[openssh.git] / sshd.c

/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Created: Fri Mar 17 17:09:28 1995 ylo
 * 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.
 */

#include "includes.h"
RCSID("$Id$");

#include <poll.h>

#include "xmalloc.h"
#include "rsa.h"
#include "ssh.h"
#include "pty.h"
#include "packet.h"
#include "buffer.h"
#include "cipher.h"
#include "mpaux.h"
#include "servconf.h"
#include "uidswap.h"
#include "compat.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

/* Local Xauthority file. */
static char *xauthfile = NULL;

/* Server configuration options. */
ServerOptions options;

/* Name of the server configuration file. */
char *config_file_name = 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 is being started from inetd. */
int inetd_flag = 0;

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

/* argv[0] without path. */
char *av0;

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

/*
 * This is set to the socket that the server is listening; this is used in
 * the SIGHUP signal handler.
 */
int listen_sock;

/*
 * 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;

/* Flags set in auth-rsa from authorized_keys flags.  These are set in auth-rsa.c. */
int no_port_forwarding_flag = 0;
int no_agent_forwarding_flag = 0;
int no_x11_forwarding_flag = 0;
int no_pty_flag = 0;

/* RSA authentication "command=" option. */
char *forced_command = NULL;

/* RSA authentication "environment=" options. */
struct envstring *custom_environment = NULL;

/* Session id for the current session. */
unsigned char session_id[16];

/*
 * 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 {
        RSA *private_key;        /* Private part of server key. */
        RSA *host_key;           /* Private part of host key. */
} sensitive_data;

/*
 * Flag indicating whether the current session key has been used.  This flag
 * is set whenever the key is used, and cleared when the key is regenerated.
 */
int key_used = 0;

/* This is set to true when SIGHUP is received. */
int received_sighup = 0;

/* Public side of the server key.  This value is regenerated regularly with
   the private key. */
RSA *public_key;

/* Prototypes for various functions defined later in this file. */
void do_connection();
void do_authentication(char *user);
void do_authloop(struct passwd * pw);
void do_fake_authloop(char *user);
void do_authenticated(struct passwd * pw);
void do_exec_pty(const char *command, int ptyfd, int ttyfd,
                 const char *ttyname, struct passwd * pw, const char *term,
                 const char *display, const char *auth_proto,
                 const char *auth_data);
void do_exec_no_pty(const char *command, struct passwd * pw,
                    const char *display, const char *auth_proto,
                    const char *auth_data);
void do_child(const char *command, struct passwd * pw, const char *term,
              const char *display, const char *auth_proto,
              const char *auth_data, const char *ttyname);

#ifdef HAVE_LIBPAM
static int pamconv(int num_msg, const struct pam_message **msg,
          struct pam_response **resp, void *appdata_ptr);
void do_pam_account(char *username, char *remote_user);
void do_pam_session(char *username, char *ttyname);
void pam_cleanup_proc(void *context);

static struct pam_conv conv = {
        pamconv,
        NULL
};
struct pam_handle_t *pamh = NULL;
const char *pampasswd = NULL;
char *pamconv_msg = NULL;

static int pamconv(int num_msg, const struct pam_message **msg,
        struct pam_response **resp, void *appdata_ptr)
{
        struct pam_response *reply;
        int count;
        size_t msg_len;
        char *p;

        /* PAM will free this later */
        reply = malloc(num_msg * sizeof(*reply));
        if (reply == NULL)
                return PAM_CONV_ERR; 

        for(count = 0; count < num_msg; count++) {
                switch (msg[count]->msg_style) {
                        case PAM_PROMPT_ECHO_OFF:
                                if (pampasswd == NULL) {
                                        free(reply);
                                        return PAM_CONV_ERR;
                                }
                                reply[count].resp_retcode = PAM_SUCCESS;
                                reply[count].resp = xstrdup(pampasswd);
                                break;

                        case PAM_TEXT_INFO:
                                reply[count].resp_retcode = PAM_SUCCESS;
                                reply[count].resp = xstrdup("");

                                if (msg[count]->msg == NULL)
                                        break;

                                debug("Adding PAM message: %s", msg[count]->msg);

                                msg_len = strlen(msg[count]->msg);
                                if (pamconv_msg) {
                                        size_t n = strlen(pamconv_msg);
                                        pamconv_msg = xrealloc(pamconv_msg, n + msg_len + 2);
                                        p = pamconv_msg + n;
                                } else {
                                        pamconv_msg = p = xmalloc(msg_len + 2);
                                }
                                memcpy(p, msg[count]->msg, msg_len);
                                p[msg_len] = '\n';
                                p[msg_len + 1] = '\0';
                                break;

                        case PAM_PROMPT_ECHO_ON:
                        case PAM_ERROR_MSG:
                        default:
                                free(reply);
                                return PAM_CONV_ERR;
                }
        }

        *resp = reply;

        return PAM_SUCCESS;
}

void pam_cleanup_proc(void *context)
{
        int pam_retval;

        if (pamh != NULL)
        {
                pam_retval = pam_close_session((pam_handle_t *)pamh, 0);
                if (pam_retval != PAM_SUCCESS) {
                        log("Cannot close PAM session: %.200s", 
                        PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                }

                pam_retval = pam_end((pam_handle_t *)pamh, pam_retval);
                if (pam_retval != PAM_SUCCESS) {
                        log("Cannot release PAM authentication: %.200s", 
                        PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                }
        }
}

void do_pam_account(char *username, char *remote_user)
{
        int pam_retval;

        debug("PAM setting rhost to \"%.200s\"", get_canonical_hostname());
        pam_retval = pam_set_item((pam_handle_t *)pamh, PAM_RHOST, 
                get_canonical_hostname());
        if (pam_retval != PAM_SUCCESS) {
                log("PAM set rhost failed: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                do_fake_authloop(username);
        }

        if (remote_user != NULL) {
                debug("PAM setting ruser to \"%.200s\"", remote_user);
                pam_retval = pam_set_item((pam_handle_t *)pamh, PAM_RUSER, remote_user);
                if (pam_retval != PAM_SUCCESS) {
                        log("PAM set ruser failed: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                        do_fake_authloop(username);
                }
        }

        pam_retval = pam_acct_mgmt((pam_handle_t *)pamh, 0);
        if (pam_retval != PAM_SUCCESS) {
                log("PAM rejected by account configuration: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                do_fake_authloop(username);
        }
}

void do_pam_session(char *username, char *ttyname)
{
        int pam_retval;

        if (ttyname != NULL) {
                debug("PAM setting tty to \"%.200s\"", ttyname);
                pam_retval = pam_set_item((pam_handle_t *)pamh, PAM_TTY, ttyname);
                if (pam_retval != PAM_SUCCESS)
                        fatal("PAM set tty failed: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
        }

        pam_retval = pam_open_session((pam_handle_t *)pamh, 0);
        if (pam_retval != PAM_SUCCESS)
                fatal("PAM session setup failed: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
}
#endif /* HAVE_LIBPAM */

/*
 * 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).
 */
void 
sighup_handler(int sig)
{
        received_sighup = 1;
        signal(SIGHUP, sighup_handler);
}

/*
 * Called from the main program after receiving SIGHUP.
 * Restarts the server.
 */
void 
sighup_restart()
{
        log("Received SIGHUP; restarting.");
        close(listen_sock);
        execv(saved_argv[0], saved_argv);
        log("RESTART FAILED: av0='%s', error: %s.", av0, strerror(errno));
        exit(1);
}

/*
 * Generic signal handler for terminating signals in the master daemon.
 * These close the listen socket; not closing it seems to cause "Address
 * already in use" problems on some machines, which is inconvenient.
 */
void 
sigterm_handler(int sig)
{
        log("Received signal %d; terminating.", sig);
        close(listen_sock);
        exit(255);
}

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

        while (waitpid(-1, &status, WNOHANG) > 0)
                ;

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

/*
 * Signal handler for the alarm after the login grace period has expired.
 */
void 
grace_alarm_handler(int sig)
{
        /* Close the connection. */
        packet_close();

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

/*
 * convert ssh auth msg type into description
 */
char *
get_authname(int type)
{
        switch (type) {
        case SSH_CMSG_AUTH_PASSWORD:
                return "password";
        case SSH_CMSG_AUTH_RSA:
                return "rsa";
        case SSH_CMSG_AUTH_RHOSTS_RSA:
                return "rhosts-rsa";
        case SSH_CMSG_AUTH_RHOSTS:
                return "rhosts";
#ifdef KRB4
        case SSH_CMSG_AUTH_KERBEROS:
                return "kerberos";
#endif
#ifdef SKEY
        case SSH_CMSG_AUTH_TIS_RESPONSE:
                return "s/key";
#endif
        }
        fatal("get_authname: unknown auth %d: internal error", type);
        return NULL;
}

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

        /* Check if we should generate a new key. */
        if (key_used) {
                /* This should really be done in the background. */
                log("Generating new %d bit RSA key.", options.server_key_bits);

                if (sensitive_data.private_key != NULL)
                        RSA_free(sensitive_data.private_key);
                sensitive_data.private_key = RSA_new();

                if (public_key != NULL)
                        RSA_free(public_key);
                public_key = RSA_new();

                rsa_generate_key(sensitive_data.private_key, public_key,
                                 options.server_key_bits);
                arc4random_stir();
                key_used = 0;
                log("RSA key generation complete.");
        }
        /* Reschedule the alarm. */
        signal(SIGALRM, key_regeneration_alarm);
        alarm(options.key_regeneration_time);
        errno = save_errno;
}

/*
 * Main program for the daemon.
 */
int
main(int ac, char **av)
{
        extern char *optarg;
        extern int optind;
        int opt, aux, sock_in, sock_out, newsock, i, pid, on = 1;
        int remote_major, remote_minor;
        int silentrsa = 0;
        struct pollfd fds;
        struct sockaddr_in sin;
        char buf[100];                  /* Must not be larger than remote_version. */
        char remote_version[100];       /* Must be at least as big as buf. */
        const char *remote_ip;
        int remote_port;
        char *comment;
        FILE *f;
        struct linger linger;

        /* Save argv[0]. */
        saved_argv = av;
        if (strchr(av[0], '/'))
                av0 = strrchr(av[0], '/') + 1;
        else
                av0 = av[0];

