- djm@cvs.openbsd.org 2006/08/18 14:40:34

[openssh-git.git] / sshd.c

/* $OpenBSD: sshd.c,v 1.347 2006/08/18 09:15:20 markus Exp $ */
/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * This program is the ssh daemon.  It listens for connections from clients,
 * and performs authentication, executes use commands or shell, and forwards
 * information to/from the application to the user client over an encrypted
 * connection.  This can also handle forwarding of X11, TCP/IP, and
 * authentication agent connections.
 *
 * As far as I am concerned, the code I have written for this software
 * can be used freely for any purpose.  Any derived versions of this
 * software must be clearly marked as such, and if the derived work is
 * incompatible with the protocol description in the RFC file, it must be
 * called by a name other than "ssh" or "Secure Shell".
 *
 * SSH2 implementation:
 * Privilege Separation:
 *
 * Copyright (c) 2000, 2001, 2002 Markus Friedl.  All rights reserved.
 * Copyright (c) 2002 Niels Provos.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include "includes.h"

#include <sys/types.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#ifdef HAVE_SYS_STAT_H
# include <sys/stat.h>
#endif
#ifdef HAVE_SYS_TIME_H
# include <sys/time.h>
#endif
#include "openbsd-compat/sys-tree.h"
#include <sys/wait.h>

#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#ifdef HAVE_PATHS_H
#include <paths.h>
#endif
#include <grp.h>
#include <pwd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <openssl/dh.h>
#include <openssl/bn.h>
#include <openssl/md5.h>
#include <openssl/rand.h>
#ifdef HAVE_SECUREWARE
#include <sys/security.h>
#include <prot.h>
#endif

#include "xmalloc.h"
#include "ssh.h"
#include "ssh1.h"
#include "ssh2.h"
#include "rsa.h"
#include "sshpty.h"
#include "packet.h"
#include "log.h"
#include "buffer.h"
#include "servconf.h"
#include "uidswap.h"
#include "compat.h"
#include "cipher.h"
#include "key.h"
#include "kex.h"
#include "dh.h"
#include "myproposal.h"
#include "authfile.h"
#include "pathnames.h"
#include "atomicio.h"
#include "canohost.h"
#include "hostfile.h"
#include "auth.h"
#include "misc.h"
#include "msg.h"
#include "dispatch.h"
#include "channels.h"
#include "session.h"
#include "monitor_mm.h"
#include "monitor.h"
#ifdef GSSAPI
#include "ssh-gss.h"
#endif
#include "monitor_wrap.h"
#include "monitor_fdpass.h"
#include "version.h"

#ifdef LIBWRAP
#include <tcpd.h>
#include <syslog.h>
int allow_severity = LOG_INFO;
int deny_severity = LOG_WARNING;
#endif /* LIBWRAP */

#ifndef O_NOCTTY
#define O_NOCTTY        0
#endif

/* Re-exec fds */
#define REEXEC_DEVCRYPTO_RESERVED_FD    (STDERR_FILENO + 1)
#define REEXEC_STARTUP_PIPE_FD          (STDERR_FILENO + 2)
#define REEXEC_CONFIG_PASS_FD           (STDERR_FILENO + 3)
#define REEXEC_MIN_FREE_FD              (STDERR_FILENO + 4)

extern char *__progname;

/* Server configuration options. */
ServerOptions options;

/* Name of the server configuration file. */
char *config_file_name = _PATH_SERVER_CONFIG_FILE;

/*
 * Debug mode flag.  This can be set on the command line.  If debug
 * mode is enabled, extra debugging output will be sent to the system
 * log, the daemon will not go to background, and will exit after processing
 * the first connection.
 */
int debug_flag = 0;

/* Flag indicating that the daemon should only test the configuration and keys. */
int test_flag = 0;

/* Flag indicating that the daemon is being started from inetd. */
int inetd_flag = 0;

/* Flag indicating that sshd should not detach and become a daemon. */
int no_daemon_flag = 0;

/* debug goes to stderr unless inetd_flag is set */
int log_stderr = 0;

/* Saved arguments to main(). */
char **saved_argv;
int saved_argc;

/* re-exec */
int rexeced_flag = 0;
int rexec_flag = 1;
int rexec_argc = 0;
char **rexec_argv;

/*
 * The sockets that the server is listening; this is used in the SIGHUP
 * signal handler.
 */
#define MAX_LISTEN_SOCKS        16
int listen_socks[MAX_LISTEN_SOCKS];
int num_listen_socks = 0;

/*
 * the client's version string, passed by sshd2 in compat mode. if != NULL,
 * sshd will skip the version-number exchange
 */
char *client_version_string = NULL;
char *server_version_string = NULL;

/* for rekeying XXX fixme */
Kex *xxx_kex;

/*
 * Any really sensitive data in the application is contained in this
 * structure. The idea is that this structure could be locked into memory so
 * that the pages do not get written into swap.  However, there are some
 * problems. The private key contains BIGNUMs, and we do not (in principle)
 * have access to the internals of them, and locking just the structure is
 * not very useful.  Currently, memory locking is not implemented.
 */
struct {
        Key     *server_key;            /* ephemeral server key */
        Key     *ssh1_host_key;         /* ssh1 host key */
        Key     **host_keys;            /* all private host keys */
        int     have_ssh1_key;
        int     have_ssh2_key;
        u_char  ssh1_cookie[SSH_SESSION_KEY_LENGTH];
} sensitive_data;

/*
 * Flag indicating whether the RSA server key needs to be regenerated.
 * Is set in the SIGALRM handler and cleared when the key is regenerated.
 */
static volatile sig_atomic_t key_do_regen = 0;

/* This is set to true when a signal is received. */
static volatile sig_atomic_t received_sighup = 0;
static volatile sig_atomic_t received_sigterm = 0;

/* session identifier, used by RSA-auth */
u_char session_id[16];

/* same for ssh2 */
u_char *session_id2 = NULL;
u_int session_id2_len = 0;

/* record remote hostname or ip */
u_int utmp_len = MAXHOSTNAMELEN;

/* options.max_startup sized array of fd ints */
int *startup_pipes = NULL;
int startup_pipe;               /* in child */

/* variables used for privilege separation */
int use_privsep = -1;
struct monitor *pmonitor = NULL;

/* global authentication context */
Authctxt *the_authctxt = NULL;

/* sshd_config buffer */
Buffer cfg;

/* message to be displayed after login */
Buffer loginmsg;

/* Prototypes for various functions defined later in this file. */
void destroy_sensitive_data(void);
void demote_sensitive_data(void);

static void do_ssh1_kex(void);
static void do_ssh2_kex(void);

/*
 * Close all listening sockets
 */
static void
close_listen_socks(void)
{
        int i;

        for (i = 0; i < num_listen_socks; i++)
                close(listen_socks[i]);
        num_listen_socks = -1;
}

static void
close_startup_pipes(void)
{
        int i;

        if (startup_pipes)
                for (i = 0; i < options.max_startups; i++)
                        if (startup_pipes[i] != -1)
                                close(startup_pipes[i]);
}

/*
 * Signal handler for SIGHUP.  Sshd execs itself when it receives SIGHUP;
 * the effect is to reread the configuration file (and to regenerate
 * the server key).
 */

/*ARGSUSED*/
static void
sighup_handler(int sig)
{
        int save_errno = errno;

        received_sighup = 1;
        signal(SIGHUP, sighup_handler);
        errno = save_errno;
}

/*
 * Called from the main program after receiving SIGHUP.
 * Restarts the server.
 */
static void
sighup_restart(void)
{
        logit("Received SIGHUP; restarting.");
        close_listen_socks();
        close_startup_pipes();
        execv(saved_argv[0], saved_argv);
        logit("RESTART FAILED: av[0]='%.100s', error: %.100s.", saved_argv[0],
            strerror(errno));
        exit(1);
}

/*
 * Generic signal handler for terminating signals in the master daemon.
 */
/*ARGSUSED*/
static void
sigterm_handler(int sig)
{
        received_sigterm = sig;
}

