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

/*

clientloop.c

Author: Tatu Ylonen <ylo@cs.hut.fi>

Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
                   All rights reserved


Created: Sat Sep 23 12:23:57 1995 ylo

The main loop for the interactive session (client side).

*/

#include "includes.h"
RCSID("$Id: clientloop.c,v 1.6 1999/09/30 08:34:24 deraadt Exp $");

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

/* Flag indicating whether quiet mode is on. */
extern int quiet_flag;

/* Flag indicating that stdin should be redirected from /dev/null. */
extern int stdin_null_flag;

/* 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. */
extern char *host;

/* 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. */
static volatile int received_window_change_signal = 0;

/* Terminal modes, as saved by enter_raw_mode. */
static struct termios saved_tio;

/* Flag indicating whether we are in raw mode.  This is used by enter_raw_mode
   and leave_raw_mode. */
static int in_raw_mode = 0;

/* Flag indicating whether the user\'s terminal is in non-blocking mode. */
static int in_non_blocking_mode = 0;

/* Common data for the client loop code. */
static int escape_pending;  /* Last character was the escape character */
static int last_was_cr; /* Last character was a newline. */
static int exit_status; /* Used to store the exit status of the command. */
static int stdin_eof; /* EOF has been encountered on standard error. */
static Buffer stdin_buffer;  /* Buffer for stdin data. */
static Buffer stdout_buffer; /* Buffer for stdout data. */
static Buffer stderr_buffer; /* Buffer for stderr data. */
static unsigned int buffer_high; /* Soft max buffer size. */
static int max_fd; /* Maximum file descriptor number in select(). */
static int connection_in; /* Connection to server (input). */
static int connection_out; /* Connection to server (output). */
static unsigned long stdin_bytes, stdout_bytes, stderr_bytes;
static int quit_pending; /* Set to non-zero to quit the client loop. */
static int escape_char; /* Escape character. */

/* Returns the user\'s terminal to normal mode if it had been put in raw 
   mode. */

void leave_raw_mode()
{
  if (!in_raw_mode)
    return;
  in_raw_mode = 0;
  if (tcsetattr(fileno(stdin), TCSADRAIN, &saved_tio) < 0)
    perror("tcsetattr");

  fatal_remove_cleanup((void (*)(void *))leave_raw_mode, NULL);
}

/* Puts the user\'s terminal in raw mode. */

void enter_raw_mode()
{
  struct termios tio;

  if (tcgetattr(fileno(stdin), &tio) < 0)
    perror("tcgetattr");
  saved_tio = tio;
  tio.c_iflag |= IGNPAR;
  tio.c_iflag &= ~(ISTRIP|INLCR|IGNCR|ICRNL|IXON|IXANY|IXOFF);
  tio.c_lflag &= ~(ISIG|ICANON|ECHO|ECHOE|ECHOK|ECHONL);
#ifdef IEXTEN
  tio.c_lflag &= ~IEXTEN;
#endif /* IEXTEN */
  tio.c_oflag &= ~OPOST;
  tio.c_cc[VMIN] = 1;
  tio.c_cc[VTIME] = 0;
  if (tcsetattr(fileno(stdin), TCSADRAIN, &tio) < 0)
    perror("tcsetattr");
  in_raw_mode = 1;

  fatal_add_cleanup((void (*)(void *))leave_raw_mode, NULL);
}  

/* Puts stdin terminal in non-blocking mode. */

/* Restores stdin to blocking mode. */

void leave_non_blocking()
{
  if (in_non_blocking_mode)
    {
      (void)fcntl(fileno(stdin), F_SETFL, 0);
      in_non_blocking_mode = 0;
      fatal_remove_cleanup((void (*)(void *))leave_non_blocking, NULL);
    }
}

void enter_non_blocking()
{
  in_non_blocking_mode = 1;
  (void)fcntl(fileno(stdin), F_SETFL, O_NONBLOCK);
  fatal_add_cleanup((void (*)(void *))leave_non_blocking, NULL);
}

