telnet.c

/*
 * Telnet backend.
 */

#include <stdio.h>
#include <stdlib.h>

#include "putty.h"

#ifndef FALSE
#define FALSE 0
#endif
#ifndef TRUE
#define TRUE 1
#endif

#define	IAC	255		       /* interpret as command: */
#define	DONT	254		       /* you are not to use option */
#define	DO	253		       /* please, you use option */
#define	WONT	252		       /* I won't use option */
#define	WILL	251		       /* I will use option */
#define	SB	250		       /* interpret as subnegotiation */
#define	SE	240		       /* end sub negotiation */

#define GA      249		       /* you may reverse the line */
#define EL      248		       /* erase the current line */
#define EC      247		       /* erase the current character */
#define	AYT	246		       /* are you there */
#define	AO	245		       /* abort output--but let prog finish */
#define	IP	244		       /* interrupt process--permanently */
#define	BREAK	243		       /* break */
#define DM      242		       /* data mark--for connect. cleaning */
#define NOP     241		       /* nop */
#define EOR     239		       /* end of record (transparent mode) */
#define ABORT   238		       /* Abort process */
#define SUSP    237		       /* Suspend process */
#define xEOF    236		       /* End of file: EOF is already used... */

#define TELOPTS(X) \
    X(BINARY, 0)                       /* 8-bit data path */ \
    X(ECHO, 1)                         /* echo */ \
    X(RCP, 2)                          /* prepare to reconnect */ \
    X(SGA, 3)                          /* suppress go ahead */ \
    X(NAMS, 4)                         /* approximate message size */ \
    X(STATUS, 5)                       /* give status */ \
    X(TM, 6)                           /* timing mark */ \
    X(RCTE, 7)                         /* remote controlled transmission and echo */ \
    X(NAOL, 8)                         /* negotiate about output line width */ \
    X(NAOP, 9)                         /* negotiate about output page size */ \
    X(NAOCRD, 10)                      /* negotiate about CR disposition */ \
    X(NAOHTS, 11)                      /* negotiate about horizontal tabstops */ \
    X(NAOHTD, 12)                      /* negotiate about horizontal tab disposition */ \
    X(NAOFFD, 13)                      /* negotiate about formfeed disposition */ \
    X(NAOVTS, 14)                      /* negotiate about vertical tab stops */ \
    X(NAOVTD, 15)                      /* negotiate about vertical tab disposition */ \
    X(NAOLFD, 16)                      /* negotiate about output LF disposition */ \
    X(XASCII, 17)                      /* extended ascic character set */ \
    X(LOGOUT, 18)                      /* force logout */ \
    X(BM, 19)                          /* byte macro */ \
    X(DET, 20)                         /* data entry terminal */ \
    X(SUPDUP, 21)                      /* supdup protocol */ \
    X(SUPDUPOUTPUT, 22)                /* supdup output */ \
    X(SNDLOC, 23)                      /* send location */ \
    X(TTYPE, 24)                       /* terminal type */ \
    X(EOR, 25)                         /* end or record */ \
    X(TUID, 26)                        /* TACACS user identification */ \
    X(OUTMRK, 27)                      /* output marking */ \
    X(TTYLOC, 28)                      /* terminal location number */ \
    X(3270REGIME, 29)                  /* 3270 regime */ \
    X(X3PAD, 30)                       /* X.3 PAD */ \
    X(NAWS, 31)                        /* window size */ \
    X(TSPEED, 32)                      /* terminal speed */ \
    X(LFLOW, 33)                       /* remote flow control */ \
    X(LINEMODE, 34)                    /* Linemode option */ \
    X(XDISPLOC, 35)                    /* X Display Location */ \
    X(OLD_ENVIRON, 36)                 /* Old - Environment variables */ \
    X(AUTHENTICATION, 37)              /* Authenticate */ \
    X(ENCRYPT, 38)                     /* Encryption option */ \
    X(NEW_ENVIRON, 39)                 /* New - Environment variables */ \
    X(TN3270E, 40)                     /* TN3270 enhancements */ \
    X(XAUTH, 41)                       \
    X(CHARSET, 42)                     /* Character set */ \
    X(RSP, 43)                         /* Remote serial port */ \
    X(COM_PORT_OPTION, 44)             /* Com port control */ \
    X(SLE, 45)                         /* Suppress local echo */ \
    X(STARTTLS, 46)                    /* Start TLS */ \
    X(KERMIT, 47)                      /* Automatic Kermit file transfer */ \
    X(SEND_URL, 48)                    \
    X(FORWARD_X, 49)                   \
    X(PRAGMA_LOGON, 138)               \
    X(SSPI_LOGON, 139)                 \
    X(PRAGMA_HEARTBEAT, 140)           \
    X(EXOPL, 255)                      /* extended-options-list */

