/* 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 source tree.
 *
 * ---------------------------------------------------------------------------
 * (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
 */
//config:config TELNETD
//config:       bool "telnetd (12 kb)"
//config:       default y
//config:       select FEATURE_SYSLOG
//config:       help
//config:       A daemon for the TELNET protocol, allowing you to log onto the host
//config:       running the daemon. Please keep in mind that the TELNET protocol
//config:       sends passwords in plain text. If you can't afford the space for an
//config:       SSH daemon and you trust your network, you may say 'y' here. As a
//config:       more secure alternative, you should seriously consider installing the
//config:       very small Dropbear SSH daemon instead:
//config:               http://matt.ucc.asn.au/dropbear/dropbear.html
//config:
//config:       Note that for busybox telnetd to work you need several things:
//config:       First of all, your kernel needs:
//config:                 CONFIG_UNIX98_PTYS=y
//config:
//config:       Next, you need a /dev/pts directory on your root filesystem:
//config:
//config:                 $ ls -ld /dev/pts
//config:                 drwxr-xr-x  2 root root 0 Sep 23 13:21 /dev/pts/
//config:
//config:       Next you need the pseudo terminal master multiplexer /dev/ptmx:
//config:
//config:                 $ ls -la /dev/ptmx
//config:                 crw-rw-rw-  1 root tty 5, 2 Sep 23 13:55 /dev/ptmx
//config:
//config:       Any /dev/ttyp[0-9]* files you may have can be removed.
//config:       Next, you need to mount the devpts filesystem on /dev/pts using:
//config:
//config:                 mount -t devpts devpts /dev/pts
//config:
//config:       You need to be sure that busybox has LOGIN and
//config:       FEATURE_SUID enabled. And finally, you should make
//config:       certain that busybox has been installed setuid root:
//config:
//config:               chown root.root /bin/busybox
//config:               chmod 4755 /bin/busybox
//config:
//config:       with all that done, telnetd _should_ work....
//config:
//config:config FEATURE_TELNETD_STANDALONE
//config:       bool "Support standalone telnetd (not inetd only)"
//config:       default y
//config:       depends on TELNETD
//config:       help
//config:       Selecting this will make telnetd able to run standalone.
//config:
//config:config FEATURE_TELNETD_INETD_WAIT
//config:       bool "Support -w SEC option (inetd wait mode)"
//config:       default y
//config:       depends on FEATURE_TELNETD_STANDALONE
//config:       help
//config:       This option allows you to run telnetd in "inet wait" mode.
//config:       Example inetd.conf line (note "wait", not usual "nowait"):
//config:
//config:       telnet stream tcp wait root /bin/telnetd telnetd -w10
//config:
//config:       In this example, inetd passes _listening_ socket_ as fd 0
//config:       to telnetd when connection appears.
//config:       telnetd will wait for connections until all existing
//config:       connections are closed, and no new connections
//config:       appear during 10 seconds. Then it exits, and inetd continues
//config:       to listen for new connections.
//config:
//config:       This option is rarely used. "tcp nowait" is much more usual
//config:       way of running tcp services, including telnetd.
//config:       You most probably want to say N here.

//applet:IF_TELNETD(APPLET(telnetd, BB_DIR_USR_SBIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_TELNETD) += telnetd.o

//usage:#define telnetd_trivial_usage
//usage:       "[OPTIONS]"
//usage:#define telnetd_full_usage "\n\n"
//usage:       "Handle incoming telnet connections"
//usage:        IF_NOT_FEATURE_TELNETD_STANDALONE(" via inetd") "\n"
//usage:     "\n        -l LOGIN        Exec LOGIN on connect"
//usage:     "\n        -f ISSUE_FILE   Display ISSUE_FILE instead of /etc/issue"
//usage:     "\n        -K              Close connection as soon as login exits"
//usage:     "\n                        (normally wait until all programs close slave pty)"
//usage:        IF_FEATURE_TELNETD_STANDALONE(
//usage:     "\n        -p PORT         Port to listen on"
//usage:     "\n        -b ADDR[:PORT]  Address to bind to"
//usage:     "\n        -F              Run in foreground"
//usage:     "\n        -i              Inetd mode"
//usage:        IF_FEATURE_TELNETD_INETD_WAIT(
//usage:     "\n        -w SEC          Inetd 'wait' mode, linger time SEC"
//usage:     "\n        -S              Log to syslog (implied by -i or without -F and -w)"
//usage:        )
//usage:        )

