programs/test/udp_proxy.c

/*
 *  UDP proxy: emulate an unreliable UDP connection for DTLS testing
 *
 *  Copyright The Mbed TLS Contributors
 *  SPDX-License-Identifier: Apache-2.0
 *
 *  Licensed under the Apache License, Version 2.0 (the "License"); you may
 *  not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 *  WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

/*
 * Warning: this is an internal utility program we use for tests.
 * It does break some abstractions from the NET layer, and is thus NOT an
 * example of good general usage.
 */

#define MBEDTLS_ALLOW_PRIVATE_ACCESS

#include "mbedtls/build_info.h"

#if defined(MBEDTLS_PLATFORM_C)
#include "mbedtls/platform.h"
#else
#include <stdio.h>
#include <stdlib.h>
#if defined(MBEDTLS_HAVE_TIME)
#include <time.h>
#define mbedtls_time            time
#define mbedtls_time_t          time_t
#endif
#define mbedtls_printf          printf
#define mbedtls_calloc          calloc
#define mbedtls_free            free
#define mbedtls_exit            exit
#define MBEDTLS_EXIT_SUCCESS    EXIT_SUCCESS
#define MBEDTLS_EXIT_FAILURE    EXIT_FAILURE
#endif /* MBEDTLS_PLATFORM_C */

#if !defined(MBEDTLS_NET_C)
int main(void)
{
    mbedtls_printf("MBEDTLS_NET_C not defined.\n");
    mbedtls_exit(0);
}
#else

#include "mbedtls/net_sockets.h"
#include "mbedtls/error.h"
#include "mbedtls/ssl.h"
#include "mbedtls/timing.h"

#include <string.h>

/* For select() */
#if (defined(_WIN32) || defined(_WIN32_WCE)) && !defined(EFIX64) && \
    !defined(EFI32)
#include <winsock2.h>
#include <windows.h>
#if defined(_MSC_VER)
#if defined(_WIN32_WCE)
#pragma comment( lib, "ws2.lib" )
#else
#pragma comment( lib, "ws2_32.lib" )
#endif
#endif /* _MSC_VER */
#else /* ( _WIN32 || _WIN32_WCE ) && !EFIX64 && !EFI32 */
#if defined(MBEDTLS_HAVE_TIME) || (defined(MBEDTLS_TIMING_C) && !defined(MBEDTLS_TIMING_ALT))
#include <sys/time.h>
#endif
#include <sys/select.h>
#include <sys/types.h>
#include <unistd.h>
#endif /* ( _WIN32 || _WIN32_WCE ) && !EFIX64 && !EFI32 */

#define MAX_MSG_SIZE            16384 + 2048 /* max record/datagram size */

#define DFL_SERVER_ADDR         "localhost"
#define DFL_SERVER_PORT         "4433"
#define DFL_LISTEN_ADDR         "localhost"
#define DFL_LISTEN_PORT         "5556"
#define DFL_PACK                0

#if defined(MBEDTLS_TIMING_C)
#define USAGE_PACK                                                          \
    "    pack=%%d             default: 0     (don't pack)\n"                \
    "                         options: t > 0 (pack for t milliseconds)\n"
#else
#define USAGE_PACK
#endif

