=====================================================================
  libtelnet - TELNET protocol handling library
=====================================================================

 http://github.com/elanthis/libtelnet

 Sean Middleditch
 sean@sourcemud.org

---------------------------------------------------------------------
The author or authors of this code dedicate any and all copyright
interest in this code to the public domain. We make this dedication
for the benefit of the public at large and to the detriment of our
heirs and successors. We intend this dedication to be an overt act of
relinquishment in perpetuity of all present and future rights to this
code under copyright law. 
---------------------------------------------------------------------

*** TODO ***

 - RFC 1143 option negotiation algorithm
 - automatic MCCP2 handling (controllable by host app)
 - efficient one-byte sub-requests
 ? MCCP1
 ? ZMP parsing
 ? MSSP parsing
 ? ENVIRON/NEW-ENVIRON parsing
 ? telnet-status testing tool
 ? few options to make telnet-proxy even more useful

I. INTRODUCTION
=====================================================================

libtelnet provides safe and correct handling of the core TELNET
protocol.  It does not include any "smarts," and all use of the
protocol (such as deciding which options to support, enabling
and disabling options, or processing subrequests) must be implemented
by the application author.

For more information on the TELNET protocol, see:

 http://www.faqs.org/rfcs/rfc854.html

II. LIBTELNET API
=====================================================================

The libtelnet API contains several distinct parts.  The first part is
the basic initialization and deinitialization routines.  The second
part is a single function for pushing received data into the
libtelnet processor.  The third part is the libtelnet_send_*()
functions, which generate TELNET commands and ensure data is properly
formatted before sending over the wire.  The final part is the
callback structure libtelnet_cb_t.

IIa. Initialization

 struct libtelnet_t;
   This structure represents the state of the TELNET protocol for a
   single connection.  Each connection utilizing TELNET must have
   its own libtelnet_t structure, which is passed to all libtelnet
   API calls.

 struct libtelnet_cb_t;
   An instance of this structure must be initialized and have all
   mandatory and desired optional callbacks set.  See section IId
   for more information.

 void libtelnet_init(struct libtelnet_t *telnet,
     struct libtelnet_cb_t *cb, enum libtelnet_mode_t mode);
   The libtelnet_init() function is responsible for initializing
   the data in a libtelnet_t structure.  It must be called
   immediately after establishing a connection and before any other
   libtelnet API calls are made.

   The cb parameter must be a pointer to a fully initialized
   instance of libtelnet_cb_t.  A single instance of the structure
   can be shared between any number of libtelnet_t instances.

   The mode parameter must be one of LIBTELNET_MODE_SERVER or
   LIBTELNET_MODE_CLIENT.  These slightly alter the behavior of
   libtelnet in certain instances.  If you are implementing a
   TELNET server, use the SERVER mode.  If you are implementing a
   client, use the CLIENT mode.
 
 boid libtelnet_free(struct libtelnet_t *telnet);
   Releases any internal memory allocated by libtelnet.  This must
   be called whenever a connection is closed, or you will incur
   memory leaks.

IIb. Receiving Data

 void libtelnet_push(struct libtelnet_t *telnet,
     unsigned char *buffer, unsigned int size, void *user_data);
   When your application receives data over the socket from the
   remote end, it must pass the received bytes into this function.
   Callback functions will be invoked as the buffer is processed,
   and the user_data parameter will be passed to each callback.

IIc. Sending Data

 Note that all of the libtelnet_send_*() functions will invoke
 the send callback function attached to the libtelnet_t instance.
 The user_data parameter to each of these functions is passed
 through to the callback.

 void libtelnet_send_command(struct libtelnet_t *telnet,
     unsigned char cmd, void *user_data);
   Sends a single "simple" TELNET command, such as the GO-AHEAD
   commands (255 249).

 void libtelnet_send_negotiate(struct libtelnet_t *telnet,
     unsigned char cmd, unsigned char opt, void *user_data);
   Sends a TELNET negotiation command.  The cmd parameter must be
   one of LIBTELNET_WILL, LIBTELNET_DONT, LIBTELNET_DO, or
   LIBTELNET_DONT.  The opt parameter is the option to
   negotiate.

 void libtelnet_send_data(struct libtelnet_t *telnet,
     unsigned char *buffer, unsigned int size, void *user_data);
   Sends raw data, which would be either the process output from
   a server or the user input from a client.

 void libtelnet_send_subnegotiation(struct libtelnet_t *telnet,
     unsigned char opt, unsigned char *buffer, unsigned int size,
     void *user_data);
   Sends a TELNET sub-negotiation command.  The opt parameter
   is the sub-negotiation option.

IId. Callbacks

 The libtelnet_cb_t structure containers a number of callback
 entry points.  Of these, only the send and data callbacks are
 absolutely required.  All others are optional.  The declarations
 below show the signature of the callback functions.

 An example of initializing a libtelnet_cb_t structure:

  /* illustrative data callback */
  void my_data_cb(libtelnet_t *telnet, unsigned char *buffer,
      unsigned int size, void *user_data) {
    /* print number of bytes received and then show the
     * whole buffer */
    printf("RECV(%d): %.*s\n", size, size, buffer);
  }

  /* illustrative variable definitions */
  libtelnet_t conn;
  libtelnet_cb_t callbacks;

  /* clear all callbacks and set just the ones we want */
  memset(&callbacks, 0, sizeof(callbacks));
  callbacks->send = my_send_cb;
  callbacks->data = my_data_cb;

  /* initialize the connection with our callbacks */
  libtelnet_init(&conn, &callbacks, LIBTELNET_MODE_SERVER);

 Remember that a single libtelnet_cb_t structure can be shared
 between any number of libtelnet_t instances.  There is no reason
 to make multiple copies of the data if all of your connections
 use the same callback functions.

 void libtelnet_cb_t->data(struct libtelnet_t *telnet,
     unsigned char *buffer, unsigned int size, void *user_data);
   Regular data has been received by the remote end.  For a server,
   this would be input typed by the client; for a client, this is
   process output generated by the server.