#define DEBUG 0

#include "libbb.h"
#include "common_bufsiz.h"
#include <syslog.h>

#if DEBUG
# define TELCMDS
# define TELOPTS
#endif
#include <arpa/telnet.h>


struct tsession {
        struct tsession *next;
        pid_t shell_pid;
        int sockfd_read;
        int sockfd_write;
        int ptyfd;
        smallint buffered_IAC_for_pty;

        /* two circular buffers */
        /*char *buf1, *buf2;*/
/*#define TS_BUF1(ts) ts->buf1*/
/*#define TS_BUF2(ts) TS_BUF2(ts)*/
#define TS_BUF1(ts) ((unsigned char*)(ts + 1))
#define TS_BUF2(ts) (((unsigned char*)(ts + 1)) + BUFSIZE)
        int rdidx1, wridx1, size1;
        int rdidx2, wridx2, size2;
};

/* Two buffers are directly after tsession in malloced memory.
 * Make whole thing fit in 4k */
enum { BUFSIZE = (4 * 1024 - sizeof(struct tsession)) / 2 };


/* Globals */
struct globals {
        struct tsession *sessions;
        const char *loginpath;
        const char *issuefile;
        int maxfd;
} FIX_ALIASING;
#define G (*(struct globals*)bb_common_bufsiz1)
#define INIT_G() do { \
        setup_common_bufsiz(); \
        G.loginpath = "/bin/login"; \
        G.issuefile = "/etc/issue.net"; \
} while (0)


/* Write some buf1 data to pty, processing IACs.
 * Update wridx1 and size1. Return < 0 on error.
 * Buggy if IAC is present but incomplete: skips them.
 */
static ssize_t
safe_write_to_pty_decode_iac(struct tsession *ts)
{
        unsigned wr;
        ssize_t rc;
        unsigned char *buf;
        unsigned char *found;

        buf = TS_BUF1(ts) + ts->wridx1;
        wr = MIN(BUFSIZE - ts->wridx1, ts->size1);
        /* wr is at least 1 here */

        if (ts->buffered_IAC_for_pty) {
                /* Last time we stopped on a "dangling" IAC byte.
                 * We removed it from the buffer back then.
                 * Now pretend it's still there, and jump to IAC processing.
                 */
                ts->buffered_IAC_for_pty = 0;
                wr++;
                ts->size1++;
                buf--; /* Yes, this can point before the buffer. It's ok */
                ts->wridx1--;
                goto handle_iac;
        }

        found = memchr(buf, IAC, wr);
        if (found != buf) {
                /* There is a "prefix" of non-IAC chars.
                 * Write only them, and return.
                 */
                if (found)
                        wr = found - buf;

                /* We map \r\n ==> \r for pragmatic reasons:
                 * many client implementations send \r\n when
                 * the user hits the CarriageReturn key.
                 * See RFC 1123 3.3.1 Telnet End-of-Line Convention.
                 */
                rc = wr;
                found = memchr(buf, '\r', wr);
                if (found)
                        rc = found - buf + 1;
                rc = safe_write(ts->ptyfd, buf, rc);
                if (rc <= 0)
                        return rc;
                if (rc < wr /* don't look past available data */
                 && buf[rc-1] == '\r' /* need this: imagine that write was _short_ */
                 && (buf[rc] == '\n' || buf[rc] == '\0')
                ) {
                        rc++;
                }
                goto update_and_return;
        }

