networking/telnetd.c

/* vi: set sw=4 ts=4: */
/*
 * Simple telnet server
 * Bjorn Wesen, Axis Communications AB (bjornw@axis.com)
 *
 * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
 *
 * ---------------------------------------------------------------------------
 * (C) Copyright 2000, Axis Communications AB, LUND, SWEDEN
 ****************************************************************************
 *
 * The telnetd manpage says it all:
 *
 *   Telnetd operates by allocating a pseudo-terminal device (see pty(4))  for
 *   a client, then creating a login process which has the slave side of the
 *   pseudo-terminal as stdin, stdout, and stderr. Telnetd manipulates the
 *   master side of the pseudo-terminal, implementing the telnet protocol and
 *   passing characters between the remote client and the login process.
 *
 * Vladimir Oleynik <dzo@simtreas.ru> 2001
 *     Set process group corrections, initial busybox port
 */

/*#define DEBUG 1 */
#undef DEBUG

#include "busybox.h"

#ifdef DEBUG
#define TELCMDS
#define TELOPTS
#endif
#include <arpa/telnet.h>
#include <sys/syslog.h>


#define BUFSIZE 4000

#ifdef CONFIG_FEATURE_IPV6
#define SOCKET_TYPE	AF_INET6
typedef struct sockaddr_in6 sockaddr_type;
#else
#define SOCKET_TYPE	AF_INET
typedef struct sockaddr_in sockaddr_type;
#endif


#ifdef CONFIG_LOGIN
static const char *loginpath = "/bin/login";
#else
static const char *loginpath;
#endif
static const char *issuefile = "/etc/issue.net";

/* shell name and arguments */

static const char *argv_init[] = {NULL, NULL};

/* structure that describes a session */

struct tsession {
#ifdef CONFIG_FEATURE_TELNETD_INETD
	int sockfd_read, sockfd_write, ptyfd;
#else /* CONFIG_FEATURE_TELNETD_INETD */
	struct tsession *next;
	int sockfd, ptyfd;
#endif /* CONFIG_FEATURE_TELNETD_INETD */
	int shell_pid;
	/* two circular buffers */
	char *buf1, *buf2;
	int rdidx1, wridx1, size1;
	int rdidx2, wridx2, size2;
};

/*

   This is how the buffers are used. The arrows indicate the movement
   of data.

   +-------+     wridx1++     +------+     rdidx1++     +----------+
   |       | <--------------  | buf1 | <--------------  |          |
   |       |     size1--      +------+     size1++      |          |
   |  pty  |                                            |  socket  |
   |       |     rdidx2++     +------+     wridx2++     |          |
   |       |  --------------> | buf2 |  --------------> |          |
   +-------+     size2++      +------+     size2--      +----------+

   Each session has got two buffers.

*/

static int maxfd;

static struct tsession *sessions;


/*

   Remove all IAC's from the buffer pointed to by bf (received IACs are ignored
   and must be removed so as to not be interpreted by the terminal).  Make an
   uninterrupted string of characters fit for the terminal.  Do this by packing
   all characters meant for the terminal sequentially towards the end of bf.

   Return a pointer to the beginning of the characters meant for the terminal.
   and make *num_totty the number of characters that should be sent to
   the terminal.

   Note - If an IAC (3 byte quantity) starts before (bf + len) but extends
   past (bf + len) then that IAC will be left unprocessed and *processed will be
   less than len.

   FIXME - if we mean to send 0xFF to the terminal then it will be escaped,
   what is the escape character?  We aren't handling that situation here.

   CR-LF ->'s CR mapping is also done here, for convenience

  */
static char *
remove_iacs(struct tsession *ts, int *pnum_totty) {
	unsigned char *ptr0 = (unsigned char *)ts->buf1 + ts->wridx1;
	unsigned char *ptr = ptr0;
	unsigned char *totty = ptr;
	unsigned char *end = ptr + MIN(BUFSIZE - ts->wridx1, ts->size1);
	int processed;
	int num_totty;

	while (ptr < end) {
		if (*ptr != IAC) {
			int c = *ptr;
			*totty++ = *ptr++;
			/* We now map \r\n ==> \r for pragmatic reasons.
			 * Many client implementations send \r\n when
			 * the user hits the CarriageReturn key.
			 */
			if (c == '\r' && (*ptr == '\n' || *ptr == 0) && ptr < end)
				ptr++;
		}
		else {
			/*
			 * TELOPT_NAWS support!
			 */
			if ((ptr+2) >= end) {
				/* only the beginning of the IAC is in the
				buffer we were asked to process, we can't
				process this char. */
				break;
			}