#define USAGE                                                               \
    "\n usage: udp_proxy param=<>...\n"                                     \
    "\n acceptable parameters:\n"                                           \
    "    server_addr=%%s      default: localhost\n"                         \
    "    server_port=%%d      default: 4433\n"                              \
    "    listen_addr=%%s      default: localhost\n"                         \
    "    listen_port=%%d      default: 4433\n"                              \
    "\n"                                                                    \
    "    duplicate=%%d        default: 0 (no duplication)\n"                \
    "                        duplicate about 1:N packets randomly\n"        \
    "    delay=%%d            default: 0 (no delayed packets)\n"            \
    "                        delay about 1:N packets randomly\n"            \
    "    delay_ccs=0/1       default: 0 (don't delay ChangeCipherSpec)\n"   \
    "    delay_cli=%%s        Handshake message from client that should be\n" \
    "                        delayed. Possible values are 'ClientHello',\n" \
    "                        'Certificate', 'CertificateVerify', and\n"     \
    "                        'ClientKeyExchange'.\n"                        \
    "                        May be used multiple times, even for the same\n" \
    "                        message, in which case the respective message\n" \
    "                        gets delayed multiple times.\n"                 \
    "    delay_srv=%%s        Handshake message from server that should be\n" \
    "                        delayed. Possible values are 'HelloRequest',\n" \
    "                        'ServerHello', 'ServerHelloDone', 'Certificate'\n" \
    "                        'ServerKeyExchange', 'NewSessionTicket',\n" \
    "                        'HelloVerifyRequest' and ''CertificateRequest'.\n" \
    "                        May be used multiple times, even for the same\n" \
    "                        message, in which case the respective message\n" \
    "                        gets delayed multiple times.\n"                 \
    "    drop=%%d             default: 0 (no dropped packets)\n"            \
    "                        drop about 1:N packets randomly\n"             \
    "    mtu=%%d              default: 0 (unlimited)\n"                     \
    "                        drop packets larger than N bytes\n"            \
    "    bad_ad=0/1          default: 0 (don't add bad ApplicationData)\n"  \
    "    bad_cid=%%d          default: 0 (don't corrupt Connection IDs)\n"   \
    "                        duplicate 1:N packets containing a CID,\n" \
    "                        modifying CID in first instance of the packet.\n" \
    "    protect_hvr=0/1     default: 0 (don't protect HelloVerifyRequest)\n" \
    "    protect_len=%%d      default: (don't protect packets of this size)\n" \
    "    inject_clihlo=0/1   default: 0 (don't inject fake ClientHello)\n"  \
    "\n"                                                                    \
    "    seed=%%d             default: (use current time)\n"                \
    USAGE_PACK                                                              \
    "\n"

/*
 * global options
 */

#define MAX_DELAYED_HS 10

static struct options {
    const char *server_addr;    /* address to forward packets to            */
    const char *server_port;    /* port to forward packets to               */
    const char *listen_addr;    /* address for accepting client connections */
    const char *listen_port;    /* port for accepting client connections    */

    int duplicate;              /* duplicate 1 in N packets (none if 0)     */
    int delay;                  /* delay 1 packet in N (none if 0)          */
    int delay_ccs;              /* delay ChangeCipherSpec                   */
    char *delay_cli[MAX_DELAYED_HS];  /* handshake types of messages from
                                       * client that should be delayed.     */
    uint8_t delay_cli_cnt;      /* Number of entries in delay_cli.          */
    char *delay_srv[MAX_DELAYED_HS];  /* handshake types of messages from
                                       * server that should be delayed.     */
    uint8_t delay_srv_cnt;      /* Number of entries in delay_srv.          */
    int drop;                   /* drop 1 packet in N (none if 0)           */
    int mtu;                    /* drop packets larger than this            */
    int bad_ad;                 /* inject corrupted ApplicationData record  */
    unsigned bad_cid;           /* inject corrupted CID record              */
    int protect_hvr;            /* never drop or delay HelloVerifyRequest   */
    int protect_len;            /* never drop/delay packet of the given size*/
    int inject_clihlo;          /* inject fake ClientHello after handshake  */
    unsigned pack;              /* merge packets into single datagram for
                                 * at most \c merge milliseconds if > 0     */
    unsigned int seed;          /* seed for "random" events                 */
} opt;

static void exit_usage(const char *name, const char *value)
{
    if (value == NULL) {
        mbedtls_printf(" unknown option or missing value: %s\n", name);
    } else {
        mbedtls_printf(" option %s: illegal value: %s\n", name, value);
    }

    mbedtls_printf(USAGE);
    mbedtls_exit(1);
}

static void get_options(int argc, char *argv[])
{
    int i;
    char *p, *q;

    opt.server_addr    = DFL_SERVER_ADDR;
    opt.server_port    = DFL_SERVER_PORT;
    opt.listen_addr    = DFL_LISTEN_ADDR;
    opt.listen_port    = DFL_LISTEN_PORT;
    opt.pack           = DFL_PACK;
    /* Other members default to 0 */

