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The Common Gateway Interface (CGI) Version 1.1
RFC 3875

Document type: RFC - Informational (October 2004; Errata)
Document stream: ISE
Last updated: 2013-03-20
Other versions: plain text, pdf, html

ISE State: (None)
Document shepherd: No shepherd assigned

IESG State: RFC 3875 (Informational)
Responsible AD: Ted Hardie
Send notices to: No addresses provided

Network Working Group                                        D. Robinson
Request for Comments: 3875                                       K. Coar
Category: Informational                   The Apache Software Foundation
                                                            October 2004

             The Common Gateway Interface (CGI) Version 1.1

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

IESG Note

   This document is not a candidate for any level of Internet Standard.
   The IETF disclaims any knowledge of the fitness of this document for
   any purpose, and in particular notes that it has not had IETF review
   for such things as security, congestion control or inappropriate
   interaction with deployed protocols.  The RFC Editor has chosen to
   publish this document at its discretion.  Readers of this document
   should exercise caution in evaluating its value for implementation
   and deployment.

Abstract

   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 (WWW) 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.  This document also defines
   the use of the CGI/1.1 interface on UNIX(R) and other, similar
   systems.

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Table of Contents

   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.1. Purpose  . . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2. Requirements . . . . . . . . . . . . . . . . . . . . . .   4
       1.3. Specifications . . . . . . . . . . . . . . . . . . . . .   4
       1.4. Terminology  . . . . . . . . . . . . . . . . . . . . . .   5

   2.  Notational Conventions and Generic Grammar. . . . . . . . . .   5
       2.1. Augmented BNF  . . . . . . . . . . . . . . . . . . . . .   5
       2.2. Basic Rules  . . . . . . . . . . . . . . . . . . . . . .   6
       2.3. URL Encoding . . . . . . . . . . . . . . . . . . . . . .   7

   3.  Invoking the Script . . . . . . . . . . . . . . . . . . . . .   8
       3.1. Server Responsibilities  . . . . . . . . . . . . . . . .   8
       3.2. Script Selection . . . . . . . . . . . . . . . . . . . .   9
       3.3. The Script-URI . . . . . . . . . . . . . . . . . . . . .   9
       3.4. Execution  . . . . . . . . . . . . . . . . . . . . . . .  10

   4.  The CGI Request . . . . . . . . . . . . . . . . . . . . . . .  10
       4.1. Request Meta-Variables . . . . . . . . . . . . . . . . .  10
            4.1.1.  AUTH_TYPE. . . . . . . . . . . . . . . . . . . .  11
            4.1.2.  CONTENT_LENGTH . . . . . . . . . . . . . . . . .  12
            4.1.3.  CONTENT_TYPE . . . . . . . . . . . . . . . . . .  12
            4.1.4.  GATEWAY_INTERFACE. . . . . . . . . . . . . . . .  13
            4.1.5.  PATH_INFO. . . . . . . . . . . . . . . . . . . .  13
            4.1.6.  PATH_TRANSLATED. . . . . . . . . . . . . . . . .  14
            4.1.7.  QUERY_STRING . . . . . . . . . . . . . . . . . .  15
            4.1.8.  REMOTE_ADDR. . . . . . . . . . . . . . . . . . .  15
            4.1.9.  REMOTE_HOST. . . . . . . . . . . . . . . . . . .  16
            4.1.10. REMOTE_IDENT . . . . . . . . . . . . . . . . . .  16
            4.1.11. REMOTE_USER. . . . . . . . . . . . . . . . . . .  16
            4.1.12. REQUEST_METHOD . . . . . . . . . . . . . . . . .  17
            4.1.13. SCRIPT_NAME. . . . . . . . . . . . . . . . . . .  17
            4.1.14. SERVER_NAME. . . . . . . . . . . . . . . . . . .  17
            4.1.15. SERVER_PORT. . . . . . . . . . . . . . . . . . .  18
            4.1.16. SERVER_PROTOCOL. . . . . . . . . . . . . . . . .  18
            4.1.17. SERVER_SOFTWARE. . . . . . . . . . . . . . . . .  19
            4.1.18. Protocol-Specific Meta-Variables . . . . . . . .  19
       4.2. Request Message-Body . . . . . . . . . . . . . . . . . .  20
       4.3. Request Methods  . . . . . . . . . . . . . . . . . . . .  20
            4.3.1.  GET. . . . . . . . . . . . . . . . . . . . . . .  20
            4.3.2.  POST . . . . . . . . . . . . . . . . . . . . . .  21
            4.3.3.  HEAD . . . . . . . . . . . . . . . . . . . . . .  21
            4.3.4.  Protocol-Specific Methods. . . . . . . . . . . .  21
       4.4. The Script Command Line. . . . . . . . . . . . . . . . .  21

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   5.  NPH Scripts . . . . . . . . . . . . . . . . . . . . . . . . .  22
       5.1. Identification . . . . . . . . . . . . . . . . . . . . .  22
       5.2. NPH Response . . . . . . . . . . . . . . . . . . . . . .  22

   6.  CGI Response. . . . . . . . . . . . . . . . . . . . . . . . .  23
       6.1. Response Handling. . . . . . . . . . . . . . . . . . . .  23
       6.2. Response Types . . . . . . . . . . . . . . . . . . . . .  23
            6.2.1.  Document Response. . . . . . . . . . . . . . . .  23
            6.2.2.  Local Redirect Response. . . . . . . . . . . . .  24
            6.2.3.  Client Redirect Response . . . . . . . . . . . .  24
            6.2.4.  Client Redirect Response with Document . . . . .  24
       6.3. Response Header Fields . . . . . . . . . . . . . . . . .  25
            6.3.1.  Content-Type . . . . . . . . . . . . . . . . . .  25
            6.3.2.  Location . . . . . . . . . . . . . . . . . . . .  26
            6.3.3.  Status . . . . . . . . . . . . . . . . . . . . .  26
            6.3.4.  Protocol-Specific Header Fields. . . . . . . . .  27
            6.3.5.  Extension Header Fields. . . . . . . . . . . . .  27
       6.4. Response Message-Body. . . . . . . . . . . . . . . . . .  28