#define telnet_enum(x,y) TELOPT_##x = y,
enum { TELOPTS(telnet_enum) dummy=0 };
#undef telnet_enum

#define	TELQUAL_IS	0	       /* option is... */
#define	TELQUAL_SEND	1	       /* send option */
#define	TELQUAL_INFO	2	       /* ENVIRON: informational version of IS */
#define BSD_VAR 1
#define BSD_VALUE 0
#define RFC_VAR 0
#define RFC_VALUE 1

#define CR 13
#define LF 10
#define NUL 0

#define iswritable(x) \
	( (x) != IAC && \
	      (telnet->opt_states[o_we_bin.index] == ACTIVE || (x) != CR))

static char *telopt(int opt)
{
#define telnet_str(x,y) case TELOPT_##x: return #x;
    switch (opt) {
	TELOPTS(telnet_str)
      default:
	return "<unknown>";
    }
#undef telnet_str
}

static void telnet_size(void *handle, int width, int height);

struct Opt {
    int send;			       /* what we initially send */
    int nsend;			       /* -ve send if requested to stop it */
    int ack, nak;		       /* +ve and -ve acknowledgements */
    int option;			       /* the option code */
    int index;			       /* index into telnet->opt_states[] */
    enum {
	REQUESTED, ACTIVE, INACTIVE, REALLY_INACTIVE
    } initial_state;
};

enum {
    OPTINDEX_NAWS,
    OPTINDEX_TSPEED,
    OPTINDEX_TTYPE,
    OPTINDEX_OENV,
    OPTINDEX_NENV,
    OPTINDEX_ECHO,
    OPTINDEX_WE_SGA,
    OPTINDEX_THEY_SGA,
    OPTINDEX_WE_BIN,
    OPTINDEX_THEY_BIN,
    NUM_OPTS
};

static const struct Opt o_naws =
    { WILL, WONT, DO, DONT, TELOPT_NAWS, OPTINDEX_NAWS, REQUESTED };
static const struct Opt o_tspeed =
    { WILL, WONT, DO, DONT, TELOPT_TSPEED, OPTINDEX_TSPEED, REQUESTED };
static const struct Opt o_ttype =
    { WILL, WONT, DO, DONT, TELOPT_TTYPE, OPTINDEX_TTYPE, REQUESTED };
static const struct Opt o_oenv =
    { WILL, WONT, DO, DONT, TELOPT_OLD_ENVIRON, OPTINDEX_OENV, INACTIVE };
static const struct Opt o_nenv =
    { WILL, WONT, DO, DONT, TELOPT_NEW_ENVIRON, OPTINDEX_NENV, REQUESTED };
static const struct Opt o_echo =
    { DO, DONT, WILL, WONT, TELOPT_ECHO, OPTINDEX_ECHO, REQUESTED };
static const struct Opt o_we_sga =
    { WILL, WONT, DO, DONT, TELOPT_SGA, OPTINDEX_WE_SGA, REQUESTED };
static const struct Opt o_they_sga =
    { DO, DONT, WILL, WONT, TELOPT_SGA, OPTINDEX_THEY_SGA, REQUESTED };
static const struct Opt o_we_bin =
    { WILL, WONT, DO, DONT, TELOPT_BINARY, OPTINDEX_WE_BIN, INACTIVE };
static const struct Opt o_they_bin =
    { DO, DONT, WILL, WONT, TELOPT_BINARY, OPTINDEX_THEY_BIN, INACTIVE };

static const struct Opt *const opts[] = {
    &o_naws, &o_tspeed, &o_ttype, &o_oenv, &o_nenv, &o_echo,
    &o_we_sga, &o_they_sga, &o_we_bin, &o_they_bin, NULL
};

typedef struct telnet_tag {
    const struct plug_function_table *fn;
    /* the above field _must_ be first in the structure */

    Socket s;

    void *frontend;
    void *ldisc;
    int term_width, term_height;

    int opt_states[NUM_OPTS];

    int echoing, editing;
    int activated;
    int bufsize;
    int in_synch;
    int sb_opt, sb_len;
    unsigned char *sb_buf;
    int sb_size;

    enum {
	TOP_LEVEL, SEENIAC, SEENWILL, SEENWONT, SEENDO, SEENDONT,
	    SEENSB, SUBNEGOT, SUBNEG_IAC, SEENCR
    } state;

    Config cfg;

    Pinger pinger;
} *Telnet;