        /* 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:V:diqQ")) != EOF) {
                switch (opt) {
                case 'f':
                        config_file_name = optarg;
                        break;
                case 'd':
                        debug_flag = 1;
                        options.log_level = SYSLOG_LEVEL_DEBUG;
                        break;
                case 'i':
                        inetd_flag = 1;
                        break;
                case 'Q':
                        silentrsa = 1;
                        break;
                case 'q':
                        options.log_level = SYSLOG_LEVEL_QUIET;
                        break;
                case 'b':
                        options.server_key_bits = atoi(optarg);
                        break;
                case 'p':
                        options.port = atoi(optarg);
                        break;
                case 'g':
                        options.login_grace_time = atoi(optarg);
                        break;
                case 'k':
                        options.key_regeneration_time = atoi(optarg);
                        break;
                case 'h':
                        options.host_key_file = optarg;
                        break;
                case 'V':
                        client_version_string = optarg;
                        /* only makes sense with inetd_flag, i.e. no listen() */
                        inetd_flag = 1;
                        break;
                case '?':
                default:
                        fprintf(stderr, "sshd version %s\n", SSH_VERSION);
                        fprintf(stderr, "Usage: %s [options]\n", av0);
                        fprintf(stderr, "Options:\n");
                        fprintf(stderr, "  -f file    Configuration file (default %s)\n", SERVER_CONFIG_FILE);
                        fprintf(stderr, "  -d         Debugging mode\n");
                        fprintf(stderr, "  -i         Started from inetd\n");
                        fprintf(stderr, "  -q         Quiet (no logging)\n");
                        fprintf(stderr, "  -p port    Listen on the specified port (default: 22)\n");
                        fprintf(stderr, "  -k seconds Regenerate server key every this many seconds (default: 3600)\n");
                        fprintf(stderr, "  -g seconds Grace period for authentication (default: 300)\n");
                        fprintf(stderr, "  -b bits    Size of server RSA key (default: 768 bits)\n");
                        fprintf(stderr, "  -h file    File from which to read host key (default: %s)\n",
                                HOST_KEY_FILE);
                        exit(1);
                }
        }

        /* check if RSA support exists */
        if (rsa_alive() == 0) {
                if (silentrsa == 0)
                        printf("sshd: no RSA support in libssl and libcrypto -- exiting.  See ssl(8)\n");
                log("no RSA support in libssl and libcrypto -- exiting.  See ssl(8)");
                exit(1);
        }
        /* Read server configuration options from the configuration file. */
        read_server_config(&options, config_file_name);

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

        /* Check certain values for sanity. */
        if (options.server_key_bits < 512 ||
            options.server_key_bits > 32768) {
                fprintf(stderr, "Bad server key size.\n");
                exit(1);
        }
        if (options.port < 1 || options.port > 65535) {
                fprintf(stderr, "Bad port number.\n");
                exit(1);
        }
        /* Check that there are no remaining arguments. */
        if (optind < ac) {
                fprintf(stderr, "Extra argument %s.\n", av[optind]);
                exit(1);
        }
        /* Force logging to stderr while loading the private host key
           unless started from inetd */
        log_init(av0, options.log_level, options.log_facility, !inetd_flag);

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

        sensitive_data.host_key = RSA_new();
        errno = 0;
        /* Load the host key.  It must have empty passphrase. */
        if (!load_private_key(options.host_key_file, "",
                              sensitive_data.host_key, &comment)) {
                error("Could not load host key: %.200s: %.100s",
                      options.host_key_file, strerror(errno));
                exit(1);
        }
        xfree(comment);

        /* Initialize the log (it is reinitialized below in case we
           forked). */
        if (debug_flag && !inetd_flag)
                log_stderr = 1;
        log_init(av0, 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) {
#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("/dev/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(av0, options.log_level, options.log_facility, log_stderr);

        /* 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.host_key->n) - SSH_KEY_BITS_RESERVED &&
            options.server_key_bits <
        BN_num_bits(sensitive_data.host_key->n) + SSH_KEY_BITS_RESERVED) {
                options.server_key_bits =
                        BN_num_bits(sensitive_data.host_key->n) + SSH_KEY_BITS_RESERVED;
                debug("Forcing server key to %d bits to make it differ from host key.",
                      options.server_key_bits);
        }
        /* Do not display messages to stdout in RSA code. */
        rsa_set_verbose(0);

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

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

        /* Close connection cleanly after attack. */
        cipher_attack_detected = packet_disconnect;

        /* Start listening for a socket, unless started from inetd. */
        if (inetd_flag) {
                int s1, s2;
                s1 = dup(0);    /* Make sure descriptors 0, 1, and 2 are in use. */
                s2 = dup(s1);
                sock_in = dup(0);
                sock_out = dup(1);
                /* 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. */
                debug("inetd sockets after dupping: %d, %d", sock_in, sock_out);

                public_key = RSA_new();
                sensitive_data.private_key = RSA_new();

                log("Generating %d bit RSA key.", options.server_key_bits);
                rsa_generate_key(sensitive_data.private_key, public_key,
                                 options.server_key_bits);
                arc4random_stir();
                log("RSA key generation complete.");
        } else {
                /* Create socket for listening. */
                listen_sock = socket(AF_INET, SOCK_STREAM, 0);
                if (listen_sock < 0)
                        fatal("socket: %.100s", strerror(errno));

                /* Set socket options.  We try to make the port reusable
                   and have it close as fast as possible without waiting
                   in unnecessary wait states on close. */
                setsockopt(listen_sock, SOL_SOCKET, SO_REUSEADDR, (void *) &on,
                           sizeof(on));
                linger.l_onoff = 1;
                linger.l_linger = 5;
                setsockopt(listen_sock, SOL_SOCKET, SO_LINGER, (void *) &linger,
                           sizeof(linger));

                memset(&sin, 0, sizeof(sin));
                sin.sin_family = AF_INET;
                sin.sin_addr = options.listen_addr;
                sin.sin_port = htons(options.port);

                if (bind(listen_sock, (struct sockaddr *) & sin, sizeof(sin)) < 0) {
                        error("bind: %.100s", strerror(errno));
                        shutdown(listen_sock, SHUT_RDWR);
                        close(listen_sock);
                        fatal("Bind to port %d failed.", options.port);
                }
                if (!debug_flag) {
                        /*
                         * Record our pid in /etc/sshd_pid to make it easier
                         * to kill the correct sshd.  We don\'t want to do
                         * this before the bind above because the bind will
                         * fail if there already is a daemon, and this will
                         * overwrite any old pid in the file.
                         */
                        f = fopen(SSH_DAEMON_PID_FILE, "w");
                        if (f) {
                                fprintf(f, "%u\n", (unsigned int) getpid());
                                fclose(f);
                        }
                }

                log("Server listening on port %d.", options.port);
                if (listen(listen_sock, 5) < 0)
                        fatal("listen: %.100s", strerror(errno));

                public_key = RSA_new();
                sensitive_data.private_key = RSA_new();

                log("Generating %d bit RSA key.", options.server_key_bits);
                rsa_generate_key(sensitive_data.private_key, public_key,
                                 options.server_key_bits);
                arc4random_stir();
                log("RSA key generation complete.");

                /* Schedule server key regeneration alarm. */
                signal(SIGALRM, key_regeneration_alarm);
                alarm(options.key_regeneration_time);

                /* Arrange to restart on SIGHUP.  The handler needs listen_sock. */
                signal(SIGHUP, sighup_handler);
                signal(SIGTERM, sigterm_handler);
                signal(SIGQUIT, sigterm_handler);

                /* Arrange SIGCHLD to be caught. */
                signal(SIGCHLD, main_sigchld_handler);

                /*
                 * Stay listening for connections until the system crashes or
                 * the daemon is killed with a signal.
                 */
                for (;;) {
                        if (received_sighup)
                                sighup_restart();
                        /* Wait in poll until there is a connection. */
                        memset(&fds, 0, sizeof(fds));
                        fds.fd = listen_sock;
                        fds.events = POLLIN;
                        if (poll(&fds, 1, -1) == -1) {
                                if (errno == EINTR)
                                        continue;
                                fatal("poll: %.100s", strerror(errno));
                                /*NOTREACHED*/
                        }
                        if (fds.revents == 0)
                                continue;
                        aux = sizeof(sin);
                        newsock = accept(listen_sock, (struct sockaddr *) & sin, &aux);
                        if (received_sighup)
                                sighup_restart();
                        if (newsock < 0) {
                                if (errno == EINTR)
                                        continue;
                                error("accept: %.100s", strerror(errno));
                                continue;
                        }
                        /*
                         * 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_sock);
                                sock_in = newsock;
                                sock_out = newsock;
                                pid = getpid();
                                break;
                        } else {
                                /*
                                 * Normal production daemon.  Fork, and have
                                 * the child process the connection. The
                                 * parent continues listening.
                                 */
                                if ((pid = fork()) == 0) {
                                        /*
                                         * Child.  Close the listening socket, and start using the
                                         * accepted socket.  Reinitialize logging (since our pid has
                                         * changed).  We break out of the loop to handle the connection.
                                         */
                                        close(listen_sock);
                                        sock_in = newsock;
                                        sock_out = newsock;
                                        log_init(av0, options.log_level, options.log_facility, log_stderr);
                                        break;
                                }
                        }

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

                        /* Mark that the key has been used (it was "given" to the child). */
                        key_used = 1;

                        arc4random_stir();

                        /* Close the new socket (the child is now taking care of it). */
                        close(newsock);
                }
        }

        /* This is the child processing a new connection. */

        /*
         * 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);

        /*
         * Set socket options for the connection.  We want the socket to
         * close as fast as possible without waiting for anything.  If the
         * connection is not a socket, these will do nothing.
         */
        /* setsockopt(sock_in, SOL_SOCKET, SO_REUSEADDR, (void *)&on, sizeof(on)); */
        linger.l_onoff = 1;
        linger.l_linger = 5;
        setsockopt(sock_in, SOL_SOCKET, SO_LINGER, (void *) &linger, sizeof(linger));

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

        remote_port = get_remote_port();
        remote_ip = get_remote_ipaddr();