        /* buf starts with IAC char. Process that sequence.
         * Example: we get this from our own (bbox) telnet client:
         * read(5, "\377\374\1""\377\373\37""\377\372\37\0\262\0@\377\360""\377\375\1""\377\375\3"):
         * IAC WONT ECHO, IAC WILL NAWS, IAC SB NAWS <cols> <rows> IAC SE, IAC DO SGA
         * Another example (telnet-0.17 from old-netkit):
         * read(4, "\377\375\3""\377\373\30""\377\373\37""\377\373 ""\377\373!""\377\373\"""\377\373'"
         * "\377\375\5""\377\373#""\377\374\1""\377\372\37\0\257\0I\377\360""\377\375\1"):
         * IAC DO SGA, IAC WILL TTYPE, IAC WILL NAWS, IAC WILL TSPEED, IAC WILL LFLOW, IAC WILL LINEMODE, IAC WILL NEW_ENVIRON,
         * IAC DO STATUS, IAC WILL XDISPLOC, IAC WONT ECHO, IAC SB NAWS <cols> <rows> IAC SE, IAC DO ECHO
         */
        if (wr <= 1) {
                /* Only the single IAC byte is in the buffer, eat it
                 * and set a flag "process the rest of the sequence
                 * next time we are here".
                 */
                //bb_error_msg("dangling IAC!");
                ts->buffered_IAC_for_pty = 1;
                rc = 1;
                goto update_and_return;
        }

 handle_iac:
        /* 2-byte commands (240..250 and 255):
         * IAC IAC (255) Literal 255. Supported.
         * IAC SE  (240) End of subnegotiation. Treated as NOP.
         * IAC NOP (241) NOP. Supported.
         * IAC BRK (243) Break. Like serial line break. TODO via tcsendbreak()?
         * IAC AYT (246) Are you there. Send back evidence that AYT was seen. TODO (send NOP back)?
         *  These don't look useful:
         * IAC DM  (242) Data mark. What is this?
         * IAC IP  (244) Suspend, interrupt or abort the process. (Ancient cousin of ^C).
         * IAC AO  (245) Abort output. "You can continue running, but do not send me the output".
         * IAC EC  (247) Erase character. The receiver should delete the last received char.
         * IAC EL  (248) Erase line. The receiver should delete everything up tp last newline.
         * IAC GA  (249) Go ahead. For half-duplex lines: "now you talk".
         *  Implemented only as part of NAWS:
         * IAC SB  (250) Subnegotiation of an option follows.
         */
        if (buf[1] == IAC) {
                /* Literal 255 (emacs M-DEL) */
                //bb_error_msg("255!");
                rc = safe_write(ts->ptyfd, &buf[1], 1);
                /*
                 * If we went through buffered_IAC_for_pty==1 path,
                 * bailing out on error like below messes up the buffer.
                 * EAGAIN is highly unlikely here, other errors will be
                 * repeated on next write, let's just skip error check.
                 */
#if 0
                if (rc <= 0)
                        return rc;
#endif
                rc = 2;
                goto update_and_return;
        }
        if (buf[1] >= 240 && buf[1] <= 249) {
                /* NOP (241). Ignore (putty keepalive, etc) */
                /* All other 2-byte commands also treated as NOPs here */
                rc = 2;
                goto update_and_return;
        }

        if (wr <= 2) {
/* BUG: only 2 bytes of the IAC is in the buffer, we just eat them.
 * This is not a practical problem since >2 byte IACs are seen only
 * in initial negotiation, when buffer is empty
 */
                rc = 2;
                goto update_and_return;
        }

        if (buf[1] == SB) {
                if (buf[2] == TELOPT_NAWS) {
                        /* IAC SB, TELOPT_NAWS, 4-byte, IAC SE */
                        struct winsize ws;
                        if (wr <= 6) {
/* BUG: incomplete, can't process */
                                rc = wr;
                                goto update_and_return;
                        }
                        memset(&ws, 0, sizeof(ws)); /* pixel sizes are set to 0 */
                        ws.ws_col = (buf[3] << 8) | buf[4];
                        ws.ws_row = (buf[5] << 8) | buf[6];
                        ioctl(ts->ptyfd, TIOCSWINSZ, (char *)&ws);
                        rc = 7;
                        /* trailing IAC SE will be eaten separately, as 2-byte NOP */
                        goto update_and_return;
                }
                /* else: other subnegs not supported yet */
        }