   7.  System Specifications . . . . . . . . . . . . . . . . . . . .  28
       7.1. AmigaDOS . . . . . . . . . . . . . . . . . . . . . . . .  28
       7.2. UNIX . . . . . . . . . . . . . . . . . . . . . . . . . .  28
       7.3. EBCDIC/POSIX . . . . . . . . . . . . . . . . . . . . . .  29

   8.  Implementation. . . . . . . . . . . . . . . . . . . . . . . .  29
       8.1. Recommendations for Servers. . . . . . . . . . . . . . .  29
       8.2. Recommendations for Scripts. . . . . . . . . . . . . . .  30

   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  30
       9.1. Safe Methods . . . . . . . . . . . . . . . . . . . . . .  30
       9.2. Header Fields Containing Sensitive Information . . . . .  31
       9.3. Data Privacy . . . . . . . . . . . . . . . . . . . . . .  31
       9.4. Information Security Model . . . . . . . . . . . . . . .  31
       9.5. Script Interference with the Server. . . . . . . . . . .  31
       9.6. Data Length and Buffering Considerations . . . . . . . .  32
       9.7. Stateless Processing . . . . . . . . . . . . . . . . . .  32
       9.8. Relative Paths . . . . . . . . . . . . . . . . . . . . .  33
       9.9. Non-parsed Header Output . . . . . . . . . . . . . . . .  33

   10. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  33

   11. References. . . . . . . . . . . . . . . . . . . . . . . . . .  33
       11.1. Normative References. . . . . . . . . . . . . . . . . .  33
       11.2. Informative References. . . . . . . . . . . . . . . . .  34

   12. Authors' Addresses. . . . . . . . . . . . . . . . . . . . . .  35

   13. Full Copyright Statement. . . . . . . . . . . . . . . . . . .  36

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1.  Introduction

1.1.  Purpose

   The Common Gateway Interface (CGI) [22] allows an HTTP [1], [4]
   server and a CGI script to share responsibility for responding to
   client requests.  The client request comprises a Uniform Resource
   Identifier (URI) [11], a request method and various ancillary
   information about the request provided by the transport protocol.

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

   The server is responsible for managing connection, data transfer,
   transport and network issues related to the client request, whereas
   the CGI script handles the application issues, such as data access
   and document processing.

1.2.  Requirements

   The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT',
   'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY' and 'OPTIONAL' in this
   document are to be interpreted as described in BCP 14, RFC 2119 [3].

   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 that are not specified:

   'system-defined'
      The feature may differ between systems, but must be the same for
      different implementations using the same system.  A system will
      usually identify a class of operating systems.  Some systems are
      defined in section 7 of this document.  New systems may be defined
      by new specifications without revision of this document.

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   'implementation-defined'
      The behaviour of the feature may vary from implementation to
      implementation; a particular implementation must document its
      behaviour.

1.4.  Terminology

   This specification uses many terms defined in the HTTP/1.1
   specification [4]; 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.

   'meta-variable'
      A named parameter which carries information from the server to the
      script.  It is not necessarily a variable in the operating
      system's environment, although that is the most common
      implementation.

   'script'
      The software that is invoked by the server according to this
      interface.  It need not be a standalone program, but could be a
      dynamically-loaded or shared library, or even a subroutine in the
      server.  It might be a set of statements interpreted at run-time,
      as the term 'script' is frequently understood, but that is not a
      requirement and within the context of this specification the term
      has the broader definition stated.

   'server'
      The application program that invokes the script in order to
      service requests from the client.

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 [13].  Unless stated otherwise, the elements are
   case-sensitive.  This augmented BNF contains the following
   constructs:

   name = definition
      The name of a rule and its definition are separated by the equals
      character ('=').  Whitespace is only significant in that
      continuation lines of a definition are indented.

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   "literal"
      Double quotation marks (") surround literal text, except for a
      literal quotation mark, which is surrounded by angle-brackets ('<'
      and '>').

   rule1 | rule2
      Alternative rules are separated by a vertical bar ('|').

   (rule1 rule2 rule3)
      Elements enclosed in parentheses are treated as a single element.

   *rule
      A rule preceded by an asterisk ('*') may have zero or more
      occurrences.  The full form is 'n*m rule' indicating at least n
      and at most m occurrences of the rule.  n and m are optional
      decimal values with default values of 0 and infinity respectively.

   [rule]
      An element enclosed in square brackets ('[' and ']') is optional,
      and is equivalent to '*1 rule'.

   N rule
      A rule preceded by a decimal number represents exactly N
      occurrences of the rule.  It is equivalent to 'N*N rule'.

2.2.  Basic Rules

   This specification uses a BNF-like grammar defined in terms of
   characters.  Unlike many specifications which define the bytes
   allowed by a protocol, here each literal in the grammar corresponds
   to the character it represents.  How these characters are represented
   in terms of bits and bytes within a system are either system-defined
   or specified in the particular context.  The single exception is the
   rule 'OCTET', defined below.

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

      alpha         = lowalpha | hialpha
      lowalpha      = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                      "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                      "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                      "y" | "z"
      hialpha       = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
                      "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
                      "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
                      "Y" | "Z"

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      digit         = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                      "8" | "9"
      alphanum      = alpha | digit
      OCTET         = <any 8-bit byte>
      CHAR          = alpha | digit | separator | "!" | "#" | "$" |
                      "%" | "&" | "'" | "*" | "+" | "-" | "." | "`" |
                      "^" | "_" | "{" | "|" | "}" | "~" | CTL
      CTL           = <any control character>
      SP            = <space character>
      HT            = <horizontal tab character>
      NL            = <newline>
      LWSP          = SP | HT | NL
      separator     = "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" |
                      "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" |
                      "}" | SP | HT
      token         = 1*<any CHAR except CTLs or separators>
      quoted-string = <"> *qdtext <">
      qdtext        = <any CHAR except <"> and CTLs but including LWSP>
      TEXT          = <any printable character>

   Note that newline (NL) need not be a single control character, but
   can be a sequence of control characters.  A system MAY define TEXT to
   be a larger set of characters than <any CHAR excluding CTLs but
   including LWSP>.