#define TELNET_MAX_BACKLOG 4096

#define SB_DELTA 1024

static void c_write(Telnet telnet, char *buf, int len)
{
    int backlog;
    backlog = from_backend(telnet->frontend, 0, buf, len);
    sk_set_frozen(telnet->s, backlog > TELNET_MAX_BACKLOG);
    exec_autocmd(telnet, &telnet->cfg, buf, len, telnet_backend.send);
}

static void log_option(Telnet telnet, char *sender, int cmd, int option)
{
    char *buf;
    /*
     * The strange-looking "<?""?>" below is there to avoid a
     * trigraph - a double question mark followed by > maps to a
     * closing brace character!
     */
    buf = dupprintf("%s:\t%s %s", sender,
		    (cmd == WILL ? "WILL" : cmd == WONT ? "WONT" :
		     cmd == DO ? "DO" : cmd == DONT ? "DONT" : "<?""?>"),
		    telopt(option));
    logevent(telnet->frontend, buf);
    sfree(buf);
}

static void send_opt(Telnet telnet, int cmd, int option)
{
    unsigned char b[3];

    b[0] = IAC;
    b[1] = cmd;
    b[2] = option;
    telnet->bufsize = sk_write(telnet->s, (char *)b, 3);
    log_option(telnet, "client", cmd, option);
}

static void deactivate_option(Telnet telnet, const struct Opt *o)
{
    if (telnet->opt_states[o->index] == REQUESTED ||
	telnet->opt_states[o->index] == ACTIVE)
	send_opt(telnet, o->nsend, o->option);
    telnet->opt_states[o->index] = REALLY_INACTIVE;
}

/*
 * Generate side effects of enabling or disabling an option.
 */
static void option_side_effects(Telnet telnet, const struct Opt *o, int enabled)
{
    if (o->option == TELOPT_ECHO && o->send == DO)
	telnet->echoing = !enabled;
    else if (o->option == TELOPT_SGA && o->send == DO)
	telnet->editing = !enabled;
    if (telnet->ldisc)		       /* cause ldisc to notice the change */
	ldisc_send(telnet->ldisc, NULL, 0, 0);

    /* Ensure we get the minimum options */
    if (!telnet->activated) {
	if (telnet->opt_states[o_echo.index] == INACTIVE) {
	    telnet->opt_states[o_echo.index] = REQUESTED;
	    send_opt(telnet, o_echo.send, o_echo.option);
	}
	if (telnet->opt_states[o_we_sga.index] == INACTIVE) {
	    telnet->opt_states[o_we_sga.index] = REQUESTED;
	    send_opt(telnet, o_we_sga.send, o_we_sga.option);
	}
	if (telnet->opt_states[o_they_sga.index] == INACTIVE) {
	    telnet->opt_states[o_they_sga.index] = REQUESTED;
	    send_opt(telnet, o_they_sga.send, o_they_sga.option);
	}
	telnet->activated = TRUE;
    }
}

static void activate_option(Telnet telnet, const struct Opt *o)
{
    if (o->send == WILL && o->option == TELOPT_NAWS)
	telnet_size(telnet, telnet->term_width, telnet->term_height);
    if (o->send == WILL &&
	(o->option == TELOPT_NEW_ENVIRON ||
	 o->option == TELOPT_OLD_ENVIRON)) {
	/*
	 * We may only have one kind of ENVIRON going at a time.
	 * This is a hack, but who cares.
	 */
	deactivate_option(telnet, o->option ==
			  TELOPT_NEW_ENVIRON ? &o_oenv : &o_nenv);
    }
    option_side_effects(telnet, o, 1);
}

static void refused_option(Telnet telnet, const struct Opt *o)
{
    if (o->send == WILL && o->option == TELOPT_NEW_ENVIRON &&
	telnet->opt_states[o_oenv.index] == INACTIVE) {
	send_opt(telnet, WILL, TELOPT_OLD_ENVIRON);
	telnet->opt_states[o_oenv.index] = REQUESTED;
    }
    option_side_effects(telnet, o, 0);
}

static void proc_rec_opt(Telnet telnet, int cmd, int option)
{
    const struct Opt *const *o;