/*
 * SIGCHLD handler.  This is called whenever a child dies.  This will then
 * reap any zombies left by exited children.
 */
/*ARGSUSED*/
static void
main_sigchld_handler(int sig)
{
        int save_errno = errno;
        pid_t pid;
        int status;

        while ((pid = waitpid(-1, &status, WNOHANG)) > 0 ||
            (pid < 0 && errno == EINTR))
                ;

        signal(SIGCHLD, main_sigchld_handler);
        errno = save_errno;
}

/*
 * Signal handler for the alarm after the login grace period has expired.
 */
/*ARGSUSED*/
static void
grace_alarm_handler(int sig)
{
        if (use_privsep && pmonitor != NULL && pmonitor->m_pid > 0)
                kill(pmonitor->m_pid, SIGALRM);

        /* Log error and exit. */
        sigdie("Timeout before authentication for %s", get_remote_ipaddr());
}

/*
 * Signal handler for the key regeneration alarm.  Note that this
 * alarm only occurs in the daemon waiting for connections, and it does not
 * do anything with the private key or random state before forking.
 * Thus there should be no concurrency control/asynchronous execution
 * problems.
 */
static void
generate_ephemeral_server_key(void)
{
        u_int32_t rnd = 0;
        int i;

        verbose("Generating %s%d bit RSA key.",
            sensitive_data.server_key ? "new " : "", options.server_key_bits);
        if (sensitive_data.server_key != NULL)
                key_free(sensitive_data.server_key);
        sensitive_data.server_key = key_generate(KEY_RSA1,
            options.server_key_bits);
        verbose("RSA key generation complete.");

        for (i = 0; i < SSH_SESSION_KEY_LENGTH; i++) {
                if (i % 4 == 0)
                        rnd = arc4random();
                sensitive_data.ssh1_cookie[i] = rnd & 0xff;
                rnd >>= 8;
        }
        arc4random_stir();
}

/*ARGSUSED*/
static void
key_regeneration_alarm(int sig)
{
        int save_errno = errno;

        signal(SIGALRM, SIG_DFL);
        errno = save_errno;
        key_do_regen = 1;
}

static void
sshd_exchange_identification(int sock_in, int sock_out)
{
        u_int i;
        int mismatch;
        int remote_major, remote_minor;
        int major, minor;
        char *s;
        char buf[256];                  /* Must not be larger than remote_version. */
        char remote_version[256];       /* Must be at least as big as buf. */

        if ((options.protocol & SSH_PROTO_1) &&
            (options.protocol & SSH_PROTO_2)) {
                major = PROTOCOL_MAJOR_1;
                minor = 99;
        } else if (options.protocol & SSH_PROTO_2) {
                major = PROTOCOL_MAJOR_2;
                minor = PROTOCOL_MINOR_2;
        } else {
                major = PROTOCOL_MAJOR_1;
                minor = PROTOCOL_MINOR_1;
        }
        snprintf(buf, sizeof buf, "SSH-%d.%d-%.100s\n", major, minor, SSH_VERSION);
        server_version_string = xstrdup(buf);

        /* Send our protocol version identification. */
        if (atomicio(vwrite, sock_out, server_version_string,
            strlen(server_version_string))
            != strlen(server_version_string)) {
                logit("Could not write ident string to %s", get_remote_ipaddr());
                cleanup_exit(255);
        }

        /* Read other sides version identification. */
        memset(buf, 0, sizeof(buf));
        for (i = 0; i < sizeof(buf) - 1; i++) {
                if (atomicio(read, sock_in, &buf[i], 1) != 1) {
                        logit("Did not receive identification string from %s",
                            get_remote_ipaddr());
                        cleanup_exit(255);
                }
                if (buf[i] == '\r') {
                        buf[i] = 0;
                        /* Kludge for F-Secure Macintosh < 1.0.2 */
                        if (i == 12 &&
                            strncmp(buf, "SSH-1.5-W1.0", 12) == 0)
                                break;
                        continue;
                }
                if (buf[i] == '\n') {
                        buf[i] = 0;
                        break;
                }
        }
        buf[sizeof(buf) - 1] = 0;
        client_version_string = xstrdup(buf);

        /*
         * Check that the versions match.  In future this might accept
         * several versions and set appropriate flags to handle them.
         */
        if (sscanf(client_version_string, "SSH-%d.%d-%[^\n]\n",
            &remote_major, &remote_minor, remote_version) != 3) {
                s = "Protocol mismatch.\n";
                (void) atomicio(vwrite, sock_out, s, strlen(s));
                close(sock_in);
                close(sock_out);
                logit("Bad protocol version identification '%.100s' from %s",
                    client_version_string, get_remote_ipaddr());
                cleanup_exit(255);
        }
        debug("Client protocol version %d.%d; client software version %.100s",
            remote_major, remote_minor, remote_version);

        compat_datafellows(remote_version);

        if (datafellows & SSH_BUG_PROBE) {
                logit("probed from %s with %s.  Don't panic.",
                    get_remote_ipaddr(), client_version_string);
                cleanup_exit(255);
        }

        if (datafellows & SSH_BUG_SCANNER) {
                logit("scanned from %s with %s.  Don't panic.",
                    get_remote_ipaddr(), client_version_string);
                cleanup_exit(255);
        }

        mismatch = 0;
        switch (remote_major) {
        case 1:
                if (remote_minor == 99) {
                        if (options.protocol & SSH_PROTO_2)
                                enable_compat20();
                        else
                                mismatch = 1;
                        break;
                }
                if (!(options.protocol & SSH_PROTO_1)) {
                        mismatch = 1;
                        break;
                }
                if (remote_minor < 3) {
                        packet_disconnect("Your ssh version is too old and "
                            "is no longer supported.  Please install a newer version.");
                } else if (remote_minor == 3) {
                        /* note that this disables agent-forwarding */
                        enable_compat13();
                }
                break;
        case 2:
                if (options.protocol & SSH_PROTO_2) {
                        enable_compat20();
                        break;
                }
                /* FALLTHROUGH */
        default:
                mismatch = 1;
                break;
        }
        chop(server_version_string);
        debug("Local version string %.200s", server_version_string);

        if (mismatch) {
                s = "Protocol major versions differ.\n";
                (void) atomicio(vwrite, sock_out, s, strlen(s));
                close(sock_in);
                close(sock_out);
                logit("Protocol major versions differ for %s: %.200s vs. %.200s",
                    get_remote_ipaddr(),
                    server_version_string, client_version_string);
                cleanup_exit(255);
        }
}

/* Destroy the host and server keys.  They will no longer be needed. */
void
destroy_sensitive_data(void)
{
        int i;

        if (sensitive_data.server_key) {
                key_free(sensitive_data.server_key);
                sensitive_data.server_key = NULL;
        }
        for (i = 0; i < options.num_host_key_files; i++) {
                if (sensitive_data.host_keys[i]) {
                        key_free(sensitive_data.host_keys[i]);
                        sensitive_data.host_keys[i] = NULL;
                }
        }
        sensitive_data.ssh1_host_key = NULL;
        memset(sensitive_data.ssh1_cookie, 0, SSH_SESSION_KEY_LENGTH);
}

/* Demote private to public keys for network child */
void
demote_sensitive_data(void)
{
        Key *tmp;
        int i;

        if (sensitive_data.server_key) {
                tmp = key_demote(sensitive_data.server_key);
                key_free(sensitive_data.server_key);
                sensitive_data.server_key = tmp;
        }

        for (i = 0; i < options.num_host_key_files; i++) {
                if (sensitive_data.host_keys[i]) {
                        tmp = key_demote(sensitive_data.host_keys[i]);
                        key_free(sensitive_data.host_keys[i]);
                        sensitive_data.host_keys[i] = tmp;
                        if (tmp->type == KEY_RSA1)
                                sensitive_data.ssh1_host_key = tmp;
                }
        }

        /* We do not clear ssh1_host key and cookie.  XXX - Okay Niels? */
}

static void
privsep_preauth_child(void)
{
        u_int32_t rnd[256];
        gid_t gidset[1];
        struct passwd *pw;
        int i;

        /* Enable challenge-response authentication for privilege separation */
        privsep_challenge_enable();

        for (i = 0; i < 256; i++)
                rnd[i] = arc4random();
        RAND_seed(rnd, sizeof(rnd));

        /* Demote the private keys to public keys. */
        demote_sensitive_data();

        if ((pw = getpwnam(SSH_PRIVSEP_USER)) == NULL)
                fatal("Privilege separation user %s does not exist",
                    SSH_PRIVSEP_USER);
        memset(pw->pw_passwd, 0, strlen(pw->pw_passwd));
        endpwent();

