putty/telnet.c

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

#include "putty.h"

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

static SOCKET s = INVALID_SOCKET;

#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 TELOPT_BINARY	0	/* 8-bit data path */
#define TELOPT_ECHO	1	/* echo */
#define	TELOPT_RCP	2	/* prepare to reconnect */
#define	TELOPT_SGA	3	/* suppress go ahead */
#define	TELOPT_NAMS	4	/* approximate message size */
#define	TELOPT_STATUS	5	/* give status */
#define	TELOPT_TM	6	/* timing mark */
#define	TELOPT_RCTE	7	/* remote controlled transmission and echo */
#define TELOPT_NAOL 	8	/* negotiate about output line width */
#define TELOPT_NAOP 	9	/* negotiate about output page size */
#define TELOPT_NAOCRD	10	/* negotiate about CR disposition */
#define TELOPT_NAOHTS	11	/* negotiate about horizontal tabstops */
#define TELOPT_NAOHTD	12	/* negotiate about horizontal tab disposition */
#define TELOPT_NAOFFD	13	/* negotiate about formfeed disposition */
#define TELOPT_NAOVTS	14	/* negotiate about vertical tab stops */
#define TELOPT_NAOVTD	15	/* negotiate about vertical tab disposition */
#define TELOPT_NAOLFD	16	/* negotiate about output LF disposition */
#define TELOPT_XASCII	17	/* extended ascic character set */
#define	TELOPT_LOGOUT	18	/* force logout */
#define	TELOPT_BM	19	/* byte macro */
#define	TELOPT_DET	20	/* data entry terminal */
#define	TELOPT_SUPDUP	21	/* supdup protocol */
#define	TELOPT_SUPDUPOUTPUT 22	/* supdup output */
#define	TELOPT_SNDLOC	23	/* send location */
#define	TELOPT_TTYPE	24	/* terminal type */
#define	TELOPT_EOR	25	/* end or record */
#define	TELOPT_TUID	26	/* TACACS user identification */
#define	TELOPT_OUTMRK	27	/* output marking */
#define	TELOPT_TTYLOC	28	/* terminal location number */
#define	TELOPT_3270REGIME 29	/* 3270 regime */
#define	TELOPT_X3PAD	30	/* X.3 PAD */
#define	TELOPT_NAWS	31	/* window size */
#define	TELOPT_TSPEED	32	/* terminal speed */
#define	TELOPT_LFLOW	33	/* remote flow control */
#define TELOPT_LINEMODE	34	/* Linemode option */
#define TELOPT_XDISPLOC	35	/* X Display Location */
#define TELOPT_OLD_ENVIRON 36	/* Old - Environment variables */
#define	TELOPT_AUTHENTICATION 37/* Authenticate */
#define	TELOPT_ENCRYPT	38	/* Encryption option */
#define TELOPT_NEW_ENVIRON 39	/* New - Environment variables */
#define	TELOPT_EXOPL	255	/* extended-options-list */

#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 && (x) != CR )

static char *telopt(int opt) {
#define i(x) if (opt == TELOPT_ ## x) return #x;
    i(BINARY); i(ECHO); i(RCP); i(SGA); i(NAMS); i(STATUS); i(TM); i(RCTE);
    i(NAOL); i(NAOP); i(NAOCRD); i(NAOHTS); i(NAOHTD); i(NAOFFD); i(NAOVTS);
    i(NAOVTD); i(NAOLFD); i(XASCII); i(LOGOUT); i(BM); i(DET); i(SUPDUP);
    i(SUPDUPOUTPUT); i(SNDLOC); i(TTYPE); i(EOR); i(TUID); i(OUTMRK);
    i(TTYLOC); i(X3PAD); i(NAWS); i(TSPEED); i(LFLOW); i(LINEMODE);
    i(XDISPLOC); i(OLD_ENVIRON); i(AUTHENTICATION); i(ENCRYPT);
    i(NEW_ENVIRON); i(EXOPL);
#undef i
    return "<unknown>";
}

static void telnet_size(void);

