MTU test -- results should be available in 3 to 4 minutes.");
        }
 
        entry = &mtu_load_test_sequence[c->c2.occ_mtu_load_n_tries++];
        if (entry->op >= 0)
        {
            c->c2.occ_op = entry->op;
            size_t payload_size =
                frame_calculate_payload_size(&c->c2.frame, &c->options, &c->c1.ks.key_type);
            size_t header_size =
                frame_calculate_protocol_header_size(&c->c1.ks.key_type, &c->options, false);
 
            c->c2.occ_mtu_load_size = payload_size + header_size;
        }
        else
        {
            msg(M_INFO, "NOTE: failed to empirically measure MTU (requires " PACKAGE_NAME
                        " 1.5 or higher at other end of connection).");
            event_timeout_clear(&c->c2.occ_mtu_load_test_interval);
            c->c2.occ_mtu_load_n_tries = 0;
        }
    }
}
 
void
check_send_occ_msg_dowork(struct context *c)
{
    bool doit = false;
 
    c->c2.buf = c->c2.buffers->aux_buf;
    ASSERT(buf_init(&c->c2.buf, c->c2.frame.buf.headroom));
    ASSERT(buf_safe(&c->c2.buf, c->c2.frame.buf.payload_size));
    ASSERT(buf_write(&c->c2.buf, occ_magic, OCC_STRING_SIZE));
 
    switch (c->c2.occ_op)
    {
        case OCC_REQUEST:
            if (!buf_write_u8(&c->c2.buf, OCC_REQUEST))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_REQUEST");
            doit = true;
            break;
 
        case OCC_REPLY:
            if (!c->c2.options_string_local)
            {
                break;
            }
            if (!buf_write_u8(&c->c2.buf, OCC_REPLY))
            {
                break;
            }
            if (!buf_write(&c->c2.buf, c->c2.options_string_local,
                           strlen(c->c2.options_string_local) + 1))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_REPLY");
            doit = true;
            break;
 
        case OCC_MTU_REQUEST:
            if (!buf_write_u8(&c->c2.buf, OCC_MTU_REQUEST))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_MTU_REQUEST");
            doit = true;
            break;
 
        case OCC_MTU_REPLY:
            if (!buf_write_u8(&c->c2.buf, OCC_MTU_REPLY))
            {
                break;
            }
            if (!buf_write_u16(&c->c2.buf, c->c2.max_recv_size_local))
            {
                break;
            }
            if (!buf_write_u16(&c->c2.buf, c->c2.max_send_size_local))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_MTU_REPLY");
            doit = true;
            break;
 
        case OCC_MTU_LOAD_REQUEST:
            if (!buf_write_u8(&c->c2.buf, OCC_MTU_LOAD_REQUEST))
            {
                break;
            }
            if (!buf_write_u16(&c->c2.buf, c->c2.occ_mtu_load_size))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_MTU_LOAD_REQUEST");
            doit = true;
            break;
 
        case OCC_MTU_LOAD:
        {
            int need_to_add;
 
            if (!buf_write_u8(&c->c2.buf, OCC_MTU_LOAD))
            {
                break;
            }
            size_t proto_hdr, payload_hdr;
            const struct key_type *kt = &c->c1.ks.key_type;
 
            /* OCC message have comp/fragment headers but not ethernet headers */
            payload_hdr = frame_calculate_payload_overhead(0, &c->options, kt);
 
            /* Since we do not know the payload size we just pass 0 as size here */
            proto_hdr = frame_calculate_protocol_header_size(kt, &c->options, false);
 
            need_to_add = min_int(c->c2.occ_mtu_load_size, c->c2.frame.buf.payload_size)
                          - OCC_STRING_SIZE - sizeof(uint8_t) /* occ opcode */
                          - payload_hdr - proto_hdr;
 
            while (need_to_add > 0)
            {
                /*
                 * Fill the load test packet with pseudo-random bytes.
                 */
                if (!buf_write_u8(&c->c2.buf, get_random() & 0xFF))
                {
                    break;
                }
                --need_to_add;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_MTU_LOAD min_int(%d,%d)-%d-%d-%d-%d) size=%d",
                 c->c2.occ_mtu_load_size, c->c2.frame.buf.payload_size, OCC_STRING_SIZE,
                 (int)sizeof(uint8_t), (int)payload_hdr, (int)proto_hdr, BLEN(&c->c2.buf));
            doit = true;
        }
        break;
 
