path: root/usr.bin/ssh/ssh.c

/*
 * Author: Tatu Ylonen <ylo@cs.hut.fi>
 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
 *                    All rights reserved
 * Created: Sat Mar 18 16:36:11 1995 ylo
 * Ssh client program.  This program can be used to log into a remote machine.
 * The software supports strong authentication, encryption, and forwarding
 * of X11, TCP/IP, and authentication connections.
 *
 * Modified to work with SSL by Niels Provos <provos@citi.umich.edu> in Canada.
 */

#include "includes.h"
RCSID("$Id: ssh.c,v 1.33 1999/11/24 19:53:52 markus Exp $");

#include "xmalloc.h"
#include "ssh.h"
#include "packet.h"
#include "buffer.h"
#include "authfd.h"
#include "readconf.h"
#include "uidswap.h"

/* Flag indicating whether debug mode is on.  This can be set on the command line. */
int debug_flag = 0;

int tty_flag = 0;

/*
 * Flag indicating that nothing should be read from stdin.  This can be set
 * on the command line.
 */
int stdin_null_flag = 0;

/*
 * Flag indicating that ssh should fork after authentication.  This is useful
 * so that the pasphrase can be entered manually, and then ssh goes to the
 * background.
 */
int fork_after_authentication_flag = 0;

/*
 * General data structure for command line options and options configurable
 * in configuration files.  See readconf.h.
 */
Options options;

/*
 * Name of the host we are connecting to.  This is the name given on the
 * command line, or the HostName specified for the user-supplied name in a
 * configuration file.
 */
char *host;

/* socket address the host resolves to */
struct sockaddr_in hostaddr;

/*
 * Flag to indicate that we have received a window change signal which has
 * not yet been processed.  This will cause a message indicating the new
 * window size to be sent to the server a little later.  This is volatile
 * because this is updated in a signal handler.
 */
volatile int received_window_change_signal = 0;

/* Value of argv[0] (set in the main program). */
char *av0;

/* Flag indicating whether we have a valid host private key loaded. */
int host_private_key_loaded = 0;

/* Host private key. */
RSA *host_private_key = NULL;

/* Original real UID. */
uid_t original_real_uid;

/* Prints a help message to the user.  This function never returns. */

void
usage()
{
	fprintf(stderr, "Usage: %s [options] host [command]\n", av0);
	fprintf(stderr, "Options:\n");
	fprintf(stderr, "  -l user     Log in using this user name.\n");
	fprintf(stderr, "  -n          Redirect input from /dev/null.\n");
	fprintf(stderr, "  -a          Disable authentication agent forwarding.\n");
#ifdef AFS
	fprintf(stderr, "  -k          Disable Kerberos ticket and AFS token forwarding.\n");
#endif				/* AFS */
	fprintf(stderr, "  -x          Disable X11 connection forwarding.\n");
	fprintf(stderr, "  -i file     Identity for RSA authentication (default: ~/.ssh/identity).\n");
	fprintf(stderr, "  -t          Tty; allocate a tty even if command is given.\n");
	fprintf(stderr, "  -v          Verbose; display verbose debugging messages.\n");
	fprintf(stderr, "  -V          Display version number only.\n");
	fprintf(stderr, "  -P          Don't allocate a privileged port.\n");
	fprintf(stderr, "  -q          Quiet; don't display any warning messages.\n");
	fprintf(stderr, "  -f          Fork into background after authentication.\n");
	fprintf(stderr, "  -e char     Set escape character; ``none'' = disable (default: ~).\n");

	fprintf(stderr, "  -c cipher   Select encryption algorithm: "
			"``3des'', "
			"``blowfish''\n");
	fprintf(stderr, "  -p port     Connect to this port.  Server must be on the same port.\n");
	fprintf(stderr, "  -L listen-port:host:port   Forward local port to remote address\n");
	fprintf(stderr, "  -R listen-port:host:port   Forward remote port to local address\n");
	fprintf(stderr, "              These cause %s to listen for connections on a port, and\n", av0);
	fprintf(stderr, "              forward them to the other side by connecting to host:port.\n");
	fprintf(stderr, "  -C          Enable compression.\n");
	fprintf(stderr, "  -g          Allow remote hosts to connect to forwarded ports.\n");
	fprintf(stderr, "  -o 'option' Process the option as if it was read from a configuration file.\n");
	exit(1);
}

