Common Gateway Interface - 1.1 *Draft 03* [http://cgi-spec.golux.com/draft-coar-cgi-v11-03-clean.html]

The Common Gateway Interface (CGI) is a simple interface for running external programs, software or gateways under an information server in a platform-independent manner. Currently, the supported information servers are HTTP servers.

The interface has been in use by the World-Wide Web since 1993. This specification defines the "current practice" parameters of the 'CGI/1.1' interface developed and documented at the U.S. National Centre for Supercomputing Applications [NCSA-CGI]. This document also defines the use of the CGI/1.1 interface on the Unix and AmigaDOS(tm) systems.

Discussion of this draft occurs on the CGI-WG mailing list; see the project Web page at <URL:http://CGI-Spec.Golux.Com/> for details on the mailing list and the status of the project.

Revision History

The revision history of this draft is being maintained using Web-based GUI notation, such as struck-through characters and colour-coded sections. The following legend describes how to determine the origin of a particular revision according to the colour of the text:

  1 Introduction..............................................TBD
   1.1 Purpose................................................TBD
   1.2 Requirements...........................................TBD
   1.3 Specifications.........................................TBD
   1.4 Terminology............................................TBD
  2 Notational Conventions and Generic Grammar................TBD
   2.1 Augmented BNF..........................................TBD
   2.2 Basic Rules............................................TBD
  3 Protocol Parameters.......................................TBD
   3.1 URL Encoding...........................................TBD
   3.2 The Script-URI.........................................TBD
  4 Invoking the Script.......................................TBD
  5 The CGI Script Command Line...............................TBD
  6 Data Input to the CGI Script..............................TBD
   6.1 Request Metadata (Metavariables).......................TBD
    6.1.1 AUTH_TYPE...........................................TBD
    6.1.2 CONTENT_LENGTH......................................TBD
    6.1.3 CONTENT_TYPE........................................TBD
    6.1.4 GATEWAY_INTERFACE...................................TBD
    6.1.5 Protocol-Specific Metavariables.....................TBD
    6.1.6 PATH_INFO...........................................TBD
    6.1.7 PATH_TRANSLATED.....................................TBD
    6.1.8 QUERY_STRING........................................TBD
    6.1.9 REMOTE_ADDR.........................................TBD
    6.1.10 REMOTE_HOST........................................TBD
    6.1.11 REMOTE_IDENT.......................................TBD
    6.1.12 REMOTE_USER........................................TBD
    6.1.13 REQUEST_METHOD.....................................TBD
    6.1.14 SCRIPT_NAME........................................TBD
    6.1.15 SERVER_NAME........................................TBD
    6.1.16 SERVER_PORT........................................TBD
    6.1.17 SERVER_PROTOCOL....................................TBD
    6.1.18 SERVER_SOFTWARE....................................TBD
    6.2 Request Message-Bodies................................TBD
  7 Data Output from the CGI Script...........................TBD
   7.1 Non-Parsed Header Output...............................TBD
   7.2 Parsed Header Output...................................TBD
    7.2.1 CGI header fields...................................TBD
     7.2.1.1 Content-Type.....................................TBD
     7.2.1.2 Location.........................................TBD
     7.2.1.3 Status...........................................TBD
     7.2.1.4 Extension header fields..........................TBD
    7.2.2 HTTP header fields..................................TBD
  8 Server Implementation.....................................TBD
   8.1 Requirements for Servers...............................TBD
    8.1.1 Script-URI..........................................TBD
    8.1.2 Request Message-body Handling.......................TBD
    8.1.3 Required Metavariables..............................TBD
    8.1.4 Response Compliance.................................TBD
   8.2 Recommendations for Servers............................TBD
   8.3 Summary of Metavariables...............................TBD
  9 Script Implementation.....................................TBD
   9.1 Requirements for Scripts...............................TBD
   9.2 Recommendations for Scripts............................TBD
  10 System Specifications....................................TBD
   10.1 AmigaDOS..............................................TBD
   10.2 Unix..................................................TBD
  11 Security Considerations..................................TBD
   11.1 Safe Methods..........................................TBD
   11.2 HTTP Header Fields Containing Sensitive Information...TBD
   11.3 Script Interference with the Server...................TBD
   11.4 Data Length and Buffering Considerations..............TBD
   11.5 Stateless Processing..................................TBD
  12 Acknowledgments..........................................TBD
  13 References...............................................TBD
  14 Authors' Addresses.......................................TBD

1. Introduction

1.1. Purpose

Together the HTTP [3,8] server and the CGI script are responsible for servicing a client request by sending back responses. The client request comprises a Universal Resource Identifier (URI) [1], a request method, and various ancillary information about the request provided by the transport mechanism.

