module Nethttpd_kernel

: sig

The protocol kernel of the HTTP daemon

This module mainly defines the http_protocol class which implements the exchange of messages with a HTTP client. The request messages are represented as sequence of req_token values. The response is encapsulated in a separate http_response class. The contents of the response are represented as sequence of resp_token values.

type fatal_error = [
| `Broken_pipe
| `Broken_pipe_ignore
| `Message_too_long
| `Timeout
| `Unix_error of Unix.error
| `Server_error

These are the serious protocol violations after that the daemon stops any further processing.

Note that `Timeout refers to a timeout in the middle of a request. `Broken_pipe_ignore is the "harmless" version of `Broken_pipe (see config_suppress_broken_pipe).

Long messages are fatal because it is suspected that they are denial of service attacks. The kernel generates `Message_too_long only for long headers, not for long bodies.

Fatal server errors can happen when exceptions are not properly handled. As last resort the HTTP daemon closes the connection without notifying the client.

val string_of_fatal_error : fatal_error -> string

Convert error to a string, for logging

type bad_request_error = [
| `Bad_header_field of string
| `Bad_header
| `Bad_trailer
| `Bad_request_line
| `Request_line_too_long
| `Protocol_not_supported
| `Unexpected_eof
| `Format_error of string

A bad request is a violation where the current request cannot be decoded, and it is not possible to accept further requests over the current connection.

val string_of_bad_request_error : bad_request_error -> string

Convert error to a string, for logging

val status_of_bad_request_error : bad_request_error -> Nethttp.http_status

Returns the best response code for the error

type data_chunk = string * int * int

A data_chunk is a substring of a string. The substring is described by the triple (s, pos, len) where s is the container, pos is the position where the substring begins, and len its length.

type status_line = int * string

= (code, phrase)

type transfer_coding = [
| `Identity
| `Chunked
type resp_token = [
| `Resp_info_line of status_line * Nethttp.http_header
| `Resp_status_line of status_line
| `Resp_header of Nethttp.http_header
| `Resp_body of data_chunk
| `Resp_trailer of Nethttp.http_trailer
| `Resp_end
| `Resp_action of unit -> unit

The resp_token represents a textual part of the response to send:

  • `Resp_info_line is an informational status line (code=100..199). There can be several informational lines, and they can be accompanied with their own headers. Such lines are only sent to HTTP/1.1 clients.
  • `Resp_status_line is the final status line to send (code >= 200)
  • `Resp_header is the whole response header to send
  • `Resp_body is the next part of the response body to send.
  • `Resp_trailer is the whole response trailer to send (currently ignored)
  • `Resp_action is special because it does not directly represent a token to send. The argument is a function which is called when the token is the next token on the active event queue. The function is also called when the event queue is dropped because of an error (the state of the response object indicates this). The function must not raise exceptions except Unix_error, and it must not block.
val resp_100_continue : resp_token

The predefined token for the "100 Continue" response

type resp_state = [
| `Inhibited
| `Queued
| `Active
| `Processed
| `Error
| `Dropped

The response state:

  • `Inhibited = it is not yet allowed to start the response
  • `Queued = the response waits on the queue for activation
  • `Active = the response is currently being transmitted
  • `Processed = the response has been completely sent
  • `Error = an error occurred during the transmission of this response
  • `Dropped = an earlier response forced to close the connection, and this response is dequeued
type front_token = [
| `Resp_wire_data of data_chunk
| `Resp_end

Tokens generated by http_response:

  • `Resp_wire_data are data tokens.
  • `Resp_end indicates the end of the response.
exception Send_queue_empty
type announcement = [
| `Ignore
| `Ocamlnet
| `Ocamlnet_and of string
| `As of string

See config

class type http_response =

Represents the action of sending the response

This class has an internal queue of response tokens that are not yet processed. One can easily add new tokens to the end of the queue (send).

The class is responsible for determining the transfer encoding:

  • When the HTTP version is 1.0, the encoding is always "identity", and the connection will be closed after the response.
  • When the HTTP version is 1.1, and there is a Content-length header, the encoding will be selected as "identity". It is checked whether the body has really this length. If too short, it is suggested to close the connection. If too long, the extra part of the message is silently dropped.
  • When the HTTP version is 1.1, and there is no Content-length header, the encoding will be selected as "chunked".

Currently, the TE request header is not taken into account. The trailer is always empty.