        /* Check whether logins are denied from this host. */
#ifdef LIBWRAP
        {
                struct request_info req;

                request_init(&req, RQ_DAEMON, av0, RQ_FILE, sock_in, NULL);
                fromhost(&req);

                if (!hosts_access(&req)) {
                        close(sock_in);
                        close(sock_out);
                        refuse(&req);
                }
                verbose("Connection from %.500s port %d", eval_client(&req), remote_port);
        }
#else
        /* Log the connection. */
        verbose("Connection from %.500s port %d", remote_ip, remote_port);
#endif /* LIBWRAP */

        /*
         * 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);

        if (client_version_string != NULL) {
                /* we are exec'ed by sshd2, so skip exchange of protocol version */
                strlcpy(buf, client_version_string, sizeof(buf));
        } else {
                /* Send our protocol version identification. */
                snprintf(buf, sizeof buf, "SSH-%d.%d-%.100s\n",
                         PROTOCOL_MAJOR, PROTOCOL_MINOR, SSH_VERSION);
                if (atomicio(write, sock_out, buf, strlen(buf)) != strlen(buf))
                        fatal("Could not write ident string to %s.", get_remote_ipaddr());

                /* Read other side\'s version identification. */
                for (i = 0; i < sizeof(buf) - 1; i++) {
                        if (read(sock_in, &buf[i], 1) != 1)
                                fatal("Did not receive ident string from %s.", get_remote_ipaddr());
                        if (buf[i] == '\r') {
                                buf[i] = '\n';
                                buf[i + 1] = 0;
                                break;
                        }
                        if (buf[i] == '\n') {
                                /* buf[i] == '\n' */
                                buf[i + 1] = 0;
                                break;
                        }
                }
                buf[sizeof(buf) - 1] = 0;
        }

        /*
         * Check that the versions match.  In future this might accept
         * several versions and set appropriate flags to handle them.
         */
        if (sscanf(buf, "SSH-%d.%d-%[^\n]\n", &remote_major, &remote_minor,
            remote_version) != 3) {
                char *s = "Protocol mismatch.\n";

                (void) atomicio(write, sock_out, s, strlen(s));
                close(sock_in);
                close(sock_out);
                fatal("Bad protocol version identification '%.100s' from %s",
                      buf, get_remote_ipaddr());
        }
        debug("Client protocol version %d.%d; client software version %.100s",
              remote_major, remote_minor, remote_version);
        if (remote_major != PROTOCOL_MAJOR) {
                char *s = "Protocol major versions differ.\n";

                (void) atomicio(write, sock_out, s, strlen(s));
                close(sock_in);
                close(sock_out);
                fatal("Protocol major versions differ for %s: %d vs. %d",
                      get_remote_ipaddr(),
                      PROTOCOL_MAJOR, remote_major);
        }
        /* Check that the client has sufficiently high software version. */
        if (remote_major == 1 && remote_minor < 3)
                packet_disconnect("Your ssh version is too old and is no longer supported.  Please install a newer version.");

        if (remote_major == 1 && remote_minor == 3) {
                enable_compat13();
                if (strcmp(remote_version, "OpenSSH-1.1") != 0) {
                        debug("Agent forwarding disabled, remote version is not compatible.");
                        no_agent_forwarding_flag = 1;
                }
        }
        /*
         * Check that the connection comes from a privileged port.  Rhosts-
         * and Rhosts-RSA-Authentication only make sense from priviledged
         * programs.  Of course, if the intruder has root access on his local
         * machine, he can connect from any port.  So do not use these
         * authentication methods from machines that you do not trust.
         */
        if (remote_port >= IPPORT_RESERVED ||
            remote_port < IPPORT_RESERVED / 2) {
                options.rhosts_authentication = 0;
                options.rhosts_rsa_authentication = 0;
        }
        packet_set_nonblocking();

        /* Handle the connection. */
        do_connection();

#ifdef KRB4
        /* Cleanup user's ticket cache file. */
        if (options.kerberos_ticket_cleanup)
                (void) dest_tkt();
#endif /* KRB4 */

        /* Cleanup user's local Xauthority file. */
        if (xauthfile)
                unlink(xauthfile);

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

#ifdef HAVE_LIBPAM
        {
                int retval;

                if (pamh != NULL) {
                        debug("Closing PAM session.");
                        retval = pam_close_session((pam_handle_t *)pamh, 0);

                        debug("Terminating PAM library.");
                        if (pam_end((pam_handle_t *)pamh, retval) != PAM_SUCCESS)
                                log("Cannot release PAM authentication.");

                        fatal_remove_cleanup(&pam_cleanup_proc, NULL);
                }
        }
#endif /* HAVE_LIBPAM */

        packet_close();
        exit(0);
}

/*
 * Process an incoming connection.  Protocol version identifiers have already
 * been exchanged.  This sends server key and performs the key exchange.
 * Server and host keys will no longer be needed after this functions.
 */
void
do_connection()
{
        int i, len;
        BIGNUM *session_key_int;
        unsigned char session_key[SSH_SESSION_KEY_LENGTH];
        unsigned char check_bytes[8];
        char *user;
        unsigned int cipher_type, auth_mask, protocol_flags;
        int plen, slen, ulen;
        u_int32_t rand = 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)
                        rand = arc4random();
                check_bytes[i] = rand & 0xff;
                rand >>= 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(check_bytes[i]);

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

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

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

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

        /* Declare supported authentication types. */
        auth_mask = 0;
        if (options.rhosts_authentication)
                auth_mask |= 1 << SSH_AUTH_RHOSTS;
        if (options.rhosts_rsa_authentication)
                auth_mask |= 1 << SSH_AUTH_RHOSTS_RSA;
        if (options.rsa_authentication)
                auth_mask |= 1 << SSH_AUTH_RSA;
#ifdef KRB4
        if (options.kerberos_authentication)
                auth_mask |= 1 << SSH_AUTH_KERBEROS;
#endif
#ifdef AFS
        if (options.kerberos_tgt_passing)
                auth_mask |= 1 << SSH_PASS_KERBEROS_TGT;
        if (options.afs_token_passing)
                auth_mask |= 1 << SSH_PASS_AFS_TOKEN;
#endif
#ifdef SKEY
        if (options.skey_authentication == 1)
                auth_mask |= 1 << SSH_AUTH_TIS;
#endif
        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 public key and %d bit host key.",
              BN_num_bits(public_key->n), BN_num_bits(sensitive_data.host_key->n));

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

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

        if (!(cipher_mask() & (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 (check_bytes[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. */
        session_key_int = BN_new();
        packet_get_bignum(session_key_int, &slen);

        protocol_flags = packet_get_int();
        packet_set_protocol_flags(protocol_flags);

        packet_integrity_check(plen, 1 + 8 + slen + 4, SSH_CMSG_SESSION_KEY);

        /*
         * Decrypt it using our private server key and private host key (key
         * with larger modulus first).
         */
        if (BN_cmp(sensitive_data.private_key->n, sensitive_data.host_key->n) > 0) {
                /* Private key has bigger modulus. */
                if (BN_num_bits(sensitive_data.private_key->n) <
                    BN_num_bits(sensitive_data.host_key->n) + SSH_KEY_BITS_RESERVED) {
                        fatal("do_connection: %s: private_key %d < host_key %d + SSH_KEY_BITS_RESERVED %d",
                              get_remote_ipaddr(),
                              BN_num_bits(sensitive_data.private_key->n),
                              BN_num_bits(sensitive_data.host_key->n),
                              SSH_KEY_BITS_RESERVED);
                }
                rsa_private_decrypt(session_key_int, session_key_int,
                                    sensitive_data.private_key);
                rsa_private_decrypt(session_key_int, session_key_int,
                                    sensitive_data.host_key);
        } else {
                /* Host key has bigger modulus (or they are equal). */
                if (BN_num_bits(sensitive_data.host_key->n) <
                    BN_num_bits(sensitive_data.private_key->n) + SSH_KEY_BITS_RESERVED) {
                        fatal("do_connection: %s: host_key %d < private_key %d + SSH_KEY_BITS_RESERVED %d",
                              get_remote_ipaddr(),
                              BN_num_bits(sensitive_data.host_key->n),
                              BN_num_bits(sensitive_data.private_key->n),
                              SSH_KEY_BITS_RESERVED);
                }
                rsa_private_decrypt(session_key_int, session_key_int,
                                    sensitive_data.host_key);
                rsa_private_decrypt(session_key_int, session_key_int,
                                    sensitive_data.private_key);
        }

        compute_session_id(session_id, check_bytes,
                           sensitive_data.host_key->n,
                           sensitive_data.private_key->n);

        /*
         * 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.
         */
        BN_mask_bits(session_key_int, sizeof(session_key) * 8);
        len = BN_num_bytes(session_key_int);
        if (len < 0 || len > sizeof(session_key))
                fatal("do_connection: bad len from %s: session_key_int %d > sizeof(session_key) %d",
                      get_remote_ipaddr(),
                      len, sizeof(session_key));
        memset(session_key, 0, sizeof(session_key));
        BN_bn2bin(session_key_int, session_key + sizeof(session_key) - len);

        /* 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];

        /* 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 acknowledgement packet.  Note that this packet is sent encrypted. */
        packet_start(SSH_SMSG_SUCCESS);
        packet_send();
        packet_write_wait();

        /* Get the name of the user that we wish to log in as. */
        packet_read_expect(&plen, SSH_CMSG_USER);

        /* Get the user name. */
        user = packet_get_string(&ulen);
        packet_integrity_check(plen, (4 + ulen), SSH_CMSG_USER);

        /* Destroy the private and public keys.  They will no longer be needed. */
        RSA_free(public_key);
        RSA_free(sensitive_data.private_key);
        RSA_free(sensitive_data.host_key);

        setproctitle("%s", user);
        /* Do the authentication. */
        do_authentication(user);
}

/*
 * Check if the user is allowed to log in via ssh. If user is listed in
 * DenyUsers or user's primary group is listed in DenyGroups, false will
 * be returned. If AllowUsers isn't empty and user isn't listed there, or
 * if AllowGroups isn't empty and user isn't listed there, false will be
 * returned. Otherwise true is returned.
 * XXX This function should also check if user has a valid shell
 */
static int
allowed_user(struct passwd * pw)
{
        struct group *grp;
        int i;