/* Signal handler for the window change signal (SIGWINCH).  This just
   sets a flag indicating that the window has changed. */

void window_change_handler(int sig)
{
  received_window_change_signal = 1;
  signal(SIGWINCH, window_change_handler);
}

/* Signal handler for signals that cause the program to terminate.  These
   signals must be trapped to restore terminal modes. */

void signal_handler(int sig)
{
  if (in_raw_mode)
    leave_raw_mode();
  if (in_non_blocking_mode)
    leave_non_blocking();
  channel_stop_listening();
  packet_close();
  fatal("Killed by signal %d.", sig);
}

/* Returns current time in seconds from Jan 1, 1970 with the maximum available
   resolution. */

double get_current_time()
{
  struct timeval tv;
  gettimeofday(&tv, NULL);
  return (double)tv.tv_sec + (double)tv.tv_usec / 1000000.0;
}

/* This is called when the interactive is entered.  This checks if there
   is an EOF coming on stdin.  We must check this explicitly, as select()
   does not appear to wake up when redirecting from /dev/null. */

void client_check_initial_eof_on_stdin()
{
  int len;
  char buf[1];

  /* If standard input is to be "redirected from /dev/null", we simply
     mark that we have seen an EOF and send an EOF message to the server.
     Otherwise, we try to read a single character; it appears that for some
     files, such /dev/null, select() never wakes up for read for this
     descriptor, which means that we never get EOF.  This way we will get
     the EOF if stdin comes from /dev/null or similar. */
  if (stdin_null_flag)
    {
      /* Fake EOF on stdin. */
      debug("Sending eof.");
      stdin_eof = 1;
      packet_start(SSH_CMSG_EOF);
      packet_send();
    }
  else
    {
      /* Enter non-blocking mode for stdin. */
      enter_non_blocking();

      /* Check for immediate EOF on stdin. */
      len = read(fileno(stdin), buf, 1);
      if (len == 0)
	{
	  /* EOF.  Record that we have seen it and send EOF to server. */
	  debug("Sending eof.");
	  stdin_eof = 1;
	  packet_start(SSH_CMSG_EOF);
	  packet_send();
	}
      else
	if (len > 0)
	  {
	    /* Got data.  We must store the data in the buffer, and also
	       process it as an escape character if appropriate. */
	    if ((unsigned char)buf[0] == escape_char)
	      escape_pending = 1;
	    else
	      {
		buffer_append(&stdin_buffer, buf, 1);
		stdin_bytes += 1;
	      }
	  }
      
      /* Leave non-blocking mode. */
      leave_non_blocking();
    }
}

/* Get packets from the connection input buffer, and process them as long
   as there are packets available. */

void client_process_buffered_input_packets()
{
  int type;
  char *data;
  unsigned int data_len;
  int payload_len;

  /* Process any buffered packets from the server. */
  while (!quit_pending && (type = packet_read_poll(&payload_len)) != SSH_MSG_NONE)
    {
      switch (type)
	{
	  
	case SSH_SMSG_STDOUT_DATA:
	  data = packet_get_string(&data_len);
	  packet_integrity_check(payload_len, 4 + data_len, type);
	  buffer_append(&stdout_buffer, data, data_len);
	  stdout_bytes += data_len;
	  memset(data, 0, data_len);
	  xfree(data);
	  break;

	case SSH_SMSG_STDERR_DATA:
	  data = packet_get_string(&data_len);
	  packet_integrity_check(payload_len, 4 + data_len, type);
	  buffer_append(&stderr_buffer, data, data_len);
	  stdout_bytes += data_len;
	  memset(data, 0, data_len);
	  xfree(data);
	  break;

	case SSH_SMSG_EXITSTATUS:
	  packet_integrity_check(payload_len, 4, type);
	  exit_status = packet_get_int();
	  /* Acknowledge the exit. */
	  packet_start(SSH_CMSG_EXIT_CONFIRMATION);
	  packet_send();
	  /* Must wait for packet to be sent since we are exiting the
	     loop. */
	  packet_write_wait();
	  /* Flag that we want to exit. */
	  quit_pending = 1;
	  break;

	case SSH_SMSG_X11_OPEN:
	  x11_input_open(payload_len);
	  break;