    opt.delay_cli_cnt = 0;
    opt.delay_srv_cnt = 0;
    memset(opt.delay_cli, 0, sizeof(opt.delay_cli));
    memset(opt.delay_srv, 0, sizeof(opt.delay_srv));

    for (i = 1; i < argc; i++) {
        p = argv[i];
        if ((q = strchr(p, '=')) == NULL) {
            exit_usage(p, NULL);
        }
        *q++ = '\0';

        if (strcmp(p, "server_addr") == 0) {
            opt.server_addr = q;
        } else if (strcmp(p, "server_port") == 0) {
            opt.server_port = q;
        } else if (strcmp(p, "listen_addr") == 0) {
            opt.listen_addr = q;
        } else if (strcmp(p, "listen_port") == 0) {
            opt.listen_port = q;
        } else if (strcmp(p, "duplicate") == 0) {
            opt.duplicate = atoi(q);
            if (opt.duplicate < 0 || opt.duplicate > 20) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "delay") == 0) {
            opt.delay = atoi(q);
            if (opt.delay < 0 || opt.delay > 20 || opt.delay == 1) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "delay_ccs") == 0) {
            opt.delay_ccs = atoi(q);
            if (opt.delay_ccs < 0 || opt.delay_ccs > 1) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "delay_cli") == 0 ||
                   strcmp(p, "delay_srv") == 0) {
            uint8_t *delay_cnt;
            char **delay_list;
            size_t len;
            char *buf;

            if (strcmp(p, "delay_cli") == 0) {
                delay_cnt  = &opt.delay_cli_cnt;
                delay_list = opt.delay_cli;
            } else {
                delay_cnt  = &opt.delay_srv_cnt;
                delay_list = opt.delay_srv;
            }

            if (*delay_cnt == MAX_DELAYED_HS) {
                mbedtls_printf(" too many uses of %s: only %d allowed\n",
                               p, MAX_DELAYED_HS);
                exit_usage(p, NULL);
            }

            len = strlen(q);
            buf = mbedtls_calloc(1, len + 1);
            if (buf == NULL) {
                mbedtls_printf(" Allocation failure\n");
                exit(1);
            }
            memcpy(buf, q, len + 1);

            delay_list[(*delay_cnt)++] = buf;
        } else if (strcmp(p, "drop") == 0) {
            opt.drop = atoi(q);
            if (opt.drop < 0 || opt.drop > 20 || opt.drop == 1) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "pack") == 0) {
#if defined(MBEDTLS_TIMING_C)
            opt.pack = (unsigned) atoi(q);
#else
            mbedtls_printf(" option pack only defined if MBEDTLS_TIMING_C is enabled\n");
            exit(1);
#endif
        } else if (strcmp(p, "mtu") == 0) {
            opt.mtu = atoi(q);
            if (opt.mtu < 0 || opt.mtu > MAX_MSG_SIZE) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "bad_ad") == 0) {
            opt.bad_ad = atoi(q);
            if (opt.bad_ad < 0 || opt.bad_ad > 1) {
                exit_usage(p, q);
            }
        }
#if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
        else if (strcmp(p, "bad_cid") == 0) {
            opt.bad_cid = (unsigned) atoi(q);
        }
#endif /* MBEDTLS_SSL_DTLS_CONNECTION_ID */
        else if (strcmp(p, "protect_hvr") == 0) {
            opt.protect_hvr = atoi(q);
            if (opt.protect_hvr < 0 || opt.protect_hvr > 1) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "protect_len") == 0) {
            opt.protect_len = atoi(q);
            if (opt.protect_len < 0) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "inject_clihlo") == 0) {
            opt.inject_clihlo = atoi(q);
            if (opt.inject_clihlo < 0 || opt.inject_clihlo > 1) {
                exit_usage(p, q);
            }
        } else if (strcmp(p, "seed") == 0) {
            opt.seed = atoi(q);
            if (opt.seed == 0) {
                exit_usage(p, q);
            }
        } else {
            exit_usage(p, NULL);
        }
    }
}