The CGI defines the abstract parameters, known as metavariables, which describe the client's request. Together with a concrete programmer interface this specifies a platform-independent interface between the script and the HTTP server.

1.2. Requirements

This specification uses the same words as RFC 1123 [5] to define the significance of each particular requirement. These are:

An implementation is not compliant if it fails to satisfy one or more of the 'must' requirements for the protocols it implements. An implementation that satisfies all of the 'must' and all of the 'should' requirements for its features is said to be 'unconditionally compliant'; one that satisfies all of the 'must' requirements but not all of the 'should' requirements for its features is said to be 'conditionally compliant.'

1.3. Specifications

Not all of the functions and features of the CGI are defined in the main part of this specification. The following phrases are used to describe the features which are not specified:

1.4. Terminology

This specification uses many terms defined in the HTTP/1.1 specification [8]; however, the following terms are used here in a sense which may not accord with their definitions in that document, or with their common meaning.

2. Notational Conventions and Generic Grammar

2.1. Augmented BNF

All of the mechanisms specified in this document are described in both prose and an augmented Backus-Naur Form (BNF) similar to that used by RFC 822 [6]. This augmented BNF contains the following constructs:

2.2. Basic Rules

The following rules are used throughout this specification to describe basic parsing constructs.

Note that newline (NL) need not be a single character, but can be a character sequence.

3. Protocol Parameters

3.1. URL Encoding

Some variables and constructs used here are described as being 'URL-encoded'. This encoding is described in section 2 of RFC 2396 [4].

An alternate "shortcut" encoding for representing the space character exists and is in common use. Scripts MUST be prepared to recognise both '+' and '%20' as an encoded space in a URL-encoded value.

Note that some unsafe characters may have different semantics if they are encoded. The definition of which characters are unsafe depends on the context. For example, the following two URLs do not necessarily refer to the same resource:

3.2. The Script-URI

The 'Script-URI' is defined as the URI of the resource identified by the metavariables. Often, this URI will be the same as the URI requested by the client (the 'Client-URI'); however, it need not be. Instead, it could be a URI invented by the server, and so it can only be used in the context of the server and its CGI interface.

The Script-URI has the syntax of generic-RL as defined in section 2.1 of RFC 1808 [7], with the exception that object parameters and fragment identifiers are not permitted:

The various components of the Script-URI are defined by some of the metavariables (see section 4 below);

where 'protocol' is obtained from SERVER_PROTOCOL, 'enc-script' is a URL-encoded version of SCRIPT_NAME and 'enc-path-info' is a URL-encoded version of PATH_INFO. See section 4.6 for more information about the PATH_INFO metavariable.

Note that the scheme and the protocol are not identical; for instance, a resource accessed via an SSL mechanism may have a Client-URI with a scheme of "https" rather than "http". CGI/1.1 provides no means for the script to reconstruct this, and therefore the Script-URI includes the base protocol used.

4. Invoking the Script

The script is invoked in a system defined manner. Unless specified otherwise, the file containing the script will be invoked as an executable program.

5. The CGI Script Command Line

Some systems support a method for supplying an array of strings to the CGI script. This is only used in the case of an 'indexed' query. This is identified by a "GET" or "HEAD" HTTP request with a URL query string not containing any unencoded "=" characters. For such a request, servers SHOULD parse the search string into words, using the following rules:

After parsing, each word is URL-decoded, optionally encoded in a system defined manner, and then the argument list is set to the list of words.

If the server cannot create any part of the argument list, then the server SHOULD NOT generate any command line information. For example, the number of arguments may be greater than operating system or server limitations permit, or one of the words may not be representable as an argument.

Scripts SHOULD check to see if the QUERY_STRING value contains an unencoded "=" character, and SHOULD NOT use the command line arguments if it does.