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 */
    enum {
	REQUESTED, ACTIVE, INACTIVE, REALLY_INACTIVE
    } state;
};

static struct Opt o_naws = {WILL, WONT, DO, DONT, TELOPT_NAWS, REQUESTED};
static struct Opt o_tspeed = {WILL, WONT, DO, DONT, TELOPT_TSPEED, REQUESTED};
static struct Opt o_ttype = {WILL, WONT, DO, DONT, TELOPT_TTYPE, REQUESTED};
static struct Opt o_oenv = {WILL, WONT, DO, DONT, TELOPT_OLD_ENVIRON,
    INACTIVE};
static struct Opt o_nenv = {WILL, WONT, DO, DONT, TELOPT_NEW_ENVIRON,
    REQUESTED};
static struct Opt o_echo = {DO, DONT, WILL, WONT, TELOPT_ECHO, REQUESTED};
static struct Opt o_we_sga = {WILL, WONT, DO, DONT, TELOPT_SGA, REQUESTED};
static struct Opt o_they_sga = {DO, DONT, WILL, WONT, TELOPT_SGA, REQUESTED};

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

static int in_synch;
static int sb_opt, sb_len;
static char *sb_buf = NULL;
static int sb_size = 0;
#define SB_DELTA 1024

static void try_write (void) {
    while (outbuf_head != outbuf_reap) {
	int end = (outbuf_reap < outbuf_head ? outbuf_head : OUTBUF_SIZE);
	int len = end - outbuf_reap;
	int ret;

	ret = send (s, outbuf+outbuf_reap, len, 0);
	if (ret > 0)
	    outbuf_reap = (outbuf_reap + ret) & OUTBUF_MASK;
	if (ret < len)
	    return;
    }
}

static void s_write (void *buf, int len) {
    unsigned char *p = buf;
    while (len--) {
	int new_head = (outbuf_head + 1) & OUTBUF_MASK;
	if (new_head != outbuf_reap) {
	    outbuf[outbuf_head] = *p++;
	    outbuf_head = new_head;
	}
    }
    try_write();
}

static void c_write (char *buf, int len) {
    while (len--) {
	int new_head = (inbuf_head + 1) & INBUF_MASK;
	if (new_head != inbuf_reap) {
	    inbuf[inbuf_head] = *buf++;
	    inbuf_head = new_head;
	} else {
            term_out();
            if( inbuf_head == inbuf_reap ) len++; else break;
	}
    }
}

static void log_option (char *sender, int cmd, int option) {
    char buf[50];
    sprintf(buf, "%s:\t%s %s", sender,
	    (cmd == WILL ? "WILL" : cmd == WONT ? "WONT" :
	     cmd == DO ? "DO" : cmd == DONT ? "DONT" : "<??>"),
	    telopt(option));
    logevent(buf);
}

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

    b[0] = IAC; b[1] = cmd; b[2] = option;
    s_write (b, 3);
    log_option("client", cmd, option);
}

static void deactivate_option (struct Opt *o) {
    if (o->state == REQUESTED || o->state == ACTIVE)
	send_opt (o->nsend, o->option);
    o->state = REALLY_INACTIVE;
}

static void activate_option (struct Opt *o) {
    if (o->send == WILL && o->option == TELOPT_NAWS)
	telnet_size();
    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 (o->option==TELOPT_NEW_ENVIRON ? &o_oenv : &o_nenv);
    }
    if (o->option == TELOPT_ECHO && cfg.ldisc_term)
	ldisc = &ldisc_simple;
}

static void refused_option (struct Opt *o) {
    if (o->send == WILL && o->option == TELOPT_NEW_ENVIRON &&
	o_oenv.state == INACTIVE) {
	send_opt (WILL, TELOPT_OLD_ENVIRON);
	o_oenv.state = REQUESTED;
    }
    if (o->option == TELOPT_ECHO && cfg.ldisc_term)
	ldisc = &ldisc_term;
}

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

    log_option ("server", cmd, option);
    for (o = opts; *o; o++) {
	if ((*o)->option == option && (*o)->ack == cmd) {
	    switch ((*o)->state) {
	      case REQUESTED:
		(*o)->state = ACTIVE;
		activate_option (*o);
		break;
	      case ACTIVE:
		break;
	      case INACTIVE:
		(*o)->state = ACTIVE;
		send_opt ((*o)->send, option);
		activate_option (*o);
		break;
	      case REALLY_INACTIVE:
		send_opt ((*o)->nsend, option);
		break;
	    }
	    return;
	} else if ((*o)->option == option && (*o)->nak == cmd) {
	    switch ((*o)->state) {
	      case REQUESTED:
		(*o)->state = INACTIVE;
		refused_option (*o);
		break;
	      case ACTIVE:
		(*o)->state = INACTIVE;
		send_opt ((*o)->nsend, option);
		break;
	      case INACTIVE:
	      case REALLY_INACTIVE:
		break;
	    }
	    return;
	}
    }
    /*
     * If we reach here, the option was one we weren't prepared to
     * cope with. So send a negative ack.
     */
    send_opt ((cmd == WILL ? DONT : WONT), option);
}

