4  HTTP server

4 HTTP server

The HTTP server, also referred to as httpd, handles HTTP requests as described in RFC 2616 with a few exceptions, such as gateway and proxy functionality. The server supports IPv6 as long as the underlying mechanisms also do so.

The server implements numerous features, such as:

  • Secure Sockets Layer (SSL)
  • Erlang Scripting Interface (ESI)
  • Common Gateway Interface (CGI)
  • User Authentication (using Mnesia, Dets or plain text database)
  • Common Logfile Format (with or without disk_log(3) support)
  • URL Aliasing
  • Action Mappings
  • Directory Listings

The configuration of the server is provided as an Erlang property list.

As of Inets 5.0 the HTTP server is an easy to start/stop and customize web server providing the most basic web server functionality. Inets is designed for embedded systems and if you want a full-fledged web server there are other erlang open source alternatives.

Almost all server functionality has been implemented using an especially crafted server API, which is described in the Erlang Web Server API. This API can be used to enhance the core server functionality, for example with custom logging and authentication.

The following is to be put in the Erlang node application configuration file to start an HTTP server at application startup:

      [{inets, [{services, [{httpd, [{proplist_file,
                {httpd, [{proplist_file,

The server is configured using an Erlang property list. For the available properties, see httpd(3).

The available configuration properties are as follows:

     httpd_service() -> {httpd, httpd()}
     httpd()         -> [httpd_config()] 
     httpd_config()  -> {proplist_file, file()}
                        {debug, debug()} |
                        {accept_timeout, integer()}
     debug()         -> disable | [debug_options()]
     debug_options() -> {all_functions, modules()} | 
                        {exported_functions, modules()} |
                        {disable, modules()}
     modules()       -> [atom()]


{proplist_file, file()}

File containing an Erlang property list, followed by a full stop, describing the HTTP server configuration.

{debug, debug()}

Can enable trace on all functions or only exported functions on chosen modules.

{accept_timeout, integer()}

Sets the wanted time-out value for the server to set up a request connection.

Start Inets:

      1 > inets:start().

Start an HTTP server with minimal required configuration. If you specify port 0, an arbitrary available port is used, and you can use function info to find which port number that was picked:

      2 > {ok, Pid} = inets:start(httpd, [{port, 0},
      {server_name,"httpd_test"}, {server_root,"/tmp"},
      {document_root,"/tmp/htdocs"}, {bind_address, "localhost"}]).
      {ok, 0.79.0}      

Call info:

      3 >  httpd:info(Pid).
      {bind_address, {127,0,0,1}},

Reload the configuration without restarting the server:

    4 > httpd:reload_config([{port, 59408},
      {server_name,"httpd_test"}, {server_root,"/tmp/www_test"},
      {bind_address, "localhost"}], non_disturbing).

port and bind_address cannot be changed. Clients trying to access the server during the reload get a service temporary unavailable answer.

      5 >  httpd:info(Pid, [server_root, document_root]).
      6 > ok = inets:stop(httpd, Pid).


      6 > ok = inets:stop(httpd, {{127,0,0,1}, 59408}).

Notice that bind_address must be the IP address reported by function info and cannot be the hostname that is allowed when putting in bind_address.

Inets HTTP server provides two ways of creating dynamic web pages, each with its own advantages and disadvantages:

CGI scripts

Common Gateway Interface (CGI) scripts can be written in any programming language. CGI scripts are standardized and supported by most web servers. The drawback with CGI scripts is that they are resource-intensive because of their design. CGI requires the server to fork a new OS process for each executable it needs to start.


Erlang Server Interface (ESI) functions provide a tight and efficient interface to the execution of Erlang functions. This interface, on the other hand, is Inets specific.

The module mod_cgi enables execution of CGI scripts on the server. A file matching the definition of a ScriptAlias config directive is treated as a CGI script. A CGI script is executed by the server and its output is returned to the client.

The CGI script response comprises a message header and a message body, separated by a blank line. The message header contains one or more header fields. The body can be empty.


"Content-Type:text/plain\nAccept-Ranges:none\n\nsome very
	plain text"

The server interprets the message headers and most of them are transformed into HTTP headers and sent back to the client together with the message-body.