static const char *msg_type(unsigned char *msg, size_t len)
{
    if (len < 1) {
        return "Invalid";
    }
    switch (msg[0]) {
        case MBEDTLS_SSL_MSG_CHANGE_CIPHER_SPEC:    return "ChangeCipherSpec";
        case MBEDTLS_SSL_MSG_ALERT:                 return "Alert";
        case MBEDTLS_SSL_MSG_APPLICATION_DATA:      return "ApplicationData";
        case MBEDTLS_SSL_MSG_CID:                   return "CID";
        case MBEDTLS_SSL_MSG_HANDSHAKE:             break; /* See below */
        default:                            return "Unknown";
    }

    if (len < 13 + 12) {
        return "Invalid handshake";
    }

    /*
     * Our handshake message are less than 2^16 bytes long, so they should
     * have 0 as the first byte of length, frag_offset and frag_length.
     * Otherwise, assume they are encrypted.
     */
    if (msg[14] || msg[19] || msg[22]) {
        return "Encrypted handshake";
    }

    switch (msg[13]) {
        case MBEDTLS_SSL_HS_HELLO_REQUEST:          return "HelloRequest";
        case MBEDTLS_SSL_HS_CLIENT_HELLO:           return "ClientHello";
        case MBEDTLS_SSL_HS_SERVER_HELLO:           return "ServerHello";
        case MBEDTLS_SSL_HS_HELLO_VERIFY_REQUEST:   return "HelloVerifyRequest";
        case MBEDTLS_SSL_HS_NEW_SESSION_TICKET:     return "NewSessionTicket";
        case MBEDTLS_SSL_HS_CERTIFICATE:            return "Certificate";
        case MBEDTLS_SSL_HS_SERVER_KEY_EXCHANGE:    return "ServerKeyExchange";
        case MBEDTLS_SSL_HS_CERTIFICATE_REQUEST:    return "CertificateRequest";
        case MBEDTLS_SSL_HS_SERVER_HELLO_DONE:      return "ServerHelloDone";
        case MBEDTLS_SSL_HS_CERTIFICATE_VERIFY:     return "CertificateVerify";
        case MBEDTLS_SSL_HS_CLIENT_KEY_EXCHANGE:    return "ClientKeyExchange";
        case MBEDTLS_SSL_HS_FINISHED:               return "Finished";
        default:                            return "Unknown handshake";
    }
}

#if defined(MBEDTLS_TIMING_C)
/* Return elapsed time in milliseconds since the first call */
static unsigned elapsed_time(void)
{
    static int initialized = 0;
    static struct mbedtls_timing_hr_time hires;

    if (initialized == 0) {
        (void) mbedtls_timing_get_timer(&hires, 1);
        initialized = 1;
        return 0;
    }

    return mbedtls_timing_get_timer(&hires, 0);
}

typedef struct {
    mbedtls_net_context *ctx;

    const char *description;

    unsigned packet_lifetime;
    unsigned num_datagrams;

    unsigned char data[MAX_MSG_SIZE];
    size_t len;

} ctx_buffer;

static ctx_buffer outbuf[2];

static int ctx_buffer_flush(ctx_buffer *buf)
{
    int ret;

    mbedtls_printf("  %05u flush    %s: %u bytes, %u datagrams, last %u ms\n",
                   elapsed_time(), buf->description,
                   (unsigned) buf->len, buf->num_datagrams,
                   elapsed_time() - buf->packet_lifetime);

    ret = mbedtls_net_send(buf->ctx, buf->data, buf->len);

    buf->len           = 0;
    buf->num_datagrams = 0;

    return ret;
}

static unsigned ctx_buffer_time_remaining(ctx_buffer *buf)
{
    unsigned const cur_time = elapsed_time();

    if (buf->num_datagrams == 0) {
        return (unsigned) -1;
    }

    if (cur_time - buf->packet_lifetime >= opt.pack) {
        return 0;
    }

    return opt.pack - (cur_time - buf->packet_lifetime);
}

static int ctx_buffer_append(ctx_buffer *buf,
                             const unsigned char *data,
                             size_t len)
{
    int ret;

    if (len > (size_t) INT_MAX) {
        return -1;
    }

    if (len > sizeof(buf->data)) {
        mbedtls_printf("  ! buffer size %u too large (max %u)\n",
                       (unsigned) len, (unsigned) sizeof(buf->data));
        return -1;
    }