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

    switch (sb_opt) {
      case TELOPT_TSPEED:
	if (sb_len == 1 && sb_buf[0] == TELQUAL_SEND) {
	    char logbuf[sizeof(cfg.termspeed)+80];
	    b[0] = IAC; b[1] = SB; b[2] = TELOPT_TSPEED;
	    b[3] = TELQUAL_IS;
	    strcpy(b+4, cfg.termspeed);
	    n = 4 + strlen(cfg.termspeed);
	    b[n] = IAC; b[n+1] = SE;
	    s_write (b, n+2);
	    logevent("server:\tSB TSPEED SEND");
	    sprintf(logbuf, "client:\tSB TSPEED IS %s", cfg.termspeed);
	    logevent (logbuf);
	} else
	    logevent ("server:\tSB TSPEED <something weird>");
	break;
      case TELOPT_TTYPE:
	if (sb_len == 1 && sb_buf[0] == TELQUAL_SEND) {
	    char logbuf[sizeof(cfg.termtype)+80];
	    b[0] = IAC; b[1] = SB; b[2] = TELOPT_TTYPE;
	    b[3] = TELQUAL_IS;
	    for (n = 0; cfg.termtype[n]; n++)
		b[n+4] = (cfg.termtype[n] >= 'a' && cfg.termtype[n] <= 'z' ?
			  cfg.termtype[n] + 'A'-'a' : cfg.termtype[n]);
	    b[n+4] = IAC; b[n+5] = SE;
	    s_write (b, n+6);
	    b[n+4] = 0;
	    logevent("server:\tSB TTYPE SEND");
	    sprintf(logbuf, "client:\tSB TTYPE IS %s", b+4);
	    logevent(logbuf);
	} else
	    logevent("server:\tSB TTYPE <something weird>\r\n");
	break;
      case TELOPT_OLD_ENVIRON:
      case TELOPT_NEW_ENVIRON:	
	p = sb_buf;
	q = p + sb_len;
	if (p < q && *p == TELQUAL_SEND) {
	    char logbuf[50];
	    p++;
	    sprintf (logbuf, "server:\tSB %s SEND", telopt(sb_opt));
	    logevent (logbuf);
	    if (sb_opt == TELOPT_OLD_ENVIRON) {
		if (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] = sb_opt;
	    b[3] = TELQUAL_IS;
	    n = 4;
          e = 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++;
	    }
	    if (*cfg.username) {
		b[n++] = var; b[n++] = 'U'; b[n++] = 'S';
		b[n++] = 'E'; b[n++] = 'R'; b[n++] = value;
		e = cfg.username;
		while (*e) b[n++] = *e++;
	    }
	    b[n++] = IAC; b[n++] = SE;
	    s_write (b, n);
	    sprintf(logbuf, "client:\tSB %s IS %s", telopt(sb_opt),
		    n==6 ? "<nothing>" : "<stuff>");
	    logevent (logbuf);
	}
	break;
    }
}

static enum {
    TOPLEVEL, SEENIAC, SEENWILL, SEENWONT, SEENDO, SEENDONT,
    SEENSB, SUBNEGOT, SUBNEG_IAC, SEENCR
} telnet_state = TOPLEVEL;

static void do_telnet_read (char *buf, int len) {
    unsigned char b[10];

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

	switch (telnet_state) {
	  case TOPLEVEL:
	  case SEENCR:
	    if (c == NUL && telnet_state == SEENCR)
		telnet_state = TOPLEVEL;
	    else if (c == IAC)
		telnet_state = SEENIAC;
	    else {
		b[0] = c;
		if (!in_synch)
		    c_write (b, 1);