        /* Change our root directory */
        if (chroot(_PATH_PRIVSEP_CHROOT_DIR) == -1)
                fatal("chroot(\"%s\"): %s", _PATH_PRIVSEP_CHROOT_DIR,
                    strerror(errno));
        if (chdir("/") == -1)
                fatal("chdir(\"/\"): %s", strerror(errno));

        /* Drop our privileges */
        debug3("privsep user:group %u:%u", (u_int)pw->pw_uid,
            (u_int)pw->pw_gid);
#if 0
        /* XXX not ready, too heavy after chroot */
        do_setusercontext(pw);
#else
        gidset[0] = pw->pw_gid;
        if (setgroups(1, gidset) < 0)
                fatal("setgroups: %.100s", strerror(errno));
        permanently_set_uid(pw);
#endif
}

static int
privsep_preauth(Authctxt *authctxt)
{
        int status;
        pid_t pid;

        /* Set up unprivileged child process to deal with network data */
        pmonitor = monitor_init();
        /* Store a pointer to the kex for later rekeying */
        pmonitor->m_pkex = &xxx_kex;

        pid = fork();
        if (pid == -1) {
                fatal("fork of unprivileged child failed");
        } else if (pid != 0) {
                debug2("Network child is on pid %ld", (long)pid);

                close(pmonitor->m_recvfd);
                pmonitor->m_pid = pid;
                monitor_child_preauth(authctxt, pmonitor);
                close(pmonitor->m_sendfd);

                /* Sync memory */
                monitor_sync(pmonitor);

                /* Wait for the child's exit status */
                while (waitpid(pid, &status, 0) < 0)
                        if (errno != EINTR)
                                break;
                return (1);
        } else {
                /* child */

                close(pmonitor->m_sendfd);

                /* Demote the child */
                if (getuid() == 0 || geteuid() == 0)
                        privsep_preauth_child();
                setproctitle("%s", "[net]");
        }
        return (0);
}

static void
privsep_postauth(Authctxt *authctxt)
{
#ifdef DISABLE_FD_PASSING
        if (1) {
#else
        if (authctxt->pw->pw_uid == 0 || options.use_login) {
#endif
                /* File descriptor passing is broken or root login */
                use_privsep = 0;
                goto skip;
        }

        /* New socket pair */
        monitor_reinit(pmonitor);

        pmonitor->m_pid = fork();
        if (pmonitor->m_pid == -1)
                fatal("fork of unprivileged child failed");
        else if (pmonitor->m_pid != 0) {
                debug2("User child is on pid %ld", (long)pmonitor->m_pid);
                close(pmonitor->m_recvfd);
                buffer_clear(&loginmsg);
                monitor_child_postauth(pmonitor);

                /* NEVERREACHED */
                exit(0);
        }

        close(pmonitor->m_sendfd);

        /* Demote the private keys to public keys. */
        demote_sensitive_data();

        /* Drop privileges */
        do_setusercontext(authctxt->pw);

 skip:
        /* It is safe now to apply the key state */
        monitor_apply_keystate(pmonitor);

        /*
         * Tell the packet layer that authentication was successful, since
         * this information is not part of the key state.
         */
        packet_set_authenticated();
}

static char *
list_hostkey_types(void)
{
        Buffer b;
        const char *p;
        char *ret;
        int i;

        buffer_init(&b);
        for (i = 0; i < options.num_host_key_files; i++) {
                Key *key = sensitive_data.host_keys[i];
                if (key == NULL)
                        continue;
                switch (key->type) {
                case KEY_RSA:
                case KEY_DSA:
                        if (buffer_len(&b) > 0)
                                buffer_append(&b, ",", 1);
                        p = key_ssh_name(key);
                        buffer_append(&b, p, strlen(p));
                        break;
                }
        }
        buffer_append(&b, "\0", 1);
        ret = xstrdup(buffer_ptr(&b));
        buffer_free(&b);
        debug("list_hostkey_types: %s", ret);
        return ret;
}

Key *
get_hostkey_by_type(int type)
{
        int i;

        for (i = 0; i < options.num_host_key_files; i++) {
                Key *key = sensitive_data.host_keys[i];
                if (key != NULL && key->type == type)
                        return key;
        }
        return NULL;
}

Key *
get_hostkey_by_index(int ind)
{
        if (ind < 0 || ind >= options.num_host_key_files)
                return (NULL);
        return (sensitive_data.host_keys[ind]);
}

int
get_hostkey_index(Key *key)
{
        int i;

        for (i = 0; i < options.num_host_key_files; i++) {
                if (key == sensitive_data.host_keys[i])
                        return (i);
        }
        return (-1);
}

/*
 * returns 1 if connection should be dropped, 0 otherwise.
 * dropping starts at connection #max_startups_begin with a probability
 * of (max_startups_rate/100). the probability increases linearly until
 * all connections are dropped for startups > max_startups
 */
static int
drop_connection(int startups)
{
        int p, r;

        if (startups < options.max_startups_begin)
                return 0;
        if (startups >= options.max_startups)
                return 1;
        if (options.max_startups_rate == 100)
                return 1;

        p  = 100 - options.max_startups_rate;
        p *= startups - options.max_startups_begin;
        p /= options.max_startups - options.max_startups_begin;
        p += options.max_startups_rate;
        r = arc4random() % 100;

        debug("drop_connection: p %d, r %d", p, r);
        return (r < p) ? 1 : 0;
}

static void
usage(void)
{
        fprintf(stderr, "%s, %s\n",
            SSH_RELEASE, SSLeay_version(SSLEAY_VERSION));
        fprintf(stderr,
"usage: sshd [-46Ddeiqt] [-b bits] [-f config_file] [-g login_grace_time]\n"
"            [-h host_key_file] [-k key_gen_time] [-o option] [-p port] [-u len]\n"
        );
        exit(1);
}

static void
send_rexec_state(int fd, Buffer *conf)
{
        Buffer m;

        debug3("%s: entering fd = %d config len %d", __func__, fd,
            buffer_len(conf));

        /*
         * Protocol from reexec master to child:
         *      string  configuration
         *      u_int   ephemeral_key_follows
         *      bignum  e               (only if ephemeral_key_follows == 1)
         *      bignum  n                       "
         *      bignum  d                       "
         *      bignum  iqmp                    "
         *      bignum  p                       "
         *      bignum  q                       "
         *      string rngseed          (only if OpenSSL is not self-seeded)
         */
        buffer_init(&m);
        buffer_put_cstring(&m, buffer_ptr(conf));

        if (sensitive_data.server_key != NULL &&
            sensitive_data.server_key->type == KEY_RSA1) {
                buffer_put_int(&m, 1);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->e);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->n);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->d);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->iqmp);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->p);
                buffer_put_bignum(&m, sensitive_data.server_key->rsa->q);
        } else
                buffer_put_int(&m, 0);

#ifndef OPENSSL_PRNG_ONLY
        rexec_send_rng_seed(&m);
#endif

        if (ssh_msg_send(fd, 0, &m) == -1)
                fatal("%s: ssh_msg_send failed", __func__);

        buffer_free(&m);

        debug3("%s: done", __func__);
}

static void
recv_rexec_state(int fd, Buffer *conf)
{
        Buffer m;
        char *cp;
        u_int len;

        debug3("%s: entering fd = %d", __func__, fd);

        buffer_init(&m);

        if (ssh_msg_recv(fd, &m) == -1)
                fatal("%s: ssh_msg_recv failed", __func__);
        if (buffer_get_char(&m) != 0)
                fatal("%s: rexec version mismatch", __func__);

        cp = buffer_get_string(&m, &len);
        if (conf != NULL)
                buffer_append(conf, cp, len + 1);
        xfree(cp);

        if (buffer_get_int(&m)) {
                if (sensitive_data.server_key != NULL)
                        key_free(sensitive_data.server_key);
                sensitive_data.server_key = key_new_private(KEY_RSA1);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->e);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->n);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->d);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->iqmp);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->p);
                buffer_get_bignum(&m, sensitive_data.server_key->rsa->q);
                rsa_generate_additional_parameters(
                    sensitive_data.server_key->rsa);
        }