	case SSH_MSG_PORT_OPEN:
	  channel_input_port_open(payload_len);
	  break;

	case SSH_SMSG_AGENT_OPEN:
	  packet_integrity_check(payload_len, 4, type);
	  auth_input_open_request();
	  break;

	case SSH_MSG_CHANNEL_OPEN_CONFIRMATION:
	  packet_integrity_check(payload_len, 4 + 4, type);
	  channel_input_open_confirmation();
	  break;

	case SSH_MSG_CHANNEL_OPEN_FAILURE:
	  packet_integrity_check(payload_len, 4, type);
	  channel_input_open_failure();
	  break;

	case SSH_MSG_CHANNEL_DATA:
	  channel_input_data(payload_len);
	  break;

	case SSH_MSG_CHANNEL_CLOSE:
	  packet_integrity_check(payload_len, 4, type);
	  channel_input_close();
	  break;

	case SSH_MSG_CHANNEL_CLOSE_CONFIRMATION:
	  packet_integrity_check(payload_len, 4, type);
	  channel_input_close_confirmation();
	  break;

	default:
	  /* Any unknown packets received during the actual session
	     cause the session to terminate.  This is intended to make
	     debugging easier since no confirmations are sent.  Any
	     compatible protocol extensions must be negotiated during
	     the preparatory phase. */
	  packet_disconnect("Protocol error during session: type %d",
			    type);
	}
    }
}

/* Make packets from buffered stdin data, and buffer them for sending to
   the connection. */

void client_make_packets_from_stdin_data()
{
  unsigned int len;

  /* Send buffered stdin data to the server. */
  while (buffer_len(&stdin_buffer) > 0 && 
	 packet_not_very_much_data_to_write())
    {
      len = buffer_len(&stdin_buffer);
      if (len > 32768)
	len = 32768;  /* Keep the packets at reasonable size. */
      packet_start(SSH_CMSG_STDIN_DATA);
      packet_put_string(buffer_ptr(&stdin_buffer), len);
      packet_send();
      buffer_consume(&stdin_buffer, len);
      /* If we have a pending EOF, send it now. */
      if (stdin_eof && buffer_len(&stdin_buffer) == 0)
	{
	  packet_start(SSH_CMSG_EOF);
	  packet_send();
	}
    }
}

/* Checks if the client window has changed, and sends a packet about it to
   the server if so.  The actual change is detected elsewhere (by a software
   interrupt on Unix); this just checks the flag and sends a message if
   appropriate. */

void client_check_window_change()
{
  /* Send possible window change message to the server. */
  if (received_window_change_signal)
    {
      struct winsize ws;

      /* Clear the window change indicator. */
      received_window_change_signal = 0;

      /* Read new window size. */
      if (ioctl(fileno(stdin), TIOCGWINSZ, &ws) >= 0)
	{
	  /* Successful, send the packet now. */
	  packet_start(SSH_CMSG_WINDOW_SIZE);
	  packet_put_int(ws.ws_row);
	  packet_put_int(ws.ws_col);
	  packet_put_int(ws.ws_xpixel);
	  packet_put_int(ws.ws_ypixel);
	  packet_send();
	}
    }
}

/* Waits until the client can do something (some data becomes available on
   one of the file descriptors). */

void client_wait_until_can_do_something(fd_set *readset, fd_set *writeset)
{
  /* Initialize select masks. */
  FD_ZERO(readset);
  
  /* Read from the connection, unless our buffers are full. */
  if (buffer_len(&stdout_buffer) < buffer_high &&
      buffer_len(&stderr_buffer) < buffer_high &&
      channel_not_very_much_buffered_data())
    FD_SET(connection_in, readset);

  /* Read from stdin, unless we have seen EOF or have very much buffered
     data to send to the server. */
  if (!stdin_eof && packet_not_very_much_data_to_write())
    FD_SET(fileno(stdin), readset);
  
  FD_ZERO(writeset);
  
  /* Add any selections by the channel mechanism. */
  channel_prepare_select(readset, writeset);
  
  /* Select server connection if have data to write to the server. */
  if (packet_have_data_to_write())
    FD_SET(connection_out, writeset);