6. Data Input to the CGI Script

Information about a request comes from two different sources: the request header, and any associated message-body. Servers MUST make portions of this information available to scripts.

6.1. Request Metadata (Metavariables)

Each CGI server implementation MUST define a mechanism to pass data about the request from the server to the script. The metavariables containing these data are accessed by the script in a system defined manner. The representation of the characters in the metavariables is system defined.

This specification does not distinguish between the representation of null values and missing ones. Whether null or missing values (such as a query component of "?" or "", respectively) are represented by undefined metavariables or by metavariables with values of "" is implementation-defined.

Case is not significant in the metavariable names, in that there cannot be two different variables whose names differ in case only. Here they are shown using a canonical representation of capitals plus underscore ("_"). The actual representation of the names is system defined; for a particular system the representation MAY be defined differently than this.

Metavariable values MUST be considered case-sensitive except as noted otherwise.

Metavariables with names beginning with the protocol name (e.g., "HTTP_ACCEPT") are also canonical in their description of request header fields. The number and meaning of these fields may change independently of this specification. (See also section 6.1.5.)

6.1.1. AUTH_TYPE

If the Script-URI required access authentication for external access, then the server MUST set the value of this variable from the 'auth-scheme' token in the request's "Authorization" header field. Otherwise it is set to NULL.

HTTP access authentication schemes are described in section 11 of the HTTP/1.1 specification [8]. The auth-scheme is not case-sensitive.

Servers MUST provide this metavariable to scripts if the request header included an "Authorization" field that was authenticated.

6.1.2. CONTENT_LENGTH

This metavariable is set to the size of the message-body entity attached to the request, if any, in decimal number of octets. If no data are attached, then this metavariable is either NULL or not defined. The syntax is the same as for the HTTP "Content-Length" header field (section 14.14, HTTP/1.1 specification [8]).

Servers MUST provide this metavariable to scripts if the request was accompanied by a message-body entity.

6.1.3. CONTENT_TYPE

If the request includes a message-body, CONTENT_TYPE is set to the Internet Media Type [9] of the attached entity if the type was provided via a "Content-type" field in the request header, or if the server can determine it in the absence of a supplied "Content-type" field. The syntax is the same as for the HTTP "Content-Type" header field.

The type, subtype, and parameter attribute names are not case-sensitive. Parameter values MAY be case sensitive. Media types and their use in HTTP are described in section 3.7 of the HTTP/1.1 specification [8].

There is no default value for this variable. If and only if it is unset, then the script MAY attempt to determine the media type from the data received. If the type remains unknown, then the script MAY choose to either assume a content-type of application/octet-stream or reject the request with a 415 ("Unsupported Media Type") error. See section 7.2.1.3 for more information about returning error status values.

Servers MUST provide this metavariable to scripts if a "Content-Type" field was present in the original request header. If the server receives a request with an attached entity but no "Content-Type" header field, it MAY attempt to determine the correct datatype, or it MAY omit this metavariable when communicating the request information to the script.

6.1.4. GATEWAY_INTERFACE

This metavariable is set to the dialect of CGI being used by the server to communicate with the script. Syntax:

Note that the major and minor numbers are treated as separate integers and hence each may be more than a single digit. Thus CGI/2.4 is a lower version than CGI/2.13 which in turn is lower than CGI/12.3. Leading zeros in either the major or the minor number MUST be ignored by scripts and SHOULD NOT be generated by servers.

6.1.5. Protocol-Specific Metavariables

These metavariables are specific to the protocol via which the request is made. Interpretation of these variables depends on the value of the SERVER_PROTOCOL metavariable (see section 6.1.17).

Metavariables with names beginning with "HTTP_" contain values from the request header, if the scheme used was HTTP. Each HTTP header field name is converted to upper case, has all occurrences of "-" replaced with "_", and has "HTTP_" prepended to form the metavariable name. Similar transformations are applied for other protocols. The header data MAY be presented as sent by the client, or MAY be rewritten in ways which do not change its semantics. If multiple header fields with the same field-name are received then the server MUST rewrite them as though they had been received as a single header field having the same semantics before being represented in a metavariable. Similarly, a header field that is received on more than one line MUST be merged into a single line. The server MUST, if necessary, change the representation of the data (for example, the character set) to be appropriate for a CGI metavariable.