Support for CGI-1.1 is implemented in accordance with RFC 3875.

The Erlang server interface is implemented by module mod_esi.

ERL Scheme

The erl scheme is designed to mimic plain CGI, but without the extra overhead. An URL that calls an Erlang erl function has the following syntax (regular expression):


*** depends on how the ErlScriptAlias config directive has been used.

The module Module referred to must be found in the code path, and it must define a function Function with an arity of two or three. It is preferable to implement a function with arity three, as it permits to send chunks of the web page to the client during the generation phase instead of first generating the whole web page and then sending it to the client. The option to implement a function with arity two is only kept for backwards compatibility reasons. For implementation details of the ESI callback function, see mod_esi(3).

Three types of logs are supported: transfer logs, security logs, and error logs. The de-facto standard Common Logfile Format is used for the transfer and security logging. There are numerous statistics programs available to analyze Common Logfile Format. The Common Logfile Format looks as follows:

remotehost rfc931 authuser [date] "request" status bytes


Remote hostname.
The client remote username (RFC 931).
The username used for authentication.
Date and time of the request (RFC 1123).
The request line exactly as it came from the client (RFC 1945).
The HTTP status code returned to the client (RFC 1945).
The content-length of the document transferred.

Internal server errors are recorded in the error log file. The format of this file is a more unplanned format than the logs using Common Logfile Format, but conforms to the following syntax:

[date] access to path failed for remotehost, reason: reason

The process of handling an HTTP request involves several steps, such as:

  • Setting up connections, sending and receiving data.
  • URI to filename translation.
  • Authentication/access checks.
  • Retrieving/generating the response.
  • Logging.

To provide customization and extensibility of the request handling of the HTTP servers, most of these steps are handled by one or more modules. These modules can be replaced or removed at runtime and new ones can be added. For each request, all modules are traversed in the order specified by the module directive in the server configuration file. Some parts, mainly the communication- related steps, are considered server core functionality and are not implemented using the Erlang web server API. A description of functionality implemented by the Erlang webserver API is described in Section Inets Web Server Modules.

A module can use data generated by previous modules in the Erlang webserver API module sequence or generate data to be used by consecutive Erlang Web Server API modules. This is possible owing to an internal list of key-value tuples, referred to as interaction data.


Interaction data enforces module dependencies and is to be avoided if possible. This means that the order of modules in the modules property is significant.

Each module that implements server functionality using the Erlang web server API is to implement the following call back functions:

  • do/1 (mandatory) - the function called when a request is to be handled
  • load/2
  • store/2
  • remove/1

The latter functions are needed only when new config directives are to be introduced. For details, see httpd(3).

The convention is that all modules implementing some web server functionality has the name mod_*. When configuring the web server, an appropriate selection of these modules is to be present in the module directive. Notice that there are some interaction dependencies to take into account, so the order of the modules cannot be random.

This module runs CGI scripts whenever a file of a certain type or HTTP method (see RFC 1945) is requested.

Uses the following Erlang Web Server API interaction data:

Exports the following Erlang Web Server API interaction data, if possible:

{new_request_uri, RequestURI}
An alternative RequestURI has been generated.

The mod_alias module makes it possible to map different parts of the host file system into the document tree, that is, creates aliases and redirections.

Exports the following Erlang Web Server API interaction data, if possible:

{real_name, PathData}
PathData is the argument used for API function mod_alias:path/3.

The mod_auth(3) module provides for basic user authentication using textual files, Dets databases as well as Mnesia databases.

Uses the following Erlang Web Server API interaction data:

Exports the following Erlang Web Server API interaction data:

{remote_user, User}
The username used for authentication.
Mnesia As Authentication Database

If Mnesia is used as storage method, Mnesia must be started before the HTTP server. The first time Mnesia is started, the schema and the tables must be created before Mnesia is started. A simple example of a module with two functions that creates and start Mnesia is provided here. Function first_start/0 is to be used the first time. It creates the schema and the tables. start/0 is to be used in consecutive startups. start/0 starts Mnesia and waits for the tables to be initiated. This function must only be used when the schema and the tables are already created.


first_start() ->
                        [{type, bag},
                         {disc_copies, [node()]},
                         {attributes, record_info(fields, 
                        [{type, bag},
                         {disc_copies, [node()]},          
                         {attributes, record_info(fields, 
    mnesia:wait_for_tables([httpd_user, httpd_group], 60000).

start() ->
    mnesia:wait_for_tables([httpd_user, httpd_group], 60000).  