#ifndef OPENSSL_PRNG_ONLY
        rexec_recv_rng_seed(&m);
#endif

        buffer_free(&m);

        debug3("%s: done", __func__);
}

/* Accept a connection from inetd */
static void
server_accept_inetd(int *sock_in, int *sock_out)
{
        int fd;

        startup_pipe = -1;
        if (rexeced_flag) {
                close(REEXEC_CONFIG_PASS_FD);
                *sock_in = *sock_out = dup(STDIN_FILENO);
                if (!debug_flag) {
                        startup_pipe = dup(REEXEC_STARTUP_PIPE_FD);
                        close(REEXEC_STARTUP_PIPE_FD);
                }
        } else {
                *sock_in = dup(STDIN_FILENO);
                *sock_out = dup(STDOUT_FILENO);
        }
        /*
         * We intentionally do not close the descriptors 0, 1, and 2
         * as our code for setting the descriptors won't work if
         * ttyfd happens to be one of those.
         */
        if ((fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
                dup2(fd, STDIN_FILENO);
                dup2(fd, STDOUT_FILENO);
                if (fd > STDOUT_FILENO)
                        close(fd);
        }
        debug("inetd sockets after dupping: %d, %d", *sock_in, *sock_out);
}

/*
 * Listen for TCP connections
 */
static void
server_listen(void)
{
        int ret, listen_sock, on = 1;
        struct addrinfo *ai;
        char ntop[NI_MAXHOST], strport[NI_MAXSERV];

        for (ai = options.listen_addrs; ai; ai = ai->ai_next) {
                if (ai->ai_family != AF_INET && ai->ai_family != AF_INET6)
                        continue;
                if (num_listen_socks >= MAX_LISTEN_SOCKS)
                        fatal("Too many listen sockets. "
                            "Enlarge MAX_LISTEN_SOCKS");
                if ((ret = getnameinfo(ai->ai_addr, ai->ai_addrlen,
                    ntop, sizeof(ntop), strport, sizeof(strport),
                    NI_NUMERICHOST|NI_NUMERICSERV)) != 0) {
                        error("getnameinfo failed: %.100s",
                            (ret != EAI_SYSTEM) ? gai_strerror(ret) :
                            strerror(errno));
                        continue;
                }
                /* Create socket for listening. */
                listen_sock = socket(ai->ai_family, ai->ai_socktype,
                    ai->ai_protocol);
                if (listen_sock < 0) {
                        /* kernel may not support ipv6 */
                        verbose("socket: %.100s", strerror(errno));
                        continue;
                }
                if (set_nonblock(listen_sock) == -1) {
                        close(listen_sock);
                        continue;
                }
                /*
                 * Set socket options.
                 * Allow local port reuse in TIME_WAIT.
                 */
                if (setsockopt(listen_sock, SOL_SOCKET, SO_REUSEADDR,
                    &on, sizeof(on)) == -1)
                        error("setsockopt SO_REUSEADDR: %s", strerror(errno));

                debug("Bind to port %s on %s.", strport, ntop);

                /* Bind the socket to the desired port. */
                if (bind(listen_sock, ai->ai_addr, ai->ai_addrlen) < 0) {
                        error("Bind to port %s on %s failed: %.200s.",
                            strport, ntop, strerror(errno));
                        close(listen_sock);
                        continue;
                }
                listen_socks[num_listen_socks] = listen_sock;
                num_listen_socks++;

                /* Start listening on the port. */
                if (listen(listen_sock, SSH_LISTEN_BACKLOG) < 0)
                        fatal("listen on [%s]:%s: %.100s",
                            ntop, strport, strerror(errno));
                logit("Server listening on %s port %s.", ntop, strport);
        }
        freeaddrinfo(options.listen_addrs);

        if (!num_listen_socks)
                fatal("Cannot bind any address.");
}

/*
 * The main TCP accept loop. Note that, for the non-debug case, returns
 * from this function are in a forked subprocess.
 */
static void
server_accept_loop(int *sock_in, int *sock_out, int *newsock, int *config_s)
{
        fd_set *fdset;
        int i, j, ret, maxfd;
        int key_used = 0, startups = 0;
        int startup_p[2] = { -1 , -1 };
        struct sockaddr_storage from;
        socklen_t fromlen;
        pid_t pid;

        /* setup fd set for accept */
        fdset = NULL;
        maxfd = 0;
        for (i = 0; i < num_listen_socks; i++)
                if (listen_socks[i] > maxfd)
                        maxfd = listen_socks[i];
        /* pipes connected to unauthenticated childs */
        startup_pipes = xcalloc(options.max_startups, sizeof(int));
        for (i = 0; i < options.max_startups; i++)
                startup_pipes[i] = -1;

        /*
         * Stay listening for connections until the system crashes or
         * the daemon is killed with a signal.
         */
        for (;;) {
                if (received_sighup)
                        sighup_restart();
                if (fdset != NULL)
                        xfree(fdset);
                fdset = (fd_set *)xcalloc(howmany(maxfd + 1, NFDBITS),
                    sizeof(fd_mask));

                for (i = 0; i < num_listen_socks; i++)
                        FD_SET(listen_socks[i], fdset);
                for (i = 0; i < options.max_startups; i++)
                        if (startup_pipes[i] != -1)
                                FD_SET(startup_pipes[i], fdset);

                /* Wait in select until there is a connection. */
                ret = select(maxfd+1, fdset, NULL, NULL, NULL);
                if (ret < 0 && errno != EINTR)
                        error("select: %.100s", strerror(errno));
                if (received_sigterm) {
                        logit("Received signal %d; terminating.",
                            (int) received_sigterm);
                        close_listen_socks();
                        unlink(options.pid_file);
                        exit(255);
                }
                if (key_used && key_do_regen) {
                        generate_ephemeral_server_key();
                        key_used = 0;
                        key_do_regen = 0;
                }
                if (ret < 0)
                        continue;

                for (i = 0; i < options.max_startups; i++)
                        if (startup_pipes[i] != -1 &&
                            FD_ISSET(startup_pipes[i], fdset)) {
                                /*
                                 * the read end of the pipe is ready
                                 * if the child has closed the pipe
                                 * after successful authentication
                                 * or if the child has died
                                 */
                                close(startup_pipes[i]);
                                startup_pipes[i] = -1;
                                startups--;
                        }
                for (i = 0; i < num_listen_socks; i++) {
                        if (!FD_ISSET(listen_socks[i], fdset))
                                continue;
                        fromlen = sizeof(from);
                        *newsock = accept(listen_socks[i],
                            (struct sockaddr *)&from, &fromlen);
                        if (*newsock < 0) {
                                if (errno != EINTR && errno != EWOULDBLOCK)
                                        error("accept: %.100s", strerror(errno));
                                continue;
                        }
                        if (unset_nonblock(*newsock) == -1) {
                                close(*newsock);
                                continue;
                        }
                        if (drop_connection(startups) == 1) {
                                debug("drop connection #%d", startups);
                                close(*newsock);
                                continue;
                        }
                        if (pipe(startup_p) == -1) {
                                close(*newsock);
                                continue;
                        }

                        if (rexec_flag && socketpair(AF_UNIX,
                            SOCK_STREAM, 0, config_s) == -1) {
                                error("reexec socketpair: %s",
                                    strerror(errno));
                                close(*newsock);
                                close(startup_p[0]);
                                close(startup_p[1]);
                                continue;
                        }

                        for (j = 0; j < options.max_startups; j++)
                                if (startup_pipes[j] == -1) {
                                        startup_pipes[j] = startup_p[0];
                                        if (maxfd < startup_p[0])
                                                maxfd = startup_p[0];
                                        startups++;
                                        break;
                                }

                        /*
                         * Got connection.  Fork a child to handle it, unless
                         * we are in debugging mode.
                         */
                        if (debug_flag) {
                                /*
                                 * In debugging mode.  Close the listening
                                 * socket, and start processing the
                                 * connection without forking.
                                 */
                                debug("Server will not fork when running in debugging mode.");
                                close_listen_socks();
                                *sock_in = *newsock;
                                *sock_out = *newsock;
                                close(startup_p[0]);
                                close(startup_p[1]);
                                startup_pipe = -1;
                                pid = getpid();
                                if (rexec_flag) {
                                        send_rexec_state(config_s[0],
                                            &cfg);
                                        close(config_s[0]);
                                }
                                break;
                        }

                        /*
                         * Normal production daemon.  Fork, and have
                         * the child process the connection. The
                         * parent continues listening.
                         */
                        if ((pid = fork()) == 0) {
                                /*
                                 * Child.  Close the listening and
                                 * max_startup sockets.  Start using
                                 * the accepted socket. Reinitialize
                                 * logging (since our pid has changed).
                                 * We break out of the loop to handle
                                 * the connection.
                                 */
                                startup_pipe = startup_p[1];
                                close_startup_pipes();
                                close_listen_socks();
                                *sock_in = *newsock;
                                *sock_out = *newsock;
                                log_init(__progname,
                                    options.log_level,
                                    options.log_facility,
                                    log_stderr);
                                if (rexec_flag)
                                        close(config_s[0]);
                                break;
                        }