        /* Shouldn't be called if pw is NULL, but better safe than sorry... */
        if (!pw)
                return 0;

        /* XXX Should check for valid login shell */

        /* Return false if user is listed in DenyUsers */
        if (options.num_deny_users > 0) {
                if (!pw->pw_name)
                        return 0;
                for (i = 0; i < options.num_deny_users; i++)
                        if (match_pattern(pw->pw_name, options.deny_users[i]))
                                return 0;
        }
        /* Return false if AllowUsers isn't empty and user isn't listed there */
        if (options.num_allow_users > 0) {
                if (!pw->pw_name)
                        return 0;
                for (i = 0; i < options.num_allow_users; i++)
                        if (match_pattern(pw->pw_name, options.allow_users[i]))
                                break;
                /* i < options.num_allow_users iff we break for loop */
                if (i >= options.num_allow_users)
                        return 0;
        }
        /* Get the primary group name if we need it. Return false if it fails */
        if (options.num_deny_groups > 0 || options.num_allow_groups > 0) {
                grp = getgrgid(pw->pw_gid);
                if (!grp)
                        return 0;

                /* Return false if user's group is listed in DenyGroups */
                if (options.num_deny_groups > 0) {
                        if (!grp->gr_name)
                                return 0;
                        for (i = 0; i < options.num_deny_groups; i++)
                                if (match_pattern(grp->gr_name, options.deny_groups[i]))
                                        return 0;
                }
                /*
                 * Return false if AllowGroups isn't empty and user's group
                 * isn't listed there
                 */
                if (options.num_allow_groups > 0) {
                        if (!grp->gr_name)
                                return 0;
                        for (i = 0; i < options.num_allow_groups; i++)
                                if (match_pattern(grp->gr_name, options.allow_groups[i]))
                                        break;
                        /* i < options.num_allow_groups iff we break for
                           loop */
                        if (i >= options.num_allow_groups)
                                return 0;
                }
        }
        /* We found no reason not to let this user try to log on... */
        return 1;
}

/*
 * Performs authentication of an incoming connection.  Session key has already
 * been exchanged and encryption is enabled.  User is the user name to log
 * in as (received from the client).
 */
void
do_authentication(char *user)
{
        struct passwd *pw, pwcopy;

#ifdef AFS
        /* If machine has AFS, set process authentication group. */
        if (k_hasafs()) {
                k_setpag();
                k_unlog();
        }
#endif /* AFS */

        /* Verify that the user is a valid user. */
        pw = getpwnam(user);
        if (!pw || !allowed_user(pw))
                do_fake_authloop(user);

        /* Take a copy of the returned structure. */
        memset(&pwcopy, 0, sizeof(pwcopy));
        pwcopy.pw_name = xstrdup(pw->pw_name);
        pwcopy.pw_passwd = xstrdup(pw->pw_passwd);
        pwcopy.pw_uid = pw->pw_uid;
        pwcopy.pw_gid = pw->pw_gid;
        pwcopy.pw_dir = xstrdup(pw->pw_dir);
        pwcopy.pw_shell = xstrdup(pw->pw_shell);
        pw = &pwcopy;

#ifdef HAVE_LIBPAM
        {
                int pam_retval;

                debug("Starting up PAM with username \"%.200s\"", pw->pw_name);

                pam_retval = pam_start("sshd", pw->pw_name, &conv, (pam_handle_t**)&pamh);
                if (pam_retval != PAM_SUCCESS)
                        fatal("PAM initialisation failed: %.200s", PAM_STRERROR((pam_handle_t *)pamh, pam_retval));

                fatal_add_cleanup(&pam_cleanup_proc, NULL);
        }
#endif

        /*
         * If we are not running as root, the user must have the same uid as
         * the server.
         */
        if (getuid() != 0 && pw->pw_uid != getuid())
                packet_disconnect("Cannot change user when server not running as root.");

        debug("Attempting authentication for %.100s.", user);

        /* If the user has no password, accept authentication immediately. */
        if (options.password_authentication &&
#ifdef KRB4
            (!options.kerberos_authentication || options.kerberos_or_local_passwd) &&
#endif /* KRB4 */
            auth_password(pw, "")) {
                /* Authentication with empty password succeeded. */
                log("Login for user %s from %.100s, accepted without authentication.",
                    pw->pw_name, get_remote_ipaddr());
        } else {
                /* Loop until the user has been authenticated or the
                   connection is closed, do_authloop() returns only if
                   authentication is successfull */
                do_authloop(pw);
        }

        /* Check if the user is logging in as root and root logins are disallowed. */
        if (pw->pw_uid == 0 && !options.permit_root_login) {
                if (forced_command)
                        log("Root login accepted for forced command.");
                else
                        packet_disconnect("ROOT LOGIN REFUSED FROM %.200s",
                                          get_canonical_hostname());
        }
        /* The user has been authenticated and accepted. */
        packet_start(SSH_SMSG_SUCCESS);
        packet_send();
        packet_write_wait();

        /* Perform session preparation. */
        do_authenticated(pw);
}

#define AUTH_FAIL_MAX 6
#define AUTH_FAIL_LOG (AUTH_FAIL_MAX/2)
#define AUTH_FAIL_MSG "Too many authentication failures for %.100s"

/*
 * read packets and try to authenticate local user *pw.
 * return if authentication is successfull
 */
void
do_authloop(struct passwd * pw)
{
        int attempt = 0;
        unsigned int bits;
        BIGNUM *client_host_key_e, *client_host_key_n;
        BIGNUM *n;
        char *client_user = NULL, *password = NULL;
        char user[1024];
        int plen, dlen, nlen, ulen, elen;
        int type = 0;
        void (*authlog) (const char *fmt,...) = verbose;
#ifdef HAVE_LIBPAM
        int pam_retval;
#endif /* HAVE_LIBPAM */

        /* Indicate that authentication is needed. */
        packet_start(SSH_SMSG_FAILURE);
        packet_send();
        packet_write_wait();

        for (attempt = 1;; attempt++) {
                int authenticated = 0;
                strlcpy(user, "", sizeof user);

                /* Get a packet from the client. */
                type = packet_read(&plen);

                /* Process the packet. */
                switch (type) {
#ifdef AFS
                case SSH_CMSG_HAVE_KERBEROS_TGT:
                        if (!options.kerberos_tgt_passing) {
                                /* packet_get_all(); */
                                verbose("Kerberos tgt passing disabled.");
                                break;
                        } else {
                                /* Accept Kerberos tgt. */
                                char *tgt = packet_get_string(&dlen);
                                packet_integrity_check(plen, 4 + dlen, type);
                                if (!auth_kerberos_tgt(pw, tgt))
                                        verbose("Kerberos tgt REFUSED for %s", pw->pw_name);
                                xfree(tgt);
                        }
                        continue;

                case SSH_CMSG_HAVE_AFS_TOKEN:
                        if (!options.afs_token_passing || !k_hasafs()) {
                                /* packet_get_all(); */
                                verbose("AFS token passing disabled.");
                                break;
                        } else {
                                /* Accept AFS token. */
                                char *token_string = packet_get_string(&dlen);
                                packet_integrity_check(plen, 4 + dlen, type);
                                if (!auth_afs_token(pw, token_string))
                                        verbose("AFS token REFUSED for %s", pw->pw_name);
                                xfree(token_string);
                        }
                        continue;
#endif /* AFS */
#ifdef KRB4
                case SSH_CMSG_AUTH_KERBEROS:
                        if (!options.kerberos_authentication) {
                                /* packet_get_all(); */
                                verbose("Kerberos authentication disabled.");
                                break;
                        } else {
                                /* Try Kerberos v4 authentication. */
                                KTEXT_ST auth;
                                char *tkt_user = NULL;
                                char *kdata = packet_get_string((unsigned int *) &auth.length);
                                packet_integrity_check(plen, 4 + auth.length, type);

                                if (auth.length < MAX_KTXT_LEN)
                                        memcpy(auth.dat, kdata, auth.length);
                                xfree(kdata);

                                authenticated = auth_krb4(pw->pw_name, &auth, &tkt_user);

                                if (authenticated) {
                                        snprintf(user, sizeof user, " tktuser %s", tkt_user);
                                        xfree(tkt_user);
                                }
                        }
                        break;
#endif /* KRB4 */

                case SSH_CMSG_AUTH_RHOSTS:
                        if (!options.rhosts_authentication) {
                                verbose("Rhosts authentication disabled.");
                                break;
                        }
                        /*
                         * Get client user name.  Note that we just have to
                         * trust the client; this is one reason why rhosts
                         * authentication is insecure. (Another is
                         * IP-spoofing on a local network.)
                         */
                        client_user = packet_get_string(&ulen);
                        packet_integrity_check(plen, 4 + ulen, type);

                        /* Try to authenticate using /etc/hosts.equiv and
                           .rhosts. */
                        authenticated = auth_rhosts(pw, client_user);

                        snprintf(user, sizeof user, " ruser %s", client_user);
#ifndef HAVE_LIBPAM
                        xfree(client_user);
#endif /* HAVE_LIBPAM */
                        break;

                case SSH_CMSG_AUTH_RHOSTS_RSA:
                        if (!options.rhosts_rsa_authentication) {
                                verbose("Rhosts with RSA authentication disabled.");
                                break;
                        }
                        /*
                         * Get client user name.  Note that we just have to
                         * trust the client; root on the client machine can
                         * claim to be any user.
                         */
                        client_user = packet_get_string(&ulen);

                        /* Get the client host key. */
                        client_host_key_e = BN_new();
                        client_host_key_n = BN_new();
                        bits = packet_get_int();
                        packet_get_bignum(client_host_key_e, &elen);
                        packet_get_bignum(client_host_key_n, &nlen);

                        if (bits != BN_num_bits(client_host_key_n))
                                error("Warning: keysize mismatch for client_host_key: "
                                      "actual %d, announced %d", BN_num_bits(client_host_key_n), bits);
                        packet_integrity_check(plen, (4 + ulen) + 4 + elen + nlen, type);

                        authenticated = auth_rhosts_rsa(pw, client_user,
                                   client_host_key_e, client_host_key_n);
                        BN_clear_free(client_host_key_e);
                        BN_clear_free(client_host_key_n);