Servers are not required to create metavariables for all the request header fields that they receive. In particular, they MAY decline to make available any header fields carrying authentication information, such as "Authorization", or which are available to the script via other metavariables, such as "Content-Length" and "Content-Type".

6.1.6. PATH_INFO

The PATH_INFO metavariable specifies a path to be interpreted by the CGI script. It identifies the resource or sub-resource to be returned by the CGI script, and it is derived from the portion of the URI path following the script name but preceding any query data. The syntax and semantics are similar to a decoded HTTP URL 'path' token (defined in RFC 2396 [4]), with the exception that a PATH_INFO of "/" represents a single void path segment.

The PATH_INFO string is the trailing part of the <path> component of the Script-URI (see section 3.2) that follows the SCRIPT_NAME portion of the path.

Servers MAY impose their own restrictions and limitations on what values they will accept for PATH_INFO, and MAY reject or edit any values they consider objectionable before passing them to the script.

Servers MUST make this URI component available to CGI scripts. The PATH_INFO value is case-sensitive, and the server MUST preserve the case of the PATH_INFO element of the URI when making it available to scripts.

6.1.7. PATH_TRANSLATED

PATH_TRANSLATED is derived by taking any path-info component of the request URI (see section 6.1.6), decoding it (see section 3.1), parsing it as a URI in its own right, and performing any virtual-to-physical translation appropriate to map it onto the server's document repository structure. If the request URI includes no path-info component, the PATH_TRANSLATED metavariable SHOULD NOT be defined.

the PATH_INFO component would be decoded, and the result parsed as though it were a request for the following:

This would then be translated to a location in the server's document repository, perhaps a filesystem path something like this:

The value of PATH_TRANSLATED may or may not map to a valid repository location. Servers MUST preserve the case of the path-info segment if and only if the underlying repository supports case-sensitive names. If the repository is only case-aware, case-preserving, or case-blind with regard to document names, servers are not required to preserve the case of the original segment through the translation.

The translation algorithm the server uses to derive PATH_TRANSLATED is implementation defined; CGI scripts which use this variable may suffer limited portability.

Servers SHOULD provide this metavariable to scripts if and only if the request URI includes a path-info component.

6.1.8. QUERY_STRING

Servers MUST supply this value to scripts. The QUERY_STRING value is case-sensitive. If the Script-URI does not include a query component, the QUERY_STRING metavariable MUST be defined as an empty string ("").

6.1.9. REMOTE_ADDR

The IP address of the client sending the request to the server. This is not necessarily that of the user agent (such as if the request came through a proxy).

The definitions of ipv4-address and ipv6-address are provided in Appendix B of RFC 2373 [13].

6.1.10. REMOTE_HOST

The fully qualified domain name of the client sending the request to the server, if available, otherwise NULL. (See section 6.1.9.) Fully qualified domain names take the form as described in section 3.5 of RFC 1034 [10] and section 2.1 of RFC 1123 [5]. Domain names are not case sensitive.

6.1.11. REMOTE_IDENT

The identity information reported about the connection by a RFC 1413 [11] request to the remote agent, if available. Servers MAY choose not to support this feature, or not to request the data for efficiency reasons.

The data returned may be used for authentication purposes, but the level of trust reposed in them should be minimal.

Servers MAY supply this information to scripts if the RFC1413 [11] lookup is performed.

6.1.12. REMOTE_USER

If the request required authentication using the "Basic" mechanism (i.e., the AUTH_TYPE metavariable is set to "Basic"), then the value of the REMOTE_USER metavariable is set to the user-ID supplied. In all other cases the value of this metavariable is undefined.

6.1.13. REQUEST_METHOD

The REQUEST_METHOD metavariable is set to the method with which the request was made, as described in section 5.1.1 of the HTTP/1.0 specification [3] and section 5.1.1 of the HTTP/1.1 specification [8].

The method is case sensitive. CGI/1.1 servers MAY choose to process some methods directly rather than passing them to scripts.

