## 4 HTTP server

### 4.1 Introduction

The HTTP server also refered to as `httpd` handles HTTP requests as described in RFC 2616 with a few exceptions such as gateway and proxy functionality. The same is true for servers written by NCSA and others.

The server implements numerous features such as SSL (Secure Sockets Layer), ESI (Erlang Scripting Interface), CGI (Common Gateway Interface), 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 and SSI (Server-Side Includes).

The configuration of the server is done using Apache-style configuration directives. The goal is to be plug-in compatible with Apache.

Allmost all server functionality has been implemented using an especially crafted server API, it is described in the Erlang Web Server API. This API can be used to advantage by all who wants to enhance the server core functionality, for example custom logging and authentication.

### 4.2 HTTP Server Setup

It is possible to start a number of Web servers in an embedded system using the `services` config parameter from an application config file. A minimal application config file (from now on referred to as `inets.config`) starting two HTTP servers typically looks as follows:

          [{inets,
          [{services,[{httpd,"/var/tmp/server_root/conf/8888.conf"},
          {httpd,"/var/tmp/server_root/conf/8080.conf"}]}]}].

A server config file is specified for each HTTP server to be started. The config file syntax and semantics is described in the run time configuration section.

`inets.config` can be tested by copying the example server root to a specific installation directory, as described in the run time configuration section. The example below shows a manual start of an Erlang node, using `inets.config`, and the start of two HTTP servers listening listen on ports 8888 and 8080.

          $ erl -config ./inets
          Erlang (BEAM) emulator version 4.9

          Eshell V4.9  (abort with ^G)
          1> application:start(inets).
          ok

### 4.3 Run-Time Configuration

All functionality in the server can be configured using Apache-style configuration directives stored in a configuration file. Take a look at the example config files in the conf directory (`UNIX: $INETS_ROOT/examples/server_root/conf/, Windows: %INETS_ROOT%\examples\server_root\conf\`) of the server root for a complete understanding.

An alphabetical list of all config directives:

- AccessFileName
- Action
- Alias
- allow
- AuthName
- AuthGroupFile
- AuthUserFile
- BindAddress
- DefaultType
- deny
- <Directory>
- DirectoryIndex
- DisableChunkedTransferEncodingSend
- DocumentRoot
- ErlScriptAlias
- ErlScriptNoCache
- ErlScriptTimeout
- ErrorLog
- ErrorDiskLog
- ErrorDiskLogSize
- EvalScriptAlias
- KeepAlive
- KeepAliveTimeout
- MaxBodySize
- MaxBodyAction
- MaxClients
- MaxHeaderSize
- MaxHeaderAction
- MaxKeepAliveRequests
- Modules
- Port
- require
- SecurityAuthTimeout
- SecurityBlockTime
- SecurityCallbackModule
- SecurityDataFile
- SecurityDiskLog
- SecurityDiskLogSize
- SecurityFailExpireTime
- SecurityLog
- SecurityMaxRetries
- ServerAdmin
- ServerName
- ServerRoot
- Script
- ScriptAlias
- ScriptNoCache
- ScriptTimeout
- SocketType
- SSLCACertificateFile
- SSLCertificateFile
- SSLCertificateKeyFile
- SSLCiphers
- SSLPasswordCallbackFunction
- SSLPasswordCallbackModule
- SSLVerifyClient
- SSLVerifyDepth
- TransferLog
- TransferDiskLog
- TransferDiskLogSize

### 4.4 Erlang Web Server API

Almost all server functionality has been implemented using EWSAPI (Erlang Web Server API) modules. The following modules are available:

httpd_core
  Some core features are not implemented using EWSAPI you can read about these int the virtual module httpd_core.
mod_actions
  Filetype/method-based script execution.
mod_alias
  Aliases and redirects.
mod_auth
  User authentication using text files, mnesia or dets.
mod_browser
  Tries to recognize the clients browser and operating system.
mod_cgi
  Invoking of CGI scripts.
mod_dir
  Basic directory handling.
mod_disk_log
  Standard logging in the Common Logfile Format using `disk_log(3)`.
mod_esi
  Efficient Erlang Scripting.
mod_get
  Handle HTTP GET Method.
mod_head
  Handle HTTP HEAD Method.
mod_htacceess
  User configurable user authentication.
mod_include
  Server-parsed documents.
mod_log
  Standard logging in the Common Logfile Format using text files.
mod_range
  Handles GET requests for parts of files.
mod_responsecontrol
  Controls the restrictions in the request i.e. If-Match, If-Range,If-Modified-Since, and take the appropriate action.
mod_security
  Filter authenticated requests.
mod_trace
  Handles. HTTP TRACE Method

Each module has a man page that further describe it's functionality.