/*
 * Connects to the given host using rsh (or prints an error message and exits
 * if rsh is not available).  This function never returns.
 */
void
rsh_connect(char *host, char *user, Buffer * command)
{
	char *args[10];
	int i;

	log("Using rsh.  WARNING: Connection will not be encrypted.");
	/* Build argument list for rsh. */
	i = 0;
	args[i++] = _PATH_RSH;
	/* host may have to come after user on some systems */
	args[i++] = host;
	if (user) {
		args[i++] = "-l";
		args[i++] = user;
	}
	if (buffer_len(command) > 0) {
		buffer_append(command, "\0", 1);
		args[i++] = buffer_ptr(command);
	}
	args[i++] = NULL;
	if (debug_flag) {
		for (i = 0; args[i]; i++) {
			if (i != 0)
				fprintf(stderr, " ");
			fprintf(stderr, "%s", args[i]);
		}
		fprintf(stderr, "\n");
	}
	execv(_PATH_RSH, args);
	perror(_PATH_RSH);
	exit(1);
}

/*
 * Main program for the ssh client.
 */
int
main(int ac, char **av)
{
	int i, opt, optind, type, exit_status, ok, fwd_port, fwd_host_port,
	 authfd;
	char *optarg, *cp, buf[256];
	Buffer command;
	struct winsize ws;
	struct stat st;
	struct passwd *pw, pwcopy;
	int interactive = 0, dummy;
	uid_t original_effective_uid;
	int plen;

	/*
	 * Save the original real uid.  It will be needed later (uid-swapping
	 * may clobber the real uid).
	 */
	original_real_uid = getuid();
	original_effective_uid = geteuid();

	/* If we are installed setuid root be careful to not drop core. */
	if (original_real_uid != original_effective_uid) {
		struct rlimit rlim;
		rlim.rlim_cur = rlim.rlim_max = 0;
		if (setrlimit(RLIMIT_CORE, &rlim) < 0)
			fatal("setrlimit failed: %.100s", strerror(errno));
	}
	/*
	 * Use uid-swapping to give up root privileges for the duration of
	 * option processing.  We will re-instantiate the rights when we are
	 * ready to create the privileged port, and will permanently drop
	 * them when the port has been created (actually, when the connection
	 * has been made, as we may need to create the port several times).
	 */
	temporarily_use_uid(original_real_uid);

	/*
	 * Set our umask to something reasonable, as some files are created
	 * with the default umask.  This will make them world-readable but
	 * writable only by the owner, which is ok for all files for which we
	 * don't set the modes explicitly.
	 */
	umask(022);

	/* Save our own name. */
	av0 = av[0];

	/* Initialize option structure to indicate that no values have been set. */
	initialize_options(&options);

	/* Parse command-line arguments. */
	host = NULL;

	/* If program name is not one of the standard names, use it as host name. */
	if (strchr(av0, '/'))
		cp = strrchr(av0, '/') + 1;
	else
		cp = av0;
	if (strcmp(cp, "rsh") != 0 && strcmp(cp, "ssh") != 0 &&
	    strcmp(cp, "rlogin") != 0 && strcmp(cp, "slogin") != 0)
		host = cp;

	for (optind = 1; optind < ac; optind++) {
		if (av[optind][0] != '-') {
			if (host)
				break;
			if ((cp = strchr(av[optind], '@'))) {
				options.user = av[optind];
				*cp = '\0';
				host = ++cp;
			} else
				host = av[optind];
			continue;
		}
		opt = av[optind][1];
		if (!opt)
			usage();
		if (strchr("eilcpLRo", opt)) {	/* options with arguments */
			optarg = av[optind] + 2;
			if (strcmp(optarg, "") == 0) {
				if (optind >= ac - 1)
					usage();
				optarg = av[++optind];
			}
		} else {
			if (av[optind][2])
				usage();
			optarg = NULL;
		}
		switch (opt) {
		case 'n':
			stdin_null_flag = 1;
			break;

		case 'f':
			fork_after_authentication_flag = 1;
			stdin_null_flag = 1;
			break;

		case 'x':
			options.forward_x11 = 0;
			break;

		case 'X':
			options.forward_x11 = 1;
			break;

		case 'g':
			options.gateway_ports = 1;
			break;