  /* Select stdout if have data in buffer. */
  if (buffer_len(&stdout_buffer) > 0)
    FD_SET(fileno(stdout), writeset);

  /* Select stderr if have data in buffer. */
  if (buffer_len(&stderr_buffer) > 0)
    FD_SET(fileno(stderr), writeset);

  /* Update maximum file descriptor number, if appropriate. */
  if (channel_max_fd() > max_fd)
    max_fd = channel_max_fd();

  /* Wait for something to happen.  This will suspend the process until
     some selected descriptor can be read, written, or has some other
     event pending.  Note: if you want to implement SSH_MSG_IGNORE
     messages to fool traffic analysis, this might be the place to do
     it: just have a random timeout for the select, and send a random
     SSH_MSG_IGNORE packet when the timeout expires. */
  if (select(max_fd + 1, readset, writeset, NULL, NULL) < 0)
    {
      char buf[100];
      /* Some systems fail to clear these automatically. */
      FD_ZERO(readset);
      FD_ZERO(writeset);
      if (errno == EINTR)
	return;
      /* Note: we might still have data in the buffers. */
      sprintf(buf, "select: %.100s\r\n", strerror(errno));
      buffer_append(&stderr_buffer, buf, strlen(buf));
      stderr_bytes += strlen(buf);
      quit_pending = 1;
    }
}

void client_suspend_self()
{
  struct winsize oldws, newws;

  /* Flush stdout and stderr buffers. */
  if (buffer_len(&stdout_buffer) > 0)
    write(fileno(stdout), 
	  buffer_ptr(&stdout_buffer), 
	  buffer_len(&stdout_buffer));
  if (buffer_len(&stderr_buffer) > 0)
    write(fileno(stderr), 
	  buffer_ptr(&stderr_buffer), 
	  buffer_len(&stderr_buffer));

  /* Leave raw mode. */
  leave_raw_mode();

  /* Free (and clear) the buffer to reduce the
     amount of data that gets written to swap. */
  buffer_free(&stdin_buffer);
  buffer_free(&stdout_buffer);
  buffer_free(&stderr_buffer);

  /* Save old window size. */
  ioctl(fileno(stdin), TIOCGWINSZ, &oldws);

  /* Send the suspend signal to the program
     itself. */
  kill(getpid(), SIGTSTP);

  /* Check if the window size has changed. */
  if (ioctl(fileno(stdin), TIOCGWINSZ, &newws) >= 0 &&
      (oldws.ws_row != newws.ws_row || oldws.ws_col != newws.ws_col ||
       oldws.ws_xpixel != newws.ws_xpixel || 
       oldws.ws_ypixel != newws.ws_ypixel))
    received_window_change_signal = 1;

  /* OK, we have been continued by the user. 
     Reinitialize buffers. */
  buffer_init(&stdin_buffer);
  buffer_init(&stdout_buffer);
  buffer_init(&stderr_buffer);

  /* Re-enter raw mode. */
  enter_raw_mode();
}

void client_process_input(fd_set *readset)
{
  int len, pid;
  char buf[8192], *s;

  /* Read input from the server, and add any such data to the buffer of the
     packet subsystem. */
  if (FD_ISSET(connection_in, readset))
    {
      /* Read as much as possible. */
      len = read(connection_in, buf, sizeof(buf));
      if (len == 0)
	{ 
	  /* Received EOF.  The remote host has closed the connection. */
	  sprintf(buf, "Connection to %.300s closed by remote host.\r\n",
		  host);
	  buffer_append(&stderr_buffer, buf, strlen(buf));
	  stderr_bytes += strlen(buf);
	  quit_pending = 1;
	  return;
	}

      /* There is a kernel bug on Solaris that causes select to sometimes
	 wake up even though there is no data available. */
      if (len < 0 && errno == EAGAIN)
	len = 0;

      if (len < 0)
	{
	  /* An error has encountered.  Perhaps there is a network
	     problem. */
	  sprintf(buf, "Read from remote host %.300s: %.100s\r\n", 
		  host, strerror(errno));
	  buffer_append(&stderr_buffer, buf, strlen(buf));
	  stderr_bytes += strlen(buf);
	  quit_pending = 1;
	  return;
	}
      packet_process_incoming(buf, len);
    }