    if (sizeof(buf->data) - buf->len < len) {
        if ((ret = ctx_buffer_flush(buf)) <= 0) {
            mbedtls_printf("ctx_buffer_flush failed with -%#04x", (unsigned int) -ret);
            return ret;
        }
    }

    memcpy(buf->data + buf->len, data, len);

    buf->len += len;
    if (++buf->num_datagrams == 1) {
        buf->packet_lifetime = elapsed_time();
    }

    return (int) len;
}
#endif /* MBEDTLS_TIMING_C */

static int dispatch_data(mbedtls_net_context *ctx,
                         const unsigned char *data,
                         size_t len)
{
    int ret;
#if defined(MBEDTLS_TIMING_C)
    ctx_buffer *buf = NULL;
    if (opt.pack > 0) {
        if (outbuf[0].ctx == ctx) {
            buf = &outbuf[0];
        } else if (outbuf[1].ctx == ctx) {
            buf = &outbuf[1];
        }

        if (buf == NULL) {
            return -1;
        }

        return ctx_buffer_append(buf, data, len);
    }
#endif /* MBEDTLS_TIMING_C */

    ret = mbedtls_net_send(ctx, data, len);
    if (ret < 0) {
        mbedtls_printf("net_send returned -%#04x\n", (unsigned int) -ret);
    }
    return ret;
}

typedef struct {
    mbedtls_net_context *dst;
    const char *way;
    const char *type;
    unsigned len;
    unsigned char buf[MAX_MSG_SIZE];
} packet;

/* Print packet. Outgoing packets come with a reason (forward, dupl, etc.) */
void print_packet(const packet *p, const char *why)
{
#if defined(MBEDTLS_TIMING_C)
    if (why == NULL) {
        mbedtls_printf("  %05u dispatch %s %s (%u bytes)\n",
                       elapsed_time(), p->way, p->type, p->len);
    } else {
        mbedtls_printf("  %05u dispatch %s %s (%u bytes): %s\n",
                       elapsed_time(), p->way, p->type, p->len, why);
    }
#else
    if (why == NULL) {
        mbedtls_printf("        dispatch %s %s (%u bytes)\n",
                       p->way, p->type, p->len);
    } else {
        mbedtls_printf("        dispatch %s %s (%u bytes): %s\n",
                       p->way, p->type, p->len, why);
    }
#endif

    fflush(stdout);
}

/*
 * In order to test the server's behaviour when receiving a ClientHello after
 * the connection is established (this could be a hard reset from the client,
 * but the server must not drop the existing connection before establishing
 * client reachability, see RFC 6347 Section 4.2.8), we memorize the first
 * ClientHello we see (which can't have a cookie), then replay it after the
 * first ApplicationData record - then we're done.
 *
 * This is controlled by the inject_clihlo option.
 *
 * We want an explicit state and a place to store the packet.
 */
typedef enum {
    ICH_INIT,       /* haven't seen the first ClientHello yet */
    ICH_CACHED,     /* cached the initial ClientHello */
    ICH_INJECTED,   /* ClientHello already injected, done */
} inject_clihlo_state_t;

static inject_clihlo_state_t inject_clihlo_state;
static packet initial_clihlo;

int send_packet(const packet *p, const char *why)
{
    int ret;
    mbedtls_net_context *dst = p->dst;

    /* save initial ClientHello? */
    if (opt.inject_clihlo != 0 &&
        inject_clihlo_state == ICH_INIT &&
        strcmp(p->type, "ClientHello") == 0) {
        memcpy(&initial_clihlo, p, sizeof(packet));
        inject_clihlo_state = ICH_CACHED;
    }

    /* insert corrupted CID record? */
    if (opt.bad_cid != 0 &&
        strcmp(p->type, "CID") == 0 &&
        (rand() % opt.bad_cid) == 0) {
        unsigned char buf[MAX_MSG_SIZE];
        memcpy(buf, p->buf, p->len);

        /* The CID resides at offset 11 in the DTLS record header. */
        buf[11] ^= 1;
        print_packet(p, "modified CID");

        if ((ret = dispatch_data(dst, buf, p->len)) <= 0) {
            mbedtls_printf("  ! dispatch returned %d\n", ret);
            return ret;
        }
    }