        /* Assume it is a 3-byte WILL/WONT/DO/DONT 251..254 command and skip it */
#if DEBUG
        fprintf(stderr, "Ignoring IAC %s,%s\n",
                        TELCMD(buf[1]), TELOPT(buf[2]));
#endif
        rc = 3;

 update_and_return:
        ts->wridx1 += rc;
        if (ts->wridx1 >= BUFSIZE) /* actually == BUFSIZE */
                ts->wridx1 = 0;
        ts->size1 -= rc;
        /*
         * Hack. We cannot process IACs which wrap around buffer's end.
         * Since properly fixing it requires writing bigger code,
         * we rely instead on this code making it virtually impossible
         * to have wrapped IAC (people don't type at 2k/second).
         * It also allows for bigger reads in common case.
         */
        if (ts->size1 == 0) { /* very typical */
                //bb_error_msg("zero size1");
                ts->rdidx1 = 0;
                ts->wridx1 = 0;
                return rc;
        }
        wr = ts->wridx1;
        if (wr != 0 && wr < ts->rdidx1) {
                /* Buffer is not wrapped yet.
                 * We can easily move it to the beginning.
                 */
                //bb_error_msg("moved %d", wr);
                memmove(TS_BUF1(ts), TS_BUF1(ts) + wr, ts->size1);
                ts->rdidx1 -= wr;
                ts->wridx1 = 0;
        }
        return rc;
}

/*
 * Converting single IAC into double on output
 */
static size_t safe_write_double_iac(int fd, const char *buf, size_t count)
{
        const char *IACptr;
        size_t wr, rc, total;

        total = 0;
        while (1) {
                if (count == 0)
                        return total;
                if (*buf == (char)IAC) {
                        static const char IACIAC[] ALIGN1 = { IAC, IAC };
                        rc = safe_write(fd, IACIAC, 2);
/* BUG: if partial write was only 1 byte long, we end up emitting just one IAC */
                        if (rc != 2)
                                break;
                        buf++;
                        total++;
                        count--;
                        continue;
                }
                /* count != 0, *buf != IAC */
                IACptr = memchr(buf, IAC, count);
                wr = count;
                if (IACptr)
                        wr = IACptr - buf;
                rc = safe_write(fd, buf, wr);
                if (rc != wr)
                        break;
                buf += rc;
                total += rc;
                count -= rc;
        }
        /* here: rc - result of last short write */
        if ((ssize_t)rc < 0) { /* error? */
                if (total == 0)
                        return rc;
                rc = 0;
        }
        return total + rc;
}

/* Must match getopt32 string */
enum {
        OPT_WATCHCHILD = (1 << 2), /* -K */
        OPT_INETD      = (1 << 3) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -i */
        OPT_PORT       = (1 << 4) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -p PORT */
        OPT_FOREGROUND = (1 << 6) * ENABLE_FEATURE_TELNETD_STANDALONE, /* -F */
        OPT_SYSLOG     = (1 << 7) * ENABLE_FEATURE_TELNETD_INETD_WAIT, /* -S */
        OPT_WAIT       = (1 << 8) * ENABLE_FEATURE_TELNETD_INETD_WAIT, /* -w SEC */
};

static struct tsession *
make_new_session(
                IF_FEATURE_TELNETD_STANDALONE(int sock)
                IF_NOT_FEATURE_TELNETD_STANDALONE(void)
) {
#if !ENABLE_FEATURE_TELNETD_STANDALONE
        enum { sock = 0 };
#endif
        const char *login_argv[2];
        struct termios termbuf;
        int fd, pid;
        char tty_name[GETPTY_BUFSIZE];
        struct tsession *ts = xzalloc(sizeof(struct tsession) + BUFSIZE * 2);

        /*ts->buf1 = (char *)(ts + 1);*/
        /*ts->buf2 = ts->buf1 + BUFSIZE;*/

        /* Got a new connection, set up a tty */
        fd = xgetpty(tty_name);
        if (fd > G.maxfd)
                G.maxfd = fd;
        ts->ptyfd = fd;
        ndelay_on(fd);
        close_on_exec_on(fd);