                        snprintf(user, sizeof user, " ruser %s", client_user);
#ifndef HAVE_LIBPAM
                        xfree(client_user);
#endif /* HAVE_LIBPAM */
                        break;

                case SSH_CMSG_AUTH_RSA:
                        if (!options.rsa_authentication) {
                                verbose("RSA authentication disabled.");
                                break;
                        }
                        /* RSA authentication requested. */
                        n = BN_new();
                        packet_get_bignum(n, &nlen);
                        packet_integrity_check(plen, nlen, type);
                        authenticated = auth_rsa(pw, n);
                        BN_clear_free(n);
                        break;

                case SSH_CMSG_AUTH_PASSWORD:
                        if (!options.password_authentication) {
                                verbose("Password authentication disabled.");
                                break;
                        }
                        /*
                         * Read user password.  It is in plain text, but was
                         * transmitted over the encrypted channel so it is
                         * not visible to an outside observer.
                         */
                        password = packet_get_string(&dlen);
                        packet_integrity_check(plen, 4 + dlen, type);

#ifdef HAVE_LIBPAM
                        /* Do PAM auth with password */
                        pampasswd = password;
                        pam_retval = pam_authenticate((pam_handle_t *)pamh, 0);
                        if (pam_retval == PAM_SUCCESS) {
                                log("PAM Password authentication accepted for user \"%.100s\"", pw->pw_name);
                                memset(password, 0, strlen(password));
                                xfree(password);
                                authenticated = 1;
                                break;
                        }

                        log("PAM Password authentication for \"%.100s\" failed: %s", 
                                pw->pw_name, PAM_STRERROR((pam_handle_t *)pamh, pam_retval));
                        memset(password, 0, strlen(password));
                        xfree(password);
                        break;
#else /* HAVE_LIBPAM */
                        /* Try authentication with the password. */
                        authenticated = auth_password(pw, password);

                        memset(password, 0, strlen(password));
                        xfree(password);
                        break;
#endif /* HAVE_LIBPAM */

#ifdef SKEY
                case SSH_CMSG_AUTH_TIS:
                        debug("rcvd SSH_CMSG_AUTH_TIS");
                        if (options.skey_authentication == 1) {
                                char *skeyinfo = skey_keyinfo(pw->pw_name);
                                if (skeyinfo == NULL) {
                                        debug("generating fake skeyinfo for %.100s.", pw->pw_name);
                                        skeyinfo = skey_fake_keyinfo(pw->pw_name);
                                }
                                if (skeyinfo != NULL) {
                                        /* we send our s/key- in tis-challenge messages */
                                        debug("sending challenge '%s'", skeyinfo);
                                        packet_start(SSH_SMSG_AUTH_TIS_CHALLENGE);
                                        packet_put_string(skeyinfo, strlen(skeyinfo));
                                        packet_send();
                                        packet_write_wait();
                                        continue;
                                }
                        }
                        break;
                case SSH_CMSG_AUTH_TIS_RESPONSE:
                        debug("rcvd SSH_CMSG_AUTH_TIS_RESPONSE");
                        if (options.skey_authentication == 1) {
                                char *response = packet_get_string(&dlen);
                                debug("skey response == '%s'", response);
                                packet_integrity_check(plen, 4 + dlen, type);
                                authenticated = (skey_haskey(pw->pw_name) == 0 &&
                                                 skey_passcheck(pw->pw_name, response) != -1);
                                xfree(response);
                        }
                        break;
#else
                case SSH_CMSG_AUTH_TIS:
                        /* TIS Authentication is unsupported */
                        log("TIS authentication unsupported.");
                        break;
#endif

                default:
                        /*
                         * Any unknown messages will be ignored (and failure
                         * returned) during authentication.
                         */
                        log("Unknown message during authentication: type %d", type);
                        break;
                }

                /* Raise logging level */
                if (authenticated ||
                    attempt == AUTH_FAIL_LOG ||
                    type == SSH_CMSG_AUTH_PASSWORD)
                        authlog = log;

                authlog("%s %s for %.200s from %.200s port %d%s",
                        authenticated ? "Accepted" : "Failed",
                        get_authname(type),
                        pw->pw_uid == 0 ? "ROOT" : pw->pw_name,
                        get_remote_ipaddr(),
                        get_remote_port(),
                        user);

#ifndef HAVE_LIBPAM
                if (authenticated)
                        return;

                if (attempt > AUTH_FAIL_MAX)
                        packet_disconnect(AUTH_FAIL_MSG, pw->pw_name);
#else /* HAVE_LIBPAM */
                if (authenticated) {
                        do_pam_account(pw->pw_name, client_user);

                        if (client_user != NULL)
                                xfree(client_user);

                        return;
                }

                if (attempt > AUTH_FAIL_MAX) {
                        if (client_user != NULL)
                                xfree(client_user);

                        packet_disconnect(AUTH_FAIL_MSG, pw->pw_name);
                }
#endif /* HAVE_LIBPAM */

                /* Send a message indicating that the authentication attempt failed. */
                packet_start(SSH_SMSG_FAILURE);
                packet_send();
                packet_write_wait();
        }
}

/*
 * The user does not exist or access is denied,
 * but fake indication that authentication is needed.
 */
void
do_fake_authloop(char *user)
{
        int attempt = 0;

        log("Faking authloop for illegal user %.200s from %.200s port %d",
            user,
            get_remote_ipaddr(),
            get_remote_port());

        /* Indicate that authentication is needed. */
        packet_start(SSH_SMSG_FAILURE);
        packet_send();
        packet_write_wait();

        /*
         * Keep reading packets, and always respond with a failure.  This is
         * to avoid disclosing whether such a user really exists.
         */
        for (attempt = 1;; attempt++) {
                /* Read a packet.  This will not return if the client disconnects. */
                int plen;
                int type = packet_read(&plen);
#ifdef SKEY
                int dlen;
                char *password, *skeyinfo;
                /* Try to send a fake s/key challenge. */
                if (options.skey_authentication == 1 &&
                    (skeyinfo = skey_fake_keyinfo(user)) != NULL) {
                        if (type == SSH_CMSG_AUTH_TIS) {
                                packet_start(SSH_SMSG_AUTH_TIS_CHALLENGE);
                                packet_put_string(skeyinfo, strlen(skeyinfo));
                                packet_send();
                                packet_write_wait();
                                continue;
                        } else if (type == SSH_CMSG_AUTH_PASSWORD &&
                                   options.password_authentication &&
                                   (password = packet_get_string(&dlen)) != NULL &&
                                   dlen == 5 &&
                                   strncasecmp(password, "s/key", 5) == 0 ) {
                                packet_send_debug(skeyinfo);
                        }
                }
#endif
                if (attempt > AUTH_FAIL_MAX)
                        packet_disconnect(AUTH_FAIL_MSG, user);

                /*
                 * Send failure.  This should be indistinguishable from a
                 * failed authentication.
                 */
                packet_start(SSH_SMSG_FAILURE);
                packet_send();
                packet_write_wait();
        }
        /* NOTREACHED */
        abort();
}


/*
 * Remove local Xauthority file.
 */
static void
xauthfile_cleanup_proc(void *ignore)
{
        debug("xauthfile_cleanup_proc called");

        if (xauthfile != NULL) {
                unlink(xauthfile);
                xfree(xauthfile);
                xauthfile = NULL;
        }
}

/*
 * Prepares for an interactive session.  This is called after the user has
 * been successfully authenticated.  During this message exchange, pseudo
 * terminals are allocated, X11, TCP/IP, and authentication agent forwardings
 * are requested, etc.
 */
void 
do_authenticated(struct passwd * pw)
{
        int type;
        int compression_level = 0, enable_compression_after_reply = 0;
        int have_pty = 0, ptyfd = -1, ttyfd = -1, xauthfd = -1;
        int row, col, xpixel, ypixel, screen;
        char ttyname[64];
        char *command, *term = NULL, *display = NULL, *proto = NULL,
        *data = NULL;
        struct group *grp;
        gid_t tty_gid;
        mode_t tty_mode;
        int n_bytes;

        /*
         * Cancel the alarm we set to limit the time taken for
         * authentication.
         */
        alarm(0);

        /*
         * Inform the channel mechanism that we are the server side and that
         * the client may request to connect to any port at all. (The user
         * could do it anyway, and we wouldn\'t know what is permitted except
         * by the client telling us, so we can equally well trust the client
         * not to request anything bogus.)
         */
        channel_permit_all_opens();

        /*
         * We stay in this loop until the client requests to execute a shell
         * or a command.
         */
        while (1) {
                int plen, dlen;

                /* Get a packet from the client. */
                type = packet_read(&plen);

                /* Process the packet. */
                switch (type) {
                case SSH_CMSG_REQUEST_COMPRESSION:
                        packet_integrity_check(plen, 4, type);
                        compression_level = packet_get_int();
                        if (compression_level < 1 || compression_level > 9) {
                                packet_send_debug("Received illegal compression level %d.",
                                                  compression_level);
                                goto fail;
                        }
                        /* Enable compression after we have responded with SUCCESS. */
                        enable_compression_after_reply = 1;
                        break;

                case SSH_CMSG_REQUEST_PTY:
                        if (no_pty_flag) {
                                debug("Allocating a pty not permitted for this authentication.");
                                goto fail;
                        }
                        if (have_pty)
                                packet_disconnect("Protocol error: you already have a pty.");

                        debug("Allocating pty.");

                        /* Allocate a pty and open it. */
                        if (!pty_allocate(&ptyfd, &ttyfd, ttyname,
                            sizeof(ttyname))) {
                                error("Failed to allocate pty.");
                                goto fail;
                        }
                        /* Determine the group to make the owner of the tty. */
                        grp = getgrnam("tty");
                        if (grp) {
                                tty_gid = grp->gr_gid;
                                tty_mode = S_IRUSR | S_IWUSR | S_IWGRP;
                        } else {
                                tty_gid = pw->pw_gid;
                                tty_mode = S_IRUSR | S_IWUSR | S_IWGRP | S_IWOTH;
                        }

                        /* Change ownership of the tty. */
                        if (chown(ttyname, pw->pw_uid, tty_gid) < 0)
                                fatal("chown(%.100s, %d, %d) failed: %.100s",
                                      ttyname, pw->pw_uid, tty_gid, strerror(errno));
                        if (chmod(ttyname, tty_mode) < 0)
                                fatal("chmod(%.100s, 0%o) failed: %.100s",
                                      ttyname, tty_mode, strerror(errno));