The following headers are set (or removed) by this class:

  • Transfer-Encoding
  • Trailer
  • Date
  • Connection
  • Upgrade
  • Server (it is appended to this field)

Responses for HEAD requests have the special behaviour that the body is silently dropped. The calculation of header fields is not affected by this. This means that HEAD can be easily implemented by doing the same as for GET.

Responses for other requests that must not include a body must set Content-Length to 0.

These methods can be called by the content provider:

method state : resp_state

Reports the state. The initial state is `Inhibited

method bidirectional_phase : bool

The bidrectional phase starts after "100 Continue" has been sent to the client, and stops when the response body begins. The bidirectional phase is special for the calculation of timeout values (input determines the timeout although the response has started).

method send : resp_token -> unit

Add token to the end of the send queue

method send_queue_empty : bool

Return whether the send queue is empty. When the state is `Inhibited, this method fakes an empty queue.

method protocol : Nethttp.protocol

The HTTP version of the response. This is currently always HTTP/1.1, but maybe we need to fake lower versions for buggy clients. Let's see what comes.

method close_connection : bool

Returns whether the connection should be closed after this response. This flag should be evaluated when the `Resp_end front token has been reached.

method transfer_encoding : transfer_coding

Returns the selected transfer encoding. This is valid after the header has been passed to this object with send.

method front_token : front_token

The first token of the queue, represented as data_chunk. Raises Send_queue_empty when there is currently no front token, or the state is `Inhibited. If there is a front token, it will never have length 0.

Note that Unix_error exceptions can be raised when `Resp_action tokens are processed.

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

The function will be called when either set_state changes the state, or when the send queue becomes empty. Note that the callback must never fail, it is called in situations that make it hard to recover from errors.

method body_size : int64

Accumulated size of the response body

These methods must only be called by the HTTP protocol processor:

method set_state : resp_state -> unit

Sets the state

method advance : int -> unit

Tell this object that n bytes of the front token could be really sent using Unix.write. If this means that the whole front token has been sent, the next token is pulled from the queue and is made the new front token. Otherwise, the data chunk representing the front token is modified such that the position is advanced by n, and the length is reduced by n.

Encapsultation of the HTTP response for a single request

class http_response_impl : ?close:bool option -> ?suppress_body:bool option -> int64 -> Nethttp.protocol -> announcement -> http_response

Exported for debugging and testing only

val send_static_response : http_response -> Nethttp.http_status -> Nethttp.http_header option -> string -> unit

Sends the string argument as response body, together with the given status and the header (optional). Response header fields are set as follows:

  • The Content-Length is set to the length of the string.
  • The Content-Type is set to "text/html" unless given by the header. If the header object is passed in, these modifications are done directly in this object as side effect.
val send_file_response : http_response -> Nethttp.http_status -> Nethttp.http_header option -> Unix.file_descr -> int64 -> unit

Sends the contents of a file as response body, together with the given status and the header (optional). The descriptor must be a file descriptor (that cannot block). The int64 number is the length of the body. Response header fields are set as follows:

  • The Content-Length is set to the length of the string.
  • The Content-Type is set to "text/html" unless given by the header.

Note that Content-Range is not set automatically, even if the file is only partially transferred.

If the header object is passed in, these modifications are done directly in this object as side effect.

The function does not send the file immediately, but rather sets the http_response object up that the next chunk of the file is added when the send queue becomes empty. This file will be closed when the transfer is done.

type request_line = Nethttp.http_method * Nethttp.protocol

The method (including the URI), and the HTTP version

type req_token = [
| `Req_expect_100_continue
| `Req_body of data_chunk
| `Req_trailer of Nethttp.http_trailer
| `Req_end
| `Eof
| `Fatal_error of fatal_error
| `Bad_request_error of bad_request_error * http_response
| `Timeout

A req_token represents a textual part of the received request:

  • `Req_header is the full received header. Together with the header, the corresponding http_response object is returned which must be used to transmit the response.
  • `Req_expect_100_continue is generated when the client expects that the server sends a "100 Continue" response (or a final status code) now. One should add `Resp_info_line resp_100_continue to the send queue if the header is acceptable, or otherwise generate an error response. In any case, the rest of the request must be read until `Req_end.
  • `Req_body is a part of the request body. The transfer-coding, if any, is already decoded.
  • `Req_trailer is the received trailer
  • `Req_end indicates the end of the request (the next request may begin immediately).
  • `Eof indicates the end of the stream
  • `Bad_request_error indicates that the request violated the HTTP protocol in a serious way and cannot be decoded. It is required to send a "400 Bad Request" response. The following token will be `Eof.
  • `Fatal_error indicates that the connection crashed. The following token will be `Eof.
  • `Timeout means that nothing has been received for a certain amount of time, and the protocol is in a state that the next request can begin. The following token will be `Eof.