6.1.14. SCRIPT_NAME

The SCRIPT_NAME metavariable is set to a URL path that could identify the CGI script (rather than the script's output). The syntax and semantics are identical to a decoded HTTP URL 'path' token (see RFC 2396 [4]).

The SCRIPT_NAME string is some leading part of the <path> component of the Script-URI derived in some implementation defined manner. No PATH_INFO or QUERY_STRING segments (see sections 6.1.6 and 6.1.8) are included in the SCRIPT_NAME value.

6.1.15. SERVER_NAME

The SERVER_NAME metavariable is set to the name of the server, as derived from the <host> part of the Script-URI (see section 3.2).

6.1.16. SERVER_PORT

The SERVER_PORT metavariable is set to the port on which the request was received, as used in the <port> part of the Script-URI.

If the <port> portion of the script-URI is blank, the actual port number upon which the request was received MUST be supplied.

6.1.17. SERVER_PROTOCOL

The SERVER_PROTOCOL metavariable is set to the name and revision of the information protocol with which the request arrived. This is not necessarily the same as the protocol version used by the server in its response to the client.

'protocol' is a version of the <scheme> part of the Script-URI, but is not identical to it. For example, the scheme of a request may be "https" while the protocol remains "http". The protocol is not case sensitive, but by convention, 'protocol' is in upper case.

A well-known extension token value is "INCLUDED", which signals that the current document is being included as part of a composite document, rather than being the direct target of the client request.

6.1.18. SERVER_SOFTWARE

The SERVER_SOFTWARE metavariable is set to the name and version of the information server software answering the request (and running the gateway).

6.2. Request Message-Bodies

As there may be a data entity attached to the request, there MUST be a system defined method for the script to read these data. Unless defined otherwise, this will be via the 'standard input' file descriptor.

If the CONTENT_LENGTH value (see section 6.1.2) is non-NULL, the server MUST supply at least that many bytes to scripts on the standard input stream. Scripts are not obliged to read the data. Servers MAY signal an EOF condition after CONTENT_LENGTH bytes have been read, but are not obligated to do so. Therefore, scripts MUST NOT attempt to read more than CONTENT_LENGTH bytes, even if more data are available.

For non-parsed header (NPH) scripts (see section 7.1 below), servers SHOULD attempt to ensure that the data supplied to the script are precisely as supplied by the client and unaltered by the server.

Section 8.1.2 describes the requirements of servers with regard to requests that include message-bodies.

7. Data Output from the CGI Script

There MUST be a system defined method for the script to send data back to the server or client; a script MUST always return some data. Unless defined otherwise, this will be via the 'standard output' file descriptor.

There are two forms of output that scripts can supply to servers: non-parsed header (NPH) output, and parsed header output. Servers MUST support parsed header output and MAY support NPH output. The method of distinguishing between the two types of output (or scripts) is implementation defined.

Servers MAY implement a timeout period within which data must be received from scripts. If a server implementation defines such a timeout and receives no data from a script within the timeout period, the server MAY terminate the script process and SHOULD abort the client request with either a '504 Gateway Timed Out' or a '500 Internal Server Error' response.

7.1. Non-Parsed Header Output

Scripts using the NPH output form MUST return a complete HTTP response message, as described in Section 6 of the HTTP specifications [3,8]. NPH scripts MUST use the SERVER_PROTOCOL variable to determine the appropriate format for a response.

Servers SHOULD attempt to ensure that the script output is sent directly to the client, with minimal internal and no transport-visible buffering.

7.2. Parsed Header Output

Scripts using the parsed header output form MUST supply a CGI response message to the server as follows:

The response comprises a header and a body, separated by a blank line. The body may be NULL. The header fields are either CGI header fields to be interpreted by the server, or HTTP header fields to be included in the response returned to the client if the request method is HTTP. At least one CGI-Field MUST be supplied, but no CGI field name may be used more than once in a response. If a body is supplied, then a "Content-type" header field MUST be supplied by the script, otherwise the script MUST send a "Location" or "Status" header field. If a Location CGI-Field is returned, then the script MUST NOT supply any HTTP-Fields.

Each header field in a CGI-Response MUST be specified on a single line; CGI/1.1 does not support continuation lines.

7.2.1. CGI header fields

The field-name is not case sensitive; a NULL field value is equivalent to the header field not being sent.