        /* SO_KEEPALIVE by popular demand */
        setsockopt_keepalive(sock);
#if ENABLE_FEATURE_TELNETD_STANDALONE
        ts->sockfd_read = sock;
        ndelay_on(sock);
        if (sock == 0) { /* We are called with fd 0 - we are in inetd mode */
                sock++; /* so use fd 1 for output */
                ndelay_on(sock);
        }
        ts->sockfd_write = sock;
        if (sock > G.maxfd)
                G.maxfd = sock;
#else
        /* ts->sockfd_read = 0; - done by xzalloc */
        ts->sockfd_write = 1;
        ndelay_on(0);
        ndelay_on(1);
#endif

        /* 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. */
        {
                static const char iacs_to_send[] ALIGN1 = {
                        IAC, DO, TELOPT_ECHO,
                        IAC, DO, TELOPT_NAWS,
                        /* This requires telnetd.ctrlSQ.patch (incomplete) */
                        /*IAC, DO, TELOPT_LFLOW,*/
                        IAC, WILL, TELOPT_ECHO,
                        IAC, WILL, TELOPT_SGA
                };
                /* This confuses safe_write_double_iac(), it will try to duplicate
                 * each IAC... */
                //memcpy(TS_BUF2(ts), iacs_to_send, sizeof(iacs_to_send));
                //ts->rdidx2 = sizeof(iacs_to_send);
                //ts->size2 = sizeof(iacs_to_send);
                /* So just stuff it into TCP stream! (no error check...) */
#if ENABLE_FEATURE_TELNETD_STANDALONE
                safe_write(sock, iacs_to_send, sizeof(iacs_to_send));
#else
                safe_write(1, iacs_to_send, sizeof(iacs_to_send));
#endif
                /*ts->rdidx2 = 0; - xzalloc did it */
                /*ts->size2 = 0;*/
        }

        fflush_all();
        pid = vfork(); /* NOMMU-friendly */
        if (pid < 0) {
                free(ts);
                close(fd);
                /* sock will be closed by caller */
                bb_perror_msg("vfork");
                return NULL;
        }
        if (pid > 0) {
                /* Parent */
                ts->shell_pid = pid;
                return ts;
        }

        /* Child */
        /* Careful - we are after vfork! */

        /* Restore default signal handling ASAP */
        bb_signals((1 << SIGCHLD) + (1 << SIGPIPE), SIG_DFL);

        pid = getpid();

        if (ENABLE_FEATURE_UTMP) {
                len_and_sockaddr *lsa = get_peer_lsa(sock);
                char *hostname = NULL;
                if (lsa) {
                        hostname = xmalloc_sockaddr2dotted(&lsa->u.sa);
                        free(lsa);
                }
                write_new_utmp(pid, LOGIN_PROCESS, tty_name, /*username:*/ "LOGIN", hostname);
                free(hostname);
        }

        /* Make new session and process group */
        setsid();

        /* Open the child's side of the tty */
        /* NB: setsid() disconnects from any previous ctty's. Therefore
         * we must open child's side of the tty AFTER setsid! */
        close(0);
        xopen(tty_name, O_RDWR); /* becomes our ctty */
        xdup2(0, 1);
        xdup2(0, 2);
        tcsetpgrp(0, pid); /* switch this tty's process group to us */

        /* 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_stdin_TCSANOW(&termbuf);

        /* Uses FILE-based I/O to stdout, but does fflush_all(),
         * so should be safe with vfork.
         * I fear, though, that some users will have ridiculously big
         * issue files, and they may block writing to fd 1,
         * (parent is supposed to read it, but parent waits
         * for vforked child to exec!) */
        print_login_issue(G.issuefile, tty_name);