Note that it is always allowed to send tokens to the client. The protocol implementation takes care that the response is transmitted at the right point in time.

val string_of_req_token : req_token -> string

For debugging

exception Recv_queue_empty
class type http_protocol_config =
method config_max_reqline_length : int

Maximum size of the request line. Longer lines are immediately replied with a "Request URI too long" response. Suggestion: 32768.

method config_max_header_length : int

Maximum size of the header, including the request line. Longer headers are treated as attack, and cause the fatal error `Message_too_long. Suggestion: 65536.

method config_max_trailer_length : int

Maximum size of the trailer

method config_limit_pipeline_length : int

Limits the length of the pipeline (= unreplied requests). A value of 0 disables pipelining. A value of n allows that another request is received although there are already n unreplied requests.

method config_limit_pipeline_size : int

Limits the size of the pipeline in bytes. If the buffered bytes in the input queue exceed this value, the receiver temporarily stops reading more data. The value 0 has the effect that even the read-ahead of data of the current request is disabled. The value (-1) disables the receiver completely (not recommended).

method config_announce_server : announcement

Whether to set the Server header:

  • `Ignore: The kernel does not touch the Server header.
  • `Ocamlnet: Announce this web server as "Ocamlnet/<version>"
  • `Ocamlnet_and s: Announce this web server as s and append the Ocamlnet string.
  • `As s: Announce this web server as s
method config_suppress_broken_pipe : bool

Whether to suppress `Broken_pipe errors. Instead `Broken_pipe_ignore is reported.

Configuration values for the HTTP kernel

val default_http_protocol_config : http_protocol_config

Default config:

  • config_max_reqline_length = 32768
  • config_max_header_length = 65536
  • config_max_trailer_length = 32768
  • config_limit_pipeline_length = 5
  • config_limit_pipeline_size = 65536
  • config_announce_server = `Ocamlnet
  • config_suppress_broken_pipe = false
class modify_http_protocol_config : ?config_max_reqline_length:int option -> ?config_max_header_length:int option -> ?config_max_trailer_length:int option -> ?config_limit_pipeline_length:int option -> ?config_limit_pipeline_size:int option -> ?config_announce_server:announcement option -> ?config_suppress_broken_pipe:bool option -> http_protocol_config -> http_protocol_config

Modifies the passed config object as specified by the optional arguments

class http_protocol : http_protocol_config -> Unix.file_descr ->

Exchange of HTTP messages

In fd one must pass the already connected socket. It must be in non- blocking mode.

How to use this class: Basically, one invokes cycle until the whole message exchange on fd is processed. cycle receives data from the socket and sends data to the socket. There are two internal queues:

The receive queue stores parts of received requests as req_token. One can take values from the front of this queue by calling receive.

The response queue stores http_response objects. Each of the objects corresponds to a request that was received before. This queue is handled fully automatically, but one can watch its length to see whether all responses are actually transmitted over the wire.

The basic algorithm to process messages is:

      let rec next_token () =
        if proto # recv_queue_len = 0 then (
          proto # cycle ();
          proto # receive()
      let cur_token = ref (next_token()) in
      while !cur_token <> `Eof do
        (* Process first token of next request: *)
        match !cur_token with
         | `Req_header(req_line, header, resp) ->
              (* Depending on [req_line], read further tokens until [`Req_end] *)
              (* Switch to the first token of the next message: *)
              cur_token := next_token()
         | `Timeout -> ...
         | `Bad_request_error(e,resp) -> 
               (* Generate 400 error, send it to [resp] *)
               (* Switch to the first token of the next message: *)
               cur_token := next_token()
         | `Fatal_error e -> failwith "Crash"
         | _ -> assert false
      while proto # resp_queue_len > 0 do
        proto # cycle ();
      proto # shutdown()

See the file tests/easy_daemon.ml for a complete implementation of this.