                        /* Parent.  Stay in the loop. */
                        if (pid < 0)
                                error("fork: %.100s", strerror(errno));
                        else
                                debug("Forked child %ld.", (long)pid);

                        close(startup_p[1]);

                        if (rexec_flag) {
                                send_rexec_state(config_s[0], &cfg);
                                close(config_s[0]);
                                close(config_s[1]);
                        }

                        /*
                         * Mark that the key has been used (it
                         * was "given" to the child).
                         */
                        if ((options.protocol & SSH_PROTO_1) &&
                            key_used == 0) {
                                /* Schedule server key regeneration alarm. */
                                signal(SIGALRM, key_regeneration_alarm);
                                alarm(options.key_regeneration_time);
                                key_used = 1;
                        }

                        close(*newsock);

                        /*
                         * Ensure that our random state differs
                         * from that of the child
                         */
                        arc4random_stir();
                }

                /* child process check (or debug mode) */
                if (num_listen_socks < 0)
                        break;
        }
}


/*
 * Main program for the daemon.
 */
int
main(int ac, char **av)
{
        extern char *optarg;
        extern int optind;
        int opt, i, on = 1;
        int sock_in = -1, sock_out = -1, newsock = -1;
        const char *remote_ip;
        int remote_port;
        char *line;
        int config_s[2] = { -1 , -1 };
        Key *key;
        Authctxt *authctxt;

#ifdef HAVE_SECUREWARE
        (void)set_auth_parameters(ac, av);
#endif
        __progname = ssh_get_progname(av[0]);
        init_rng();

        /* Save argv. Duplicate so setproctitle emulation doesn't clobber it */
        saved_argc = ac;
        rexec_argc = ac;
        saved_argv = xcalloc(ac + 1, sizeof(*saved_argv));
        for (i = 0; i < ac; i++)
                saved_argv[i] = xstrdup(av[i]);
        saved_argv[i] = NULL;

#ifndef HAVE_SETPROCTITLE
        /* Prepare for later setproctitle emulation */
        compat_init_setproctitle(ac, av);
        av = saved_argv;
#endif

        if (geteuid() == 0 && setgroups(0, NULL) == -1)
                debug("setgroups(): %.200s", strerror(errno));

        /* Ensure that fds 0, 1 and 2 are open or directed to /dev/null */
        sanitise_stdfd();

        /* Initialize configuration options to their default values. */
        initialize_server_options(&options);

        /* Parse command-line arguments. */
        while ((opt = getopt(ac, av, "f:p:b:k:h:g:u:o:dDeiqrtQR46")) != -1) {
                switch (opt) {
                case '4':
                        options.address_family = AF_INET;
                        break;
                case '6':
                        options.address_family = AF_INET6;
                        break;
                case 'f':
                        config_file_name = optarg;
                        break;
                case 'd':
                        if (debug_flag == 0) {
                                debug_flag = 1;
                                options.log_level = SYSLOG_LEVEL_DEBUG1;
                        } else if (options.log_level < SYSLOG_LEVEL_DEBUG3)
                                options.log_level++;
                        break;
                case 'D':
                        no_daemon_flag = 1;
                        break;
                case 'e':
                        log_stderr = 1;
                        break;
                case 'i':
                        inetd_flag = 1;
                        break;
                case 'r':
                        rexec_flag = 0;
                        break;
                case 'R':
                        rexeced_flag = 1;
                        inetd_flag = 1;
                        break;
                case 'Q':
                        /* ignored */
                        break;
                case 'q':
                        options.log_level = SYSLOG_LEVEL_QUIET;
                        break;
                case 'b':
                        options.server_key_bits = (int)strtonum(optarg, 256,
                            32768, NULL);
                        break;
                case 'p':
                        options.ports_from_cmdline = 1;
                        if (options.num_ports >= MAX_PORTS) {
                                fprintf(stderr, "too many ports.\n");
                                exit(1);
                        }
                        options.ports[options.num_ports++] = a2port(optarg);
                        if (options.ports[options.num_ports-1] == 0) {
                                fprintf(stderr, "Bad port number.\n");
                                exit(1);
                        }
                        break;
                case 'g':
                        if ((options.login_grace_time = convtime(optarg)) == -1) {
                                fprintf(stderr, "Invalid login grace time.\n");
                                exit(1);
                        }
                        break;
                case 'k':
                        if ((options.key_regeneration_time = convtime(optarg)) == -1) {
                                fprintf(stderr, "Invalid key regeneration interval.\n");
                                exit(1);
                        }
                        break;
                case 'h':
                        if (options.num_host_key_files >= MAX_HOSTKEYS) {
                                fprintf(stderr, "too many host keys.\n");
                                exit(1);
                        }
                        options.host_key_files[options.num_host_key_files++] = optarg;
                        break;
                case 't':
                        test_flag = 1;
                        break;
                case 'u':
                        utmp_len = (u_int)strtonum(optarg, 0, MAXHOSTNAMELEN+1, NULL);
                        if (utmp_len > MAXHOSTNAMELEN) {
                                fprintf(stderr, "Invalid utmp length.\n");
                                exit(1);
                        }
                        break;
                case 'o':
                        line = xstrdup(optarg);
                        if (process_server_config_line(&options, line,
                            "command-line", 0, NULL, NULL, NULL, NULL) != 0)
                                exit(1);
                        xfree(line);
                        break;
                case '?':
                default:
                        usage();
                        break;
                }
        }
        if (rexeced_flag || inetd_flag)
                rexec_flag = 0;
        if (rexec_flag && (av[0] == NULL || *av[0] != '/'))
                fatal("sshd re-exec requires execution with an absolute path");
        if (rexeced_flag)
                closefrom(REEXEC_MIN_FREE_FD);
        else
                closefrom(REEXEC_DEVCRYPTO_RESERVED_FD);

        SSLeay_add_all_algorithms();

        /*
         * Force logging to stderr until we have loaded the private host
         * key (unless started from inetd)
         */
        log_init(__progname,
            options.log_level == SYSLOG_LEVEL_NOT_SET ?
            SYSLOG_LEVEL_INFO : options.log_level,
            options.log_facility == SYSLOG_FACILITY_NOT_SET ?
            SYSLOG_FACILITY_AUTH : options.log_facility,
            log_stderr || !inetd_flag);

        /*
         * Unset KRB5CCNAME, otherwise the user's session may inherit it from
         * root's environment
         */
        if (getenv("KRB5CCNAME") != NULL)
                unsetenv("KRB5CCNAME");

#ifdef _UNICOS
        /* Cray can define user privs drop all privs now!
         * Not needed on PRIV_SU systems!
         */
        drop_cray_privs();
#endif

        sensitive_data.server_key = NULL;
        sensitive_data.ssh1_host_key = NULL;
        sensitive_data.have_ssh1_key = 0;
        sensitive_data.have_ssh2_key = 0;

        /* Fetch our configuration */
        buffer_init(&cfg);
        if (rexeced_flag)
                recv_rexec_state(REEXEC_CONFIG_PASS_FD, &cfg);
        else
                load_server_config(config_file_name, &cfg);

        parse_server_config(&options, rexeced_flag ? "rexec" : config_file_name,
            &cfg, NULL, NULL, NULL);

        seed_rng();

        /* Fill in default values for those options not explicitly set. */
        fill_default_server_options(&options);

        /* set default channel AF */
        channel_set_af(options.address_family);

        /* Check that there are no remaining arguments. */
        if (optind < ac) {
                fprintf(stderr, "Extra argument %s.\n", av[optind]);
                exit(1);
        }

        debug("sshd version %.100s", SSH_RELEASE);