   Note that data is not line-buffered by libtelnet.  A single
   line of input may be broken into pieces and given to
   consecutive calls to libtelnet_data_cb().  If you are doing
   line-based processing of data, it is your responsibility to
   buffer data and find the line breaks.

 void libtelnet_cb_t->send(struct libtelnet_t *telnet,
     unsigned char *buffer, unsigned int size, void *user_data);
   This is called whenever libtelnet has generated output to be
   send to the remote end of the connection.  In most cases this
   will be a simple wrapper arround your applications network
   output buffering/transmission code.

   You can pass socket information through the user_data
   parameter to libtelnet calls so that it is available in this
   callback.

 void libtelnet_cb_t->command(struct libtelnet_t *telnet,
     unsigned char cmd, void *user_data);
   Called whenever a "simpler" TELNET command has arrived, such
   as GO-AHEAD commands (255 249).  The necessary processing
   depends on the specific commands; see the TELNET RFC for
   more information.

 void libtelnet_cb_t->negotiate(struct libtelnet_t *telnet,
     unsigned char cmd, unsigned char opt, void *user_data);
   This function is called whenever a TELNET negotiation command
   has been received.  The cmd parameter will be one of
   LIBTELNET_WILL, LIBTELNET_WONT, LIBTELNET_DO, or LIBTELNET_DONT.
   The opt parameter is the option being negotiated.

   libtelnet does not currently manage negotiation for you.  For
   best practice in implementing TELNET negotiation, see:

    http://www.faqs.org/rfcs/rfc1143.html

 void libtelnet_cb_t->subnegotiation(struct libtelnet_t *telnet,
     unsigned char opt, unsigned char *data, unsigned int size,
     void *user_data);
   Called whenever a TELNET sub-negotiation has been received.
   Sub-negotiations include the NAWS option for communicating
   terminal size to a server, the NEW-ENVIRON and TTYPE options
   for negotiating terminal features, and MUD-centric protocols
   such as ZMP, MSSP, and MCCP2.

   The opt parameter is the option under sub-negotiation.  The
   remaining data (if any) is passed in the buffer.

 void libtelnet_cb_t->compress(struct libtelnet_t *telnet,
     char enabled, void *user_data);
   The callback is invoked whenever the COMPRESS2 (MCCP2)
   feature is enabled or disabled.  For servers, this is called
   immediately after beginning compression after a client accepts
   the COMPRESS2 option.  For clients, this is called immediately
   after a compress stream begin or ends.

   The enabled parameter is 1 if compression has begun and 0 if
   compression has ended.

III. INTEGRATING LIBTELNET
=====================================================================

FIXME: fill in notes about implementing the libtelnet_*_cb functions

IV. SAFETY AND CORRECTNESS CONSIDERATIONS
=====================================================================

Your existing application may make heavy use of its own output
buffering and transmission commands, including hand-made routines
for sending TELNET commands and sub-negotiation requests.  There are
at times subtle issues that need to be handled when communication
over the TELNET protocol, not least of which is the need to escape
any byte value 0xFF with a special TELNET command.

For these reasons, it is very important that applications making use
of libtelnet always make use of the libtelnet_send_*() family of
functions for all data being sent over the TELNET connection.

V. MCCP2 COMPRESSION
=====================================================================

The MCCP2 (COMPRESS2) TELNET extension allows for the compression of
all traffic sent from server to client.  For more information:

 http://www.mudbytes.net/index.php?a=articles&s=mccp

libtelnet transparently supports MCCP2.  For a server to support
MCCP2, the application must begin negotiation of the COMPRESS2
option using libtelnet_send_negotiate(), for example:

 libtelnet_send_negotiate(&telnet, LIBTELNET_WILL,
     LIBTELNET_OPTION_COMPRESS2, user_data);

libtelnet will automatically detect if the client responds favoribly
and will begin compressing data.  For clients, no action must be
taken, as libtelnet will automatically handle the requests.

NOTE: libtelnet will still invoke the callback functions for
negotiation and sub-negotiation commands relating to MCCP2.  Do not
respond to these.

In order for libtelnet to support MCCP2, zlib must be installed and
enabled when compiling libtelnet.  Use -DHAVE_ZLIB to enable zlib
when compiling libtelnet.c and pass -lz to the linker to link in the
zlib shared library.

VI. TELNET PROXY UTILITY
=====================================================================

The telnet-proxy utility is a small application that serves both as
a testbed for libtelnet and as a powerful debugging tool for TELNET
servers and clients.

To use telnet-proxy, you must first compile it using:

 $ make

If you do not have zlib installed and wish to disable MCCP2 support
then you must first edit the Makefile and remove the -DHAVE_ZLIB and
the -lz from the compile flags.

To run telnet-proxy, you simply give it the server's IP address
(telnet-proxy does not support hostname resolution, nor IPv6), the
server's port number, and the port number that telnet-proxy should
listen on.  For example, to connect to the server on 209.85.171.100
port 7800 and to listen on port 5000, run:

 $ ./telnet-proxy 209.85.171.100 7800 5000

You can then connect to the host telnet-proxy is running on (e.g.
127.0.0.1, or the machine's public IP) on port 500 and you will
automatically be proxied into 209.85.171.100.

telnet-proxy will display status information about the data
passing through both ends of the tunnel.  Once either end
disconnects, telnet-proxy will close.  telnet-proxy can only
support a single tunnel at a time and must be restarted for each
connection.