			/*
			 * IAC -> SB -> TELOPT_NAWS -> 4-byte -> IAC -> SE
			 */
			else if (ptr[1] == SB && ptr[2] == TELOPT_NAWS) {
				struct winsize ws;
				if ((ptr+8) >= end)
					break;	/* incomplete, can't process */
				ws.ws_col = (ptr[3] << 8) | ptr[4];
				ws.ws_row = (ptr[5] << 8) | ptr[6];
				(void) ioctl(ts->ptyfd, TIOCSWINSZ, (char *)&ws);
				ptr += 9;
			}
			else {
				/* skip 3-byte IAC non-SB cmd */
#ifdef DEBUG
				fprintf(stderr, "Ignoring IAC %s,%s\n",
					TELCMD(*(ptr+1)), TELOPT(*(ptr+2)));
#endif
				ptr += 3;
			}
		}
	}

	processed = ptr - ptr0;
	num_totty = totty - ptr0;
	/* the difference between processed and num_to tty
	   is all the iacs we removed from the stream.
	   Adjust buf1 accordingly. */
	ts->wridx1 += processed - num_totty;
	ts->size1 -= processed - num_totty;
	*pnum_totty = num_totty;
	/* move the chars meant for the terminal towards the end of the
	buffer. */
	return memmove(ptr - num_totty, ptr0, num_totty);
}


static int
getpty(char *line)
{
	int p;
#ifdef CONFIG_FEATURE_DEVPTS
	p = open("/dev/ptmx", 2);
	if (p > 0) {
		grantpt(p);
		unlockpt(p);
		strcpy(line, ptsname(p));
		return(p);
	}
#else
	struct stat stb;
	int i;
	int j;

	strcpy(line, "/dev/ptyXX");

	for (i = 0; i < 16; i++) {
		line[8] = "pqrstuvwxyzabcde"[i];
		line[9] = '0';
		if (stat(line, &stb) < 0) {
			continue;
		}
		for (j = 0; j < 16; j++) {
			line[9] = j < 10 ? j + '0' : j - 10 + 'a';
#ifdef DEBUG
			fprintf(stderr, "Trying to open device: %s\n", line);
#endif
			if ((p = open(line, O_RDWR | O_NOCTTY)) >= 0) {
				line[5] = 't';
				return p;
			}
		}
	}
#endif /* CONFIG_FEATURE_DEVPTS */
	return -1;
}


static void
send_iac(struct tsession *ts, unsigned char command, int option)
{
	/* We rely on that there is space in the buffer for now.  */
	char *b = ts->buf2 + ts->rdidx2;
	*b++ = IAC;
	*b++ = command;
	*b++ = option;
	ts->rdidx2 += 3;
	ts->size2 += 3;
}


static struct tsession *
#ifdef CONFIG_FEATURE_TELNETD_INETD
make_new_session(void)
#else /* CONFIG_FEATURE_TELNETD_INETD */
make_new_session(int sockfd)
#endif /* CONFIG_FEATURE_TELNETD_INETD */
{
	struct termios termbuf;
	int pty, pid;
	char tty_name[32];
	struct tsession *ts = xzalloc(sizeof(struct tsession) + BUFSIZE * 2);

	ts->buf1 = (char *)(&ts[1]);
	ts->buf2 = ts->buf1 + BUFSIZE;

#ifdef CONFIG_FEATURE_TELNETD_INETD
	ts->sockfd_write = 1;
#else /* CONFIG_FEATURE_TELNETD_INETD */
	ts->sockfd = sockfd;
#endif /* CONFIG_FEATURE_TELNETD_INETD */

	/* Got a new connection, set up a tty and spawn a shell.  */

	pty = getpty(tty_name);

	if (pty < 0) {
		syslog(LOG_ERR, "All terminals in use!");
		return 0;
	}

	if (pty > maxfd)
		maxfd = pty;

	ts->ptyfd = pty;