    log_option(telnet, "server", cmd, option);
    for (o = opts; *o; o++) {
	if ((*o)->option == option && (*o)->ack == cmd) {
	    switch (telnet->opt_states[(*o)->index]) {
	      case REQUESTED:
		telnet->opt_states[(*o)->index] = ACTIVE;
		activate_option(telnet, *o);
		break;
	      case ACTIVE:
		break;
	      case INACTIVE:
		telnet->opt_states[(*o)->index] = ACTIVE;
		send_opt(telnet, (*o)->send, option);
		activate_option(telnet, *o);
		break;
	      case REALLY_INACTIVE:
		send_opt(telnet, (*o)->nsend, option);
		break;
	    }
	    return;
	} else if ((*o)->option == option && (*o)->nak == cmd) {
	    switch (telnet->opt_states[(*o)->index]) {
	      case REQUESTED:
		telnet->opt_states[(*o)->index] = INACTIVE;
		refused_option(telnet, *o);
		break;
	      case ACTIVE:
		telnet->opt_states[(*o)->index] = INACTIVE;
		send_opt(telnet, (*o)->nsend, option);
		option_side_effects(telnet, *o, 0);
		break;
	      case INACTIVE:
	      case REALLY_INACTIVE:
		break;
	    }
	    return;
	}
    }
    /*
     * If we reach here, the option was one we weren't prepared to
     * cope with. If the request was positive (WILL or DO), we send
     * a negative ack to indicate refusal. If the request was
     * negative (WONT / DONT), we must do nothing.
     */
    if (cmd == WILL || cmd == DO)
        send_opt(telnet, (cmd == WILL ? DONT : WONT), option);
}

static void process_subneg(Telnet telnet)
{
    unsigned char b[2048], *p, *q;
    int var, value, n;
    char *e;

    switch (telnet->sb_opt) {
      case TELOPT_TSPEED:
	if (telnet->sb_len == 1 && telnet->sb_buf[0] == TELQUAL_SEND) {
	    char *logbuf;
	    b[0] = IAC;
	    b[1] = SB;
	    b[2] = TELOPT_TSPEED;
	    b[3] = TELQUAL_IS;
	    strcpy((char *)(b + 4), telnet->cfg.termspeed);
	    n = 4 + strlen(telnet->cfg.termspeed);
	    b[n] = IAC;
	    b[n + 1] = SE;
	    telnet->bufsize = sk_write(telnet->s, (char *)b, n + 2);
	    logevent(telnet->frontend, "server:\tSB TSPEED SEND");
	    logbuf = dupprintf("client:\tSB TSPEED IS %s", telnet->cfg.termspeed);
	    logevent(telnet->frontend, logbuf);
	    sfree(logbuf);
	} else
	    logevent(telnet->frontend, "server:\tSB TSPEED <something weird>");
	break;
      case TELOPT_TTYPE:
	if (telnet->sb_len == 1 && telnet->sb_buf[0] == TELQUAL_SEND) {
	    char *logbuf;
	    b[0] = IAC;
	    b[1] = SB;
	    b[2] = TELOPT_TTYPE;
	    b[3] = TELQUAL_IS;
	    for (n = 0; telnet->cfg.termtype[n]; n++)
		b[n + 4] = (telnet->cfg.termtype[n] >= 'a'
			    && telnet->cfg.termtype[n] <=
			    'z' ? telnet->cfg.termtype[n] + 'A' -
			    'a' : telnet->cfg.termtype[n]);
	    b[n + 4] = IAC;
	    b[n + 5] = SE;
	    telnet->bufsize = sk_write(telnet->s, (char *)b, n + 6);
	    b[n + 4] = 0;
	    logevent(telnet->frontend, "server:\tSB TTYPE SEND");
	    logbuf = dupprintf("client:\tSB TTYPE IS %s", b + 4);
	    logevent(telnet->frontend, logbuf);
	    sfree(logbuf);
	} else
	    logevent(telnet->frontend, "server:\tSB TTYPE <something weird>\r\n");
	break;
      case TELOPT_OLD_ENVIRON:
      case TELOPT_NEW_ENVIRON:
	p = telnet->sb_buf;
	q = p + telnet->sb_len;
	if (p < q && *p == TELQUAL_SEND) {
	    char *logbuf;
	    p++;
	    logbuf = dupprintf("server:\tSB %s SEND", telopt(telnet->sb_opt));
	    logevent(telnet->frontend, logbuf);
	    sfree(logbuf);
	    if (telnet->sb_opt == TELOPT_OLD_ENVIRON) {
		if (telnet->cfg.rfc_environ) {
		    value = RFC_VALUE;
		    var = RFC_VAR;
		} else {
		    value = BSD_VALUE;
		    var = BSD_VAR;
		}
		/*
		 * Try to guess the sense of VAR and VALUE.
		 */
		while (p < q) {
		    if (*p == RFC_VAR) {
			value = RFC_VALUE;
			var = RFC_VAR;
		    } else if (*p == BSD_VAR) {
			value = BSD_VALUE;
			var = BSD_VAR;
		    }
		    p++;
		}
	    } else {
		/*
		 * With NEW_ENVIRON, the sense of VAR and VALUE
		 * isn't in doubt.
		 */
		value = RFC_VALUE;
		var = RFC_VAR;
	    }
	    b[0] = IAC;
	    b[1] = SB;
	    b[2] = telnet->sb_opt;
	    b[3] = TELQUAL_IS;
	    n = 4;
	    e = telnet->cfg.environmt;
	    while (*e) {
		b[n++] = var;
		while (*e && *e != '\t')
		    b[n++] = *e++;
		if (*e == '\t')
		    e++;
		b[n++] = value;
		while (*e)
		    b[n++] = *e++;
		e++;
	    }
	    {
		char user[sizeof(telnet->cfg.username)];
		(void) get_remote_username(&telnet->cfg, user, sizeof(user));
		if (*user) {
		    b[n++] = var;
		    b[n++] = 'U';
		    b[n++] = 'S';
		    b[n++] = 'E';
		    b[n++] = 'R';
		    b[n++] = value;
		    e = user;
		    while (*e)
			b[n++] = *e++;
		}
		b[n++] = IAC;
		b[n++] = SE;
		telnet->bufsize = sk_write(telnet->s, (char *)b, n);
		logbuf = dupprintf("client:\tSB %s IS %s%s%s%s",
				   telopt(telnet->sb_opt),
				   *user ? "USER=" : "",
				   user,
				   *user ? " " : "",
				   n == 6 ? "<nothing>" :
				   (*telnet->cfg.environmt ? "<stuff>" : ""));
		logevent(telnet->frontend, logbuf);
		sfree(logbuf);
	    }
	}
	break;
    }
}