7.2.1.1. Content-Type

The Internet Media Type [9] of the entity body, which is to be sent unmodified to the client.

This is actually an HTTP-Field rather than a CGI-Field, but it is listed here because of its importance in the CGI dialogue as a member of the "one of these is required" set of header fields.

7.2.1.2. Location

This is used to specify to the server that the script is returning a reference to a document rather than an actual document.

The Location value is either an absolute URI with optional fragment, as defined in RFC 1630 [1], or an absolute path within the server's URI space (i.e., omitting the scheme and network-related fields) and optional query-string. If an absolute URI is returned by the script, then the server MUST generate a '302 redirect' HTTP response message unless the script has supplied an explicit Status response header field. Scripts returning an absolute URI MAY choose to provide a message-body. Servers MUST make any appropriate modifications to the script's output to ensure the response to the user-agent complies with the response protocol version. If the Location value is a path, then the server MUST generate the response that it would have produced in response to a request containing the URL

Note: If the request was accompanied by a message-body (such as for a POST request), and the script redirects the request with a Location field, the message-body may not be available to the resource that is the target of the redirect.

7.2.1.3. Status

The "Status" header field is used to indicate to the server what status code the server MUST use in the response message.

The valid status codes are listed in section 6.1.1 of the HTTP/1.0 specifications [3]. If the SERVER_PROTOCOL is "HTTP/1.1", then the status codes defined in the HTTP/1.1 specification [8] may be used. If the script does not return a "Status" header field, then "200 OK" SHOULD be assumed by the server.

If a script is being used to handle a particular error or condition encountered by the server, such as a '404 Not Found' error, the script SHOULD use the "Status" CGI header field to propagate the error condition back to the client. E.g., in the example mentioned it SHOULD include a "Status: 404 Not Found" in the header data returned to the server.

7.2.1.4. Extension header fields

Scripts MAY include in their CGI response header additional fields not defined in this or the HTTP specification. These are called "extension" fields, and have the syntax of a generic-field as defined in section 7.2.1. The name of an extension field MUST NOT conflict with a field name defined in this or any other specification; extension field names SHOULD begin with "X-CGI-" to ensure uniqueness.

7.2.2. HTTP header fields

The script MAY return any other header fields defined by the specification for the SERVER_PROTOCOL (HTTP/1.0 [3] or HTTP/1.1 [8]). Servers MUST resolve conflicts beteen CGI header and HTTP header formats or names (see section 8).

8. Server Implementation

This section defines the requirements that must be met by HTTP servers in order to provide a coherent and correct CGI/1.1 environment in which scripts may function. It is intended primarily for server implementors, but it is useful for script authors to be familiar with the information as well.

8.1. Requirements for Servers

In order to be considered CGI/1.1-compliant, a server must meet certain basic criteria and provide certain minimal functionality. The details of these requirements are described in the following sections.

8.1.1. Script-URI

Servers MUST support the standard mechanism (described below) which allows script authors to determine what URL to use in documents which reference the script; specifically, what URL to use in order to achieve particular settings of the metavariables. This mechanism is as follows:

The server MUST translate the header data from the CGI header field syntax to the HTTP header field syntax if these differ. For example, the character sequence for newline (such as Unix's ASCII NL) used by CGI scripts may not be the same as that used by HTTP (ASCII CR followed by LF). The server MUST also resolve any conflicts between header fields returned by the script and header fields that it would otherwise send itself.

8.1.2. Request Message-body Handling

These are the requirements for server handling of message-bodies directed to CGI/1.1 resources:

8.1.3. Required Metavariables

Servers MUST provide scripts with certain information and metavariables as described in section 8.3.

8.1.4. Response Compliance

Servers MUST ensure that responses sent to the user-agent meet all requirements of the protocol level in effect. This may involve modifying, deleting, or augmenting any header fields and/or message-body supplied by the script.

8.2. Recommendations for Servers

Servers SHOULD provide the "query" component of the script-URI as command-line arguments to scripts if it does not contain any unencoded '=' characters and the command-line arguments can be generated in an unambiguous manner. (See section 5.)

Servers SHOULD set the AUTH_TYPE metavariable to the value of the 'auth-scheme' token of the "Authorization" field if it was supplied as part of the request header. (See section 6.1.1.)