        case OCC_EXIT:
            if (!buf_write_u8(&c->c2.buf, OCC_EXIT))
            {
                break;
            }
            dmsg(D_PACKET_CONTENT, "SENT OCC_EXIT");
            doit = true;
            break;
    }
 
    if (doit)
    {
        /*
         * We will treat the packet like any other outgoing packet,
         * compress, encrypt, sign, etc.
         */
        encrypt_sign(c, true);
    }
 
    c->c2.occ_op = -1;
}
 
void
process_received_occ_msg(struct context *c)
{
    ASSERT(buf_advance(&c->c2.buf, OCC_STRING_SIZE));
    switch (buf_read_u8(&c->c2.buf))
    {
        case OCC_REQUEST:
            dmsg(D_PACKET_CONTENT, "RECEIVED OCC_REQUEST");
            c->c2.occ_op = OCC_REPLY;
            break;
 
        case OCC_MTU_REQUEST:
            dmsg(D_PACKET_CONTENT, "RECEIVED OCC_MTU_REQUEST");
            c->c2.occ_op = OCC_MTU_REPLY;
            break;
 
        case OCC_MTU_LOAD_REQUEST:
            dmsg(D_PACKET_CONTENT, "RECEIVED OCC_MTU_LOAD_REQUEST");
            c->c2.occ_mtu_load_size = buf_read_u16(&c->c2.buf);
            if (c->c2.occ_mtu_load_size >= 0)
            {
                c->c2.occ_op = OCC_MTU_LOAD;
            }
            break;
 
        case OCC_REPLY:
            dmsg(D_PACKET_CONTENT, "RECEIVED OCC_REPLY");
            if (c->options.occ && !TLS_MODE(c) && c->c2.options_string_remote)
            {
                if (!options_cmp_equal_safe((char *)BPTR(&c->c2.buf), c->c2.options_string_remote,
                                            c->c2.buf.len))
                {
                    options_warning_safe((char *)BPTR(&c->c2.buf), c->c2.options_string_remote,
                                         c->c2.buf.len);
                }
            }
            event_timeout_clear(&c->c2.occ_interval);
            break;
 
        case OCC_MTU_REPLY:
            dmsg(D_PACKET_CONTENT, "RECEIVED OCC_MTU_REPLY");
            c->c2.max_recv_size_remote = buf_read_u16(&c->c2.buf);
            c->c2.max_send_size_remote = buf_read_u16(&c->c2.buf);
            if (c->options.mtu_test && c->c2.max_recv_size_remote > 0
                && c->c2.max_send_size_remote > 0)
            {
                msg(M_INFO,
                    "NOTE: Empirical MTU test completed [Tried,Actual] local->remote=[%d,%d] remote->local=[%d,%d]",
                    c->c2.max_send_size_local, c->c2.max_recv_size_remote,
                    c->c2.max_send_size_remote, c->c2.max_recv_size_local);
                if (!c->options.ce.fragment && (proto_is_dgram(c->options.ce.proto))
                    && c->c2.max_send_size_local > TUN_MTU_MIN
                    && (c->c2.max_recv_size_remote < c->c2.max_send_size_local
                        || c->c2.max_recv_size_local < c->c2.max_send_size_remote))
                {
                    msg(M_INFO,
                        "NOTE: This connection is unable to accommodate a UDP packet size of %d. Consider using --fragment or --mssfix options as a workaround.",
                        c->c2.max_send_size_local);
                }
            }
            event_timeout_clear(&c->c2.occ_mtu_load_test_interval);
            break;
 
        case OCC_EXIT:
            dmsg(D_STREAM_ERRORS, "OCC exit message received by peer");
            register_signal(c->sig, SIGUSR1, "remote-exit");
            break;
    }
    c->c2.buf.len = 0; /* don't pass packet on */
}