    /* insert corrupted ApplicationData record? */
    if (opt.bad_ad &&
        strcmp(p->type, "ApplicationData") == 0) {
        unsigned char buf[MAX_MSG_SIZE];
        memcpy(buf, p->buf, p->len);

        if (p->len <= 13) {
            mbedtls_printf("  ! can't corrupt empty AD record");
        } else {
            ++buf[13];
            print_packet(p, "corrupted");
        }

        if ((ret = dispatch_data(dst, buf, p->len)) <= 0) {
            mbedtls_printf("  ! dispatch returned %d\n", ret);
            return ret;
        }
    }

    print_packet(p, why);
    if ((ret = dispatch_data(dst, p->buf, p->len)) <= 0) {
        mbedtls_printf("  ! dispatch returned %d\n", ret);
        return ret;
    }

    /* Don't duplicate Application Data, only handshake covered */
    if (opt.duplicate != 0 &&
        strcmp(p->type, "ApplicationData") != 0 &&
        rand() % opt.duplicate == 0) {
        print_packet(p, "duplicated");

        if ((ret = dispatch_data(dst, p->buf, p->len)) <= 0) {
            mbedtls_printf("  ! dispatch returned %d\n", ret);
            return ret;
        }
    }

    /* Inject ClientHello after first ApplicationData */
    if (opt.inject_clihlo != 0 &&
        inject_clihlo_state == ICH_CACHED &&
        strcmp(p->type, "ApplicationData") == 0) {
        print_packet(&initial_clihlo, "injected");

        if ((ret = dispatch_data(dst, initial_clihlo.buf,
                                 initial_clihlo.len)) <= 0) {
            mbedtls_printf("  ! dispatch returned %d\n", ret);
            return ret;
        }

        inject_clihlo_state = ICH_INJECTED;
    }

    return 0;
}

#define MAX_DELAYED_MSG 5
static size_t prev_len;
static packet prev[MAX_DELAYED_MSG];

void clear_pending(void)
{
    memset(&prev, 0, sizeof(prev));
    prev_len = 0;
}

void delay_packet(packet *delay)
{
    if (prev_len == MAX_DELAYED_MSG) {
        return;
    }

    memcpy(&prev[prev_len++], delay, sizeof(packet));
}

int send_delayed()
{
    uint8_t offset;
    int ret;
    for (offset = 0; offset < prev_len; offset++) {
        ret = send_packet(&prev[offset], "delayed");
        if (ret != 0) {
            return ret;
        }
    }

    clear_pending();
    return 0;
}

/*
 * Avoid dropping or delaying a packet that was already dropped or delayed
 * ("held") twice: this only results in uninteresting timeouts. We can't rely
 * on type to identify packets, since during renegotiation they're all
 * encrypted. So, rely on size mod 2048 (which is usually just size).
 *
 * We only hold packets at the level of entire datagrams, not at the level
 * of records. In particular, if the peer changes the way it packs multiple
 * records into a single datagram, we don't necessarily count the number of
 * times a record has been held correctly. However, the only known reason
 * why a peer would change datagram packing is disabling the latter on
 * retransmission, in which case we'd hold involved records at most
 * HOLD_MAX + 1 times.
 */
static unsigned char held[2048] = { 0 };
#define HOLD_MAX 2

int handle_message(const char *way,
                   mbedtls_net_context *dst,
                   mbedtls_net_context *src)
{
    int ret;
    packet cur;
    size_t id;

    uint8_t delay_idx;
    char **delay_list;
    uint8_t delay_list_len;

    /* receive packet */
    if ((ret = mbedtls_net_recv(src, cur.buf, sizeof(cur.buf))) <= 0) {
        mbedtls_printf("  ! mbedtls_net_recv returned %d\n", ret);
        return ret;
    }

    cur.len  = ret;
    cur.type = msg_type(cur.buf, cur.len);
    cur.way  = way;
    cur.dst  = dst;
    print_packet(&cur, NULL);

    id = cur.len % sizeof(held);

    if (strcmp(way, "S <- C") == 0) {
        delay_list     = opt.delay_cli;
        delay_list_len = opt.delay_cli_cnt;
    } else {
        delay_list     = opt.delay_srv;
        delay_list_len = opt.delay_srv_cnt;
    }