		case 'P':
			options.use_privileged_port = 0;
			break;

		case 'a':
			options.forward_agent = 0;
			break;
#ifdef AFS
		case 'k':
			options.kerberos_tgt_passing = 0;
			options.afs_token_passing = 0;
			break;
#endif
		case 'i':
			if (stat(optarg, &st) < 0) {
				fprintf(stderr, "Warning: Identity file %s does not exist.\n",
					optarg);
				break;
			}
			if (options.num_identity_files >= SSH_MAX_IDENTITY_FILES)
				fatal("Too many identity files specified (max %d)",
				      SSH_MAX_IDENTITY_FILES);
			options.identity_files[options.num_identity_files++] =
				xstrdup(optarg);
			break;

		case 't':
			tty_flag = 1;
			break;

		case 'v':
		case 'V':
			fprintf(stderr, "SSH Version %s, protocol version %d.%d.\n",
			    SSH_VERSION, PROTOCOL_MAJOR, PROTOCOL_MINOR);
			fprintf(stderr, "Compiled with SSL.\n");
			if (opt == 'V')
				exit(0);
			debug_flag = 1;
			options.log_level = SYSLOG_LEVEL_DEBUG;
			break;

		case 'q':
			options.log_level = SYSLOG_LEVEL_QUIET;
			break;

		case 'e':
			if (optarg[0] == '^' && optarg[2] == 0 &&
			    (unsigned char) optarg[1] >= 64 && (unsigned char) optarg[1] < 128)
				options.escape_char = (unsigned char) optarg[1] & 31;
			else if (strlen(optarg) == 1)
				options.escape_char = (unsigned char) optarg[0];
			else if (strcmp(optarg, "none") == 0)
				options.escape_char = -2;
			else {
				fprintf(stderr, "Bad escape character '%s'.\n", optarg);
				exit(1);
			}
			break;

		case 'c':
			options.cipher = cipher_number(optarg);
			if (options.cipher == -1) {
				fprintf(stderr, "Unknown cipher type '%s'\n", optarg);
				exit(1);
			}
			break;

		case 'p':
			options.port = atoi(optarg);
			if (options.port < 1 || options.port > 65535) {
				fprintf(stderr, "Bad port %s.\n", optarg);
				exit(1);
			}
			break;

		case 'l':
			options.user = optarg;
			break;

		case 'R':
			if (sscanf(optarg, "%d:%255[^:]:%d", &fwd_port, buf,
				   &fwd_host_port) != 3) {
				fprintf(stderr, "Bad forwarding specification '%s'.\n", optarg);
				usage();
				/* NOTREACHED */
			}
			add_remote_forward(&options, fwd_port, buf, fwd_host_port);
			break;

		case 'L':
			if (sscanf(optarg, "%d:%255[^:]:%d", &fwd_port, buf,
				   &fwd_host_port) != 3) {
				fprintf(stderr, "Bad forwarding specification '%s'.\n", optarg);
				usage();
				/* NOTREACHED */
			}
			add_local_forward(&options, fwd_port, buf, fwd_host_port);
			break;

		case 'C':
			options.compression = 1;
			break;

		case 'o':
			dummy = 1;
			if (process_config_line(&options, host ? host : "", optarg,
					 "command-line", 0, &dummy) != 0)
				exit(1);
			break;

		default:
			usage();
		}
	}

	/* Check that we got a host name. */
	if (!host)
		usage();

	/* check if RSA support exists */
	if (rsa_alive() == 0) {
		extern char *__progname;

		fprintf(stderr,
			"%s: no RSA support in libssl and libcrypto.  See ssl(8).\n",
			__progname);
		exit(1);
	}
	/* Initialize the command to execute on remote host. */
	buffer_init(&command);

	/*
	 * Save the command to execute on the remote host in a buffer. There
	 * is no limit on the length of the command, except by the maximum
	 * packet size.  Also sets the tty flag if there is no command.
	 */
	if (optind == ac) {
		/* No command specified - execute shell on a tty. */
		tty_flag = 1;
	} else {
		/* A command has been specified.  Store it into the
		   buffer. */
		for (i = optind; i < ac; i++) {
			if (i > optind)
				buffer_append(&command, " ", 1);
			buffer_append(&command, av[i], strlen(av[i]));
		}
	}

