module Telnet_client

: sig

Telnet client

This is a Telnet client providing the basic Telnet services. It supports sending and receiving data (asynchronously), and the negotiation of Telnet options, but it does not implement any option.

exception Telnet_protocol of exn

Wrapper for exceptions that already passed the exception handler.

type telnet_command =
# | Telnet_data of string
(*User data*)
# | Telnet_nop
(*No operation*)
# | Telnet_dm
(*data mark*)
# | Telnet_brk
# | Telnet_ip
(*interrupt process*)
# | Telnet_ao
(*abort output*)
# | Telnet_ayt
(*are you there?*)
# | Telnet_ec
(*erase character*)
# | Telnet_el
(*erase line*)
# | Telnet_ga
(*Go ahead*)
# | Telnet_sb of char
(*Begin of subnegotiation*)
# | Telnet_se
(*End of subnegotation*)
# | Telnet_will of char
(*Acknowledges that option is in effect*)
# | Telnet_wont of char
(*Acknowledges that option is rejected*)
# | Telnet_do of char
(*Requests to turn on an option*)
# | Telnet_dont of char
(*Requests to turn off an option*)
# | Telnet_unknown of char
(*Unknown command*)
# | Telnet_eof
(*End of file*)
# | Telnet_timeout
(*Timeout event*)

A telnet_command is the interpretation of the octets in a Telnet session, i.e. it is one level above the octet stream. See RFC 854 for an explanation what the commands mean. Telnet_data represents the data chunks between the commands. Note that you do not need to double octets having value 255; this is done automatically. Telnet_unknown represents any command not covered by RFC 854, for example the End-of-record-mark (introduced in RFC 885) would be Telnet_unknown '\239'. Telnet_eof represents the end of the octet stream, useable in both directions. Telnet_timeout is added to the input queue if I/O has not been happened for the configured period of time.

type telnet_options = {
# connection_timeout
: float;
# verbose_input
: bool;
# verbose_output
: bool;

telnet_options: modifies the behaviour of the client. Do not mix these options up with the options negotiated with the remote side.

  • connection_timeout: After this period of time (in seconds) a Telnet_timeout pseudo-command is added to the input queue, and the connection is aborted.
  • verbose_input: Enables printing of input events to Netlog.Debug.
  • verbose_output: Enables printing of output events to Netlog.Debug
type telnet_negotiated_option =
# | Telnet_binary
(*see RFC 856*)
# | Telnet_echo
(*see RFC 857*)
# | Telnet_suppress_GA
(*see RFC 858*)
# | Telnet_status
(*see RFC 859*)
# | Telnet_timing_mark
(*see RFC 860*)
# | Telnet_ext_opt_list
(*see RFC 861*)
# | Telnet_end_of_rec
(*see RFC 885*)
# | Telnet_window_size
(*see RFC 1073*)
# | Telnet_term_speed
(*see RFC 1079*)
# | Telnet_term_type
(*see RFC 1091*)
# | Telnet_X_display
(*see RFC 1096*)
# | Telnet_linemode
(*see RFC 1184*)
# | Telnet_flow_ctrl
(*see RFC 1372*)
# | Telnet_auth
(*see RFC 1416*)
# | Telnet_new_environ
(*see RFC 1572 and 1571*)
# | Telnet_option of int
(*all other options*)

telnet_negotiated_option: names for the most common options, and the generic name Telnet_option for other options.

type telnet_option_state =
# | Not_negotiated
# | Accepted
# | Rejected

An option has one of three states:

  • Not_negotiated: There was no negotiation about the option. This means that the option is turned off (but this client is allowed to reject it explicitly)
  • Accepted: Both sides have accepted the option.
  • Rejected: One side has rejected the option. This also means that the option is off, but the client refuses to send further acknoledgements that the option is off (to avoid endless negotiation loops).
val char_of_option : telnet_negotiated_option -> char

Converts the option name to the character representing it on the octet-stream level.

val option_of_char : char -> telnet_negotiated_option

Converts a character representing an option to the internal option name.

type telnet_connector =
# | Telnet_connect of (string * int)
# | Telnet_socket of Unix.file_descr


  • Telnet_connect(host,port): The client connects to this port.
  • Telnet_socket s: The client uses an already connected socket.

Why Telnet_socket? Telnet is a symmetrical protocol; client and servers implement the same protocol features (the only difference is the environment: a client is typically connected with a real terminal; a server is connected with a pseudo terminal). This simply means that this implementation of a client can also be used as a server implementation. You need only to add code which accepts new connections and which passes these connections over to a telnet_session object via Telnet_socket.

class telnet_session :


The telnet_session object has two queues, one for arriving data, one for data to send. Once the session object is attached to an event system, it connects to the remote peer, and processes the queues. Input is appended to the input queue; output found on the output queue is sent to the other side. If input arrives, a callback function is invoked. You may close the output side of the socket by putting Telnet_eof onto the output queue. Once the EOF marker has been received, a Telnet_eof is appended to the input queue, and the connection is closed (completely). The session object detaches from the event system automatically in this case.


Set an input handler as callback function in the session object. The input handler is called when new input data have been arrived. It should inspect the input queue, process the queue as much as possible, and it should remove the processed items from the queue. While processing, it may add new items to the output queue.

If you are not within the callback function and add items to the output queue, the session object will not detect that there are new items to send - unless you invoke the update method.