As one can see, it is essential to watch the lengths of the queues in order to figure out what has happened during cycle.

When the body of the request is empty, `Req_body tokens are omitted. Note that for requests like GET that always have an empty body, it is still possible that an errorneous client sends a body, and that `Req_body tokens arrive. One must accept and ignore these tokens.

Error handling: For serious errors, the connection is immediately aborted. In this case, receive returns a `Fatal_error token. Note that the queued responses cannot be sent! An example of this is `Broken_pipe.

There is a large class of non-serious errors, esp. format errors in the header and body. It is typical of these errors that one cannot determine the end of the request properly. For this reason, the daemon stops reading further data from the request, but the response queue is still delivered. For these errors, receive returns a `Bad_request_error token. This token contains a http_response object that must be filled with a 400 error response.

method cycle : ?block:float -> unit -> unit

Looks at the file descriptor. If there is data to read from the descriptor, and there is free space in the input buffer, additional data is read into the buffer. It is also tried to interpret the new data as req_tokens, and if possible, new req_tokens are appended to the receive queue.

If the response queue has objects, and there is really data one can send, and if the socket allows one to send data, it is tried to send as much data as possible.

The option block (default: 0) can be set to wait until data can be exchanged with the socket. This avoids busy waiting. The number is the duration in seconds to wait until the connection times out (0 means not to wait at all, -1 means to wait infinitely). When a timeout happens, and there is nothing to send, and the last request was fully processed, receive will simply return `Timeout (i.e. when waiting_for_next_message is true). Otherwise, the fatal error `Timeout is generated.

method receive : unit -> req_token

Returns the first req_token from the receive queue. Raises Recv_queue_empty when the queue is empty (= has no new data)

method peek_recv : unit -> req_token

Peeks the first token, but leaves it in the queue. Raises Recv_queue_empty when the queue is empty.

method recv_queue_len : int

Returns the length of the receive queue (number of tokens)

method resp_queue_len : int

Returns the length of the internal response queue (number of http_response objects that have not yet fully processed)

method pipeline_len : int

Returns the number of unanswered requests = Number of received `Req_end tokens minus number of responses in state `Processed. Note that pipeline_len can become -1 when bad requests are responded.

method recv_queue_byte_size : int

Returns the (estimated) size of the input queue in bytes

method waiting_for_next_message : bool

Whether the kernel is currently waiting for the beginning of a new arriving HTTP request. This is false while the request is being received.

method input_timeout_class : [
| `Normal
| `Next_message
| `None

Suggests the calculation of a timeout value for input:

  • `Normal: The normal timeout value applies
  • `Next_message: The timeout value applies while waiting for the next message
  • `None: The connection is output-driven, no input timeout value
method shutdown : unit -> unit

Shuts the socket down. Note: the descriptor is not closed.

method timeout : unit -> unit

Process a timeout condition as cycle does

method abort : fatal_error -> unit

Stops the transmission of data. The receive queue is cleared and filled with the two tokens `Fatal_error and `Eof. The response queue is cleared. The cycle method will return immediately without doing anything.

method fd : Unix.file_descr

Just returns the socket

method do_input : bool

Returns true iff the protocol engine is interested in new data from the socket. Returns false after EOF and after errors.

method do_output : bool

Returns true iff the protocol engine has data to output to the socket

method need_linger : bool

Returns true when a lingering close operation is needed to reliably shut down the socket. In many cases, this expensive operation is not necessary. See the class lingering_close below.

method config : http_protocol_config

Just returns the configuration

method test_coverage : string list

For testing: returns a list of tokens indicating into which cases the program ran.

The core event loop of the HTTP daemon

class lingering_close : ?preclose:(unit -> unit) option -> Unix.file_descr ->

Closes a file descriptor using the "lingering close" algorithm


while lc # lingering do lc # cycle ~block:true () done
method cycle : ?block:bool -> unit -> unit

Reads data from the file descriptor until EOF or until a fixed timeout is over. Finally, the descriptor is closed. If block is set, the method blocks until data is available. (Default: false)

method lingering : bool

Whether the socket is still lingering

method fd : Unix.file_descr

The file descriptor

Closes a file descriptor using the "lingering close" algorithm. The optional preclose function is called just before Unix.close.


module Debug : sig
val enable : bool Pervasives.ref

Enables [root:Netlog]-style debugging of this module