Where applicable, servers SHOULD set the current working directory to the directory in which the script is located before invoking it.

Servers MAY reject with error '404 Not Found' any requests that would result in an encoded "/" being decoded into PATH_INFO or SCRIPT_NAME, as this might represent a loss of information to the script.

Although the server and the CGI script need not be consistent in their handling of URL paths (client URLs and the PATH_INFO data, respectively), server authors may wish to impose consistency. So the server implementation SHOULD define its behaviour for the following cases:

Servers MAY generate the Script-URI in any way from the client URI, or from any other data (but the behaviour SHOULD be documented).

For non-parsed header (NPH) scripts (see section 7.1), servers SHOULD attempt to ensure that the script input comes directly from the client, with minimal buffering. For all scripts the data will be as supplied by the client.

8.3. Summary of MetaVariables

Servers MUST provide the following metavariables to scripts. See the individual descriptions for exceptions and semantics.

Servers SHOULD define the following metavariables for scripts. See the individual descriptions for exceptions and semantics.

In addition, servers SHOULD provide metavariables for all fields present in the HTTP request header, with the exception of those involved with access control. Servers MAY at their discretion provide metavariables for access control fields.

Servers MAY define the following metavariables. See the individual descriptions for exceptions and semantics.

Servers MAY at their discretion define additional implementation-specific extension metavariables provided their names do not conflict with defined header field names. Implementation-specific metavariable names SHOULD be prefixed with "X_" (e.g., "X_DBA") to avoid the potential for such conflicts.

9. Script Implementation

This section defines the requirements and recommendations for scripts that are intended to function in a CGI/1.1 environment. It is intended primarily as a reference for script authors, but server implementors should be familiar with these issues as well.

9.1. Requirements for Scripts

Scripts using the parsed-header method to communicate with servers MUST supply a response header to the server. (See section 7.)

Scripts using the NPH method to communicate with servers MUST provide complete HTTP responses, and MUST use the value of the SERVER_PROTOCOL metavariable to determine the appropriate format. (See section 7.1.)

Scripts MUST check the value of the REQUEST_METHOD metavariable in order to provide an appropriate response. (See section 6.1.13.)

Scripts MUST be prepared to handled URL-encoded values in metavariables. In addition, they MUST recognise both "+" and "%20" in URL-encoded quantities as representing the space character. (See section 3.1.)

Scripts MUST ignore leading zeros in the major and minor version numbers in the GATEWAY_INTERFACE metavariable value. (See section 6.1.4.)

When processing requests that include a message-body, scripts MUST NOT read more than CONTENT_LENGTH bytes from the input stream. (See sections 6.1.2 and 6.2.)

9.2. Recommendations for Scripts

Servers may interrupt or terminate script execution at any time and without warning, so scripts SHOULD be prepared to deal with abnormal termination.

Scripts MUST reject with error '405 Method Not Allowed' requests made using methods that they do not support. If the script does not intend processing the PATH_INFO data, then it SHOULD reject the request with '404 Not Found' if PATH_INFO is not NULL.

If a script is processing the output of a form, it SHOULD verify that the CONTENT_TYPE is "application/x-www-form-urlencoded" [2] or whatever other media type is expected.

Scripts parsing PATH_INFO, PATH_TRANSLATED, or SCRIPT_NAME SHOULD be careful of void path segments ("//") and special path segments ("." and ".."). They SHOULD either be removed from the path before use in OS system calls, or the request SHOULD be rejected with '404 Not Found'.

As it is impossible for scripts to determine the client URI that initiated a request without knowledge of the specific server in use, the script SHOULD NOT return "text/html" documents containing relative URL links without including a "<BASE>" tag in the document.

When returning header fields, scripts SHOULD try to send the CGI header fields (see section 7.2) as soon as possible, and SHOULD send them before any HTTP header fields. This may help reduce the server's memory requirements.

10. System Specifications

10.1. AmigaDOS

The implementation of the CGI on an AmigaDOS operating system platform SHOULD use environment variables as the mechanism of providing request metadata to CGI scripts.

10.2. Unix

The implementation of the CGI on a UNIX operating system platform SHOULD use environment variables as the mechanism of providing request metadata to CGI scripts.