                        /* Get TERM from the packet.  Note that the value may be of arbitrary length. */
                        term = packet_get_string(&dlen);
                        packet_integrity_check(dlen, strlen(term), type);
                        /* packet_integrity_check(plen, 4 + dlen + 4*4 + n_bytes, type); */
                        /* Remaining bytes */
                        n_bytes = plen - (4 + dlen + 4 * 4);

                        if (strcmp(term, "") == 0)
                                term = NULL;

                        /* Get window size from the packet. */
                        row = packet_get_int();
                        col = packet_get_int();
                        xpixel = packet_get_int();
                        ypixel = packet_get_int();
                        pty_change_window_size(ptyfd, row, col, xpixel, ypixel);

                        /* Get tty modes from the packet. */
                        tty_parse_modes(ttyfd, &n_bytes);
                        packet_integrity_check(plen, 4 + dlen + 4 * 4 + n_bytes, type);

                        /* Indicate that we now have a pty. */
                        have_pty = 1;

#ifdef HAVE_LIBPAM
                        /* do the pam_open_session since we have the pty */
                        do_pam_session(pw->pw_name,ttyname);
#endif /* HAVE_LIBPAM */

                        break;

                case SSH_CMSG_X11_REQUEST_FORWARDING:
                        if (!options.x11_forwarding) {
                                packet_send_debug("X11 forwarding disabled in server configuration file.");
                                goto fail;
                        }
#ifdef XAUTH_PATH
                        if (no_x11_forwarding_flag) {
                                packet_send_debug("X11 forwarding not permitted for this authentication.");
                                goto fail;
                        }
                        debug("Received request for X11 forwarding with auth spoofing.");
                        if (display)
                                packet_disconnect("Protocol error: X11 display already set.");
                        {
                                int proto_len, data_len;
                                proto = packet_get_string(&proto_len);
                                data = packet_get_string(&data_len);
                                packet_integrity_check(plen, 4 + proto_len + 4 + data_len + 4, type);
                        }
                        if (packet_get_protocol_flags() & SSH_PROTOFLAG_SCREEN_NUMBER)
                                screen = packet_get_int();
                        else
                                screen = 0;
                        display = x11_create_display_inet(screen, options.x11_display_offset);
                        if (!display)
                                goto fail;

                        /* Setup to always have a local .Xauthority. */
                        xauthfile = xmalloc(MAXPATHLEN);
                        snprintf(xauthfile, MAXPATHLEN, "/tmp/XauthXXXXXX");

                        if ((xauthfd = mkstemp(xauthfile)) != -1) {
                                fchown(xauthfd, pw->pw_uid, pw->pw_gid);
                                close(xauthfd);
                                fatal_add_cleanup(xauthfile_cleanup_proc, NULL);
                        } else {
                                xfree(xauthfile);
                                xauthfile = NULL;
                        }
                        break;
#else /* XAUTH_PATH */
                        packet_send_debug("No xauth program; cannot forward with spoofing.");
                        goto fail;
#endif /* XAUTH_PATH */

                case SSH_CMSG_AGENT_REQUEST_FORWARDING:
                        if (no_agent_forwarding_flag) {
                                debug("Authentication agent forwarding not permitted for this authentication.");
                                goto fail;
                        }
                        debug("Received authentication agent forwarding request.");
                        auth_input_request_forwarding(pw);
                        break;

                case SSH_CMSG_PORT_FORWARD_REQUEST:
                        if (no_port_forwarding_flag) {
                                debug("Port forwarding not permitted for this authentication.");
                                goto fail;
                        }
                        debug("Received TCP/IP port forwarding request.");
                        channel_input_port_forward_request(pw->pw_uid == 0);
                        break;

                case SSH_CMSG_MAX_PACKET_SIZE:
                        if (packet_set_maxsize(packet_get_int()) < 0)
                                goto fail;
                        break;

                case SSH_CMSG_EXEC_SHELL:
                        /* Set interactive/non-interactive mode. */
                        packet_set_interactive(have_pty || display != NULL,
                                               options.keepalives);

                        if (forced_command != NULL)
                                goto do_forced_command;
                        debug("Forking shell.");
                        packet_integrity_check(plen, 0, type);
                        if (have_pty)
                                do_exec_pty(NULL, ptyfd, ttyfd, ttyname, pw, term, display, proto, data);
                        else
                                do_exec_no_pty(NULL, pw, display, proto, data);
                        return;

                case SSH_CMSG_EXEC_CMD:
                        /* Set interactive/non-interactive mode. */
                        packet_set_interactive(have_pty || display != NULL,
                                               options.keepalives);

                        if (forced_command != NULL)
                                goto do_forced_command;
                        /* Get command from the packet. */
                        {
                                int dlen;
                                command = packet_get_string(&dlen);
                                debug("Executing command '%.500s'", command);
                                packet_integrity_check(plen, 4 + dlen, type);
                        }
                        if (have_pty)
                                do_exec_pty(command, ptyfd, ttyfd, ttyname, pw, term, display, proto, data);
                        else
                                do_exec_no_pty(command, pw, display, proto, data);
                        xfree(command);
                        return;

                default:
                        /*
                         * Any unknown messages in this phase are ignored,
                         * and a failure message is returned.
                         */
                        log("Unknown packet type received after authentication: %d", type);
                        goto fail;
                }

                /* The request was successfully processed. */
                packet_start(SSH_SMSG_SUCCESS);
                packet_send();
                packet_write_wait();

                /* Enable compression now that we have replied if appropriate. */
                if (enable_compression_after_reply) {
                        enable_compression_after_reply = 0;
                        packet_start_compression(compression_level);
                }
                continue;

fail:
                /* The request failed. */
                packet_start(SSH_SMSG_FAILURE);
                packet_send();
                packet_write_wait();
                continue;

do_forced_command:
                /*
                 * There is a forced command specified for this login.
                 * Execute it.
                 */
                debug("Executing forced command: %.900s", forced_command);
                if (have_pty)
                        do_exec_pty(forced_command, ptyfd, ttyfd, ttyname, pw, term, display, proto, data);
                else
                        do_exec_no_pty(forced_command, pw, display, proto, data);
                return;
        }
}

/*
 * This is called to fork and execute a command when we have no tty.  This
 * will call do_child from the child, and server_loop from the parent after
 * setting up file descriptors and such.
 */
void 
do_exec_no_pty(const char *command, struct passwd * pw,
               const char *display, const char *auth_proto,
               const char *auth_data)
{
        int pid;

#ifdef USE_PIPES
        int pin[2], pout[2], perr[2];
        /* Allocate pipes for communicating with the program. */
        if (pipe(pin) < 0 || pipe(pout) < 0 || pipe(perr) < 0)
                packet_disconnect("Could not create pipes: %.100s",
                                  strerror(errno));
#else /* USE_PIPES */
        int inout[2], err[2];
        /* Uses socket pairs to communicate with the program. */
        if (socketpair(AF_UNIX, SOCK_STREAM, 0, inout) < 0 ||
            socketpair(AF_UNIX, SOCK_STREAM, 0, err) < 0)
                packet_disconnect("Could not create socket pairs: %.100s",
                                  strerror(errno));
#endif /* USE_PIPES */

        setproctitle("%s@notty", pw->pw_name);

        /* Fork the child. */
        if ((pid = fork()) == 0) {
                /* Child.  Reinitialize the log since the pid has changed. */
                log_init(av0, options.log_level, options.log_facility, log_stderr);

                /*
                 * Create a new session and process group since the 4.4BSD
                 * setlogin() affects the entire process group.
                 */
                if (setsid() < 0)
                        error("setsid failed: %.100s", strerror(errno));

#ifdef USE_PIPES
                /*
                 * Redirect stdin.  We close the parent side of the socket
                 * pair, and make the child side the standard input.
                 */
                close(pin[1]);
                if (dup2(pin[0], 0) < 0)
                        perror("dup2 stdin");
                close(pin[0]);

                /* Redirect stdout. */
                close(pout[0]);
                if (dup2(pout[1], 1) < 0)
                        perror("dup2 stdout");
                close(pout[1]);

                /* Redirect stderr. */
                close(perr[0]);
                if (dup2(perr[1], 2) < 0)
                        perror("dup2 stderr");
                close(perr[1]);
#else /* USE_PIPES */
                /*
                 * Redirect stdin, stdout, and stderr.  Stdin and stdout will
                 * use the same socket, as some programs (particularly rdist)
                 * seem to depend on it.
                 */
                close(inout[1]);
                close(err[1]);
                if (dup2(inout[0], 0) < 0)      /* stdin */
                        perror("dup2 stdin");
                if (dup2(inout[0], 1) < 0)      /* stdout.  Note: same socket as stdin. */
                        perror("dup2 stdout");
                if (dup2(err[0], 2) < 0)        /* stderr */
                        perror("dup2 stderr");
#endif /* USE_PIPES */

                /* Do processing for the child (exec command etc). */
                do_child(command, pw, NULL, display, auth_proto, auth_data, NULL);
                /* NOTREACHED */
        }
        if (pid < 0)
                packet_disconnect("fork failed: %.100s", strerror(errno));
#ifdef USE_PIPES
        /* We are the parent.  Close the child sides of the pipes. */
        close(pin[0]);
        close(pout[1]);
        close(perr[1]);

        /* Enter the interactive session. */
        server_loop(pid, pin[1], pout[0], perr[0]);
        /* server_loop has closed pin[1], pout[1], and perr[1]. */
#else /* USE_PIPES */
        /* We are the parent.  Close the child sides of the socket pairs. */
        close(inout[0]);
        close(err[0]);

        /*
         * Enter the interactive session.  Note: server_loop must be able to
         * handle the case that fdin and fdout are the same.
         */
        server_loop(pid, inout[1], inout[1], err[1]);
        /* server_loop has closed inout[1] and err[1]. */
#endif /* USE_PIPES */
}

struct pty_cleanup_context {
        const char *ttyname;
        int pid;
};