    /* Check if message type is in the list of messages
     * that should be delayed */
    for (delay_idx = 0; delay_idx < delay_list_len; delay_idx++) {
        if (delay_list[delay_idx] == NULL) {
            continue;
        }

        if (strcmp(delay_list[delay_idx], cur.type) == 0) {
            /* Delay message */
            delay_packet(&cur);

            /* Remove entry from list */
            mbedtls_free(delay_list[delay_idx]);
            delay_list[delay_idx] = NULL;

            return 0;
        }
    }

    /* do we want to drop, delay, or forward it? */
    if ((opt.mtu != 0 &&
         cur.len > (unsigned) opt.mtu) ||
        (opt.drop != 0 &&
         strcmp(cur.type, "CID") != 0             &&
         strcmp(cur.type, "ApplicationData") != 0 &&
         !(opt.protect_hvr &&
           strcmp(cur.type, "HelloVerifyRequest") == 0) &&
         cur.len != (size_t) opt.protect_len &&
         held[id] < HOLD_MAX &&
         rand() % opt.drop == 0)) {
        ++held[id];
    } else if ((opt.delay_ccs == 1 &&
                strcmp(cur.type, "ChangeCipherSpec") == 0) ||
               (opt.delay != 0 &&
                strcmp(cur.type, "CID") != 0             &&
                strcmp(cur.type, "ApplicationData") != 0 &&
                !(opt.protect_hvr &&
                  strcmp(cur.type, "HelloVerifyRequest") == 0) &&
                cur.len != (size_t) opt.protect_len &&
                held[id] < HOLD_MAX &&
                rand() % opt.delay == 0)) {
        ++held[id];
        delay_packet(&cur);
    } else {
        /* forward and possibly duplicate */
        if ((ret = send_packet(&cur, "forwarded")) != 0) {
            return ret;
        }

        /* send previously delayed messages if any */
        ret = send_delayed();
        if (ret != 0) {
            return ret;
        }
    }

    return 0;
}

int main(int argc, char *argv[])
{
    int ret = 1;
    int exit_code = MBEDTLS_EXIT_FAILURE;
    uint8_t delay_idx;

    mbedtls_net_context listen_fd, client_fd, server_fd;

#if defined(MBEDTLS_TIMING_C)
    struct timeval tm;
#endif

    struct timeval *tm_ptr = NULL;

    int nb_fds;
    fd_set read_fds;

    mbedtls_net_init(&listen_fd);
    mbedtls_net_init(&client_fd);
    mbedtls_net_init(&server_fd);

    get_options(argc, argv);

    /*
     * Decisions to drop/delay/duplicate packets are pseudo-random: dropping
     * exactly 1 in N packets would lead to problems when a flight has exactly
     * N packets: the same packet would be dropped on every resend.
     *
     * In order to be able to reproduce problems reliably, the seed may be
     * specified explicitly.
     */
    if (opt.seed == 0) {
#if defined(MBEDTLS_HAVE_TIME)
        opt.seed = (unsigned int) mbedtls_time(NULL);
#else
        opt.seed = 1;
#endif /* MBEDTLS_HAVE_TIME */
        mbedtls_printf("  . Pseudo-random seed: %u\n", opt.seed);
    }

    srand(opt.seed);

    /*
     * 0. "Connect" to the server
     */
    mbedtls_printf("  . Connect to server on UDP/%s/%s ...",
                   opt.server_addr, opt.server_port);
    fflush(stdout);

    if ((ret = mbedtls_net_connect(&server_fd, opt.server_addr, opt.server_port,
                                   MBEDTLS_NET_PROTO_UDP)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_net_connect returned %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 1. Setup the "listening" UDP socket
     */
    mbedtls_printf("  . Bind on UDP/%s/%s ...",
                   opt.listen_addr, opt.listen_port);
    fflush(stdout);

    if ((ret = mbedtls_net_bind(&listen_fd, opt.listen_addr, opt.listen_port,
                                MBEDTLS_NET_PROTO_UDP)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_net_bind returned %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 2. Wait until a client connects
     */
accept:
    mbedtls_net_free(&client_fd);

    mbedtls_printf("  . Waiting for a remote connection ...");
    fflush(stdout);

    if ((ret = mbedtls_net_accept(&listen_fd, &client_fd,
                                  NULL, 0, NULL)) != 0) {
        mbedtls_printf(" failed\n  ! mbedtls_net_accept returned %d\n\n", ret);
        goto exit;
    }

    mbedtls_printf(" ok\n");