	/* Make the telnet client understand we will echo characters so it
	 * should not do it locally. We don't tell the client to run linemode,
	 * because we want to handle line editing and tab completion and other
	 * stuff that requires char-by-char support.
	 */

	send_iac(ts, DO, TELOPT_ECHO);
	send_iac(ts, DO, TELOPT_NAWS);
	send_iac(ts, DO, TELOPT_LFLOW);
	send_iac(ts, WILL, TELOPT_ECHO);
	send_iac(ts, WILL, TELOPT_SGA);

	if ((pid = fork()) < 0) {
		syslog(LOG_ERR, "Could not fork");
	}
	if (pid == 0) {
		/* In child, open the child's side of the tty.  */
		int i;

		for(i = 0; i <= maxfd; i++)
			close(i);
		/* make new process group */
		setsid();

		if (open(tty_name, O_RDWR /*| O_NOCTTY*/) < 0) {
			syslog(LOG_ERR, "Could not open tty");
			exit(1);
		}
		dup(0);
		dup(0);

		tcsetpgrp(0, getpid());

		/* The pseudo-terminal allocated to the client is configured to operate in
		 * cooked mode, and with XTABS CRMOD enabled (see tty(4)).
		 */

		tcgetattr(0, &termbuf);
		termbuf.c_lflag |= ECHO; /* if we use readline we dont want this */
		termbuf.c_oflag |= ONLCR|XTABS;
		termbuf.c_iflag |= ICRNL;
		termbuf.c_iflag &= ~IXOFF;
		/*termbuf.c_lflag &= ~ICANON;*/
		tcsetattr(0, TCSANOW, &termbuf);

		print_login_issue(issuefile, NULL);

		/* exec shell, with correct argv and env */
		execv(loginpath, (char *const *)argv_init);

		/* NOT REACHED */
		syslog(LOG_ERR, "execv error");
		exit(1);
	}

	ts->shell_pid = pid;

	return ts;
}

#ifndef CONFIG_FEATURE_TELNETD_INETD
static void
free_session(struct tsession *ts)
{
	struct tsession *t = sessions;

	/* Unlink this telnet session from the session list.  */
	if (t == ts)
		sessions = ts->next;
	else {
		while(t->next != ts)
			t = t->next;
		t->next = ts->next;
	}

	kill(ts->shell_pid, SIGKILL);

	wait4(ts->shell_pid, NULL, 0, NULL);

	close(ts->ptyfd);
	close(ts->sockfd);

	if (ts->ptyfd == maxfd || ts->sockfd == maxfd)
		maxfd--;
	if (ts->ptyfd == maxfd || ts->sockfd == maxfd)
		maxfd--;

	free(ts);
}
#endif /* CONFIG_FEATURE_TELNETD_INETD */

int
telnetd_main(int argc, char **argv)
{
#ifndef CONFIG_FEATURE_TELNETD_INETD
	sockaddr_type sa;
	int master_fd;
#endif /* CONFIG_FEATURE_TELNETD_INETD */
	fd_set rdfdset, wrfdset;
	int selret;
#ifndef CONFIG_FEATURE_TELNETD_INETD
	int on = 1;
	int portnbr = 23;
	struct in_addr bind_addr = { .s_addr = 0x0 };
#endif /* CONFIG_FEATURE_TELNETD_INETD */
	int c;
	static const char options[] =
#ifdef CONFIG_FEATURE_TELNETD_INETD
		"f:l:";
#else /* CONFIG_EATURE_TELNETD_INETD */
		"f:l:p:b:";
#endif /* CONFIG_FEATURE_TELNETD_INETD */
	int maxlen, w, r;

#ifndef CONFIG_LOGIN
	loginpath = DEFAULT_SHELL;
#endif

	for (;;) {
		c = getopt( argc, argv, options);
		if (c == EOF) break;
		switch (c) {
			case 'f':
				issuefile = optarg;
				break;
			case 'l':
				loginpath = optarg;
				break;
#ifndef CONFIG_FEATURE_TELNETD_INETD
			case 'p':
				portnbr = atoi(optarg);
				break;
			case 'b':
				if (inet_aton(optarg, &bind_addr) == 0)
					bb_show_usage();
				break;
#endif /* CONFIG_FEATURE_TELNETD_INETD */
			default:
				bb_show_usage();
		}
	}

	if (access(loginpath, X_OK) < 0) {
		bb_error_msg_and_die ("'%s' unavailable.", loginpath);
	}

	argv_init[0] = loginpath;

	openlog(bb_applet_name, 0, LOG_USER);