/*
 * Function to perform cleanup if we get aborted abnormally (e.g., due to a
 * dropped connection).
 */
void 
pty_cleanup_proc(void *context)
{
        struct pty_cleanup_context *cu = context;

        debug("pty_cleanup_proc called");

        /* Record that the user has logged out. */
        record_logout(cu->pid, cu->ttyname);

        /* Release the pseudo-tty. */
        pty_release(cu->ttyname);
}

/*
 * This is called to fork and execute a command when we have a tty.  This
 * will call do_child from the child, and server_loop from the parent after
 * setting up file descriptors, controlling tty, updating wtmp, utmp,
 * lastlog, and other such operations.
 */
void 
do_exec_pty(const char *command, int ptyfd, int ttyfd,
            const char *ttyname, struct passwd * pw, const char *term,
            const char *display, const char *auth_proto,
            const char *auth_data)
{
        int pid, fdout;
        const char *hostname;
        time_t last_login_time;
        char buf[100], *time_string;
        FILE *f;
        char line[256];
        struct stat st;
        int quiet_login;
        struct sockaddr_in from;
        int fromlen;
        struct pty_cleanup_context cleanup_context;

        /* Get remote host name. */
        hostname = get_canonical_hostname();

        /*
         * Get the time when the user last logged in.  Buf will be set to
         * contain the hostname the last login was from.
         */
        if (!options.use_login) {
                last_login_time = get_last_login_time(pw->pw_uid, pw->pw_name,
                                                      buf, sizeof(buf));
        }
        setproctitle("%s@%s", pw->pw_name, strrchr(ttyname, '/') + 1);

        /* Fork the child. */
        if ((pid = fork()) == 0) {
                pid = getpid();

                /* Child.  Reinitialize the log because the pid has
                   changed. */
                log_init(av0, options.log_level, options.log_facility, log_stderr);

                /* Close the master side of the pseudo tty. */
                close(ptyfd);

                /* Make the pseudo tty our controlling tty. */
                pty_make_controlling_tty(&ttyfd, ttyname);

                /* Redirect stdin from the pseudo tty. */
                if (dup2(ttyfd, fileno(stdin)) < 0)
                        error("dup2 stdin failed: %.100s", strerror(errno));

                /* Redirect stdout to the pseudo tty. */
                if (dup2(ttyfd, fileno(stdout)) < 0)
                        error("dup2 stdin failed: %.100s", strerror(errno));

                /* Redirect stderr to the pseudo tty. */
                if (dup2(ttyfd, fileno(stderr)) < 0)
                        error("dup2 stdin failed: %.100s", strerror(errno));

                /* Close the extra descriptor for the pseudo tty. */
                close(ttyfd);

                /*
                 * Get IP address of client.  This is needed because we want
                 * to record where the user logged in from.  If the
                 * connection is not a socket, let the ip address be 0.0.0.0.
                 */
                memset(&from, 0, sizeof(from));
                if (packet_get_connection_in() == packet_get_connection_out()) {
                        fromlen = sizeof(from);
                        if (getpeername(packet_get_connection_in(),
                             (struct sockaddr *) & from, &fromlen) < 0) {
                                debug("getpeername: %.100s", strerror(errno));
                                fatal_cleanup();
                        }
                }
                /* Record that there was a login on that terminal. */
                record_login(pid, ttyname, pw->pw_name, pw->pw_uid, hostname,
                             &from);

                /* Check if .hushlogin exists. */
                snprintf(line, sizeof line, "%.200s/.hushlogin", pw->pw_dir);
                quiet_login = stat(line, &st) >= 0;

#ifdef HAVE_LIBPAM
                /* output the results of the pamconv() */
                if (!quiet_login && pamconv_msg != NULL)
                        fprintf(stderr, pamconv_msg);
#endif

                /*
                 * If the user has logged in before, display the time of last
                 * login. However, don't display anything extra if a command
                 * has been specified (so that ssh can be used to execute
                 * commands on a remote machine without users knowing they
                 * are going to another machine). Login(1) will do this for
                 * us as well, so check if login(1) is used
                 */
                if (command == NULL && last_login_time != 0 && !quiet_login &&
                    !options.use_login) {
                        /* Convert the date to a string. */
                        time_string = ctime(&last_login_time);
                        /* Remove the trailing newline. */
                        if (strchr(time_string, '\n'))
                                *strchr(time_string, '\n') = 0;
                        /* Display the last login time.  Host if displayed
                           if known. */
                        if (strcmp(buf, "") == 0)
                                printf("Last login: %s\r\n", time_string);
                        else
                                printf("Last login: %s from %s\r\n", time_string, buf);
                }
                /*
                 * Print /etc/motd unless a command was specified or printing
                 * it was disabled in server options or login(1) will be
                 * used.  Note that some machines appear to print it in
                 * /etc/profile or similar.
                 */
                if (command == NULL && options.print_motd && !quiet_login &&
                    !options.use_login) {
                        /* Print /etc/motd if it exists. */
                        f = fopen("/etc/motd", "r");
                        if (f) {
                                while (fgets(line, sizeof(line), f))
                                        fputs(line, stdout);
                                fclose(f);
                        }
                }
                /* Do common processing for the child, such as execing the command. */
                do_child(command, pw, term, display, auth_proto, auth_data, ttyname);
                /* NOTREACHED */
        }
        if (pid < 0)
                packet_disconnect("fork failed: %.100s", strerror(errno));
        /* Parent.  Close the slave side of the pseudo tty. */
        close(ttyfd);

        /*
         * Create another descriptor of the pty master side for use as the
         * standard input.  We could use the original descriptor, but this
         * simplifies code in server_loop.  The descriptor is bidirectional.
         */
        fdout = dup(ptyfd);
        if (fdout < 0)
                packet_disconnect("dup failed: %.100s", strerror(errno));

        /*
         * Add a cleanup function to clear the utmp entry and record logout
         * time in case we call fatal() (e.g., the connection gets closed).
         */
        cleanup_context.pid = pid;
        cleanup_context.ttyname = ttyname;
        fatal_add_cleanup(pty_cleanup_proc, (void *) &cleanup_context);

        /* Enter interactive session. */
        server_loop(pid, ptyfd, fdout, -1);
        /* server_loop has not closed ptyfd and fdout. */

        /* Cancel the cleanup function. */
        fatal_remove_cleanup(pty_cleanup_proc, (void *) &cleanup_context);

        /* Record that the user has logged out. */
        record_logout(pid, ttyname);

        /* Release the pseudo-tty. */
        pty_release(ttyname);

        /*
         * Close the server side of the socket pairs.  We must do this after
         * the pty cleanup, so that another process doesn't get this pty
         * while we're still cleaning up.
         */
        close(ptyfd);
        close(fdout);
}

/*
 * Sets the value of the given variable in the environment.  If the variable
 * already exists, its value is overriden.
 */
void 
child_set_env(char ***envp, unsigned int *envsizep, const char *name,
              const char *value)
{
        unsigned int i, namelen;
        char **env;

        /*
         * Find the slot where the value should be stored.  If the variable
         * already exists, we reuse the slot; otherwise we append a new slot
         * at the end of the array, expanding if necessary.
         */
        env = *envp;
        namelen = strlen(name);
        for (i = 0; env[i]; i++)
                if (strncmp(env[i], name, namelen) == 0 && env[i][namelen] == '=')
                        break;
        if (env[i]) {
                /* Reuse the slot. */
                xfree(env[i]);
        } else {
                /* New variable.  Expand if necessary. */
                if (i >= (*envsizep) - 1) {
                        (*envsizep) += 50;
                        env = (*envp) = xrealloc(env, (*envsizep) * sizeof(char *));
                }
                /* Need to set the NULL pointer at end of array beyond the new slot. */
                env[i + 1] = NULL;
        }

        /* Allocate space and format the variable in the appropriate slot. */
        env[i] = xmalloc(strlen(name) + 1 + strlen(value) + 1);
        snprintf(env[i], strlen(name) + 1 + strlen(value) + 1, "%s=%s", name, value);
}

/*
 * Reads environment variables from the given file and adds/overrides them
 * into the environment.  If the file does not exist, this does nothing.
 * Otherwise, it must consist of empty lines, comments (line starts with '#')
 * and assignments of the form name=value.  No other forms are allowed.
 */
void 
read_environment_file(char ***env, unsigned int *envsize,
                      const char *filename)
{
        FILE *f;
        char buf[4096];
        char *cp, *value;

        f = fopen(filename, "r");
        if (!f)
                return;

        while (fgets(buf, sizeof(buf), f)) {
                for (cp = buf; *cp == ' ' || *cp == '\t'; cp++)
                        ;
                if (!*cp || *cp == '#' || *cp == '\n')
                        continue;
                if (strchr(cp, '\n'))
                        *strchr(cp, '\n') = '\0';
                value = strchr(cp, '=');
                if (value == NULL) {
                        fprintf(stderr, "Bad line in %.100s: %.200s\n", filename, buf);
                        continue;
                }
                /* Replace the equals sign by nul, and advance value to the value string. */
                *value = '\0';
                value++;
                child_set_env(env, envsize, cp, value);
        }
        fclose(f);
}

/*
 * Performs common processing for the child, such as setting up the
 * environment, closing extra file descriptors, setting the user and group
 * ids, and executing the command or shell.
 */
void 
do_child(const char *command, struct passwd * pw, const char *term,
         const char *display, const char *auth_proto,
         const char *auth_data, const char *ttyname)
{
        const char *shell, *cp = NULL;
        char buf[256];
        FILE *f;
        unsigned int envsize, i;
        char **env;
        extern char **environ;
        struct stat st;
        char *argv[10];

#ifndef HAVE_LIBPAM /* pam_nologin handles this */
        /* Check /etc/nologin. */
        f = fopen("/etc/nologin", "r");
        if (f) {
                /* /etc/nologin exists.  Print its contents and exit. */
                while (fgets(buf, sizeof(buf), f))
                        fputs(buf, stderr);
                fclose(f);
                if (pw->pw_uid != 0)
                        exit(254);
        }
#endif /* HAVE_LIBPAM */

#ifdef HAVE_SETLOGIN
        /* Set login name in the kernel. */
        if (setlogin(pw->pw_name) < 0)
                error("setlogin failed: %s", strerror(errno));
#endif /* HAVE_SETLOGIN */