	/* Cannot fork to background if no command. */
	if (fork_after_authentication_flag && buffer_len(&command) == 0)
		fatal("Cannot fork into background without a command to execute.");

	/* Allocate a tty by default if no command specified. */
	if (buffer_len(&command) == 0)
		tty_flag = 1;

	/* Do not allocate a tty if stdin is not a tty. */
	if (!isatty(fileno(stdin))) {
		if (tty_flag)
			fprintf(stderr, "Pseudo-terminal will not be allocated because stdin is not a terminal.\n");
		tty_flag = 0;
	}
	/* Get user data. */
	pw = getpwuid(original_real_uid);
	if (!pw) {
		fprintf(stderr, "You don't exist, go away!\n");
		exit(1);
	}
	/* 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;

	/* Initialize "log" output.  Since we are the client all output
	   actually goes to the terminal. */
	log_init(av[0], options.log_level, SYSLOG_FACILITY_USER, 0);

	/* Read per-user configuration file. */
	snprintf(buf, sizeof buf, "%.100s/%.100s", pw->pw_dir, SSH_USER_CONFFILE);
	read_config_file(buf, host, &options);

	/* Read systemwide configuration file. */
	read_config_file(HOST_CONFIG_FILE, host, &options);

	/* Fill configuration defaults. */
	fill_default_options(&options);

	/* reinit */
	log_init(av[0], options.log_level, SYSLOG_FACILITY_USER, 0);

	if (options.user == NULL)
		options.user = xstrdup(pw->pw_name);

	if (options.hostname != NULL)
		host = options.hostname;

	/* Find canonic host name. */
	if (strchr(host, '.') == 0) {
		struct hostent *hp = gethostbyname(host);
		if (hp != 0) {
			if (strchr(hp->h_name, '.') != 0)
				host = xstrdup(hp->h_name);
			else if (hp->h_aliases != 0
				 && hp->h_aliases[0] != 0
				 && strchr(hp->h_aliases[0], '.') != 0)
				host = xstrdup(hp->h_aliases[0]);
		}
	}
	/* Disable rhosts authentication if not running as root. */
	if (original_effective_uid != 0 || !options.use_privileged_port) {
		options.rhosts_authentication = 0;
		options.rhosts_rsa_authentication = 0;
	}
	/*
	 * If using rsh has been selected, exec it now (without trying
	 * anything else).  Note that we must release privileges first.
	 */
	if (options.use_rsh) {
		/*
		 * Restore our superuser privileges.  This must be done
		 * before permanently setting the uid.
		 */
		restore_uid();

		/* Switch to the original uid permanently. */
		permanently_set_uid(original_real_uid);

		/* Execute rsh. */
		rsh_connect(host, options.user, &command);
		fatal("rsh_connect returned");
	}
	/* Restore our superuser privileges. */
	restore_uid();

	/*
	 * Open a connection to the remote host.  This needs root privileges
	 * if rhosts_{rsa_}authentication is enabled.
	 */

	ok = ssh_connect(host, &hostaddr, options.port,
			 options.connection_attempts,
			 !options.rhosts_authentication &&
			 !options.rhosts_rsa_authentication,
			 original_real_uid,
			 options.proxy_command);

	/*
	 * If we successfully made the connection, load the host private key
	 * in case we will need it later for combined rsa-rhosts
	 * authentication. This must be done before releasing extra
	 * privileges, because the file is only readable by root.
	 */
	if (ok) {
		host_private_key = RSA_new();
		if (load_private_key(HOST_KEY_FILE, "", host_private_key, NULL))
			host_private_key_loaded = 1;
	}
	/*
	 * Get rid of any extra privileges that we may have.  We will no
	 * longer need them.  Also, extra privileges could make it very hard
	 * to read identity files and other non-world-readable files from the
	 * user's home directory if it happens to be on a NFS volume where
	 * root is mapped to nobody.
	 */

	/*
	 * Note that some legacy systems need to postpone the following call
	 * to permanently_set_uid() until the private hostkey is destroyed
	 * with RSA_free().  Otherwise the calling user could ptrace() the
	 * process, read the private hostkey and impersonate the host.
	 * OpenBSD does not allow ptracing of setuid processes.
	 */
	permanently_set_uid(original_real_uid);