2.3.  URL Encoding

   Some variables and constructs used here are described as being
   'URL-encoded'.  This encoding is described in section 2 of RFC 2396
   [2].  In a URL-encoded string an escape sequence consists of a
   percent character ("%") followed by two hexadecimal digits, where the
   two hexadecimal digits form an octet.  An escape sequence represents
   the graphic character that has the octet as its code within the
   US-ASCII [9] coded character set, if it exists.  Currently there is
   no provision within the URI syntax to identify which character set
   non-ASCII codes represent, so CGI handles this issue on an ad-hoc
   basis.

   Note that some unsafe (reserved) characters may have different
   semantics when encoded.  The definition of which characters are
   unsafe depends on the context; see section 2 of RFC 2396 [2], updated
   by RFC 2732 [7], for an authoritative treatment.  These reserved
   characters are generally used to provide syntactic structure to the
   character string, for example as field separators.  In all cases, the
   string is first processed with regard to any reserved characters
   present, and then the resulting data can be URL-decoded by replacing
   "%" escape sequences by their character values.

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   To encode a character string, all reserved and forbidden characters
   are replaced by the corresponding "%" escape sequences.  The string
   can then be used in assembling a URI.  The reserved characters will
   vary from context to context, but will always be drawn from this set:

      reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" | "$" |
                 "," | "[" | "]"

   The last two characters were added by RFC 2732 [7].  In any
   particular context, a sub-set of these characters will be reserved;
   the other characters from this set MUST NOT be encoded when a string
   is URL-encoded in that context.  Other basic rules used to describe
   URI syntax are:

      hex        = digit | "A" | "B" | "C" | "D" | "E" | "F" | "a" | "b"
                   | "c" | "d" | "e" | "f"
      escaped    = "%" hex hex
      unreserved = alpha | digit | mark
      mark       = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"

3.  Invoking the Script

3.1.  Server Responsibilities

   The server acts as an application gateway.  It receives the request
   from the client, selects a CGI script to handle the request, converts
   the client request to a CGI request, executes the script and converts
   the CGI response into a response for the client.  When processing the
   client request, it is responsible for implementing any protocol or
   transport level authentication and security.  The server MAY also
   function in a 'non-transparent' manner, modifying the request or
   response in order to provide some additional service, such as media
   type transformation or protocol reduction.

   The server MUST perform translations and protocol conversions on the
   client request data required by this specification.  Furthermore, the
   server retains its responsibility to the client to conform to the
   relevant network protocol even if the CGI script fails to conform to
   this specification.

   If the server is applying authentication to the request, then it MUST
   NOT execute the script unless the request passes all defined access
   controls.

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3.2.  Script Selection

   The server determines which CGI is script to be executed based on a
   generic-form URI supplied by the client.  This URI includes a
   hierarchical path with components separated by "/".  For any
   particular request, the server will identify all or a leading part of
   this path with an individual script, thus placing the script at a
   particular point in the path hierarchy.  The remainder of the path,
   if any, is a resource or sub-resource identifier to be interpreted by
   the script.

   Information about this split of the path is available to the script
   in the meta-variables, described below.  Support for non-hierarchical
   URI schemes is outside the scope of this specification.

3.3.  The Script-URI

   The mapping from client request URI to choice of script is defined by
   the particular server implementation and its configuration.  The
   server may allow the script to be identified with a set of several
   different URI path hierarchies, and therefore is permitted to replace
   the URI by other members of this set during processing and generation
   of the meta-variables.  The server

      1. MAY preserve the URI in the particular client request; or

      2. it MAY select a canonical URI from the set of possible values
         for each script; or

      3. it can implement any other selection of URI from the set.

   From the meta-variables thus generated, a URI, the 'Script-URI', can
   be constructed.  This MUST have the property that if the client had
   accessed this URI instead, then the script would have been executed
   with the same values for the SCRIPT_NAME, PATH_INFO and QUERY_STRING
   meta-variables.  The Script-URI has the structure of a generic URI as
   defined in section 3 of RFC 2396 [2], 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
   meta-variables (see below);

      script-URI = <scheme> "://" <server-name> ":" <server-port>
                   <script-path> <extra-path> "?" <query-string>

   where <scheme> is found from SERVER_PROTOCOL, <server-name>,
   <server-port> and <query-string> are the values of the respective
   meta-variables.  The SCRIPT_NAME and PATH_INFO values, URL-encoded
   with ";", "=" and "?"  reserved, give <script-path> and <extra-path>.

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   See section 4.1.5 for more information about the PATH_INFO
   meta-variable.

   The scheme and the protocol are not identical as the scheme
   identifies the access method in addition to the application protocol.
   For example, a resource accessed using Transport Layer Security (TLS)
   [14] would have a request URI with a scheme of https when using the
   HTTP protocol [19].  CGI/1.1 provides no generic means for the script
   to reconstruct this, and therefore the Script-URI as defined includes
   the base protocol used.  However, a script MAY make use of
   scheme-specific meta-variables to better deduce the URI scheme.

   Note that this definition also allows URIs to be constructed which
   would invoke the script with any permitted values for the path-info
   or query-string, by modifying the appropriate components.

3.4.  Execution

   The script is invoked in a system-defined manner.  Unless specified
   otherwise, the file containing the script will be invoked as an
   executable program.  The server prepares the CGI request as described
   in section 4; this comprises the request meta-variables (immediately
   available to the script on execution) and request message data.  The
   request data need not be immediately available to the script; the
   script can be executed before all this data has been received by the
   server from the client.  The response from the script is returned to
   the server as described in sections 5 and 6.

   In the event of an error condition, the server can interrupt or
   terminate script execution at any time and without warning.  That
   could occur, for example, in the event of a transport failure between
   the server and the client; so the script SHOULD be prepared to handle
   abnormal termination.

4.  The CGI Request

   Information about a request comes from two different sources; the
   request meta-variables and any associated message-body.

4.1.  Request Meta-Variables

   Meta-variables contain data about the request passed from the server
   to the script, and are accessed by the script in a system-defined
   manner.  Meta-variables are identified by case-insensitive names;
   there cannot be two different variables whose names differ in case
   only.  Here they are shown using a canonical representation of
   capitals plus underscore ("_").  A particular system can define a
   different representation.