        /* load private host keys */
        sensitive_data.host_keys = xcalloc(options.num_host_key_files,
            sizeof(Key *));
        for (i = 0; i < options.num_host_key_files; i++)
                sensitive_data.host_keys[i] = NULL;

        for (i = 0; i < options.num_host_key_files; i++) {
                key = key_load_private(options.host_key_files[i], "", NULL);
                sensitive_data.host_keys[i] = key;
                if (key == NULL) {
                        error("Could not load host key: %s",
                            options.host_key_files[i]);
                        sensitive_data.host_keys[i] = NULL;
                        continue;
                }
                switch (key->type) {
                case KEY_RSA1:
                        sensitive_data.ssh1_host_key = key;
                        sensitive_data.have_ssh1_key = 1;
                        break;
                case KEY_RSA:
                case KEY_DSA:
                        sensitive_data.have_ssh2_key = 1;
                        break;
                }
                debug("private host key: #%d type %d %s", i, key->type,
                    key_type(key));
        }
        if ((options.protocol & SSH_PROTO_1) && !sensitive_data.have_ssh1_key) {
                logit("Disabling protocol version 1. Could not load host key");
                options.protocol &= ~SSH_PROTO_1;
        }
        if ((options.protocol & SSH_PROTO_2) && !sensitive_data.have_ssh2_key) {
                logit("Disabling protocol version 2. Could not load host key");
                options.protocol &= ~SSH_PROTO_2;
        }
        if (!(options.protocol & (SSH_PROTO_1|SSH_PROTO_2))) {
                logit("sshd: no hostkeys available -- exiting.");
                exit(1);
        }

        /* Check certain values for sanity. */
        if (options.protocol & SSH_PROTO_1) {
                if (options.server_key_bits < 512 ||
                    options.server_key_bits > 32768) {
                        fprintf(stderr, "Bad server key size.\n");
                        exit(1);
                }
                /*
                 * Check that server and host key lengths differ sufficiently. This
                 * is necessary to make double encryption work with rsaref. Oh, I
                 * hate software patents. I dont know if this can go? Niels
                 */
                if (options.server_key_bits >
                    BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) -
                    SSH_KEY_BITS_RESERVED && options.server_key_bits <
                    BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) +
                    SSH_KEY_BITS_RESERVED) {
                        options.server_key_bits =
                            BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) +
                            SSH_KEY_BITS_RESERVED;
                        debug("Forcing server key to %d bits to make it differ from host key.",
                            options.server_key_bits);
                }
        }

        if (use_privsep) {
                struct stat st;

                if (getpwnam(SSH_PRIVSEP_USER) == NULL)
                        fatal("Privilege separation user %s does not exist",
                            SSH_PRIVSEP_USER);
                if ((stat(_PATH_PRIVSEP_CHROOT_DIR, &st) == -1) ||
                    (S_ISDIR(st.st_mode) == 0))
                        fatal("Missing privilege separation directory: %s",
                            _PATH_PRIVSEP_CHROOT_DIR);

#ifdef HAVE_CYGWIN
                if (check_ntsec(_PATH_PRIVSEP_CHROOT_DIR) &&
                    (st.st_uid != getuid () ||
                    (st.st_mode & (S_IWGRP|S_IWOTH)) != 0))
#else
                if (st.st_uid != 0 || (st.st_mode & (S_IWGRP|S_IWOTH)) != 0)
#endif
                        fatal("%s must be owned by root and not group or "
                            "world-writable.", _PATH_PRIVSEP_CHROOT_DIR);
        }

        /* Configuration looks good, so exit if in test mode. */
        if (test_flag)
                exit(0);

        /*
         * Clear out any supplemental groups we may have inherited.  This
         * prevents inadvertent creation of files with bad modes (in the
         * portable version at least, it's certainly possible for PAM
         * to create a file, and we can't control the code in every
         * module which might be used).
         */
        if (setgroups(0, NULL) < 0)
                debug("setgroups() failed: %.200s", strerror(errno));

        if (rexec_flag) {
                rexec_argv = xcalloc(rexec_argc + 2, sizeof(char *));
                for (i = 0; i < rexec_argc; i++) {
                        debug("rexec_argv[%d]='%s'", i, saved_argv[i]);
                        rexec_argv[i] = saved_argv[i];
                }
                rexec_argv[rexec_argc] = "-R";
                rexec_argv[rexec_argc + 1] = NULL;
        }

        /* Initialize the log (it is reinitialized below in case we forked). */
        if (debug_flag && (!inetd_flag || rexeced_flag))
                log_stderr = 1;
        log_init(__progname, options.log_level, options.log_facility, log_stderr);

        /*
         * If not in debugging mode, and not started from inetd, disconnect
         * from the controlling terminal, and fork.  The original process
         * exits.
         */
        if (!(debug_flag || inetd_flag || no_daemon_flag)) {
#ifdef TIOCNOTTY
                int fd;
#endif /* TIOCNOTTY */
                if (daemon(0, 0) < 0)
                        fatal("daemon() failed: %.200s", strerror(errno));

                /* Disconnect from the controlling tty. */
#ifdef TIOCNOTTY
                fd = open(_PATH_TTY, O_RDWR | O_NOCTTY);
                if (fd >= 0) {
                        (void) ioctl(fd, TIOCNOTTY, NULL);
                        close(fd);
                }
#endif /* TIOCNOTTY */
        }
        /* Reinitialize the log (because of the fork above). */
        log_init(__progname, options.log_level, options.log_facility, log_stderr);

        /* Initialize the random number generator. */
        arc4random_stir();

        /* Chdir to the root directory so that the current disk can be
           unmounted if desired. */
        chdir("/");

        /* ignore SIGPIPE */
        signal(SIGPIPE, SIG_IGN);

        /* Get a connection, either from inetd or a listening TCP socket */
        if (inetd_flag) {
                server_accept_inetd(&sock_in, &sock_out);

                if ((options.protocol & SSH_PROTO_1) &&
                    sensitive_data.server_key == NULL)
                        generate_ephemeral_server_key();
        } else {
                server_listen();

                if (options.protocol & SSH_PROTO_1)
                        generate_ephemeral_server_key();

                signal(SIGHUP, sighup_handler);
                signal(SIGCHLD, main_sigchld_handler);
                signal(SIGTERM, sigterm_handler);
                signal(SIGQUIT, sigterm_handler);

                /*
                 * Write out the pid file after the sigterm handler
                 * is setup and the listen sockets are bound
                 */
                if (!debug_flag) {
                        FILE *f = fopen(options.pid_file, "w");

                        if (f == NULL) {
                                error("Couldn't create pid file \"%s\": %s",
                                    options.pid_file, strerror(errno));
                        } else {
                                fprintf(f, "%ld\n", (long) getpid());
                                fclose(f);
                        }
                }

                /* Accept a connection and return in a forked child */
                server_accept_loop(&sock_in, &sock_out,
                    &newsock, config_s);
        }

        /* This is the child processing a new connection. */
        setproctitle("%s", "[accepted]");

        /*
         * Create a new session and process group since the 4.4BSD
         * setlogin() affects the entire process group.  We don't
         * want the child to be able to affect the parent.
         */
#if !defined(SSHD_ACQUIRES_CTTY)
        /*
         * If setsid is called, on some platforms sshd will later acquire a
         * controlling terminal which will result in "could not set
         * controlling tty" errors.
         */
        if (!debug_flag && !inetd_flag && setsid() < 0)
                error("setsid: %.100s", strerror(errno));
#endif

        if (rexec_flag) {
                int fd;

                debug("rexec start in %d out %d newsock %d pipe %d sock %d",
                    sock_in, sock_out, newsock, startup_pipe, config_s[0]);
                dup2(newsock, STDIN_FILENO);
                dup2(STDIN_FILENO, STDOUT_FILENO);
                if (startup_pipe == -1)
                        close(REEXEC_STARTUP_PIPE_FD);
                else
                        dup2(startup_pipe, REEXEC_STARTUP_PIPE_FD);

                dup2(config_s[1], REEXEC_CONFIG_PASS_FD);
                close(config_s[1]);
                if (startup_pipe != -1)
                        close(startup_pipe);

                execv(rexec_argv[0], rexec_argv);

                /* Reexec has failed, fall back and continue */
                error("rexec of %s failed: %s", rexec_argv[0], strerror(errno));
                recv_rexec_state(REEXEC_CONFIG_PASS_FD, NULL);
                log_init(__progname, options.log_level,
                    options.log_facility, log_stderr);