        /* Exec shell / login / whatever */
        login_argv[0] = G.loginpath;
        login_argv[1] = NULL;
        /* exec busybox applet (if PREFER_APPLETS=y), if that fails,
         * exec external program.
         * NB: sock is either 0 or has CLOEXEC set on it.
         * fd has CLOEXEC set on it too. These two fds will be closed here.
         */
        BB_EXECVP(G.loginpath, (char **)login_argv);
        /* _exit is safer with vfork, and we shouldn't send message
         * to remote clients anyway */
        _exit(EXIT_FAILURE); /*bb_perror_msg_and_die("execv %s", G.loginpath);*/
}

#if ENABLE_FEATURE_TELNETD_STANDALONE

static void
free_session(struct tsession *ts)
{
        struct tsession *t;

        if (option_mask32 & OPT_INETD)
                exit(EXIT_SUCCESS);

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

#if 0
        /* It was said that "normal" telnetd just closes ptyfd,
         * doesn't send SIGKILL. When we close ptyfd,
         * kernel sends SIGHUP to processes having slave side opened. */
        kill(ts->shell_pid, SIGKILL);
        waitpid(ts->shell_pid, NULL, 0);
#endif
        close(ts->ptyfd);
        close(ts->sockfd_read);
        /* We do not need to close(ts->sockfd_write), it's the same
         * as sockfd_read unless we are in inetd mode. But in inetd mode
         * we do not reach this */
        free(ts);

        /* Scan all sessions and find new maxfd */
        G.maxfd = 0;
        ts = G.sessions;
        while (ts) {
                if (G.maxfd < ts->ptyfd)
                        G.maxfd = ts->ptyfd;
                if (G.maxfd < ts->sockfd_read)
                        G.maxfd = ts->sockfd_read;
#if 0
                /* Again, sockfd_write == sockfd_read here */
                if (G.maxfd < ts->sockfd_write)
                        G.maxfd = ts->sockfd_write;
#endif
                ts = ts->next;
        }
}

#else /* !FEATURE_TELNETD_STANDALONE */

/* Used in main() only, thus "return 0" actually is exit(EXIT_SUCCESS). */
#define free_session(ts) return 0

#endif

static void handle_sigchld(int sig UNUSED_PARAM)
{
        pid_t pid;
        struct tsession *ts;
        int save_errno = errno;

        /* Looping: more than one child may have exited */
        while (1) {
                pid = wait_any_nohang(NULL);
                if (pid <= 0)
                        break;
                ts = G.sessions;
                while (ts) {
                        if (ts->shell_pid == pid) {
                                ts->shell_pid = -1;
                                update_utmp_DEAD_PROCESS(pid);
                                break;
                        }
                        ts = ts->next;
                }
        }

        errno = save_errno;
}

int telnetd_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int telnetd_main(int argc UNUSED_PARAM, char **argv)
{
        fd_set rdfdset, wrfdset;
        unsigned opt;
        int count;
        struct tsession *ts;
#if ENABLE_FEATURE_TELNETD_STANDALONE
#define IS_INETD (opt & OPT_INETD)
        int master_fd = master_fd; /* for compiler */
        int sec_linger = sec_linger;
        char *opt_bindaddr = NULL;
        char *opt_portnbr;
#else
        enum {
                IS_INETD = 1,
                master_fd = -1,
        };
#endif
        INIT_G();