The Modules config directive can be used to alter the server behavior, by alter the EWSAPI Module Sequence. An example module sequence can be found in the example config directory. If this needs to be altered read the EWSAPI Module Interaction section below.

#### 4.4.1 EWSAPI Module Programming

| The Erlang/OTP programming knowledge required to undertake an EWSAPI module is quite high and is not recommended for the average server user. It is best to only use it to add core functionality, e.g. custom authentication or a RFC 2109 implementation.

EWSAPI should only be used to add core functionality to the server. In order to generate dynamic content, for example on-the-fly generated HTML, use the standard CGI or ESI facilities instead.

As seen above the major part of the server functionality has been realized as EWSAPI modules (from now on only called modules). If you intend to write your own server extension start with examining the standard modules (`UNIX: $INETS_ROOT/src/, Windows: %INETS_ROOT%\src\`) `mod_*.erl` and note how to they are configured in the example config directory (`UNIX: $INETS_ROOT/examples/server_root/conf/, Windows: %INETS_ROOT%\examples\server_root\conf\`).

Each module implements `do/1` (mandatory), `load/2`, `store/2` and `remove/1`. The latter functions are needed only when new config directives are to be introduced.

A module can choose to export functions to be used by other modules in the EWSAPI Module Sequence (See Modules config directive). This should only be done as an exception! The goal is to keep each module self-sustained thus making it easy to alter the EWSAPI Module Sequence without any unneccesary module dependencies.

A module can furthermore use data generated by previous modules in the EWSAPI Module Sequence or generate data to be used by consecutive EWSAPI modules. This is made possible due to an internal list of key-value tuples.

| The server executes `do/1` function of each module listed in the Modules config directive. `do/1` takes the record `mod` as an argument, as described below. See httpd.hrl (`UNIX: $INETS_ROOT/src/httpd.hrl, Windows: %INETS_ROOT%\src\httpd.hrl`):

            -record(mod,{data=[],
            socket_type=ip_comm,
            socket,
            config_db,
            method,
            absolute_uri,
            request_uri,
            http_version,
            request_line,
            parsed_header=[],
            entity_body,
            connection}).

The fields of the `mod` record has the following meaning:

`data`
  Type `[{InteractionKey,InteractionValue}]` is used to propagate data between modules. Depicted `interaction_data()` in function type declarations.
`socket_type`
  `socket_type()`, Indicates whether it is a ip socket or a ssl socket.
`socket`
  The actual socket in `ip_comm` or `ssl` format depending on the `socket_type`.
`config_db`
  The config file directives stored as key-value tuples in an ETS-table. Depicted `config_db()` in function type declarations.
`method`
  Type `"GET" | "POST" | "HEAD" | "TRACE"`, that is the HTTP method.
`absolute_uri`
  If the request is a HTTP/1.1 request the URI might be in the absolute URI format. In that case httpd will save the absolute URI in this field. An Example of an absolute URI could be`"http://ServerName:Part/cgi-bin/find.pl?person=jocke"`
`request_uri`
  The `Request-URI` as defined in RFC 1945, for example `"/cgi-bin/find.pl?person=jocke"`
`http_version`
  The `HTTP` version of the request, that is "HTTP/0.9", "HTTP/1.0", or "HTTP/1.1".
`request_line`
  The `Request-Line` as defined in RFC 1945, for example `"GET /cgi-bin/find.pl?person=jocke HTTP/1.0"`.
`parsed_header`
  Type `[{HeaderKey,HeaderValue}]`, `parsed_header` contains all HTTP header fields from the HTTP-request stored in a list as key-value tuples. See RFC 2616 for a listing of all header fields. For example the date field would be stored as: `{"date","Wed, 15 Oct 1997 14:35:17 GMT"}. RFC 2616 defines that HTTP is a case insensitive protocol and the header fields may be in lowercase or upper case. Httpd will ensure that all header field names are in lowe case`.
`entity_body`
  The `Entity-Body` as defined in RFC 2616, for example data sent from a CGI-script using the POST method.
`connection`
  `true | false` If set to true the connection to the client is a persistent connections and will not be closed when the request is served.