11. Security Considerations

11.1. Safe Methods

As discussed in the security considerations of the HTTP specifications [3,8], the convention has been established that the GET and HEAD methods should be 'safe'; they should cause no side-effects and only have the significance of resource retrieval.

CGI scripts are responsible for enforcing any HTTP security considerations [3,8] with respect to the protocol version level of the request and any side effects generated by the scripts on behalf of the server. Primary among these are the considerations of safe and idempotent methods. Idempotent requests are those that may be repeated an arbitrary number of times and produce side effects identical to a single request.

11.2. HTTP Header Fields Containing Sensitive Information

Some HTTP header fields may carry sensitive information which the server SHOULD NOT pass on to the script unless explicitly configured to do so. For example, if the server protects the script using the "Basic" authentication scheme, then the client will send an "Authorization" header field containing a username and password. If the server, rather than the script, validates this information then the password SHOULD NOT be passed on to the script via the HTTP_AUTHORIZATION metavariable without careful consideration. This also applies to the Proxy-Authorization header field and the corresponding HTTP_PROXY_AUTHORIZATION metavariable.

11.3. Script Interference with the Server

The most common implementation of CGI invokes the script as a child process using the same user and group as the server process. It SHOULD therefore be ensured that the script cannot interfere with the server process, its configuration, or documents.

If the script is executed by calling a function linked in to the server software (either at compile-time or run-time) then precautions SHOULD be taken to protect the core memory of the server, or to ensure that untrusted code cannot be executed.

11.4. Data Length and Buffering Considerations

This specification places no limits on the length of message-bodies presented to the script. Scripts should not assume that statically allocated buffers of any size are sufficient to contain the entire submission at one time. Use of a fixed length buffer without careful overflow checking may result in an attacker exploiting 'stack-smashing' or 'stack-overflow' vulnerabilities of the operating system. Scripts may spool large submissions to disk or other buffering media, but a rapid succession of large submissions may result in denial of service conditions. If the CONTENT_LENGTH of a message-body is larger than resource considerations allow, scripts should respond with an error status appropriate for the protocol version; potentially applicable status codes include '503 Service Unavailable' (HTTP/1.0 and HTTP/1.1), '413 Request Entity Too Large' (HTTP/1.1), and '414 Request-URI Too Long' (HTTP/1.1).

11.5. Stateless Processing

The stateless nature of the Web makes each script execution and resource retrieval independent of all others even when multiple requests constitute a single conceptual Web transaction. Because of this, a script should not make any assumptions about the context of the user-agent submitting a request. In particular, scripts should examine data obtained from the client and verify that they are valid, both in form and content, before allowing them to be used for sensitive purposes such as input to other applications, commands, or operating system services. These uses include, but are not limited to: system call arguments, database writes, dynamically evaluated source code, and input to billing or other secure processes. It is important that applications be protected from invalid input regardless of whether the invalidity is the result of user error, logic error, or malicious action.

Authors of scripts involved in multi-request transactions should be particularly cautios about validating the state information; undesirable effects may result from the substitution of dangerous values for portions of the submission which might otherwise be presumed safe. Subversion of this type occurs when alterations are made to data from a prior stage of the transaction that were not meant to be controlled by the client (e.g., hidden HTML form elements, cookies, embedded URLs, etc.).

12. Acknowledgements

This work is based on a draft published in 1997 by David R. Robinson, which in turn was based on the original CGI interface that arose out of discussions on the www-talk mailing list. In particular, Rob McCool, John Franks, Ari Luotonen, George Phillips and Tony Sanders deserve special recognition for their efforts in defining and implementing the early versions of this interface.

This document has also greatly benefited from the comments and suggestions made by Chris Adie, Dave Kristol, Mike Meyer, David Morris, Jeremy Madea, Patrick M^cManus, Adam Donahue, Ross Patterson, and Harald Alvestrand.

INTERNET-DRAFT	Ken A L Coar
draft-coar-cgi-v11-03.{html,txt}	IBM Corporation
	D.R.T. Robinson
	E*TRADE UK Ltd.
	25 June 1999

The WWW Common Gateway Interface Version 1.1

Revision History

The WWW Common Gateway Interface
Version 1.1