If you want option negotiation, it is the simplest way to use the special option negotiation methods. Configure the options as you want (invoking enable, disable etc), but do not forget to modify the way input is processed. Every Telnet_will, _wont, _do, and _dont command must be passed to process_option_command.

method set_connection : telnet_connector -> unit

Sets the host name and the port of the remote server to contact.

method set_event_system : Unixqueue.event_system -> unit

Sets the event system to use. By default, a private event system is used.

method set_callback : (bool -> unit) -> unit

Sets the callback function. This function is called after new commands have been put onto the input queue. The argument passed to the callback function indicates whether a 'Synch' sequence was received from the remote side or not.

Note Synch: If the client sees a data mark command it will assume that it is actually a Synch sequence. The client automatically discards any Telnet_data commands from the input queue (but not Telnet_datas inside subnegotiations). The data mark command itself remains on the queue.

method set_exception_handler : (exn -> unit) -> unit

Sets the exception handler. Every known error condition is caught and passed to the exception handler. The exception handler can do whatever it wants to do. If it raises again an exception, the new exception is always propagated up to the caller (whoever this is). Often the caller is the event system scheduler (i.e. Unixqueue.run); see the documention there.

If you do not set the exception handler, a default handler is active. It first resets the session (see method reset), and then wraps the exception into the Telnet_protocol exception, and raises this exception again.

method output_queue : telnet_command Queue.t

The queue of commands to send to the remote side. If you add new commands to this queue, do not forget to invoke the update method which indicates to the event system that new data to send is available. After commands have been sent, they are removed from the queue.

method input_queue : telnet_command Queue.t

The queue of commands received from the remote side. This class only adds commands to the queue (and invokes the callback function). The user of this class is responsible for removing commands from the queue which have been processed.

method get_options : telnet_options

Get the configuration options.

method set_options : telnet_options -> unit

Set the configuration options.

method reset : unit -> unit

Closes the connection immediately and empties all queues. All negotiated options are reset, too.

Telnet options

The following methods deal with Telnet protocol options. These are negotiated between local and remote side by the Will, Won't, Do and Don't commands.

The "local" options describe the modification of the behaviour of the local side; the "remote" options describe the modifications of the remote side. Both set of options are independent. This object may track the set of accepted and rejected options if the following methods are used; but this works only if the Telnet_will, _wont, _do, and _dont commands received from the remote side are processed by process_option_command. So you need to invoke this method for the mentioned commands in your command interpretation loop.

The idea is: If you enable an option, it is possible to switch it on. If the remote side requests the option to be enabled, the request will be acknowledged. If the remote side does not request the option, it remains off.

You can also actively demand an option (offer_local_option, request_remote_option); this is of course only possible if the option is already enabled. In this case the client tries actively to switch it on.

You can also disable an option. If the option is on, the client actively rejects the option; following the Telnet protocol this is always possible (rejections cannot be rejected).

The reset methods are somewhat dangerous. They simply reset the internal state of the client, but do not negotiate. This possibility was added to allow the Timing Mark option to send again timing marks even if the previous timing marks have already been accepted. After reset, the client thinks the option was never negotiated; but nothing is done to tell the remote side about this.

option_negotiation_is_over: true if no option negotiation is pending (i.e. nothing has still to be acknowledged).

method enable_local_option : telnet_negotiated_option -> unit
method enable_remote_option : telnet_negotiated_option -> unit
method disable_local_option : telnet_negotiated_option -> unit
method disable_remote_option : telnet_negotiated_option -> unit
method offer_local_option : telnet_negotiated_option -> unit
method request_remote_option : telnet_negotiated_option -> unit
method reset_local_option : telnet_negotiated_option -> unit
method reset_remote_option : telnet_negotiated_option -> unit
method get_local_option : telnet_negotiated_option -> telnet_option_state
method get_remote_option : telnet_negotiated_option -> telnet_option_state
method option_negotiation_is_over : bool
method process_option_command : telnet_command -> unit
method fetch_subnegotiation : string option

This method should be called as follows: If you find a Telnet_sb at the beginning of the input queue, remove this command Queue.take, and invoke fetch_subnegotiation. This method scans the queue and looks for the associated Telnet_se command. If it does not find it, None is returned. If Telnet_se is found, the parameter enclosed by the two commands is returned as Some s where s is the parameter string. Furthermore, in the latter case the data items and the closing Telnet_se are removed from the queue.

Running the session

method attach : unit -> unit

Attach to the event system. After being attached, the client is ready to work.

method run : unit -> unit

Run the event system

method update : unit -> unit

If there are commands in the output queue, the event system is signaled that this client wants to do network I/O.

method send_synch : telnet_command list -> unit

At the next output oppurtunity, a Synch sequence is sent to the remote peer. This means that the passed commands, extended by an additional Data Mark command, are sent to the peer as urgent data.

Sending a Synch sequence has higher priority than the output queue; processing of the output queue is deferred until the Synch sequence has been completely sent.

method expect_input : bool -> unit

Set whether the timeout value is to be applied to the input side of the connection. This is true by default.

A telnet session


module Debug : sig
val enable : bool Pervasives.ref

Enables [root:Netlog]-style debugging of this module By default, the exchanged Telnet commands are logged. This can be extended by setting the verbose_input and verbose_output options.