static void do_telnet_read(Telnet telnet, char *buf, int len)
{
    char *outbuf = NULL;
    int outbuflen = 0, outbufsize = 0;

#define ADDTOBUF(c) do { \
    if (outbuflen >= outbufsize) { \
	outbufsize = outbuflen + 256; \
        outbuf = sresize(outbuf, outbufsize, char); \
    } \
    outbuf[outbuflen++] = (c); \
} while (0)

    while (len--) {
	int c = (unsigned char) *buf++;

	switch (telnet->state) {
	  case TOP_LEVEL:
	  case SEENCR:
	    if (c == NUL && telnet->state == SEENCR)
		telnet->state = TOP_LEVEL;
	    else if (c == IAC)
		telnet->state = SEENIAC;
	    else {
		if (!telnet->in_synch)
		    ADDTOBUF(c);

#if 1
		/* I can't get the F***ing winsock to insert the urgent IAC
		 * into the right position! Even with SO_OOBINLINE it gives
		 * it to recv too soon. And of course the DM byte (that
		 * arrives in the same packet!) appears several K later!!
		 *
		 * Oh well, we do get the DM in the right place so I'll
		 * just stop hiding on the next 0xf2 and hope for the best.
		 */
		else if (c == DM)
		    telnet->in_synch = 0;
#endif
		if (c == CR && telnet->opt_states[o_they_bin.index] != ACTIVE)
		    telnet->state = SEENCR;
		else
		    telnet->state = TOP_LEVEL;
	    }
	    break;
	  case SEENIAC:
	    if (c == DO)
		telnet->state = SEENDO;
	    else if (c == DONT)
		telnet->state = SEENDONT;
	    else if (c == WILL)
		telnet->state = SEENWILL;
	    else if (c == WONT)
		telnet->state = SEENWONT;
	    else if (c == SB)
		telnet->state = SEENSB;
	    else if (c == DM) {
		telnet->in_synch = 0;
		telnet->state = TOP_LEVEL;
	    } else {
		/* ignore everything else; print it if it's IAC */
		if (c == IAC) {
		    ADDTOBUF(c);
		}
		telnet->state = TOP_LEVEL;
	    }
	    break;
	  case SEENWILL:
	    proc_rec_opt(telnet, WILL, c);
	    telnet->state = TOP_LEVEL;
	    break;
	  case SEENWONT:
	    proc_rec_opt(telnet, WONT, c);
	    telnet->state = TOP_LEVEL;
	    break;
	  case SEENDO:
	    proc_rec_opt(telnet, DO, c);
	    telnet->state = TOP_LEVEL;
	    break;
	  case SEENDONT:
	    proc_rec_opt(telnet, DONT, c);
	    telnet->state = TOP_LEVEL;
	    break;
	  case SEENSB:
	    telnet->sb_opt = c;
	    telnet->sb_len = 0;
	    telnet->state = SUBNEGOT;
	    break;
	  case SUBNEGOT:
	    if (c == IAC)
		telnet->state = SUBNEG_IAC;
	    else {
	      subneg_addchar:
		if (telnet->sb_len >= telnet->sb_size) {
		    telnet->sb_size += SB_DELTA;
		    telnet->sb_buf = sresize(telnet->sb_buf, telnet->sb_size,
					     unsigned char);
		}
		telnet->sb_buf[telnet->sb_len++] = c;
		telnet->state = SUBNEGOT;	/* in case we came here by goto */
	    }
	    break;
	  case SUBNEG_IAC:
	    if (c != SE)
		goto subneg_addchar;   /* yes, it's a hack, I know, but... */
	    else {
		process_subneg(telnet);
		telnet->state = TOP_LEVEL;
	    }
	    break;
	}
    }