        /* Set uid, gid, and groups. */
        /* Login(1) does this as well, and it needs uid 0 for the "-h"
           switch, so we let login(1) to this for us. */
        if (!options.use_login) {
                if (getuid() == 0 || geteuid() == 0) {
                        if (setgid(pw->pw_gid) < 0) {
                                perror("setgid");
                                exit(1);
                        }
                        /* Initialize the group list. */
                        if (initgroups(pw->pw_name, pw->pw_gid) < 0) {
                                perror("initgroups");
                                exit(1);
                        }
                        endgrent();

                        /* Permanently switch to the desired uid. */
                        permanently_set_uid(pw->pw_uid);
                }
                if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid)
                        fatal("Failed to set uids to %d.", (int) pw->pw_uid);
        }
        /*
         * Get the shell from the password data.  An empty shell field is
         * legal, and means /bin/sh.
         */
        shell = (pw->pw_shell[0] == '\0') ? _PATH_BSHELL : pw->pw_shell;

#ifdef AFS
        /* Try to get AFS tokens for the local cell. */
        if (k_hasafs()) {
                char cell[64];

                if (k_afs_cell_of_file(pw->pw_dir, cell, sizeof(cell)) == 0)
                        krb_afslog(cell, 0);

                krb_afslog(0, 0);
        }
#endif /* AFS */

        /* Initialize the environment. */
        envsize = 100;
        env = xmalloc(envsize * sizeof(char *));
        env[0] = NULL;

        if (!options.use_login) {
                /* Set basic environment. */
                child_set_env(&env, &envsize, "USER", pw->pw_name);
                child_set_env(&env, &envsize, "LOGNAME", pw->pw_name);
                child_set_env(&env, &envsize, "HOME", pw->pw_dir);
                child_set_env(&env, &envsize, "PATH", _PATH_STDPATH);

                snprintf(buf, sizeof buf, "%.200s/%.50s",
                         _PATH_MAILDIR, pw->pw_name);
                child_set_env(&env, &envsize, "MAIL", buf);

                /* Normal systems set SHELL by default. */
                child_set_env(&env, &envsize, "SHELL", shell);
        }
        if (getenv("TZ"))
                child_set_env(&env, &envsize, "TZ", getenv("TZ"));

        /* Set custom environment options from RSA authentication. */
        while (custom_environment) {
                struct envstring *ce = custom_environment;
                char *s = ce->s;
                int i;
                for (i = 0; s[i] != '=' && s[i]; i++);
                if (s[i] == '=') {
                        s[i] = 0;
                        child_set_env(&env, &envsize, s, s + i + 1);
                }
                custom_environment = ce->next;
                xfree(ce->s);
                xfree(ce);
        }

        snprintf(buf, sizeof buf, "%.50s %d %d",
                 get_remote_ipaddr(), get_remote_port(), options.port);
        child_set_env(&env, &envsize, "SSH_CLIENT", buf);

        if (ttyname)
                child_set_env(&env, &envsize, "SSH_TTY", ttyname);
        if (term)
                child_set_env(&env, &envsize, "TERM", term);
        if (display)
                child_set_env(&env, &envsize, "DISPLAY", display);

#ifdef KRB4
        {
                extern char *ticket;

                if (ticket)
                        child_set_env(&env, &envsize, "KRBTKFILE", ticket);
        }
#endif /* KRB4 */

#ifdef HAVE_LIBPAM
        /* Pull in any environment variables that may have been set by PAM. */
        {
                char *equals, var_name[512], var_val[512];
                char **pam_env = pam_getenvlist((pam_handle_t *)pamh);
                int i;
                for(i = 0; pam_env && pam_env[i]; i++) {
                        equals = strstr(pam_env[i], "=");
                        if ((strlen(pam_env[i]) < (sizeof(var_name) - 1)) && (equals != NULL))
                        {
                                debug("PAM environment: %s=%s", var_name, var_val);
                                memset(var_name, '\0', sizeof(var_name));
                                memset(var_val, '\0', sizeof(var_val));
                                strncpy(var_name, pam_env[i], equals - pam_env[i]);
                                strcpy(var_val, equals + 1);
                                child_set_env(&env, &envsize, var_name, var_val);
                        }
                }
        }
#endif /* HAVE_LIBPAM */

        if (xauthfile)
                child_set_env(&env, &envsize, "XAUTHORITY", xauthfile);

        if (auth_get_socket_name() != NULL)
                child_set_env(&env, &envsize, SSH_AUTHSOCKET_ENV_NAME,
                              auth_get_socket_name());

        /* read $HOME/.ssh/environment. */
        if (!options.use_login) {
                snprintf(buf, sizeof buf, "%.200s/.ssh/environment", pw->pw_dir);
                read_environment_file(&env, &envsize, buf);
        }
        if (debug_flag) {
                /* dump the environment */
                fprintf(stderr, "Environment:\n");
                for (i = 0; env[i]; i++)
                        fprintf(stderr, "  %.200s\n", env[i]);
        }
        /*
         * Close the connection descriptors; note that this is the child, and
         * the server will still have the socket open, and it is important
         * that we do not shutdown it.  Note that the descriptors cannot be
         * closed before building the environment, as we call
         * get_remote_ipaddr there.
         */
        if (packet_get_connection_in() == packet_get_connection_out())
                close(packet_get_connection_in());
        else {
                close(packet_get_connection_in());
                close(packet_get_connection_out());
        }
        /*
         * Close all descriptors related to channels.  They will still remain
         * open in the parent.
         */
        /* XXX better use close-on-exec? -markus */
        channel_close_all();

        /*
         * Close any extra file descriptors.  Note that there may still be
         * descriptors left by system functions.  They will be closed later.
         */
        endpwent();
        endhostent();

        /*
         * Close any extra open file descriptors so that we don\'t have them
         * hanging around in clients.  Note that we want to do this after
         * initgroups, because at least on Solaris 2.3 it leaves file
         * descriptors open.
         */
        for (i = 3; i < 64; i++)
                close(i);

        /* Change current directory to the user\'s home directory. */
        if (chdir(pw->pw_dir) < 0)
                fprintf(stderr, "Could not chdir to home directory %s: %s\n",
                        pw->pw_dir, strerror(errno));

        /*
         * Must take new environment into use so that .ssh/rc, /etc/sshrc and
         * xauth are run in the proper environment.
         */
        environ = env;

        /*
         * Run $HOME/.ssh/rc, /etc/sshrc, or xauth (whichever is found first
         * in this order).
         */
        if (!options.use_login) {
                if (stat(SSH_USER_RC, &st) >= 0) {
                        if (debug_flag)
                                fprintf(stderr, "Running /bin/sh %s\n", SSH_USER_RC);

                        f = popen("/bin/sh " SSH_USER_RC, "w");
                        if (f) {
                                if (auth_proto != NULL && auth_data != NULL)
                                        fprintf(f, "%s %s\n", auth_proto, auth_data);
                                pclose(f);
                        } else
                                fprintf(stderr, "Could not run %s\n", SSH_USER_RC);
                } else if (stat(SSH_SYSTEM_RC, &st) >= 0) {
                        if (debug_flag)
                                fprintf(stderr, "Running /bin/sh %s\n", SSH_SYSTEM_RC);

                        f = popen("/bin/sh " SSH_SYSTEM_RC, "w");
                        if (f) {
                                if (auth_proto != NULL && auth_data != NULL)
                                        fprintf(f, "%s %s\n", auth_proto, auth_data);
                                pclose(f);
                        } else
                                fprintf(stderr, "Could not run %s\n", SSH_SYSTEM_RC);
                }
#ifdef XAUTH_PATH
                else {
                        /* Add authority data to .Xauthority if appropriate. */
                        if (auth_proto != NULL && auth_data != NULL) {
                                if (debug_flag)
                                        fprintf(stderr, "Running %.100s add %.100s %.100s %.100s\n",
                                                XAUTH_PATH, display, auth_proto, auth_data);

                                f = popen(XAUTH_PATH " -q -", "w");
                                if (f) {
                                        fprintf(f, "add %s %s %s\n", display, auth_proto, auth_data);
                                        fclose(f);
                                } else
                                        fprintf(stderr, "Could not run %s -q -\n", XAUTH_PATH);
                        }
                }
#endif /* XAUTH_PATH */

                /* Get the last component of the shell name. */
                cp = strrchr(shell, '/');
                if (cp)
                        cp++;
                else
                        cp = shell;
        }
        /*
         * If we have no command, execute the shell.  In this case, the shell
         * name to be passed in argv[0] is preceded by '-' to indicate that
         * this is a login shell.
         */
        if (!command) {
                if (!options.use_login) {
                        char buf[256];

                        /*
                         * Check for mail if we have a tty and it was enabled
                         * in server options.
                         */
                        if (ttyname && options.check_mail) {
                                char *mailbox;
                                struct stat mailstat;
                                mailbox = getenv("MAIL");
                                if (mailbox != NULL) {
                                        if (stat(mailbox, &mailstat) != 0 || mailstat.st_size == 0)
                                                printf("No mail.\n");
                                        else if (mailstat.st_mtime < mailstat.st_atime)
                                                printf("You have mail.\n");
                                        else
                                                printf("You have new mail.\n");
                                }
                        }
                        /* Start the shell.  Set initial character to '-'. */
                        buf[0] = '-';
                        strncpy(buf + 1, cp, sizeof(buf) - 1);
                        buf[sizeof(buf) - 1] = 0;

                        /* Execute the shell. */
                        argv[0] = buf;
                        argv[1] = NULL;
                        execve(shell, argv, env);

                        /* Executing the shell failed. */
                        perror(shell);
                        exit(1);

                } else {
                        /* Launch login(1). */

                        execl(LOGIN_PROGRAM, "login", "-h", get_remote_ipaddr(),
                              "-p", "-f", "--", pw->pw_name, NULL);

                        /* Login couldn't be executed, die. */

                        perror("login");
                        exit(1);
                }
        }
        /*
         * Execute the command using the user's shell.  This uses the -c
         * option to execute the command.
         */
        argv[0] = (char *) cp;
        argv[1] = "-c";
        argv[2] = (char *) command;
        argv[3] = NULL;
        execve(shell, argv, env);
        perror(shell);
        exit(1);
}