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      meta-variable-name = "AUTH_TYPE" | "CONTENT_LENGTH" |
                           "CONTENT_TYPE" | "GATEWAY_INTERFACE" |
                           "PATH_INFO" | "PATH_TRANSLATED" |
                           "QUERY_STRING" | "REMOTE_ADDR" |
                           "REMOTE_HOST" | "REMOTE_IDENT" |
                           "REMOTE_USER" | "REQUEST_METHOD" |
                           "SCRIPT_NAME" | "SERVER_NAME" |
                           "SERVER_PORT" | "SERVER_PROTOCOL" |
                           "SERVER_SOFTWARE" | scheme |
                           protocol-var-name | extension-var-name
      protocol-var-name  = ( protocol | scheme ) "_" var-name
      scheme             = alpha *( alpha | digit | "+" | "-" | "." )
      var-name           = token
      extension-var-name = token

   Meta-variables with the same name as a scheme, and names beginning
   with the name of a protocol or scheme (e.g., HTTP_ACCEPT) are also
   defined.  The number and meaning of these variables may change
   independently of this specification.  (See also section 4.1.18.)

   The server MAY set additional implementation-defined extension meta-
   variables, whose names SHOULD be prefixed with "X_".

   This specification does not distinguish between zero-length (NULL)
   values and missing values.  For example, a script cannot distinguish
   between the two requests http://host/script and http://host/script?
   as in both cases the QUERY_STRING meta-variable would be NULL.

      meta-variable-value = "" | 1*<TEXT, CHAR or tokens of value>

   An optional meta-variable may be omitted (left unset) if its value is
   NULL.  Meta-variable values MUST be considered case-sensitive except
   as noted otherwise.  The representation of the characters in the
   meta-variables is system-defined; the server MUST convert values to
   that representation.

4.1.1.  AUTH_TYPE

   The AUTH_TYPE variable identifies any mechanism used by the server to
   authenticate the user.  It contains a case-insensitive value defined
   by the client protocol or server implementation.

   For HTTP, if the client request required authentication for external
   access, then the server MUST set the value of this variable from the
   'auth-scheme' token in the request Authorization header field.

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      AUTH_TYPE      = "" | auth-scheme
      auth-scheme    = "Basic" | "Digest" | extension-auth
      extension-auth = token

   HTTP access authentication schemes are described in RFC 2617 [5].

4.1.2.  CONTENT_LENGTH

   The CONTENT_LENGTH variable contains the size of the message-body
   attached to the request, if any, in decimal number of octets.  If no
   data is attached, then NULL (or unset).

      CONTENT_LENGTH = "" | 1*digit

   The server MUST set this meta-variable if and only if the request is
   accompanied by a message-body entity.  The CONTENT_LENGTH value must
   reflect the length of the message-body after the server has removed
   any transfer-codings or content-codings.

4.1.3.  CONTENT_TYPE

   If the request includes a message-body, the CONTENT_TYPE variable is
   set to the Internet Media Type [6] of the message-body.

      CONTENT_TYPE = "" | media-type
      media-type   = type "/" subtype *( ";" parameter )
      type         = token
      subtype      = token
      parameter    = attribute "=" value
      attribute    = token
      value        = token | quoted-string

   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 section 3.7 of the HTTP/1.1
   specification [4].

   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 assume a type of application/octet-stream or it may reject
   the request with an error (as described in section 6.3.3).

   Each media-type defines a set of optional and mandatory parameters.
   This may include a charset parameter with a case-insensitive value
   defining the coded character set for the message-body.  If the

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   charset parameter is omitted, then the default value should be
   derived according to whichever of the following rules is the first to
   apply:

      1. There MAY be a system-defined default charset for some
         media-types.

      2. The default for media-types of type "text" is ISO-8859-1 [4].

      3. Any default defined in the media-type specification.

      4. The default is US-ASCII.

   The server MUST set this meta-variable if an HTTP Content-Type field
   is present in the client 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 content type, otherwise it
   should omit this meta-variable.

4.1.4.  GATEWAY_INTERFACE

   The GATEWAY_INTERFACE variable MUST be set to the dialect of CGI
   being used by the server to communicate with the script.  Syntax:

      GATEWAY_INTERFACE = "CGI" "/" 1*digit "." 1*digit

   Note that the major and minor numbers are treated as separate
   integers and hence each may be incremented higher 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 MUST be ignored by the script
   and MUST NOT be generated by the server.

   This document defines the 1.1 version of the CGI interface.

4.1.5.  PATH_INFO

   The PATH_INFO variable 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 is derived from the portion of the URI path
   hierarchy following the part that identifies the script itself.
   Unlike a URI path, the PATH_INFO is not URL-encoded, and cannot
   contain path-segment parameters.  A PATH_INFO of "/" represents a
   single void path segment.

      PATH_INFO = "" | ( "/" path )
      path      = lsegment *( "/" lsegment )
      lsegment  = *lchar
      lchar     = <any TEXT or CTL except "/">

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   The value is considered case-sensitive and the server MUST preserve
   the case of the path as presented in the request URI.  The server MAY
   impose restrictions and limitations on what values it permits for
   PATH_INFO, and MAY reject the request with an error if it encounters
   any values considered objectionable.  That MAY include any requests
   that would result in an encoded "/" being decoded into PATH_INFO, as
   this might represent a loss of information to the script.  Similarly,
   treatment of non US-ASCII characters in the path is system-defined.

   URL-encoded, the PATH_INFO string forms the extra-path component of
   the Script-URI (see section 3.3) which follows the SCRIPT_NAME part
   of that path.

4.1.6.  PATH_TRANSLATED

   The PATH_TRANSLATED variable is derived by taking the PATH_INFO
   value, parsing it as a local URI in its own right, and performing any
   virtual-to-physical translation appropriate to map it onto the
   server's document repository structure.  The set of characters
   permitted in the result is system-defined.

      PATH_TRANSLATED = *<any character>

   This is the file location that would be accessed by a request for

      <scheme> "://" <server-name> ":" <server-port> <extra-path>

   where <scheme> is the scheme for the original client request and
   <extra-path> is a URL-encoded version of PATH_INFO, with ";", "=" and
   "?"  reserved.  For example, a request such as the following:

      http://somehost.com/cgi-bin/somescript/this%2eis%2epath%3binfo

   would result in a PATH_INFO value of

      /this.is.the.path;info

   An internal URI is constructed from the scheme, server location and
   the URL-encoded PATH_INFO:

      http://somehost.com/this.is.the.path%3binfo

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

      /usr/local/www/htdocs/this.is.the.path;info

   The value of PATH_TRANSLATED is the result of the translation.