    if (outbuflen)
	c_write(telnet, outbuf, outbuflen);
    sfree(outbuf);
}

static void telnet_log(Plug plug, int type, SockAddr addr, int port,
		       const char *error_msg, int error_code)
{
    Telnet telnet = (Telnet) plug;
    char addrbuf[256], *msg;

    sk_getaddr(addr, addrbuf, lenof(addrbuf));

    if (type == 0)
	msg = dupprintf("Connecting to %s port %d", addrbuf, port);
    else
	msg = dupprintf("Failed to connect to %s: %s", addrbuf, error_msg);

    logevent(telnet->frontend, msg);
}

static int telnet_closing(Plug plug, const char *error_msg, int error_code,
			  int calling_back)
{
    Telnet telnet = (Telnet) plug;

    if (telnet->s) {
        sk_close(telnet->s);
        telnet->s = NULL;
	notify_remote_exit(telnet->frontend);
    }
    if (error_msg) {
	logevent(telnet->frontend, error_msg);
	connection_fatal(telnet->frontend, "%s", error_msg);
    }
    /* Otherwise, the remote side closed the connection normally. */
    return 0;
}

static int telnet_receive(Plug plug, int urgent, char *data, int len)
{
    Telnet telnet = (Telnet) plug;
    if (urgent)
	telnet->in_synch = TRUE;
    do_telnet_read(telnet, data, len);
    return 1;
}

static void telnet_sent(Plug plug, int bufsize)
{
    Telnet telnet = (Telnet) plug;
    telnet->bufsize = bufsize;
}

/*
 * Called to set up the Telnet connection.
 *
 * Returns an error message, or NULL on success.
 *
 * Also places the canonical host name into `realhost'. It must be
 * freed by the caller.
 */
static const char *telnet_init(void *frontend_handle, void **backend_handle,
			       Config *cfg,
			       char *host, int port, char **realhost,
			       int nodelay, int keepalive)
{
    static const struct plug_function_table fn_table = {
	telnet_log,
	telnet_closing,
	telnet_receive,
	telnet_sent
    };
    SockAddr addr;
    const char *err;
    Telnet telnet;

    telnet = snew(struct telnet_tag);
    telnet->fn = &fn_table;
    telnet->cfg = *cfg;		       /* STRUCTURE COPY */
    telnet->s = NULL;
    telnet->echoing = TRUE;
    telnet->editing = TRUE;
    telnet->activated = FALSE;
    telnet->sb_buf = NULL;
    telnet->sb_size = 0;
    telnet->frontend = frontend_handle;
    telnet->term_width = telnet->cfg.width;
    telnet->term_height = telnet->cfg.height;
    telnet->state = TOP_LEVEL;
    telnet->ldisc = NULL;
    telnet->pinger = NULL;
    *backend_handle = telnet;
    autocmd_init(&telnet->cfg);

    /*
     * Try to find host.
     */
    {
	char *buf;
	buf = dupprintf("Looking up host \"%s\"%s", host,
			(cfg->addressfamily == ADDRTYPE_IPV4 ? " (IPv4)" :
			 (cfg->addressfamily == ADDRTYPE_IPV6 ? " (IPv6)" :
			  "")));
	logevent(telnet->frontend, buf);
	sfree(buf);
    }
    addr = name_lookup(host, port, realhost, &telnet->cfg, cfg->addressfamily);
    if ((err = sk_addr_error(addr)) != NULL) {
	sk_addr_free(addr);
	return err;
    }

    if (port < 0)
	port = 23;		       /* default telnet port */

    /*
     * Open socket.
     */
    telnet->s = new_connection(addr, *realhost, port, 0, 1,
			       nodelay, keepalive, (Plug) telnet, &telnet->cfg);
    if ((err = sk_socket_error(telnet->s)) != NULL)
	return err;

    telnet->pinger = pinger_new(&telnet->cfg, &telnet_backend, telnet);