	/*
	 * Now that we are back to our own permissions, create ~/.ssh
	 * directory if it doesn\'t already exist.
	 */
	snprintf(buf, sizeof buf, "%.100s/%.100s", pw->pw_dir, SSH_USER_DIR);
	if (stat(buf, &st) < 0)
		if (mkdir(buf, 0755) < 0)
			error("Could not create directory '%.200s'.", buf);

	/* Check if the connection failed, and try "rsh" if appropriate. */
	if (!ok) {
		if (options.port != 0)
			log("Secure connection to %.100s on port %d refused%.100s.",
			    host, options.port,
			    options.fallback_to_rsh ? "; reverting to insecure method" : "");
		else
			log("Secure connection to %.100s refused%.100s.", host,
			    options.fallback_to_rsh ? "; reverting to insecure method" : "");

		if (options.fallback_to_rsh) {
			rsh_connect(host, options.user, &command);
			fatal("rsh_connect returned");
		}
		exit(1);
	}
	/* Expand ~ in options.identity_files. */
	for (i = 0; i < options.num_identity_files; i++)
		options.identity_files[i] =
			tilde_expand_filename(options.identity_files[i], original_real_uid);

	/* Expand ~ in known host file names. */
	options.system_hostfile = tilde_expand_filename(options.system_hostfile,
						        original_real_uid);
	options.user_hostfile = tilde_expand_filename(options.user_hostfile,
						      original_real_uid);

	/* Log into the remote system.  This never returns if the login fails. */
	ssh_login(host_private_key_loaded, host_private_key,
		  host, &hostaddr, original_real_uid);

	/* We no longer need the host private key.  Clear it now. */
	if (host_private_key_loaded)
		RSA_free(host_private_key);	/* Destroys contents safely */

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

	/* If requested, fork and let ssh continue in the background. */
	if (fork_after_authentication_flag) {
		int ret = fork();
		if (ret == -1)
			fatal("fork failed: %.100s", strerror(errno));
		if (ret != 0)
			exit(0);
		setsid();
	}
	/* Enable compression if requested. */
	if (options.compression) {
		debug("Requesting compression at level %d.", options.compression_level);

		if (options.compression_level < 1 || options.compression_level > 9)
			fatal("Compression level must be from 1 (fast) to 9 (slow, best).");

		/* Send the request. */
		packet_start(SSH_CMSG_REQUEST_COMPRESSION);
		packet_put_int(options.compression_level);
		packet_send();
		packet_write_wait();
		type = packet_read(&plen);
		if (type == SSH_SMSG_SUCCESS)
			packet_start_compression(options.compression_level);
		else if (type == SSH_SMSG_FAILURE)
			log("Warning: Remote host refused compression.");
		else
			packet_disconnect("Protocol error waiting for compression response.");
	}
	/* Allocate a pseudo tty if appropriate. */
	if (tty_flag) {
		debug("Requesting pty.");

		/* Start the packet. */
		packet_start(SSH_CMSG_REQUEST_PTY);

		/* Store TERM in the packet.  There is no limit on the
		   length of the string. */
		cp = getenv("TERM");
		if (!cp)
			cp = "";
		packet_put_string(cp, strlen(cp));

		/* Store window size in the packet. */
		if (ioctl(fileno(stdin), TIOCGWINSZ, &ws) < 0)
			memset(&ws, 0, sizeof(ws));
		packet_put_int(ws.ws_row);
		packet_put_int(ws.ws_col);
		packet_put_int(ws.ws_xpixel);
		packet_put_int(ws.ws_ypixel);

		/* Store tty modes in the packet. */
		tty_make_modes(fileno(stdin));

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

		/* Read response from the server. */
		type = packet_read(&plen);
		if (type == SSH_SMSG_SUCCESS)
			interactive = 1;
		else if (type == SSH_SMSG_FAILURE)
			log("Warning: Remote host failed or refused to allocate a pseudo tty.");
		else
			packet_disconnect("Protocol error waiting for pty request response.");
	}
	/* Request X11 forwarding if enabled and DISPLAY is set. */
	if (options.forward_x11 && getenv("DISPLAY") != NULL) {
		char line[512], proto[512], data[512];
		FILE *f;
		int forwarded = 0, got_data = 0, i;

