httpd

An implementation of an HTTP 1.1 compliant Web server, as defined in RFC 2616.

HTTP (Hypertext Transfer Protocol) is an application-level protocol with the lightness and speed necessary for distributed, collaborative and hyper-media information systems. The httpd module 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.

All server functionality has been implemented using an especially crafted server API; EWSAPI (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.

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:

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

httpd_core: Core features.
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.

Types:

ConfigFile = string()
ServerRet = {ok,Pid} | ignore | {error,EReason} | {stop,SReason}
Pid = pid()
EReason = {already_started, Pid} | term()
SReason = string()

start/1 and start_link/1 starts a server as specified in the given ConfigFile. The ConfigFile supports a number of config directives specified below.

start/0 and start/0 starts a server as specified in a hard-wired config file, that is start("/var/tmp/server_root/conf/8888.conf"). Before utilizing start/0 or start_link/0, copy the example server root (UNIX: $INETS_ROOT/examples/server_root/, Windows: %INETS_ROOT%\examples\server_root\) to a specific installation directory (UNIX: /var/tmp/, Windows: X:\var\tmp\) and you have a server running in no time.

If you copy the example server root to the specific installation directory it is furthermore easy to start an SSL enabled server, that is start("/var/tmp/server_root/conf/ssl.conf").

Types:

Port = integer()
Address = {A,B,C,D} | string() | undefined
ConfigFile = string()
Reason = term()

restart restarts the server and reloads its config file.

The follwing directives cannot be changed: BindAddress, Port and SocketType. If these should be changed, then a new server should be started instead.

Note!
Before the restart function can be called the server must be blocked. After restart has been called, the server must be unblocked.

Types:

Port = integer()
Address = {A,B,C,D} | string() | undefined
ConfigFile = string()
ServerRet = ok | not_started

stop/2 stops the server which listens to the specified Port on Address. stop(integer()) stops a server which listens to a specific Port. stop(string()) extracts BindAddress and Port from the config file and stops the server which listens to the specified Port on Address. stop/0 stops a server which listens to port 8888, that is stop(8888).

Types:

Port = integer()
Address = {A,B,C,D} | string() | undefined
ConfigFile = string()
Mode = disturbing | non_disturbing
Timeout = integer()
Reason = term()

This function is used to block a server. The blocking can be done in two ways, disturbing or non-disturbing.

By performing a disturbing block, the server is blocked forcefully and all ongoing requests are terminated. No new connections are accepted. If a timeout time is given then on-going requests are given this much time to complete before the server is forcefully blocked. In this case no new connections is accepted.

A non-disturbing block is more gracefull. No new connections are accepted, but the ongoing requests are allowed to complete. If a timeout time is given, it waits this long before giving up (the block operation is aborted and the server state is once more not-blocked)

Default mode is disturbing.

Default port is 8888

Types:

Port = integer()
Address = {A,B,C,D} | string() | undefined
ConfigFile = string()
Reason = term()

Unblocks a server. If the server is already unblocked this is a no-op. If a block is ongoing, then it is aborted (this will have no effect on ongoing requests).

Types:

QueryString = string()
ServerRet = [{Key,Value}]
Key = Value = string()

parse_query/1 parses incoming data to erl and eval scripts (See mod_esi(3)) as defined in the standard URL format, that is '+' becomes 'space' and decoding of hexadecimal characters (%xx).

Types:

Info = mod()
OldData = list()
NewData = [{response,{StatusCode,Body}}] | [{response,{response,Head,Body2}}] | [{response,{already_sent,Statuscode,Size}]
StausCode = integer()
Body = String
Head = [HeaderOption]
HeaderOption = {Key, Value} | {code, StatusCode}
Key = allow | cache_control | content_MD5 | content_encoding | content_encoding | content_language,Value | content_length | content_location | content_range | content_type | date | etag | expires | last_modified | location | pragma | retry_after | server | trailer | transfer_encoding
Value = string()
Body2 = {Fun,Arg} | Body | nobody
Fun = fun( Arg )->sent| close | Body
Arg = [term()]

Info is a record of type mod, this record is defined in httpd.hrl see EWSAPI Module programming for more information.

When a valid request reaches httpd it calls do/1 in each module defined by the Modules configuration directive. The function may generate data for other modules or a response that can be sent back to the client.

The field data in Info is a list. This list will be the list returned from the from the last call to do/1.

Body is the body of the http-response that will be sent back to the client an appropriate header will be appended to the message. StatusCode will be the status code of the response see RFC2616 for the appropriate values.

Head is a key value list of HTTP header fields. the server will construct a HTTP header from this data. See RFC 2616 for the appropriate value for each header field. If the client is a HTTP/1.0 client then the server will filter the list so that only HTTP/1.0 header fields will be sent back to the client.

If Body2 is returned and equal to {Fun,Arg} The Web server will try apply/2. on Fun with Arg as argument and excpect that the fun either returns a list (Body) that is a HTTP-repsonse or the atom sent if the HTTP-response is sent back to the client. If close is returned from the fun something has gone wrong and the server will signal this to the client by closing the connection.

Types:

Line = string()
Context = NewContext = DirectiveList = [Directive]
Directive = {DirectiveKey , DirectiveValue}
DirectiveKey = DirectiveValue = term()
Reason = term()

load/2 takes a row Line from the configuration file and tries to convert it to a key value tuple. If a directive is dependent on other directives, the directive may create a context. If the directive is not dependent on other directives return {ok, [], Directive}, otherwise return a new context, that is {ok, NewContext} or {ok, Context Directive}. If {error, Reason} is returned the configuration directive is assumed to be invalid.

Types:

DirectiveList = [{DirectiveKey, DirectiveValue}]
DirectiveKey = DirecitveValue = term()
Context = NewContext = DirectiveList = [Directive]
Directive = {Key , Value}
Reason = term()

When all rows in the configuration file is read the function store/2 is called for each configuration directive. This makes it possible for a directive to alter other configuration directives. DirectiveList is a list of all configuration directives read in from load. If a directive may update other configuration directives then use this function.

Types:

ConfigDB = ets_table()
Reason = term()

When httpd shutdown it will try to execute remove/1 in each ewsapi module. The ewsapi programmer may use this to close ets tables, save data, or close down background processes.

Note!
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, see EWSAPI Module Configuration.

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, see EWSAPI Module Interaction.

Note!
The server executes do/1 (using apply/1) for 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 (See EWSAPI Module Interaction below). 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 send the client might hang if the body is not send 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...".

Warning!
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).

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([$T,$r,$a,$n,$s,$f,$e,$r,$L,$o,$g,$ |TransferLog],[]) ->
  {ok,[],{transfer_log,httpd_conf:clean(TransferLog)}};
load([$E,$r,$r,$o,$r,$L,$o,$g,$ |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.

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.

Note!
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).

If a Web browser connect itself to an SSL enabled server using a URL not starting with https:// the server will hang due to an ugly bug in the SSLeay package!

httpd_core(3), httpd_conf(3), httpd_socket(3), httpd_util(3), inets(6),

Joakim Greben� - support@erlang.ericsson.se
Torbj�rn T�rnkvist - support@erlang.ericsson.se
Joe Armstrong - support@erlang.ericsson.se

httpd

MODULE

MODULE SUMMARY

DESCRIPTION

RUN-TIME CONFIGURATION

EWSAPI MODULES

EXPORTS

ESWAPI CALLBACK FUNCTIONS

EXPORTS

EWSAPI MODULE PROGRAMMING

EWSAPI MODULE CONFIGURATION

EWSAPI MODULE INTERACTION

BUGS

SEE ALSO

AUTHORS