#ifdef CONFIG_FEATURE_TELNETD_INETD
	maxfd = 1;
	sessions = make_new_session();
#else /* CONFIG_EATURE_TELNETD_INETD */
	sessions = 0;

	/* Grab a TCP socket.  */

	master_fd = xsocket(SOCKET_TYPE, SOCK_STREAM, 0);
	(void)setsockopt(master_fd, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on));

	/* Set it to listen to specified port.  */

	memset((void *)&sa, 0, sizeof(sa));
#ifdef CONFIG_FEATURE_IPV6
	sa.sin6_family = AF_INET6;
	sa.sin6_port = htons(portnbr);
	/* sa.sin6_addr = bind_addr6; */
#else
	sa.sin_family = AF_INET;
	sa.sin_port = htons(portnbr);
	sa.sin_addr = bind_addr;
#endif

	xbind(master_fd, (struct sockaddr *) &sa, sizeof(sa));
	xlisten(master_fd, 1);
	xdaemon(0, 0);

	maxfd = master_fd;
#endif /* CONFIG_FEATURE_TELNETD_INETD */

	do {
		struct tsession *ts;

		FD_ZERO(&rdfdset);
		FD_ZERO(&wrfdset);

		/* select on the master socket, all telnet sockets and their
		 * ptys if there is room in their respective session buffers.
		 */

#ifndef CONFIG_FEATURE_TELNETD_INETD
		FD_SET(master_fd, &rdfdset);
#endif /* CONFIG_FEATURE_TELNETD_INETD */

		ts = sessions;
#ifndef CONFIG_FEATURE_TELNETD_INETD
		while (ts) {
#endif /* CONFIG_FEATURE_TELNETD_INETD */
			/* buf1 is used from socket to pty
			 * buf2 is used from pty to socket
			 */
			if (ts->size1 > 0) {
				FD_SET(ts->ptyfd, &wrfdset);  /* can write to pty */
			}
			if (ts->size1 < BUFSIZE) {
#ifdef CONFIG_FEATURE_TELNETD_INETD
				FD_SET(ts->sockfd_read, &rdfdset); /* can read from socket */
#else /* CONFIG_FEATURE_TELNETD_INETD */
				FD_SET(ts->sockfd, &rdfdset); /* can read from socket */
#endif /* CONFIG_FEATURE_TELNETD_INETD */
			}
			if (ts->size2 > 0) {
#ifdef CONFIG_FEATURE_TELNETD_INETD
				FD_SET(ts->sockfd_write, &wrfdset); /* can write to socket */
#else /* CONFIG_FEATURE_TELNETD_INETD */
				FD_SET(ts->sockfd, &wrfdset); /* can write to socket */
#endif /* CONFIG_FEATURE_TELNETD_INETD */
			}
			if (ts->size2 < BUFSIZE) {
				FD_SET(ts->ptyfd, &rdfdset);  /* can read from pty */
			}
#ifndef CONFIG_FEATURE_TELNETD_INETD
			ts = ts->next;
		}
#endif /* CONFIG_FEATURE_TELNETD_INETD */

		selret = select(maxfd + 1, &rdfdset, &wrfdset, 0, 0);

		if (!selret)
			break;

#ifndef CONFIG_FEATURE_TELNETD_INETD
		/* First check for and accept new sessions.  */
		if (FD_ISSET(master_fd, &rdfdset)) {
			int fd;
			socklen_t salen;

			salen = sizeof(sa);
			if ((fd = accept(master_fd, (struct sockaddr *)&sa,
						&salen)) < 0) {
				continue;
			} else {
				/* Create a new session and link it into
					our active list.  */
				struct tsession *new_ts = make_new_session(fd);
				if (new_ts) {
					new_ts->next = sessions;
					sessions = new_ts;
					if (fd > maxfd)
						maxfd = fd;
				} else {
					close(fd);
				}
			}
		}