    /*
     * Initialise option states.
     */
    if (telnet->cfg.passive_telnet) {
	const struct Opt *const *o;

	for (o = opts; *o; o++)
	    telnet->opt_states[(*o)->index] = INACTIVE;
    } else {
	const struct Opt *const *o;

	for (o = opts; *o; o++) {
	    telnet->opt_states[(*o)->index] = (*o)->initial_state;
	    if (telnet->opt_states[(*o)->index] == REQUESTED)
		send_opt(telnet, (*o)->send, (*o)->option);
	}
	telnet->activated = TRUE;
    }

    /*
     * Set up SYNCH state.
     */
    telnet->in_synch = FALSE;

    /*
     * We can send special commands from the start.
     */
    update_specials_menu(telnet->frontend);

    /*
     * loghost overrides realhost, if specified.
     */
    if (*telnet->cfg.loghost) {
	char *colon;

	sfree(*realhost);
	*realhost = dupstr(telnet->cfg.loghost);
	colon = strrchr(*realhost, ':');
	if (colon) {
	    /*
	     * FIXME: if we ever update this aspect of ssh.c for
	     * IPv6 literal management, this should change in line
	     * with it.
	     */
	    *colon++ = '\0';
	}
    }

    return NULL;
}

static void telnet_free(void *handle)
{
    Telnet telnet = (Telnet) handle;

    sfree(telnet->sb_buf);
    if (telnet->s)
	sk_close(telnet->s);
    if (telnet->pinger)
	pinger_free(telnet->pinger);
    sfree(telnet);
}
/*
 * Reconfigure the Telnet backend. There's no immediate action
 * necessary, in this backend: we just save the fresh config for
 * any subsequent negotiations.
 */
static void telnet_reconfig(void *handle, Config *cfg)
{
    Telnet telnet = (Telnet) handle;
    pinger_reconfig(telnet->pinger, &telnet->cfg, cfg);
    telnet->cfg = *cfg;		       /* STRUCTURE COPY */
}

/*
 * Called to send data down the Telnet connection.
 */
static int telnet_send(void *handle, char *buf, int len)
{
    Telnet telnet = (Telnet) handle;
    unsigned char *p, *end;
    static const unsigned char iac[2] = { IAC, IAC };
    static const unsigned char cr[2] = { CR, NUL };
#if 0
    static const unsigned char nl[2] = { CR, LF };
#endif

    if (telnet->s == NULL)
	return 0;

    p = (unsigned char *)buf;
    end = (unsigned char *)(buf + len);
    while (p < end) {
	unsigned char *q = p;

	while (p < end && iswritable(*p))
	    p++;
	telnet->bufsize = sk_write(telnet->s, (char *)q, p - q);

	while (p < end && !iswritable(*p)) {
	    telnet->bufsize = 
		sk_write(telnet->s, (char *)(*p == IAC ? iac : cr), 2);
	    p++;
	}
    }

    return telnet->bufsize;
}

/*
 * Called to query the current socket sendability status.
 */
static int telnet_sendbuffer(void *handle)
{
    Telnet telnet = (Telnet) handle;
    return telnet->bufsize;
}

/*
 * Called to set the size of the window from Telnet's POV.
 */
static void telnet_size(void *handle, int width, int height)
{
    Telnet telnet = (Telnet) handle;
    unsigned char b[24];
    int n;
    char *logbuf;

    telnet->term_width = width;
    telnet->term_height = height;

    if (telnet->s == NULL || telnet->opt_states[o_naws.index] != ACTIVE)
	return;
    n = 0;
    b[n++] = IAC;
    b[n++] = SB;
    b[n++] = TELOPT_NAWS;
    b[n++] = telnet->term_width >> 8;
    if (b[n-1] == IAC) b[n++] = IAC;   /* duplicate any IAC byte occurs */
    b[n++] = telnet->term_width & 0xFF;
    if (b[n-1] == IAC) b[n++] = IAC;   /* duplicate any IAC byte occurs */
    b[n++] = telnet->term_height >> 8;
    if (b[n-1] == IAC) b[n++] = IAC;   /* duplicate any IAC byte occurs */
    b[n++] = telnet->term_height & 0xFF;
    if (b[n-1] == IAC) b[n++] = IAC;   /* duplicate any IAC byte occurs */
    b[n++] = IAC;
    b[n++] = SE;
    telnet->bufsize = sk_write(telnet->s, (char *)b, n);
    logbuf = dupprintf("client:\tSB NAWS %d,%d",
		       telnet->term_width, telnet->term_height);
    logevent(telnet->frontend, logbuf);
    sfree(logbuf);
}