To create the Mnesia tables, we use two records defined in mod_auth.hrl, so that file must be included. first_start/0 creates a schema that specifies on which nodes the database is to reside. Then it starts Mnesia and creates the tables. The first argument is the name of the tables, the second argument is a list of options of how to create the table, see mnesia(3), documentation for more information. As the implementation of the mod_auth_mnesia saves one row for each user, the type must be bag. When the schema and the tables are created, function mnesia:start/0 is used to start Mnesia and waits for the tables to be loaded. Mnesia uses the directory specified as mnesia_dir at startup if specified, otherwise Mnesia uses the current directory. For security reasons, ensure that the Mnesia tables are stored outside the document tree of the HTTP server. If they are placed in the directory which it protects, clients can download the tables. Only the Dets and Mnesia storage methods allow writing of dynamic user data to disk. plain is a read only method.

This module handles invoking of CGI scripts.

This module generates an HTML directory listing (Apache-style) if a client sends a request for a directory instead of a file. This module must be removed from the Modules config directive if directory listings is unwanted.

Uses the following Erlang Web Server API interaction data:

Exports the following Erlang Web Server API interaction data:

{mime_type, MimeType}
The file suffix of the incoming URL mapped into a MimeType.

Standard logging using the "Common Logfile Format" and disk_log(3).

Uses the following Erlang Web Server API interaction data:

  • remote_user - from mod_auth

The mod_esi(3) module implements the Erlang Server Interface (ESI) providing a tight and efficient interface to the execution of Erlang functions.

Uses the following Erlang web server API interaction data:

  • remote_user - from mod_auth

Exports the following Erlang web server API interaction data:

{mime_type, MimeType}
The file suffix of the incoming URL mapped into a MimeType

This module is responsible for handling GET requests to regular files. GET requests for parts of files is handled by mod_range.

Uses the following Erlang web server API interaction data:

This module is responsible for handling HEAD requests to regular files. HEAD requests for dynamic content is handled by each module responsible for dynamic content.

Uses the following Erlang Web Server API interaction data:

Standard logging using the "Common Logfile Format" and text files.

Uses the following Erlang Web Server API interaction data:

  • remote_user - from mod_auth

This module responses to requests for one or many ranges of a file. This is especially useful when downloading large files, as a broken download can be resumed.

Notice that request for multiple parts of a document report a size of zero to the log file.

Uses the following Erlang Web Server API interaction data:

This module controls that the conditions in the requests are fulfilled. For example, a request can specify that the answer only is of interest if the content is unchanged since the last retrieval. If the content is changed, the range request is to be converted to a request for the whole file instead.

If a client sends more than one of the header fields that restricts the servers right to respond, the standard does not specify how this is to be handled. httpd(3) controls each field in the following order and if one of the fields does not match the current state, the request is rejected with a proper response:





Uses the following Erlang Web Server API interaction data:

Exports the following Erlang Web Server API interaction data:

{if_range, send_file}
The conditions for the range request are not fulfilled. The response must not be treated as a range request, instead it must be treated as an ordinary get request.

The mod_security module serves as a filter for authenticated requests handled in mod_auth(3). It provides a possibility to restrict users from access for a specified amount of time if they fail to authenticate several times. It logs failed authentication as well as blocking of users, and it calls a configurable callback module when the events occur.

There is also an API to block or unblock users manually. This API can also list blocked users or users who have been authenticated within a configurable amount of time.

mod_trace is responsible for handling of TRACE requests. Trace is a new request method in HTTP/1.1. The intended use of trace requests is for testing. The body of the trace response is the request message that the responding web server or proxy received.