    /*
     * 3. Forward packets forever (kill the process to terminate it)
     */
    clear_pending();
    memset(held, 0, sizeof(held));

    nb_fds = client_fd.fd;
    if (nb_fds < server_fd.fd) {
        nb_fds = server_fd.fd;
    }
    if (nb_fds < listen_fd.fd) {
        nb_fds = listen_fd.fd;
    }
    ++nb_fds;

#if defined(MBEDTLS_TIMING_C)
    if (opt.pack > 0) {
        outbuf[0].ctx = &server_fd;
        outbuf[0].description = "S <- C";
        outbuf[0].num_datagrams = 0;
        outbuf[0].len = 0;

        outbuf[1].ctx = &client_fd;
        outbuf[1].description = "S -> C";
        outbuf[1].num_datagrams = 0;
        outbuf[1].len = 0;
    }
#endif /* MBEDTLS_TIMING_C */

    while (1) {
#if defined(MBEDTLS_TIMING_C)
        if (opt.pack > 0) {
            unsigned max_wait_server, max_wait_client, max_wait;
            max_wait_server = ctx_buffer_time_remaining(&outbuf[0]);
            max_wait_client = ctx_buffer_time_remaining(&outbuf[1]);

            max_wait = (unsigned) -1;

            if (max_wait_server == 0) {
                ctx_buffer_flush(&outbuf[0]);
            } else {
                max_wait = max_wait_server;
            }

            if (max_wait_client == 0) {
                ctx_buffer_flush(&outbuf[1]);
            } else {
                if (max_wait_client < max_wait) {
                    max_wait = max_wait_client;
                }
            }

            if (max_wait != (unsigned) -1) {
                tm.tv_sec  = max_wait / 1000;
                tm.tv_usec = (max_wait % 1000) * 1000;

                tm_ptr = &tm;
            } else {
                tm_ptr = NULL;
            }
        }
#endif /* MBEDTLS_TIMING_C */

        FD_ZERO(&read_fds);
        FD_SET(server_fd.fd, &read_fds);
        FD_SET(client_fd.fd, &read_fds);
        FD_SET(listen_fd.fd, &read_fds);

        if ((ret = select(nb_fds, &read_fds, NULL, NULL, tm_ptr)) < 0) {
            perror("select");
            goto exit;
        }

        if (FD_ISSET(listen_fd.fd, &read_fds)) {
            goto accept;
        }

        if (FD_ISSET(client_fd.fd, &read_fds)) {
            if ((ret = handle_message("S <- C",
                                      &server_fd, &client_fd)) != 0) {
                goto accept;
            }
        }

        if (FD_ISSET(server_fd.fd, &read_fds)) {
            if ((ret = handle_message("S -> C",
                                      &client_fd, &server_fd)) != 0) {
                goto accept;
            }
        }

    }

    exit_code = MBEDTLS_EXIT_SUCCESS;

exit:

#ifdef MBEDTLS_ERROR_C
    if (exit_code != MBEDTLS_EXIT_SUCCESS) {
        char error_buf[100];
        mbedtls_strerror(ret, error_buf, 100);
        mbedtls_printf("Last error was: -0x%04X - %s\n\n", (unsigned int) -ret, error_buf);
        fflush(stdout);
    }
#endif

    for (delay_idx = 0; delay_idx < MAX_DELAYED_HS; delay_idx++) {
        mbedtls_free(opt.delay_cli[delay_idx]);
        mbedtls_free(opt.delay_srv[delay_idx]);
    }

    mbedtls_net_free(&client_fd);
    mbedtls_net_free(&server_fd);
    mbedtls_net_free(&listen_fd);

    mbedtls_exit(exit_code);
}

#endif /* MBEDTLS_NET_C */