  /* Read input from stdin. */
  if (FD_ISSET(fileno(stdin), readset))
    {
      /* Read as much as possible. */
      len = read(fileno(stdin), buf, sizeof(buf));
      if (len <= 0)
	{
	  /* Received EOF or error.  They are treated similarly,
	     except that an error message is printed if it was
	     an error condition. */
	  if (len < 0)
	    {
	      sprintf(buf, "read: %.100s\r\n", strerror(errno));
	      buffer_append(&stderr_buffer, buf, strlen(buf));
	      stderr_bytes += strlen(buf);
	    }
	  /* Mark that we have seen EOF. */
	  stdin_eof = 1;
	  /* Send an EOF message to the server unless there is data
	     in the buffer.  If there is data in the buffer, no message
	     will be sent now.  Code elsewhere will send the EOF
	     when the buffer becomes empty if stdin_eof is set. */
	  if (buffer_len(&stdin_buffer) == 0)
	    {
	      packet_start(SSH_CMSG_EOF);
	      packet_send();
	    }
	}
      else
	if (escape_char == -1)
	  {
	    /* Normal successful read, and no escape character.  Just 
	       append the data to buffer. */
	    buffer_append(&stdin_buffer, buf, len);
	    stdin_bytes += len;
	  }
	else
	  {
	    /* Normal, successful read.  But we have an escape character
	       and have to process the characters one by one. */
	    unsigned int i;
	    for (i = 0; i < len; i++)
	      {
		unsigned char ch;
		/* Get one character at a time. */
		ch = buf[i];
		
		/* Check if we have a pending escape character. */
		if (escape_pending)
		  {
		    /* We have previously seen an escape character. */
		    /* Clear the flag now. */
		    escape_pending = 0;
		    /* Process the escaped character. */
		    switch (ch)
		      {
		      case '.':
			/* Terminate the connection. */
			sprintf(buf, "%c.\r\n", escape_char);
			buffer_append(&stderr_buffer, buf, strlen(buf));
			stderr_bytes += strlen(buf);
			quit_pending = 1;
			return;

		      case 'Z' - 64:
			  /* Suspend the program. */
			  /* Print a message to that effect to the user. */
			  sprintf(buf, "%c^Z\r\n", escape_char);
			  buffer_append(&stderr_buffer, buf, strlen(buf));
			  stderr_bytes += strlen(buf);

			  /* Restore terminal modes and suspend. */
			  client_suspend_self();

			  /* We have been continued. */
			  continue;
			
		      case '&':
			/* Detach the program (continue to serve connections,
			   but put in background and no more new 
			   connections). */
			if (!stdin_eof)
			  {
			    /* Sending SSH_CMSG_EOF alone does not always
			       appear to be enough.  So we try to send an
			       EOF character first. */
			    packet_start(SSH_CMSG_STDIN_DATA);
			    packet_put_string("\004", 1);
			    packet_send();
			    /* Close stdin. */
			    stdin_eof = 1;
			    if (buffer_len(&stdin_buffer) == 0)
			      {
				packet_start(SSH_CMSG_EOF);
				packet_send();
			      }
			  }
			/* Restore tty modes. */
			leave_raw_mode();

			/* Stop listening for new connections. */
			channel_stop_listening();

			printf("%c& [backgrounded]\n", escape_char);
			
			/* Fork into background. */
			pid = fork();
			if (pid < 0)
			  {
			    error("fork: %.100s", strerror(errno));
			    continue;
			  }
			if (pid != 0)
			  { /* This is the parent. */
			    /* The parent just exits. */
			    exit(0);
			  }