		/* Then check for data tunneling.  */

		ts = sessions;
		while (ts) { /* For all sessions...  */
#endif /* CONFIG_FEATURE_TELNETD_INETD */
#ifndef CONFIG_FEATURE_TELNETD_INETD
			struct tsession *next = ts->next; /* in case we free ts. */
#endif /* CONFIG_FEATURE_TELNETD_INETD */

			if (ts->size1 && FD_ISSET(ts->ptyfd, &wrfdset)) {
				int num_totty;
				char *ptr;
				/* Write to pty from buffer 1.  */

				ptr = remove_iacs(ts, &num_totty);

				w = write(ts->ptyfd, ptr, num_totty);
				if (w < 0) {
#ifdef CONFIG_FEATURE_TELNETD_INETD
					exit(0);
#else /* CONFIG_FEATURE_TELNETD_INETD */
					free_session(ts);
					ts = next;
					continue;
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				}
				ts->wridx1 += w;
				ts->size1 -= w;
				if (ts->wridx1 == BUFSIZE)
					ts->wridx1 = 0;
			}

#ifdef CONFIG_FEATURE_TELNETD_INETD
			if (ts->size2 && FD_ISSET(ts->sockfd_write, &wrfdset)) {
#else /* CONFIG_FEATURE_TELNETD_INETD */
			if (ts->size2 && FD_ISSET(ts->sockfd, &wrfdset)) {
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				/* Write to socket from buffer 2.  */
				maxlen = MIN(BUFSIZE - ts->wridx2, ts->size2);
#ifdef CONFIG_FEATURE_TELNETD_INETD
				w = write(ts->sockfd_write, ts->buf2 + ts->wridx2, maxlen);
				if (w < 0)
					exit(0);
#else /* CONFIG_FEATURE_TELNETD_INETD */
				w = write(ts->sockfd, ts->buf2 + ts->wridx2, maxlen);
				if (w < 0) {
					free_session(ts);
					ts = next;
					continue;
				}
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				ts->wridx2 += w;
				ts->size2 -= w;
				if (ts->wridx2 == BUFSIZE)
					ts->wridx2 = 0;
			}

#ifdef CONFIG_FEATURE_TELNETD_INETD
			if (ts->size1 < BUFSIZE && FD_ISSET(ts->sockfd_read, &rdfdset)) {
#else /* CONFIG_FEATURE_TELNETD_INETD */
			if (ts->size1 < BUFSIZE && FD_ISSET(ts->sockfd, &rdfdset)) {
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				/* Read from socket to buffer 1. */
				maxlen = MIN(BUFSIZE - ts->rdidx1,
						BUFSIZE - ts->size1);
#ifdef CONFIG_FEATURE_TELNETD_INETD
				r = read(ts->sockfd_read, ts->buf1 + ts->rdidx1, maxlen);
				if (!r || (r < 0 && errno != EINTR))
					exit(0);
#else /* CONFIG_FEATURE_TELNETD_INETD */
				r = read(ts->sockfd, ts->buf1 + ts->rdidx1, maxlen);
				if (!r || (r < 0 && errno != EINTR)) {
					free_session(ts);
					ts = next;
					continue;
				}
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				if (!*(ts->buf1 + ts->rdidx1 + r - 1)) {
					r--;
					if (!r)
						continue;
				}
				ts->rdidx1 += r;
				ts->size1 += r;
				if (ts->rdidx1 == BUFSIZE)
					ts->rdidx1 = 0;
			}

			if (ts->size2 < BUFSIZE && FD_ISSET(ts->ptyfd, &rdfdset)) {
				/* Read from pty to buffer 2.  */
				maxlen = MIN(BUFSIZE - ts->rdidx2,
						BUFSIZE - ts->size2);
				r = read(ts->ptyfd, ts->buf2 + ts->rdidx2, maxlen);
				if (!r || (r < 0 && errno != EINTR)) {
#ifdef CONFIG_FEATURE_TELNETD_INETD
					exit(0);
#else /* CONFIG_FEATURE_TELNETD_INETD */
					free_session(ts);
					ts = next;
					continue;
#endif /* CONFIG_FEATURE_TELNETD_INETD */
				}
				ts->rdidx2 += r;
				ts->size2 += r;
				if (ts->rdidx2 == BUFSIZE)
					ts->rdidx2 = 0;
			}

			if (ts->size1 == 0) {
				ts->rdidx1 = 0;
				ts->wridx1 = 0;
			}
			if (ts->size2 == 0) {
				ts->rdidx2 = 0;
				ts->wridx2 = 0;
			}
#ifndef CONFIG_FEATURE_TELNETD_INETD
			ts = next;
		}
#endif /* CONFIG_FEATURE_TELNETD_INETD */

	} while (1);

	return 0;
}