        /* Even if !STANDALONE, we accept (and ignore) -i, thus people
         * don't need to guess whether it's ok to pass -i to us */
        opt = getopt32(argv, "^"
                        "f:l:Ki"
                        IF_FEATURE_TELNETD_STANDALONE("p:b:F")
                        IF_FEATURE_TELNETD_INETD_WAIT("Sw:+") /* -w NUM */
                        "\0"
                        /* -w implies -F. -w and -i don't mix */
                        IF_FEATURE_TELNETD_INETD_WAIT("wF:i--w:w--i"),
                        &G.issuefile, &G.loginpath
                        IF_FEATURE_TELNETD_STANDALONE(, &opt_portnbr, &opt_bindaddr)
                        IF_FEATURE_TELNETD_INETD_WAIT(, &sec_linger)
        );
        if (!IS_INETD /*&& !re_execed*/) {
                /* inform that we start in standalone mode?
                 * May be useful when people forget to give -i */
                /*bb_error_msg("listening for connections");*/
                if (!(opt & OPT_FOREGROUND)) {
                        /* DAEMON_CHDIR_ROOT was giving inconsistent
                         * behavior with/without -F, -i */
                        bb_daemonize_or_rexec(0 /*was DAEMON_CHDIR_ROOT*/, argv);
                }
        }
        /* Redirect log to syslog early, if needed */
        if (IS_INETD || (opt & OPT_SYSLOG) || !(opt & OPT_FOREGROUND)) {
                openlog(applet_name, LOG_PID, LOG_DAEMON);
                logmode = LOGMODE_SYSLOG;
        }
#if ENABLE_FEATURE_TELNETD_STANDALONE
        if (IS_INETD) {
                G.sessions = make_new_session(0);
                if (!G.sessions) /* pty opening or vfork problem, exit */
                        return 1; /* make_new_session printed error message */
        } else {
                master_fd = 0;
                if (!(opt & OPT_WAIT)) {
                        unsigned portnbr = 23;
                        if (opt & OPT_PORT)
                                portnbr = xatou16(opt_portnbr);
                        master_fd = create_and_bind_stream_or_die(opt_bindaddr, portnbr);
                        xlisten(master_fd, 1);
                }
                close_on_exec_on(master_fd);
        }
#else
        G.sessions = make_new_session();
        if (!G.sessions) /* pty opening or vfork problem, exit */
                return 1; /* make_new_session printed error message */
#endif

        /* We don't want to die if just one session is broken */
        signal(SIGPIPE, SIG_IGN);

        if (opt & OPT_WATCHCHILD)
                signal(SIGCHLD, handle_sigchld);
        else /* prevent dead children from becoming zombies */
                signal(SIGCHLD, SIG_IGN);

/*
   This is how the buffers are used. The arrows indicate data flow.

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

   size1: "how many bytes are buffered for pty between rdidx1 and wridx1?"
   size2: "how many bytes are buffered for socket between rdidx2 and wridx2?"

   Each session has got two buffers. Buffers are circular. If sizeN == 0,
   buffer is empty. If sizeN == BUFSIZE, buffer is full. In both these cases
   rdidxN == wridxN.
*/
 again:
        FD_ZERO(&rdfdset);
        FD_ZERO(&wrfdset);

        /* Select on the master socket, all telnet sockets and their
         * ptys if there is room in their session buffers.
         * NB: scalability problem: we recalculate entire bitmap
         * before each select. Can be a problem with 500+ connections. */
        ts = G.sessions;
        while (ts) {
                struct tsession *next = ts->next; /* in case we free ts */
                if (ts->shell_pid == -1) {
                        /* Child died and we detected that */
                        free_session(ts);
                } else {
                        if (ts->size1 > 0)       /* can write to pty */
                                FD_SET(ts->ptyfd, &wrfdset);
                        if (ts->size1 < BUFSIZE) /* can read from socket */
                                FD_SET(ts->sockfd_read, &rdfdset);
                        if (ts->size2 > 0)       /* can write to socket */
                                FD_SET(ts->sockfd_write, &wrfdset);
                        if (ts->size2 < BUFSIZE) /* can read from pty */
                                FD_SET(ts->ptyfd, &rdfdset);
                }
                ts = next;
        }
        if (!IS_INETD) {
                FD_SET(master_fd, &rdfdset);
                /* This is needed because free_session() does not
                 * take master_fd into account when it finds new
                 * maxfd among remaining fd's */
                if (master_fd > G.maxfd)
                        G.maxfd = master_fd;
        }

        {
                struct timeval *tv_ptr = NULL;
#if ENABLE_FEATURE_TELNETD_INETD_WAIT
                struct timeval tv;
                if ((opt & OPT_WAIT) && !G.sessions) {
                        tv.tv_sec = sec_linger;
                        tv.tv_usec = 0;
                        tv_ptr = &tv;
                }
#endif
                count = select(G.maxfd + 1, &rdfdset, &wrfdset, NULL, tv_ptr);
        }
        if (count == 0) /* "telnetd -w SEC" timed out */
                return 0;
        if (count < 0)
                goto again; /* EINTR or ENOMEM */