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   The value is derived in this way irrespective of whether it maps to a
   valid repository location.  The server MUST preserve the case of the
   extra-path segment unless the underlying repository supports case-
   insensitive names.  If the repository is only case-aware, case-
   preserving, or case-blind with regard to document names, the server
   is 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.

   The server SHOULD set this meta-variable if the request URI includes
   a path-info component.  If PATH_INFO is NULL, then the
   PATH_TRANSLATED variable MUST be set to NULL (or unset).

4.1.7.  QUERY_STRING

   The QUERY_STRING variable contains a URL-encoded search or parameter
   string; it provides information to the CGI script to affect or refine
   the document to be returned by the script.

   The URL syntax for a search string is described in section 3 of RFC
   2396 [2].  The QUERY_STRING value is case-sensitive.

      QUERY_STRING = query-string
      query-string = *uric
      uric         = reserved | unreserved | escaped

   When parsing and decoding the query string, the details of the
   parsing, reserved characters and support for non US-ASCII characters
   depends on the context.  For example, form submission from an HTML
   document [18] uses application/x-www-form-urlencoded encoding, in
   which the characters "+", "&" and "=" are reserved, and the ISO
   8859-1 encoding may be used for non US-ASCII characters.

   The QUERY_STRING value provides the query-string part of the
   Script-URI.  (See section 3.3).

   The server MUST set this variable; if the Script-URI does not include
   a query component, the QUERY_STRING MUST be defined as an empty
   string ("").

4.1.8.  REMOTE_ADDR

   The REMOTE_ADDR variable MUST be set to the network address of the
   client sending the request to the server.

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      REMOTE_ADDR  = hostnumber
      hostnumber   = ipv4-address | ipv6-address
      ipv4-address = 1*3digit "." 1*3digit "." 1*3digit "." 1*3digit
      ipv6-address = hexpart [ ":" ipv4-address ]
      hexpart      = hexseq | ( [ hexseq ] "::" [ hexseq ] )
      hexseq       = 1*4hex *( ":" 1*4hex )

   The format of an IPv6 address is described in RFC 3513 [15].

4.1.9.  REMOTE_HOST

   The REMOTE_HOST variable contains the fully qualified domain name of
   the client sending the request to the server, if available, otherwise
   NULL.  Fully qualified domain names take the form as described in
   section 3.5 of RFC 1034 [17] and section 2.1 of RFC 1123 [12].
   Domain names are not case sensitive.

      REMOTE_HOST   = "" | hostname | hostnumber
      hostname      = *( domainlabel "." ) toplabel [ "." ]
      domainlabel   = alphanum [ *alphahypdigit alphanum ]
      toplabel      = alpha [ *alphahypdigit alphanum ]
      alphahypdigit = alphanum | "-"

   The server SHOULD set this variable.  If the hostname is not
   available for performance reasons or otherwise, the server MAY
   substitute the REMOTE_ADDR value.

4.1.10.  REMOTE_IDENT

   The REMOTE_IDENT variable MAY be used to provide identity information
   reported about the connection by an RFC 1413 [20] request to the
   remote agent, if available.  The server may choose not to support
   this feature, or not to request the data for efficiency reasons, or
   not to return available identity data.

      REMOTE_IDENT = *TEXT

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

4.1.11.  REMOTE_USER

   The REMOTE_USER variable provides a user identification string
   supplied by client as part of user authentication.

      REMOTE_USER = *TEXT

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   If the client request required HTTP Authentication [5] (e.g., the
   AUTH_TYPE meta-variable is set to "Basic" or "Digest"), then the
   value of the REMOTE_USER meta-variable MUST be set to the user-ID
   supplied.

4.1.12.  REQUEST_METHOD

   The REQUEST_METHOD meta-variable MUST be set to the method which
   should be used by the script to process the request, as described in
   section 4.3.

      REQUEST_METHOD   = method
      method           = "GET" | "POST" | "HEAD" | extension-method
      extension-method = "PUT" | "DELETE" | token

   The method is case sensitive.  The HTTP methods are described in
   section 5.1.1 of the HTTP/1.0 specification [1] and section 5.1.1 of
   the HTTP/1.1 specification [4].

4.1.13.  SCRIPT_NAME

   The SCRIPT_NAME variable MUST be set to a URI path (not URL-encoded)
   which could identify the CGI script (rather than the script's
   output).  The syntax is the same as for PATH_INFO (section 4.1.5)

      SCRIPT_NAME = "" | ( "/" path )

   The leading "/" is not part of the path.  It is optional if the path
   is NULL; however, the variable MUST still be set in that case.

   The SCRIPT_NAME string forms some leading part of the path component
   of the Script-URI derived in some implementation-defined manner.  No
   PATH_INFO segment (see section 4.1.5) is included in the SCRIPT_NAME
   value.

4.1.14.  SERVER_NAME

   The SERVER_NAME variable MUST be set to the name of the server host
   to which the client request is directed.  It is a case-insensitive
   hostname or network address.  It forms the host part of the
   Script-URI.

      SERVER_NAME = server-name
      server-name = hostname | ipv4-address | ( "[" ipv6-address "]" )

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   A deployed server can have more than one possible value for this
   variable, where several HTTP virtual hosts share the same IP address.
   In that case, the server would use the contents of the request's Host
   header field to select the correct virtual host.

4.1.15.  SERVER_PORT

   The SERVER_PORT variable MUST be set to the TCP/IP port number on
   which this request is received from the client.  This value is used
   in the port part of the Script-URI.

      SERVER_PORT = server-port
      server-port = 1*digit

   Note that this variable MUST be set, even if the port is the default
   port for the scheme and could otherwise be omitted from a URI.

4.1.16.  SERVER_PROTOCOL

   The SERVER_PROTOCOL variable MUST be set to the name and version of
   the application protocol used for this CGI request.  This MAY differ
   from the protocol version used by the server in its communication
   with the client.