/*
 * Send Telnet special codes.
 */
static void telnet_special(void *handle, Telnet_Special code)
{
    Telnet telnet = (Telnet) handle;
    unsigned char b[2];

    if (telnet->s == NULL)
	return;

    b[0] = IAC;
    switch (code) {
      case TS_AYT:
	b[1] = AYT;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_BRK:
	b[1] = BREAK;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_EC:
	b[1] = EC;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_EL:
	b[1] = EL;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_GA:
	b[1] = GA;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_NOP:
	b[1] = NOP;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_ABORT:
	b[1] = ABORT;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_AO:
	b[1] = AO;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_IP:
	b[1] = IP;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_SUSP:
	b[1] = SUSP;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_EOR:
	b[1] = EOR;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_EOF:
	b[1] = xEOF;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	break;
      case TS_EOL:
	/* In BINARY mode, CR-LF becomes just CR -
	 * and without the NUL suffix too. */
	if (telnet->opt_states[o_we_bin.index] == ACTIVE)
	    telnet->bufsize = sk_write(telnet->s, "\r", 1);
	else
	    telnet->bufsize = sk_write(telnet->s, "\r\n", 2);
	break;
      case TS_SYNCH:
	b[1] = DM;
	telnet->bufsize = sk_write(telnet->s, (char *)b, 1);
	telnet->bufsize = sk_write_oob(telnet->s, (char *)(b + 1), 1);
	break;
      case TS_RECHO:
	if (telnet->opt_states[o_echo.index] == INACTIVE ||
	    telnet->opt_states[o_echo.index] == REALLY_INACTIVE) {
	    telnet->opt_states[o_echo.index] = REQUESTED;
	    send_opt(telnet, o_echo.send, o_echo.option);
	}
	break;
      case TS_LECHO:
	if (telnet->opt_states[o_echo.index] == ACTIVE) {
	    telnet->opt_states[o_echo.index] = REQUESTED;
	    send_opt(telnet, o_echo.nsend, o_echo.option);
	}
	break;
      case TS_PING:
	if (telnet->opt_states[o_they_sga.index] == ACTIVE) {
	    b[1] = NOP;
	    telnet->bufsize = sk_write(telnet->s, (char *)b, 2);
	}
	break;
      default:
	break;	/* never heard of it */
    }
}

static const struct telnet_special *telnet_get_specials(void *handle)
{
    static const struct telnet_special specials[] = {
	{"Are You There", TS_AYT},
	{"Break", TS_BRK},
	{"Synch", TS_SYNCH},
	{"Erase Character", TS_EC},
	{"Erase Line", TS_EL},
	{"Go Ahead", TS_GA},
	{"No Operation", TS_NOP},
	{NULL, TS_SEP},
	{"Abort Process", TS_ABORT},
	{"Abort Output", TS_AO},
	{"Interrupt Process", TS_IP},
	{"Suspend Process", TS_SUSP},
	{NULL, TS_SEP},
	{"End Of Record", TS_EOR},
	{"End Of File", TS_EOF},
	{NULL, TS_EXITMENU}
    };
    return specials;
}

static int telnet_connected(void *handle)
{
    Telnet telnet = (Telnet) handle;
    return telnet->s != NULL;
}

static int telnet_sendok(void *handle)
{
    /* Telnet telnet = (Telnet) handle; */
    return 1;
}

static void telnet_unthrottle(void *handle, int backlog)
{
    Telnet telnet = (Telnet) handle;
    sk_set_frozen(telnet->s, backlog > TELNET_MAX_BACKLOG);
}

static int telnet_ldisc(void *handle, int option)
{
    Telnet telnet = (Telnet) handle;
    if (option == LD_ECHO)
	return telnet->echoing;
    if (option == LD_EDIT)
	return telnet->editing;
    return FALSE;
}

static void telnet_provide_ldisc(void *handle, void *ldisc)
{
    Telnet telnet = (Telnet) handle;
    telnet->ldisc = ldisc;
}

static void telnet_provide_logctx(void *handle, void *logctx)
{
    /* This is a stub. */
}

static int telnet_exitcode(void *handle)
{
    Telnet telnet = (Telnet) handle;
    if (telnet->s != NULL)
        return -1;                     /* still connected */
    else
        /* Telnet doesn't transmit exit codes back to the client */
        return 0;
}

/*
 * cfg_info for Telnet does nothing at all.
 */
static int telnet_cfg_info(void *handle)
{
    return 0;
}

Backend telnet_backend = {
    telnet_init,
    telnet_free,
    telnet_reconfig,
    telnet_send,
    telnet_sendbuffer,
    telnet_size,
    telnet_special,
    telnet_get_specials,
    telnet_connected,
    telnet_exitcode,
    telnet_sendok,
    telnet_ldisc,
    telnet_provide_ldisc,
    telnet_provide_logctx,
    telnet_unthrottle,
    telnet_cfg_info,
    "telnet",
    PROT_TELNET,
    23
};