#ifdef XAUTH_PATH
		/* Try to get Xauthority information for the display. */
		snprintf(line, sizeof line, "%.100s list %.200s 2>/dev/null",
			 XAUTH_PATH, getenv("DISPLAY"));
		f = popen(line, "r");
		if (f && fgets(line, sizeof(line), f) &&
		    sscanf(line, "%*s %s %s", proto, data) == 2)
			got_data = 1;
		if (f)
			pclose(f);
#endif /* XAUTH_PATH */
		/*
		 * If we didn't get authentication data, just make up some
		 * data.  The forwarding code will check the validity of the
		 * response anyway, and substitute this data.  The X11
		 * server, however, will ignore this fake data and use
		 * whatever authentication mechanisms it was using otherwise
		 * for the local connection.
		 */
		if (!got_data) {
			u_int32_t rand = 0;

			strlcpy(proto, "MIT-MAGIC-COOKIE-1", sizeof proto);
			for (i = 0; i < 16; i++) {
				if (i % 4 == 0)
					rand = arc4random();
				snprintf(data + 2 * i, sizeof data - 2 * i, "%02x", rand & 0xff);
				rand >>= 8;
			}
		}
		/*
		 * Got local authentication reasonable information. Request
		 * forwarding with authentication spoofing.
		 */
		debug("Requesting X11 forwarding with authentication spoofing.");
		x11_request_forwarding_with_spoofing(proto, data);

		/* Read response from the server. */
		type = packet_read(&plen);
		if (type == SSH_SMSG_SUCCESS) {
			forwarded = 1;
			interactive = 1;
		} else if (type == SSH_SMSG_FAILURE)
			log("Warning: Remote host denied X11 forwarding.");
		else
			packet_disconnect("Protocol error waiting for X11 forwarding");
	}
	/* Tell the packet module whether this is an interactive session. */
	packet_set_interactive(interactive, options.keepalives);

	/* Clear agent forwarding if we don\'t have an agent. */
	authfd = ssh_get_authentication_socket();
	if (authfd < 0)
		options.forward_agent = 0;
	else
		ssh_close_authentication_socket(authfd);

	/* Request authentication agent forwarding if appropriate. */
	if (options.forward_agent) {
		debug("Requesting authentication agent forwarding.");
		auth_request_forwarding();

		/* Read response from the server. */
		type = packet_read(&plen);
		packet_integrity_check(plen, 0, type);
		if (type != SSH_SMSG_SUCCESS)
			log("Warning: Remote host denied authentication agent forwarding.");
	}
	/* Initiate local TCP/IP port forwardings. */
	for (i = 0; i < options.num_local_forwards; i++) {
		debug("Connections to local port %d forwarded to remote address %.200s:%d",
		      options.local_forwards[i].port,
		      options.local_forwards[i].host,
		      options.local_forwards[i].host_port);
		channel_request_local_forwarding(options.local_forwards[i].port,
					  	 options.local_forwards[i].host,
						 options.local_forwards[i].host_port);
	}

	/* Initiate remote TCP/IP port forwardings. */
	for (i = 0; i < options.num_remote_forwards; i++) {
		debug("Connections to remote port %d forwarded to local address %.200s:%d",
		      options.remote_forwards[i].port,
		      options.remote_forwards[i].host,
		      options.remote_forwards[i].host_port);
		channel_request_remote_forwarding(options.remote_forwards[i].port,
						  options.remote_forwards[i].host,
				   		  options.remote_forwards[i].host_port);
	}

	/*
	 * If a command was specified on the command line, execute the
	 * command now. Otherwise request the server to start a shell.
	 */
	if (buffer_len(&command) > 0) {
		int len = buffer_len(&command);
		if (len > 900)
			len = 900;
		debug("Sending command: %.*s", len, buffer_ptr(&command));
		packet_start(SSH_CMSG_EXEC_CMD);
		packet_put_string(buffer_ptr(&command), buffer_len(&command));
		packet_send();
		packet_write_wait();
	} else {
		debug("Requesting shell.");
		packet_start(SSH_CMSG_EXEC_SHELL);
		packet_send();
		packet_write_wait();
	}

	/* Enter the interactive session. */
	exit_status = client_loop(tty_flag, tty_flag ? options.escape_char : -1);

	/* Close the connection to the remote host. */
	packet_close();

	/* Exit with the status returned by the program on the remote side. */
	exit(exit_status);
}