      SERVER_PROTOCOL   = HTTP-Version | "INCLUDED" | extension-version
      HTTP-Version      = "HTTP" "/" 1*digit "." 1*digit
      extension-version = protocol [ "/" 1*digit "." 1*digit ]
      protocol          = token

   Here, 'protocol' defines the syntax of some of the information
   passing between the server and the script (the 'protocol-specific'
   features).  It is not case sensitive and is usually presented in
   upper case.  The protocol is not the same as the scheme part of the
   script URI, which defines the overall access mechanism used by the
   client to communicate with the server.  For example, a request that
   reaches the script with a protocol of "HTTP" may have used an "https"
   scheme.

   A well-known value for SERVER_PROTOCOL which the server MAY use 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.  The script should treat this as an HTTP/1.0
   request.

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4.1.17.  SERVER_SOFTWARE

   The SERVER_SOFTWARE meta-variable MUST be set to the name and version
   of the information server software making the CGI request (and
   running the gateway).  It SHOULD be the same as the server
   description reported to the client, if any.

      SERVER_SOFTWARE = 1*( product | comment )
      product         = token [ "/" product-version ]
      product-version = token
      comment         = "(" *( ctext | comment ) ")"
      ctext           = <any TEXT excluding "(" and ")">

4.1.18.  Protocol-Specific Meta-Variables

   The server SHOULD set meta-variables specific to the protocol and
   scheme for the request.  Interpretation of protocol-specific
   variables depends on the protocol version in SERVER_PROTOCOL.  The
   server MAY set a meta-variable with the name of the scheme to a
   non-NULL value if the scheme is not the same as the protocol.  The
   presence of such a variable indicates to a script which scheme is
   used by the request.

   Meta-variables with names beginning with "HTTP_" contain values read
   from the client request header fields, if the protocol used is HTTP.
   The HTTP header field name is converted to upper case, has all
   occurrences of "-" replaced with "_" and has "HTTP_" prepended to
   give the meta-variable name.  The header data can be presented as
   sent by the client, or can 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 a single value
   having the same semantics.  Similarly, a header field that spans
   multiple lines MUST be merged onto 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 meta-variable.

   The server is not required to create meta-variables for all the
   header fields that it receives.  In particular, it SHOULD remove any
   header fields carrying authentication information, such as
   'Authorization'; or that are available to the script in other
   variables, such as 'Content-Length' and 'Content-Type'.  The server
   MAY remove header fields that relate solely to client-side
   communication issues, such as 'Connection'.

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4.2.  Request Message-Body

   Request data is accessed by the script in a system-defined method;
   unless defined otherwise, this will be by reading the 'standard
   input' file descriptor or file handle.

      Request-Data   = [ request-body ] [ extension-data ]
      request-body   = <CONTENT_LENGTH>OCTET
      extension-data = *OCTET

   A request-body is supplied with the request if the CONTENT_LENGTH is
   not NULL.  The server MUST make at least that many bytes available
   for the script to read.  The server MAY signal an end-of-file
   condition after CONTENT_LENGTH bytes have been read or it MAY supply
   extension data.  Therefore, the script MUST NOT attempt to read more
   than CONTENT_LENGTH bytes, even if more data is available.  However,
   it is not obliged to read any of the data.

   For non-parsed header (NPH) scripts (section 5), the server SHOULD
   attempt to ensure that the data supplied to the script is precisely
   as supplied by the client and is unaltered by the server.

   As transfer-codings are not supported on the request-body, the server
   MUST remove any such codings from the message-body, and recalculate
   the CONTENT_LENGTH.  If this is not possible (for example, because of
   large buffering requirements), the server SHOULD reject the client
   request.  It MAY also remove content-codings from the message-body.

4.3.  Request Methods

   The Request Method, as supplied in the REQUEST_METHOD meta-variable,
   identifies the processing method to be applied by the script in
   producing a response.  The script author can choose to implement the
   methods most appropriate for the particular application.  If the
   script receives a request with a method it does not support it SHOULD
   reject it with an error (see section 6.3.3).

4.3.1.  GET

   The GET method indicates that the script should produce a document
   based on the meta-variable values.  By convention, the GET method is
   'safe' and 'idempotent' and SHOULD NOT have the significance of
   taking an action other than producing a document.

   The meaning of the GET method may be modified and refined by
   protocol-specific meta-variables.

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4.3.2.  POST

   The POST method is used to request the script perform processing and
   produce a document based on the data in the request message-body, in
   addition to meta-variable values.  A common use is form submission in
   HTML [18], intended to initiate processing by the script that has a
   permanent affect, such a change in a database.

   The script MUST check the value of the CONTENT_LENGTH variable before
   reading the attached message-body, and SHOULD check the CONTENT_TYPE
   value before processing it.

4.3.3.  HEAD

   The HEAD method requests the script to do sufficient processing to
   return the response header fields, without providing a response
   message-body.  The script MUST NOT provide a response message-body
   for a HEAD request.  If it does, then the server MUST discard the
   message-body when reading the response from the script.

4.3.4.  Protocol-Specific Methods

   The script MAY implement any protocol-specific method, such as
   HTTP/1.1 PUT and DELETE; it SHOULD check the value of SERVER_PROTOCOL
   when doing so.

   The server MAY decide that some methods are not appropriate or
   permitted for a script, and may handle the methods itself or return
   an error to the client.

4.4.  The 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' HTTP
   query, which is identified by a 'GET' or 'HEAD' request with a URI
   query string that does not contain any unencoded "=" characters.  For
   such a request, the server SHOULD treat the query-string as a
   search-string and parse it into words, using the rules

      search-string = search-word *( "+" search-word )
      search-word   = 1*schar
      schar         = unreserved | escaped | xreserved
      xreserved     = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "," |
                      "$"

   After parsing, each search-word is URL-decoded, optionally encoded in
   a system-defined manner and then added to the command line argument
   list.

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

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

5.  NPH Scripts

5.1.  Identification

   The server MAY support NPH (Non-Parsed Header) scripts; these are
   scripts to which the server passes all responsibility for response
   processing.