A `do/1` function typically uses a restricted set of the `mod` record's fields to do its stuff and then returns a term depending on the outcome, The outcome is either `{proceed,NewData} | {break,NewData} | done`. Which has the following meaning:

`{proceed,OldData}`
  Proceed to next module as nothing happened. `OldData` refers to the `data` field in the incoming `mod` record.
`{proceed,[{response,{StatusCode,Response}}|OldData]}`
  A generated response (`Response`) should be sent back to the client including a status code (`StatusCode`) as defined in RFC 2616.
`{proceed,[{response,{response,Head,Body}}|OldData]}`
  Head is a list of key/value tuples. Each HTTP-header field that will be in the response header must be in the list. The following atoms are allowed header field keys:

                  code,
                  allow,
                  cache_control,
                  content_MD5,
                  content_encoding,
                  content_encoding,
                  content_language,
                  content_length,
                  content_location,
                  content_range,
                  content_type,
                  date,
                  etag,
                  expires,
                  last_modified
                  location,
                  pragma,
                  retry_after,
                  server,
                  trailer,
                  transfer_encoding,

  The key code is a special case since the value to this key is a integer and not a string. The value will be used as status code for the response.
  The benefit of this method is that the same request may be generated for both HTTP/1.1 and HTTP/1.0 clients since the list of header fields will be filtered due to the version of the request. `Body` is either the tuple {Fun,Arg} a list or the atom nobody. If Body is {Fun,Arg} Fun is assumed to be a fun that returns either close, sent or {ok,Body}. If close is returned the connection to the client will be closed. If sent is returned the connection to the client will be maintained if the connection is persitent. If {ok,Body} is returned the Body is sent back to the client as the response body.
  This is the preffered response since it makes it a lot easier to generate a response that can be sent back to both HTTP/1.0 and HTTP/1.1 clients. A warning might be in place that if content_length is not sent to the client, it might hang unless the body is sent with chunked encoding.
`{proceed,[{response,{already_sent,StatusCode,Size}}|OldData]}`
  A generated response has already manually been sent back to the client, using the `socket` provided by the `mod` record (see above), including a valid status code (`StatusCode`) as defined in RFC 1945 and the size (`Size`) of the response in bytes.
`{proceed,[{status,{StatusCode,PhraseArgs,Reason}}}|OldData]}`
  A generic status message should be sent back to the client (if the next module in the EWSAPI Module Sequence does not think otherwise!) including at status code (`StatusCode`) as defined in RFC 1945, a term describing how the client will be informed (`PhraseArgs`) and a reason (`Reason`) to why it happened. Read more about `PhraseArgs` in httpd_util:message/3.
`{break,NewData}`
  Has the same semantics as `proceed` above but with one important exception; No more modules in the EWSAPI Module Sequence are executed. Use with care!
`done`
  No more modules in the EWSAPI Module Sequence are executed and no response should be sent back to the client. If no response is sent back to the client, using the `socket` provided by the `mod` record, the client will typically get a "Document contains no data...".

| Each consecutive module in the EWSAPI Module Sequence can choose to ignore data returned from the previous module either by trashing it or by "enhancing" it.

Keep in mind that there exist numerous utility functions to help you as an EWSAPI module programmer, e.g. nifty lookup of data in ETS-tables/key-value lists and socket utilities. You are well advised to read httpd_util(3) and httpd_socket(3).

#### 4.4.2 EWSAPI Module Configuration

An EWSAPI module can define new config directives thus making it configurable for a server end-user. This is done by implementing `load/2` (mandatory), `store/2` and `remove/1`.