#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) in_synch = 0;
#endif
		if (c == CR)
		    telnet_state = SEENCR;
		else
		    telnet_state = TOPLEVEL;
	    }
	    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) {
	        in_synch = 0;
		telnet_state = TOPLEVEL;
	    } 
	    else {
		/* ignore everything else; print it if it's IAC */
		if (c == IAC) {
		    b[0] = c;
		    c_write(b,1);
		}
		telnet_state = TOPLEVEL;
	    }
	    break;
	  case SEENWILL:
	    proc_rec_opt (WILL, c);
	    telnet_state = TOPLEVEL;
	    break;
	  case SEENWONT:
	    proc_rec_opt (WONT, c);
	    telnet_state = TOPLEVEL;
	    break;
	  case SEENDO:
	    proc_rec_opt (DO, c);
	    telnet_state = TOPLEVEL;
	    break;
	  case SEENDONT:
	    proc_rec_opt (DONT, c);
	    telnet_state = TOPLEVEL;
	    break;
	  case SEENSB:
	    sb_opt = c;
	    sb_len = 0;
	    telnet_state = SUBNEGOT;
	    break;
	  case SUBNEGOT:
	    if (c == IAC)
		telnet_state = SUBNEG_IAC;
	    else {
		subneg_addchar:
		if (sb_len >= sb_size) {
		    char *newbuf;
		    sb_size += SB_DELTA;
		    newbuf = (sb_buf ?
			      realloc(sb_buf, sb_size) :
			      malloc(sb_size));
		    if (newbuf)
			sb_buf = newbuf;
		    else
			sb_size -= SB_DELTA;
		}
		if (sb_len < sb_size)
		    sb_buf[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_state = TOPLEVEL;
	    }
	    break;
	}
    }
}

/*
 * Called to set up the Telnet connection. Will arrange for
 * WM_NETEVENT messages to be passed to the specified window, whose
 * window procedure should then call telnet_msg().
 *
 * Returns an error message, or NULL on success.
 *
 * Also places the canonical host name into `realhost'.
 */
static char *telnet_init (HWND hwnd, char *host, int port, char **realhost) {
    SOCKADDR_IN addr;
    struct hostent *h;
    unsigned long a;

    /*
     * Try to find host.
     */
    if ( (a = inet_addr(host)) == (unsigned long) INADDR_NONE) {
	if ( (h = gethostbyname(host)) == NULL)
	    switch (WSAGetLastError()) {
	      case WSAENETDOWN: return "Network is down";
	      case WSAHOST_NOT_FOUND: case WSANO_DATA:
		return "Host does not exist";
	      case WSATRY_AGAIN: return "Host not found";
	      default: return "gethostbyname: unknown error";
	    }
	memcpy (&a, h->h_addr, sizeof(a));
	*realhost = h->h_name;
    } else
	*realhost = host;
    a = ntohl(a);

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

    /*
     * Open socket.
     */
    s = socket(AF_INET, SOCK_STREAM, 0);
    if (s == INVALID_SOCKET)
	switch (WSAGetLastError()) {
	  case WSAENETDOWN: return "Network is down";
	  case WSAEAFNOSUPPORT: return "TCP/IP support not present";
	  default: return "socket(): unknown error";
	}

    {
	BOOL b = TRUE;
	setsockopt (s, SOL_SOCKET, SO_OOBINLINE, (void *)&b, sizeof(b));
    }

    /*
     * Bind to local address.
     */
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(0);
    if (bind (s, (struct sockaddr *)&addr, sizeof(addr)) == SOCKET_ERROR)
	switch (WSAGetLastError()) {
	  case WSAENETDOWN: return "Network is down";
	  default: return "bind(): unknown error";
	}

    /*
     * Connect to remote address.
     */
    addr.sin_addr.s_addr = htonl(a);
    addr.sin_port = htons((short)port);
    if (connect (s, (struct sockaddr *)&addr, sizeof(addr)) == SOCKET_ERROR)
	switch (WSAGetLastError()) {
	  case WSAENETDOWN: return "Network is down";
	  case WSAECONNREFUSED: return "Connection refused";
	  case WSAENETUNREACH: return "Network is unreachable";
	  case WSAEHOSTUNREACH: return "No route to host";
	  default: return "connect(): unknown error";
	}

    if (WSAAsyncSelect (s, hwnd, WM_NETEVENT, FD_READ |
			FD_WRITE | FD_OOB | FD_CLOSE) == SOCKET_ERROR)
	switch (WSAGetLastError()) {
	  case WSAENETDOWN: return "Network is down";
	  default: return "WSAAsyncSelect(): unknown error";
	}

    /*
     * Initialise option states.
     */
    if( cfg.ldisc_term )
    {
	struct Opt **o;

	for (o = opts; *o; o++)
	    if ((*o)->state == REQUESTED)
		(*o)->state = INACTIVE;
    }
    else
    {
	struct Opt **o;

	for (o = opts; *o; o++)
	    if ((*o)->state == REQUESTED)
		send_opt ((*o)->send, (*o)->option);
    }

    /*
     * Set up SYNCH state.
     */
    in_synch = FALSE;
    return NULL;
}

/*
 * Process a WM_NETEVENT message. Will return 0 if the connection
 * has closed, or <0 for a socket error.
 */
static int telnet_msg (WPARAM wParam, LPARAM lParam) {
    int ret;
    char buf[256];