                /* Clean up fds */
                startup_pipe = REEXEC_STARTUP_PIPE_FD;
                close(config_s[1]);
                close(REEXEC_CONFIG_PASS_FD);
                newsock = sock_out = sock_in = dup(STDIN_FILENO);
                if ((fd = open(_PATH_DEVNULL, O_RDWR, 0)) != -1) {
                        dup2(fd, STDIN_FILENO);
                        dup2(fd, STDOUT_FILENO);
                        if (fd > STDERR_FILENO)
                                close(fd);
                }
                debug("rexec cleanup in %d out %d newsock %d pipe %d sock %d",
                    sock_in, sock_out, newsock, startup_pipe, config_s[0]);
        }

        /*
         * Disable the key regeneration alarm.  We will not regenerate the
         * key since we are no longer in a position to give it to anyone. We
         * will not restart on SIGHUP since it no longer makes sense.
         */
        alarm(0);
        signal(SIGALRM, SIG_DFL);
        signal(SIGHUP, SIG_DFL);
        signal(SIGTERM, SIG_DFL);
        signal(SIGQUIT, SIG_DFL);
        signal(SIGCHLD, SIG_DFL);
        signal(SIGINT, SIG_DFL);

        /*
         * Register our connection.  This turns encryption off because we do
         * not have a key.
         */
        packet_set_connection(sock_in, sock_out);
        packet_set_server();

        /* Set SO_KEEPALIVE if requested. */
        if (options.tcp_keep_alive && packet_connection_is_on_socket() &&
            setsockopt(sock_in, SOL_SOCKET, SO_KEEPALIVE, &on, sizeof(on)) < 0)
                error("setsockopt SO_KEEPALIVE: %.100s", strerror(errno));

        if ((remote_port = get_remote_port()) < 0) {
                debug("get_remote_port failed");
                cleanup_exit(255);
        }

        /*
         * We use get_canonical_hostname with usedns = 0 instead of
         * get_remote_ipaddr here so IP options will be checked.
         */
        (void) get_canonical_hostname(0);
        /*
         * The rest of the code depends on the fact that
         * get_remote_ipaddr() caches the remote ip, even if
         * the socket goes away.
         */
        remote_ip = get_remote_ipaddr();

#ifdef SSH_AUDIT_EVENTS
        audit_connection_from(remote_ip, remote_port);
#endif
#ifdef LIBWRAP
        /* Check whether logins are denied from this host. */
        if (packet_connection_is_on_socket()) {
                struct request_info req;

                request_init(&req, RQ_DAEMON, __progname, RQ_FILE, sock_in, 0);
                fromhost(&req);

                if (!hosts_access(&req)) {
                        debug("Connection refused by tcp wrapper");
                        refuse(&req);
                        /* NOTREACHED */
                        fatal("libwrap refuse returns");
                }
        }
#endif /* LIBWRAP */

        /* Log the connection. */
        verbose("Connection from %.500s port %d", remote_ip, remote_port);

        /*
         * We don't want to listen forever unless the other side
         * successfully authenticates itself.  So we set up an alarm which is
         * cleared after successful authentication.  A limit of zero
         * indicates no limit. Note that we don't set the alarm in debugging
         * mode; it is just annoying to have the server exit just when you
         * are about to discover the bug.
         */
        signal(SIGALRM, grace_alarm_handler);
        if (!debug_flag)
                alarm(options.login_grace_time);

        sshd_exchange_identification(sock_in, sock_out);

        packet_set_nonblocking();

        /* allocate authentication context */
        authctxt = xcalloc(1, sizeof(*authctxt));

        authctxt->loginmsg = &loginmsg;

        /* XXX global for cleanup, access from other modules */
        the_authctxt = authctxt;

        /* prepare buffer to collect messages to display to user after login */
        buffer_init(&loginmsg);

        if (use_privsep)
                if (privsep_preauth(authctxt) == 1)
                        goto authenticated;

        /* perform the key exchange */
        /* authenticate user and start session */
        if (compat20) {
                do_ssh2_kex();
                do_authentication2(authctxt);
        } else {
                do_ssh1_kex();
                do_authentication(authctxt);
        }
        /*
         * If we use privilege separation, the unprivileged child transfers
         * the current keystate and exits
         */
        if (use_privsep) {
                mm_send_keystate(pmonitor);
                exit(0);
        }

 authenticated:
        /*
         * Cancel the alarm we set to limit the time taken for
         * authentication.
         */
        alarm(0);
        signal(SIGALRM, SIG_DFL);
        authctxt->authenticated = 1;
        if (startup_pipe != -1) {
                close(startup_pipe);
                startup_pipe = -1;
        }

#ifdef SSH_AUDIT_EVENTS
        audit_event(SSH_AUTH_SUCCESS);
#endif

        /*
         * In privilege separation, we fork another child and prepare
         * file descriptor passing.
         */
        if (use_privsep) {
                privsep_postauth(authctxt);
                /* the monitor process [priv] will not return */
                if (!compat20)
                        destroy_sensitive_data();
        }

        /* Start session. */
        do_authenticated(authctxt);

        /* The connection has been terminated. */
        verbose("Closing connection to %.100s", remote_ip);

#ifdef USE_PAM
        if (options.use_pam)
                finish_pam();
#endif /* USE_PAM */

#ifdef SSH_AUDIT_EVENTS
        PRIVSEP(audit_event(SSH_CONNECTION_CLOSE));
#endif

        packet_close();

        if (use_privsep)
                mm_terminate();

        exit(0);
}

/*
 * Decrypt session_key_int using our private server key and private host key
 * (key with larger modulus first).
 */
int
ssh1_session_key(BIGNUM *session_key_int)
{
        int rsafail = 0;

        if (BN_cmp(sensitive_data.server_key->rsa->n,
            sensitive_data.ssh1_host_key->rsa->n) > 0) {
                /* Server key has bigger modulus. */
                if (BN_num_bits(sensitive_data.server_key->rsa->n) <
                    BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) +
                    SSH_KEY_BITS_RESERVED) {
                        fatal("do_connection: %s: "
                            "server_key %d < host_key %d + SSH_KEY_BITS_RESERVED %d",
                            get_remote_ipaddr(),
                            BN_num_bits(sensitive_data.server_key->rsa->n),
                            BN_num_bits(sensitive_data.ssh1_host_key->rsa->n),
                            SSH_KEY_BITS_RESERVED);
                }
                if (rsa_private_decrypt(session_key_int, session_key_int,
                    sensitive_data.server_key->rsa) <= 0)
                        rsafail++;
                if (rsa_private_decrypt(session_key_int, session_key_int,
                    sensitive_data.ssh1_host_key->rsa) <= 0)
                        rsafail++;
        } else {
                /* Host key has bigger modulus (or they are equal). */
                if (BN_num_bits(sensitive_data.ssh1_host_key->rsa->n) <
                    BN_num_bits(sensitive_data.server_key->rsa->n) +
                    SSH_KEY_BITS_RESERVED) {
                        fatal("do_connection: %s: "
                            "host_key %d < server_key %d + SSH_KEY_BITS_RESERVED %d",
                            get_remote_ipaddr(),
                            BN_num_bits(sensitive_data.ssh1_host_key->rsa->n),
                            BN_num_bits(sensitive_data.server_key->rsa->n),
                            SSH_KEY_BITS_RESERVED);
                }
                if (rsa_private_decrypt(session_key_int, session_key_int,
                    sensitive_data.ssh1_host_key->rsa) < 0)
                        rsafail++;
                if (rsa_private_decrypt(session_key_int, session_key_int,
                    sensitive_data.server_key->rsa) < 0)
                        rsafail++;
        }
        return (rsafail);
}
/*
 * SSH1 key exchange
 */
static void
do_ssh1_kex(void)
{
        int i, len;
        int rsafail = 0;
        BIGNUM *session_key_int;
        u_char session_key[SSH_SESSION_KEY_LENGTH];
        u_char cookie[8];
        u_int cipher_type, auth_mask, protocol_flags;
        u_int32_t rnd = 0;

        /*
         * Generate check bytes that the client must send back in the user
         * packet in order for it to be accepted; this is used to defy ip
         * spoofing attacks.  Note that this only works against somebody
         * doing IP spoofing from a remote machine; any machine on the local
         * network can still see outgoing packets and catch the random
         * cookie.  This only affects rhosts authentication, and this is one
         * of the reasons why it is inherently insecure.
         */
        for (i = 0; i < 8; i++) {
                if (i % 4 == 0)
                        rnd = arc4random();
                cookie[i] = rnd & 0xff;
                rnd >>= 8;
        }