The config file is scanned twice (`load/2` and `store/2`) and a cleanup is done (`remove/1`) during server shutdown. The reason for this is: "A directive A can be dependent upon another directive B which occur either before or after directive A in the config file". If a directive does not depend upon other directives; `store/2` can be left out. Even `remove/1` can be left out if neither `load/2` nor `store/2` open files or create ETS-tables etc.

`load/2` takes two arguments. The first being a row from the config file, that is a config directive in string format such as `"Port 80"`. The second being a list of key-value tuples (which can be empty!) defining a context. A context is needed because there are directives which defines inner contexts, that is directives within directives, such as <Directory>. `load/2` is expected to return:

`eof`
  End-of-file found.
`ok`
  Ignore the directive.
`{ok,ContextList}`
  Introduces a new context by adding a tuple to the context list or reverts to a previous context by removing a tuple from the context list. See <Directory> which introduces a new context and </Directory> which reverts to a previous one (Advice: Look at the source code for `mod_auth:load/2`).
`{ok,ContextList,{DirectiveKey,DirectiveValue}}`
  Introduces a new context (see above) and defines a new config directive, e.g. `{port,80}`.
`{ok,ContextList,[{DirectiveKey,DirectiveValue}]}`
  Introduces a new context (see above) and defines a several new config directives, e.g. `[{port,80},{foo,on}]`.
`{error,Reason}`
  An invalid directive.

An example of a load function from `mod_log.erl`:

            load("TransferLog " ++ TransferLog, []) ->
               {ok, [], {transfer_log, httpd_conf:clean(TransferLog)}};

            load("ErrorLog " ++ ErrorLog, []) ->
               {ok, [], {error_log, httpd_conf:clean(ErrorLog)}}.

`store/2` takes two arguments. The first being a tuple describing a directive (`{DirectiveKey,DirectiveValue}`) and the second argument a list of tuples describing all directives (`[{DirectiveKey,DirectiveValue}]`). This makes it possible for directive A to be dependent upon the value of directive B. `store/2` is expected to return:

`{ok,{DirectiveKey,NewDirectiveValue}}`
  Introduces a new value for the specified directive replacing the old one generated by `load/2`.
`{ok,[{DirectiveKey,NewDirectiveValue}]}`
  Introduces new values for the specified directives replacing the old ones generated by `load/2`.
`{error,Reason}`
  An invalid directive.

An example of a store function from `mod_log.erl`:

            store({error_log, ErrorLog}, ConfigList) ->
                  case create_log(ErrorLog, ConfigList) of
                      {ok, ErrorLogStream} ->
                          {ok, {error_log, ErrorLogStream}};
                      {error, Reason} -> {error, Reason}
                  end.

`remove/1` takes the ETS-table representation of the config-file as input. It is up to you to cleanup anything you opened or created in `load/2` or `store/2`. `remove/1` is expected to return:

`ok`
  If the cleanup was successful.
`{error,Reason}`
  If the cleanup failed.

A naive example from `mod_log.erl`:

            remove(ConfigDB) ->
               lists:foreach(fun([Stream]) -> file:close(Stream) end,
                             ets:match(ConfigDB,{transfer_log,'$1'})),
               lists:foreach(fun([Stream]) -> file:close(Stream) end,
                             ets:match(ConfigDB,{error_log,'$1'})),
               ok.

#### 4.4.3 EWSAPI Module Interaction

Modules in the EWSAPI Module Sequence uses the `mod` record's `data` field to propagate responses and status messages, as seen above. This data type can be used in a more versatile fashion. A module can prepare data to be used by subsequent EWSAPI modules, for example the mod_alias module appends the tuple `{real_name,string()}` to inform subsequent modules about the actual file system location for the current URL.

Before altering the EWSAPI Modules Sequence you are well advised to observe what types of data each module uses and propagates. Read the "EWSAPI Interaction" section for each module.

An EWSAPI module can furthermore export functions to be used by other EWSAPI modules but also for other purposes, for example mod_alias:path/3 and mod_auth:add_user/5. These functions should be described in the module documentation.

| When designing an EWSAPI module try to make it self-contained, that is avoid being dependent on other modules both concerning exchange of interaction data and the use of exported functions. If you are dependent on other modules do state this clearly in the module documentation!

You are well advised to read httpd_util(3) and httpd_conf(3).

Copyright © 1991-2005 Ericsson AB