			/* The child continues serving connections. */
			continue;

		      case '?':
			sprintf(buf, "%c?\r\n\
Supported escape sequences:\r\n\
~.  - terminate connection\r\n\
~^Z - suspend ssh\r\n\
~#  - list forwarded connections\r\n\
~&  - background ssh (when waiting for connections to terminate)\r\n\
~?  - this message\r\n\
~~  - send the escape character by typing it twice\r\n\
(Note that escapes are only recognized immediately after newline.)\r\n",
				escape_char);
			buffer_append(&stderr_buffer, buf, strlen(buf));
			continue;

		      case '#':
			sprintf(buf, "%c#\r\n", escape_char);
			buffer_append(&stderr_buffer, buf, strlen(buf));
			s = channel_open_message();
			buffer_append(&stderr_buffer, s, strlen(s));
			xfree(s);
			continue;

		      default:
			if (ch != escape_char)
			  {
			    /* Escape character followed by non-special
			       character.  Append both to the input
			       buffer. */
			    buf[0] = escape_char;
			    buf[1] = ch;
			    buffer_append(&stdin_buffer, buf, 2);
			    stdin_bytes += 2;
			    continue;
			  }
			/* Note that escape character typed twice falls through
			   here; the latter gets processed as a normal
			   character below. */
			break;
		      }
		  }
		else
		  {
		    /* The previous character was not an escape char. 
		       Check if this is an escape. */
		    if (last_was_cr && ch == escape_char)
		      {
			/* It is. Set the flag and continue to next
			   character. */
			escape_pending = 1;
			continue;
		      }
		  }

		/* Normal character.  Record whether it was a newline,
		   and append it to the buffer. */
		last_was_cr = (ch == '\r' || ch == '\n');
		buf[0] = ch;
		buffer_append(&stdin_buffer, buf, 1);
		stdin_bytes += 1;
		continue;
	      }
	  }
    }
}

void client_process_output(fd_set *writeset)
{
  int len;
  char buf[100];

  /* Write buffered output to stdout. */
  if (FD_ISSET(fileno(stdout), writeset))
    {
      /* Write as much data as possible. */
      len = write(fileno(stdout), buffer_ptr(&stdout_buffer),
		  buffer_len(&stdout_buffer));
      if (len <= 0)
	{
	  if (errno == EAGAIN)
	    len = 0;
	  else
	    {
	      /* An error or EOF was encountered.  Put an error message
		 to stderr buffer. */
	      sprintf(buf, "write stdout: %.50s\r\n", strerror(errno));
	      buffer_append(&stderr_buffer, buf, strlen(buf));
	      stderr_bytes += strlen(buf);
	      quit_pending = 1;
	      return;
	    }
	}
      /* Consume printed data from the buffer. */
      buffer_consume(&stdout_buffer, len);
    }

  /* Write buffered output to stderr. */
  if (FD_ISSET(fileno(stderr), writeset))
    {
      /* Write as much data as possible. */
      len = write(fileno(stderr), buffer_ptr(&stderr_buffer),
		  buffer_len(&stderr_buffer));
      if (len <= 0) {
	if (errno == EAGAIN)
	  len = 0;
	else
	  {
	    /* EOF or error, but can't even print error message. */
	    quit_pending = 1;
	    return;
	  }
      }
      /* Consume printed characters from the buffer. */
      buffer_consume(&stderr_buffer, len);
    }
}

/* Implements the interactive session with the server.  This is called
   after the user has been authenticated, and a command has been
   started on the remote host.  If escape_char != -1, it is the character
   used as an escape character for terminating or suspending the
   session. */

int client_loop(int have_pty, int escape_char_arg)
{
  double start_time, total_time;
  int len;
  char buf[100];

  debug("Entering interactive session.");

  start_time = get_current_time();

  /* Initialize variables. */
  escape_pending = 0;
  last_was_cr = 1;
  exit_status = -1;
  stdin_eof = 0;
  buffer_high = 64 * 1024;
  connection_in = packet_get_connection_in();
  connection_out = packet_get_connection_out();
  max_fd = connection_in;
  if (connection_out > max_fd)
    max_fd = connection_out;
  stdin_bytes = 0;
  stdout_bytes = 0;
  stderr_bytes = 0;
  quit_pending = 0;
  escape_char = escape_char_arg;

  /* Initialize buffers. */
  buffer_init(&stdin_buffer);
  buffer_init(&stdout_buffer);
  buffer_init(&stderr_buffer);