        /*
         * Send our public key.  We include in the packet 64 bits of random
         * data that must be matched in the reply in order to prevent IP
         * spoofing.
         */
        packet_start(SSH_SMSG_PUBLIC_KEY);
        for (i = 0; i < 8; i++)
                packet_put_char(cookie[i]);

        /* Store our public server RSA key. */
        packet_put_int(BN_num_bits(sensitive_data.server_key->rsa->n));
        packet_put_bignum(sensitive_data.server_key->rsa->e);
        packet_put_bignum(sensitive_data.server_key->rsa->n);

        /* Store our public host RSA key. */
        packet_put_int(BN_num_bits(sensitive_data.ssh1_host_key->rsa->n));
        packet_put_bignum(sensitive_data.ssh1_host_key->rsa->e);
        packet_put_bignum(sensitive_data.ssh1_host_key->rsa->n);

        /* Put protocol flags. */
        packet_put_int(SSH_PROTOFLAG_HOST_IN_FWD_OPEN);

        /* Declare which ciphers we support. */
        packet_put_int(cipher_mask_ssh1(0));

        /* Declare supported authentication types. */
        auth_mask = 0;
        if (options.rhosts_rsa_authentication)
                auth_mask |= 1 << SSH_AUTH_RHOSTS_RSA;
        if (options.rsa_authentication)
                auth_mask |= 1 << SSH_AUTH_RSA;
        if (options.challenge_response_authentication == 1)
                auth_mask |= 1 << SSH_AUTH_TIS;
        if (options.password_authentication)
                auth_mask |= 1 << SSH_AUTH_PASSWORD;
        packet_put_int(auth_mask);

        /* Send the packet and wait for it to be sent. */
        packet_send();
        packet_write_wait();

        debug("Sent %d bit server key and %d bit host key.",
            BN_num_bits(sensitive_data.server_key->rsa->n),
            BN_num_bits(sensitive_data.ssh1_host_key->rsa->n));

        /* Read clients reply (cipher type and session key). */
        packet_read_expect(SSH_CMSG_SESSION_KEY);

        /* Get cipher type and check whether we accept this. */
        cipher_type = packet_get_char();

        if (!(cipher_mask_ssh1(0) & (1 << cipher_type)))
                packet_disconnect("Warning: client selects unsupported cipher.");

        /* Get check bytes from the packet.  These must match those we
           sent earlier with the public key packet. */
        for (i = 0; i < 8; i++)
                if (cookie[i] != packet_get_char())
                        packet_disconnect("IP Spoofing check bytes do not match.");

        debug("Encryption type: %.200s", cipher_name(cipher_type));

        /* Get the encrypted integer. */
        if ((session_key_int = BN_new()) == NULL)
                fatal("do_ssh1_kex: BN_new failed");
        packet_get_bignum(session_key_int);

        protocol_flags = packet_get_int();
        packet_set_protocol_flags(protocol_flags);
        packet_check_eom();

        /* Decrypt session_key_int using host/server keys */
        rsafail = PRIVSEP(ssh1_session_key(session_key_int));

        /*
         * Extract session key from the decrypted integer.  The key is in the
         * least significant 256 bits of the integer; the first byte of the
         * key is in the highest bits.
         */
        if (!rsafail) {
                BN_mask_bits(session_key_int, sizeof(session_key) * 8);
                len = BN_num_bytes(session_key_int);
                if (len < 0 || (u_int)len > sizeof(session_key)) {
                        error("do_connection: bad session key len from %s: "
                            "session_key_int %d > sizeof(session_key) %lu",
                            get_remote_ipaddr(), len, (u_long)sizeof(session_key));
                        rsafail++;
                } else {
                        memset(session_key, 0, sizeof(session_key));
                        BN_bn2bin(session_key_int,
                            session_key + sizeof(session_key) - len);

                        derive_ssh1_session_id(
                            sensitive_data.ssh1_host_key->rsa->n,
                            sensitive_data.server_key->rsa->n,
                            cookie, session_id);
                        /*
                         * Xor the first 16 bytes of the session key with the
                         * session id.
                         */
                        for (i = 0; i < 16; i++)
                                session_key[i] ^= session_id[i];
                }
        }
        if (rsafail) {
                int bytes = BN_num_bytes(session_key_int);
                u_char *buf = xmalloc(bytes);
                MD5_CTX md;

                logit("do_connection: generating a fake encryption key");
                BN_bn2bin(session_key_int, buf);
                MD5_Init(&md);
                MD5_Update(&md, buf, bytes);
                MD5_Update(&md, sensitive_data.ssh1_cookie, SSH_SESSION_KEY_LENGTH);
                MD5_Final(session_key, &md);
                MD5_Init(&md);
                MD5_Update(&md, session_key, 16);
                MD5_Update(&md, buf, bytes);
                MD5_Update(&md, sensitive_data.ssh1_cookie, SSH_SESSION_KEY_LENGTH);
                MD5_Final(session_key + 16, &md);
                memset(buf, 0, bytes);
                xfree(buf);
                for (i = 0; i < 16; i++)
                        session_id[i] = session_key[i] ^ session_key[i + 16];
        }
        /* Destroy the private and public keys. No longer. */
        destroy_sensitive_data();

        if (use_privsep)
                mm_ssh1_session_id(session_id);

        /* Destroy the decrypted integer.  It is no longer needed. */
        BN_clear_free(session_key_int);

        /* Set the session key.  From this on all communications will be encrypted. */
        packet_set_encryption_key(session_key, SSH_SESSION_KEY_LENGTH, cipher_type);

        /* Destroy our copy of the session key.  It is no longer needed. */
        memset(session_key, 0, sizeof(session_key));

        debug("Received session key; encryption turned on.");

        /* Send an acknowledgment packet.  Note that this packet is sent encrypted. */
        packet_start(SSH_SMSG_SUCCESS);
        packet_send();
        packet_write_wait();
}

/*
 * SSH2 key exchange: diffie-hellman-group1-sha1
 */
static void
do_ssh2_kex(void)
{
        Kex *kex;

        if (options.ciphers != NULL) {
                myproposal[PROPOSAL_ENC_ALGS_CTOS] =
                myproposal[PROPOSAL_ENC_ALGS_STOC] = options.ciphers;
        }
        myproposal[PROPOSAL_ENC_ALGS_CTOS] =
            compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_CTOS]);
        myproposal[PROPOSAL_ENC_ALGS_STOC] =
            compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_STOC]);

        if (options.macs != NULL) {
                myproposal[PROPOSAL_MAC_ALGS_CTOS] =
                myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
        }
        if (options.compression == COMP_NONE) {
                myproposal[PROPOSAL_COMP_ALGS_CTOS] =
                myproposal[PROPOSAL_COMP_ALGS_STOC] = "none";
        } else if (options.compression == COMP_DELAYED) {
                myproposal[PROPOSAL_COMP_ALGS_CTOS] =
                myproposal[PROPOSAL_COMP_ALGS_STOC] = "none,zlib@openssh.com";
        }

        myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = list_hostkey_types();

        /* start key exchange */
        kex = kex_setup(myproposal);
        kex->kex[KEX_DH_GRP1_SHA1] = kexdh_server;
        kex->kex[KEX_DH_GRP14_SHA1] = kexdh_server;
        kex->kex[KEX_DH_GEX_SHA1] = kexgex_server;
        kex->kex[KEX_DH_GEX_SHA256] = kexgex_server;
        kex->server = 1;
        kex->client_version_string=client_version_string;
        kex->server_version_string=server_version_string;
        kex->load_host_key=&get_hostkey_by_type;
        kex->host_key_index=&get_hostkey_index;

        xxx_kex = kex;

        dispatch_run(DISPATCH_BLOCK, &kex->done, kex);

        session_id2 = kex->session_id;
        session_id2_len = kex->session_id_len;

#ifdef DEBUG_KEXDH
        /* send 1st encrypted/maced/compressed message */
        packet_start(SSH2_MSG_IGNORE);
        packet_put_cstring("markus");
        packet_send();
        packet_write_wait();
#endif
        debug("KEX done");
}

/* server specific fatal cleanup */
void
cleanup_exit(int i)
{
        if (the_authctxt)
                do_cleanup(the_authctxt);
#ifdef SSH_AUDIT_EVENTS
        /* done after do_cleanup so it can cancel the PAM auth 'thread' */
        if (!use_privsep || mm_is_monitor())
                audit_event(SSH_CONNECTION_ABANDON);
#endif
        _exit(i);
}