#if ENABLE_FEATURE_TELNETD_STANDALONE
        /* Check for and accept new sessions */
        if (!IS_INETD && FD_ISSET(master_fd, &rdfdset)) {
                int fd;
                struct tsession *new_ts;

                fd = accept(master_fd, NULL, NULL);
                if (fd < 0)
                        goto again;
                close_on_exec_on(fd);

                /* Create a new session and link it into active list */
                new_ts = make_new_session(fd);
                if (new_ts) {
                        new_ts->next = G.sessions;
                        G.sessions = new_ts;
                } else {
                        close(fd);
                }
        }
#endif

        /* Then check for data tunneling */
        ts = G.sessions;
        while (ts) { /* For all sessions... */
                struct tsession *next = ts->next; /* in case we free ts */

                if (/*ts->size1 &&*/ FD_ISSET(ts->ptyfd, &wrfdset)) {
                        /* Write to pty from buffer 1 */
                        count = safe_write_to_pty_decode_iac(ts);
                        if (count < 0) {
                                if (errno == EAGAIN)
                                        goto skip1;
                                goto kill_session;
                        }
                }
 skip1:
                if (/*ts->size2 &&*/ FD_ISSET(ts->sockfd_write, &wrfdset)) {
                        /* Write to socket from buffer 2 */
                        count = MIN(BUFSIZE - ts->wridx2, ts->size2);
                        count = safe_write_double_iac(ts->sockfd_write, (void*)(TS_BUF2(ts) + ts->wridx2), count);
                        if (count < 0) {
                                if (errno == EAGAIN)
                                        goto skip2;
                                goto kill_session;
                        }
                        ts->wridx2 += count;
                        if (ts->wridx2 >= BUFSIZE) /* actually == BUFSIZE */
                                ts->wridx2 = 0;
                        ts->size2 -= count;
                        if (ts->size2 == 0) {
                                ts->rdidx2 = 0;
                                ts->wridx2 = 0;
                        }
                }
 skip2:

                if (/*ts->size1 < BUFSIZE &&*/ FD_ISSET(ts->sockfd_read, &rdfdset)) {
                        /* Read from socket to buffer 1 */
                        count = MIN(BUFSIZE - ts->rdidx1, BUFSIZE - ts->size1);
                        count = safe_read(ts->sockfd_read, TS_BUF1(ts) + ts->rdidx1, count);
                        if (count <= 0) {
                                if (count < 0 && errno == EAGAIN)
                                        goto skip3;
                                goto kill_session;
                        }
                        /* Ignore trailing NUL if it is there */
                        if (!TS_BUF1(ts)[ts->rdidx1 + count - 1]) {
                                --count;
                        }
                        ts->size1 += count;
                        ts->rdidx1 += count;
                        if (ts->rdidx1 >= BUFSIZE) /* actually == BUFSIZE */
                                ts->rdidx1 = 0;
                }
 skip3:
                if (/*ts->size2 < BUFSIZE &&*/ FD_ISSET(ts->ptyfd, &rdfdset)) {
                        /* Read from pty to buffer 2 */
                        count = MIN(BUFSIZE - ts->rdidx2, BUFSIZE - ts->size2);
                        count = safe_read(ts->ptyfd, TS_BUF2(ts) + ts->rdidx2, count);
                        if (count <= 0) {
                                if (count < 0 && errno == EAGAIN)
                                        goto skip4;
                                goto kill_session;
                        }
                        ts->size2 += count;
                        ts->rdidx2 += count;
                        if (ts->rdidx2 >= BUFSIZE) /* actually == BUFSIZE */
                                ts->rdidx2 = 0;
                }
 skip4:
                ts = next;
                continue;
 kill_session:
                if (ts->shell_pid > 0)
                        update_utmp_DEAD_PROCESS(ts->shell_pid);
                free_session(ts);
                ts = next;
        }

        goto again;
}