   This specification provides no mechanism for an NPH script to be
   identified on the basis of its output data alone.  By convention,
   therefore, any particular script can only ever provide output of one
   type (NPH or CGI) and hence the script itself is described as an 'NPH
   script'.  A server with NPH support MUST provide an implementation-
   defined mechanism for identifying NPH scripts, perhaps based on the
   name or location of the script.

5.2.  NPH Response

   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 the same as for conventional
   CGI scripts.

   Currently, NPH scripts are only defined for HTTP client requests.  An
   (HTTP) NPH script MUST return a complete HTTP response message,
   currently described in section 6 of the HTTP specifications [1], [4].
   The script MUST use the SERVER_PROTOCOL variable to determine the
   appropriate format for a response.  It MUST also take account of any
   generic or protocol-specific meta-variables in the request as might
   be mandated by the particular protocol specification.

   The server MUST ensure that the script output is sent to the client
   unmodified.  Note that this requires the script to use the correct
   character set (US-ASCII [9] and ISO 8859-1 [10] for HTTP) in the
   header fields.  The server SHOULD attempt to ensure that the script
   output is sent directly to the client, with minimal internal and no
   transport-visible buffering.

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   Unless the implementation defines otherwise, the script MUST NOT
   indicate in its response that the client can send further requests
   over the same connection.

6.  CGI Response

6.1.  Response Handling

   A script MUST always provide a non-empty response, and so there is a
   system-defined method for it to send this data back to the server.
   Unless defined otherwise, this will be via the 'standard output' file
   descriptor.

   The script MUST check the REQUEST_METHOD variable when processing the
   request and preparing its response.

   The server MAY implement a timeout period within which data must be
   received from the script.  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.

6.2.  Response Types

   The 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 may be NULL.

      generic-response = 1*header-field NL [ response-body ]

   The script MUST return one of either a document response, a local
   redirect response or a client redirect (with optional document)
   response.  In the response definitions below, the order of header
   fields in a response is not significant (despite appearing so in the
   BNF).  The header fields are defined in section 6.3.

      CGI-Response = document-response | local-redir-response |
                     client-redir-response | client-redirdoc-response

6.2.1.  Document Response

   The CGI script can return a document to the user in a document
   response, with an optional error code indicating the success status
   of the response.

      document-response = Content-Type [ Status ] *other-field NL
                          response-body

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   The script MUST return a Content-Type header field.  A Status header
   field is optional, and status 200 'OK' is assumed if it is omitted.
   The server MUST make any appropriate modifications to the script's
   output to ensure that the response to the client complies with the
   response protocol version.

6.2.2.  Local Redirect Response

   The CGI script can return a URI path and query-string
   ('local-pathquery') for a local resource in a Location header field.
   This indicates to the server that it should reprocess the request
   using the path specified.

      local-redir-response = local-Location NL

   The script MUST NOT return any other header fields or a message-body,
   and the server MUST generate the response that it would have produced
   in response to a request containing the URL

      scheme "://" server-name ":" server-port local-pathquery

6.2.3.  Client Redirect Response

   The CGI script can return an absolute URI path in a Location header
   field, to indicate to the client that it should reprocess the request
   using the URI specified.

      client-redir-response = client-Location *extension-field NL

   The script MUST not provide any other header fields, except for
   server-defined CGI extension fields.  For an HTTP client request, the
   server MUST generate a 302 'Found' HTTP response message.

6.2.4.  Client Redirect Response with Document

   The CGI script can return an absolute URI path in a Location header
   field together with an attached document, to indicate to the client
   that it should reprocess the request using the URI specified.

      client-redirdoc-response = client-Location Status Content-Type
                                 *other-field NL response-body

   The Status header field MUST be supplied and MUST contain a status
   value of 302 'Found', or it MAY contain an extension-code, that is,
   another valid status code that means client redirection.  The server
   MUST make any appropriate modifications to the script's output to
   ensure that the response to the client complies with the response
   protocol version.

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6.3.  Response Header Fields

   The response header fields are either CGI or extension header fields
   to be interpreted by the server, or protocol-specific header fields
   to be included in the response returned to the client.  At least one
   CGI field MUST be supplied; each CGI field MUST NOT appear more than
   once in the response.  The response header fields have the syntax:

      header-field    = CGI-field | other-field
      CGI-field       = Content-Type | Location | Status
      other-field     = protocol-field | extension-field
      protocol-field  = generic-field
      extension-field = generic-field
      generic-field   = field-name ":" [ field-value ] NL
      field-name      = token
      field-value     = *( field-content | LWSP )
      field-content   = *( token | separator | quoted-string )

   The field-name is not case sensitive.  A NULL field value is
   equivalent to a field not being sent.  Note that each header field in
   a CGI-Response MUST be specified on a single line; CGI/1.1 does not
   support continuation lines.  Whitespace is permitted between the ":"
   and the field-value (but not between the field-name and the ":"), and
   also between tokens in the field-value.

6.3.1.  Content-Type

   The Content-Type response field sets the Internet Media Type [6] of
   the entity body.

      Content-Type = "Content-Type:" media-type NL

   If an entity body is returned, the script MUST supply a Content-Type
   field in the response.  If it fails to do so, the server SHOULD NOT
   attempt to determine the correct content type.  The value SHOULD be
   sent unmodified to the client, except for any charset parameter
   changes.

   Unless it is otherwise system-defined, the default charset assumed by
   the client for text media-types is ISO-8859-1 if the protocol is HTTP
   and US-ASCII otherwise.  Hence the script SHOULD include a charset
   parameter.  See section 3.4.1 of the HTTP/1.1 specification [4] for a
   discussion of this issue.

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6.3.2.  Location

   The Location header field is used to specify to the server that the
   script is returning a reference to a document rather than an actual
   document (see sections 6.2.3 and 6.2.4).  It is either an absolute
   URI (optionally with a fragment identifier), indicating that the
   client is to fetch the referenced document, or a local URI path
   (optionally with a query string), indicating that the server is to
   fetch the referenced document and return it to the client as the
   response.