  /* Set signal handlers to restore non-blocking mode.  */
  signal(SIGINT, signal_handler);
  signal(SIGQUIT, signal_handler);
  signal(SIGTERM, signal_handler);
  signal(SIGPIPE, SIG_IGN);
  if (have_pty)
    signal(SIGWINCH, window_change_handler);

  /* Enter raw mode if have a pseudo terminal. */
  if (have_pty)
    enter_raw_mode();

  /* Check if we should immediately send of on stdin. */
  client_check_initial_eof_on_stdin();

  /* Main loop of the client for the interactive session mode. */
  while (!quit_pending)
    {
      fd_set readset, writeset;

      /* Precess buffered packets sent by the server. */
      client_process_buffered_input_packets();

      /* Make packets of buffered stdin data, and buffer them for sending
	 to the server. */
      client_make_packets_from_stdin_data();

      /* Make packets from buffered channel data, and buffer them for sending
	 to the server. */
      if (packet_not_very_much_data_to_write())
	channel_output_poll();

      /* Check if the window size has changed, and buffer a message about
	 it to the server if so. */
      client_check_window_change();

      if (quit_pending)
	break;

      /* Wait until we have something to do (something becomes available
	 on one of the descriptors). */
      client_wait_until_can_do_something(&readset, &writeset);

      if (quit_pending)
	break;

      /* Do channel operations. */
      channel_after_select(&readset, &writeset);

      /* Process input from the connection and from stdin.  Buffer any data
         that is available. */
      client_process_input(&readset);

      /* Process output to stdout and stderr.   Output to the connection
         is processed elsewhere (above). */
      client_process_output(&writeset);

      /* Send as much buffered packet data as possible to the sender. */
      if (FD_ISSET(connection_out, &writeset))
	packet_write_poll();
    }

  /* Terminate the session. */

  /* Stop watching for window change. */
  if (have_pty)
    signal(SIGWINCH, SIG_DFL);

  /* Stop listening for connections. */
  channel_stop_listening();

  /* In interactive mode (with pseudo tty) display a message indicating that
     the connection has been closed. */
  if (have_pty && !quiet_flag)
    {
      sprintf(buf, "Connection to %.64s closed.\r\n", host);
      buffer_append(&stderr_buffer, buf, strlen(buf));
      stderr_bytes += strlen(buf);
    }

  /* Output any buffered data for stdout. */
  while (buffer_len(&stdout_buffer) > 0)
    {
      len = write(fileno(stdout), buffer_ptr(&stdout_buffer), 
		  buffer_len(&stdout_buffer));
      if (len <= 0)
	{
	  error("Write failed flushing stdout buffer.");
	  break;
	}
      buffer_consume(&stdout_buffer, len);
    }

  /* Output any buffered data for stderr. */
  while (buffer_len(&stderr_buffer) > 0)
    {
      len = write(fileno(stderr), buffer_ptr(&stderr_buffer), 
		  buffer_len(&stderr_buffer));
      if (len <= 0)
	{
	  error("Write failed flushing stderr buffer.");
	  break;
	}
      buffer_consume(&stderr_buffer, len);
    }

  /* Leave raw mode. */
  if (have_pty)
    leave_raw_mode();

  /* Clear and free any buffers. */
  memset(buf, 0, sizeof(buf));
  buffer_free(&stdin_buffer);
  buffer_free(&stdout_buffer);
  buffer_free(&stderr_buffer);

  /* Report bytes transferred, and transfer rates. */
  total_time = get_current_time() - start_time;
  debug("Transferred: stdin %lu, stdout %lu, stderr %lu bytes in %.1f seconds",
	stdin_bytes, stdout_bytes, stderr_bytes, total_time);
  if (total_time > 0)
    debug("Bytes per second: stdin %.1f, stdout %.1f, stderr %.1f",
	  stdin_bytes / total_time, stdout_bytes / total_time,
	  stderr_bytes / total_time);

  /* Return the exit status of the program. */
  debug("Exit status %d", exit_status);
  return exit_status;
}