    /*
     * Because reading less than the whole of the available pending
     * data can generate an FD_READ event, we need to allow for the
     * possibility that FD_READ may arrive with FD_CLOSE already in
     * the queue; so it's possible that we can get here even with s
     * invalid. If so, we return 1 and don't worry about it.
     */
    if (s == INVALID_SOCKET)
	return 1;

    if (WSAGETSELECTERROR(lParam) != 0)
	return -WSAGETSELECTERROR(lParam);

    switch (WSAGETSELECTEVENT(lParam)) {
      case FD_READ:
      case FD_CLOSE:
	{
	    int clear_of_oob = 1;
	    if (ioctlsocket (s, SIOCATMARK, &clear_of_oob) < 0 )
		return -20000-WSAGetLastError();

	    in_synch = !clear_of_oob;

	    do {
		ret = recv(s, buf, sizeof(buf), 0);
		if (ret < 0 && WSAGetLastError() == WSAEWOULDBLOCK)
		    return 1;
		if (ret < 0)		       /* any _other_ error */
		    return -10000-WSAGetLastError();
		if (ret == 0) {
		    s = INVALID_SOCKET;
		    return 0;
		}

	        do_telnet_read (buf, ret);
	    } while (in_synch);
	}
	return 1;
      case FD_WRITE:
	if (outbuf_head != outbuf_reap)
	    try_write();
	return 1;
    }
    return 1;			       /* shouldn't happen, but WTF */
}

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

    if (s == INVALID_SOCKET)
	return;

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

	while (iswritable((unsigned char)*p) && p < buf+len) p++;
	s_write (q, p-q);

	while (p < buf+len && !iswritable((unsigned char)*p)) {
	    s_write ((unsigned char)*p == IAC ? iac : nl, 2);
	    p++;
	}
    }
}

/*
 * Called to set the size of the window from Telnet's POV.
 */
static void telnet_size(void) {
    unsigned char b[16];
    char logbuf[50];

    if (s == INVALID_SOCKET || o_naws.state != ACTIVE)
	return;
    b[0] = IAC; b[1] = SB; b[2] = TELOPT_NAWS;
    b[3] = cols >> 8; b[4] = cols & 0xFF;
    b[5] = rows >> 8; b[6] = rows & 0xFF;
    b[7] = IAC; b[8] = SE;
    s_write (b, 9);
    sprintf(logbuf, "client:\tSB NAWS %d,%d",
	    ((unsigned char)b[3] << 8) + (unsigned char)b[4],
	    ((unsigned char)b[5] << 8) + (unsigned char)b[6]);
    logevent (logbuf);
}

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

    if (s == INVALID_SOCKET)
	return;

    b[0] = IAC;
    switch (code) {
      case TS_AYT: b[1] = AYT; s_write (b, 2); break;
      case TS_BRK: b[1] = BREAK; s_write (b, 2); break;
      case TS_EC: b[1] = EC; s_write (b, 2); break;
      case TS_EL: b[1] = EL; s_write (b, 2); break;
      case TS_GA: b[1] = GA; s_write (b, 2); break;
      case TS_NOP: b[1] = NOP; s_write (b, 2); break;
      case TS_ABORT: b[1] = ABORT; s_write (b, 2); break;
      case TS_AO: b[1] = AO; s_write (b, 2); break;
      case TS_IP: b[1] = IP; s_write (b, 2); break;
      case TS_SUSP: b[1] = SUSP; s_write (b, 2); break;
      case TS_EOR: b[1] = EOR; s_write (b, 2); break;
      case TS_EOF: b[1] = xEOF; s_write (b, 2); break;
      case TS_SYNCH:
	outbuf_head = outbuf_reap = 0;
	b[1] = DM;
	send (s, b, 2, MSG_OOB);
	break;
      case TS_RECHO:
	if (o_echo.state == INACTIVE || o_echo.state == REALLY_INACTIVE) {
	    o_echo.state = REQUESTED;
	    send_opt (o_echo.send, o_echo.option);
	}
	break;
      case TS_LECHO:
	if (o_echo.state == ACTIVE) {
	    o_echo.state = REQUESTED;
	    send_opt (o_echo.nsend, o_echo.option);
	}
	break;
    }
}

Backend telnet_backend = {
    telnet_init,
    telnet_msg,
    telnet_send,
    telnet_size,
    telnet_special
};