      Location        = local-Location | client-Location
      client-Location = "Location:" fragment-URI NL
      local-Location  = "Location:" local-pathquery NL
      fragment-URI    = absoluteURI [ "#" fragment ]
      fragment        = *uric
      local-pathquery = abs-path [ "?" query-string ]
      abs-path        = "/" path-segments
      path-segments   = segment *( "/" segment )
      segment         = *pchar
      pchar           = unreserved | escaped | extra
      extra           = ":" | "@" | "&" | "=" | "+" | "$" | ","

   The syntax of an absoluteURI is incorporated into this document from
   that specified in RFC 2396 [2] and RFC 2732 [7].  A valid absoluteURI
   always starts with the name of scheme followed by ":"; scheme names
   start with a letter and continue with alphanumerics, "+", "-" or ".".
   The local URI path and query must be an absolute path, and not a
   relative path or NULL, and hence must start with a "/".

   Note that any message-body attached to the request (such as for a
   POST request) may not be available to the resource that is the target
   of the redirect.

6.3.3.  Status

   The Status header field contains a 3-digit integer result code that
   indicates the level of success of the script's attempt to handle the
   request.

      Status         = "Status:" status-code SP reason-phrase NL
      status-code    = "200" | "302" | "400" | "501" | extension-code
      extension-code = 3digit
      reason-phrase  = *TEXT

   Status code 200 'OK' indicates success, and is the default value
   assumed for a document response.  Status code 302 'Found' is used
   with a Location header field and response message-body.  Status code

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   400 'Bad Request' may be used for an unknown request format, such as
   a missing CONTENT_TYPE.  Status code 501 'Not Implemented' may be
   returned by a script if it receives an unsupported REQUEST_METHOD.

   Other valid status codes are listed in section 6.1.1 of the HTTP
   specifications [1], [4], and also the IANA HTTP Status Code Registry
   [8] and MAY be used in addition to or instead of the ones listed
   above.  The script SHOULD check the value of SERVER_PROTOCOL before
   using HTTP/1.1 status codes.  The script MAY reject with error 405
   'Method Not Allowed' HTTP/1.1 requests made using a method it does
   not support.

   Note that returning an error status code does not have to mean an
   error condition with the script itself.  For example, a script that
   is invoked as an error handler by the server should return the code
   appropriate to the server's error condition.

   The reason-phrase is a textual description of the error to be
   returned to the client for human consumption.

6.3.4.  Protocol-Specific Header Fields

   The script MAY return any other header fields that relate to the
   response message defined by the specification for the SERVER_PROTOCOL
   (HTTP/1.0 [1] or HTTP/1.1 [4]).  The server MUST translate the header
   data from the CGI header syntax to the HTTP header syntax if these
   differ.  For example, the character sequence for newline (such as
   UNIX's US-ASCII LF) used by CGI scripts may not be the same as that
   used by HTTP (US-ASCII CR followed by LF).

   The script MUST NOT return any header fields that relate to
   client-side communication issues and could affect the server's
   ability to send the response to the client.  The server MAY remove
   any such header fields returned by the client.  It SHOULD resolve any
   conflicts between header fields returned by the script and header
   fields that it would otherwise send itself.

6.3.5.  Extension Header Fields

   There may be additional implementation-defined CGI header fields,
   whose field names SHOULD begin with "X-CGI-".  The server MAY ignore
   (and delete) any unrecognised header fields with names beginning "X-
   CGI-" that are received from the script.

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6.4.  Response Message-Body

   The response message-body is an attached document to be returned to
   the client by the server.  The server MUST read all the data provided
   by the script, until the script signals the end of the message-body
   by way of an end-of-file condition.  The message-body SHOULD be sent
   unmodified to the client, except for HEAD requests or any required
   transfer-codings, content-codings or charset conversions.

      response-body = *OCTET

7.  System Specifications

7.1.  AmigaDOS

   Meta-Variables
      Meta-variables are passed to the script in identically named
      environment variables.  These are accessed by the DOS library
      routine GetVar().  The flags argument SHOULD be 0.  Case is
      ignored, but upper case is recommended for compatibility with
      case-sensitive systems.

   The current working directory
      The current working directory for the script is set to the
      directory containing the script.

   Character set
      The US-ASCII character set [9] is used for the definition of
      meta-variables, header fields and values; the newline (NL)
      sequence is LF; servers SHOULD also accept CR LF as a newline.

7.2.  UNIX

   For UNIX compatible operating systems, the following are defined:

   Meta-Variables
      Meta-variables are passed to the script in identically named
      environment variables.  These are accessed by the C library
      routine getenv() or variable environ.

   The command line
      This is accessed using the argc and argv arguments to main().  The
      words have any characters which are 'active' in the Bourne shell
      escaped with a backslash.

   The current working directory
      The current working directory for the script SHOULD be set to the
      directory containing the script.

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   Character set
      The US-ASCII character set [9], excluding NUL, is used for the
      definition of meta-variables, header fields and CHAR values; TEXT
      values use ISO-8859-1.  The PATH_TRANSLATED value can contain any
      8-bit byte except NUL.  The newline (NL) sequence is LF; servers
      should also accept CR LF as a newline.

7.3.  EBCDIC/POSIX

   For POSIX compatible operating systems using the EBCDIC character
   set, the following are defined:

   Meta-Variables
      Meta-variables are passed to the script in identically named
      environment variables.  These are accessed by the C library
      routine getenv().

   The command line
      This is accessed using the argc and argv arguments to main().  The
      words have any characters which are 'active' in the Bourne shell
      escaped with a backslash.

   The current working directory
      The current working directory for the script SHOULD be set to the
      directory containing the script.

   Character set
      The IBM1047 character set [21], excluding NUL, is used for the
      definition of meta-variables, header fields, values, TEXT strings
      and the PATH_TRANSLATED value.  The newline (NL) sequence is LF;
      servers should also accept CR LF as a newline.

   media-type charset default
      The default charset value for text (and other implementation-
      defined) media types is IBM1047.

8.  Implementation

8.1.  Recommendations for Servers

   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 specify its behaviour for the following
   cases:

      1. define any restrictions on allowed path segments, in particular
         whether non-terminal NULL segments are permitted;

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      2. define the behaviour for "." or ".." path segments; i.e.,
         whether they are prohibited, treated as ordinary path segments
         or interpreted in accordance with the relative URL
         specification [2];

      3. define any limits of the implementation, including limits on
         path or search string lengths, and limits on the volume of
         header fields the server will parse.

8.2.  Recommendations for Scripts

   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 the output of a form is being processed, check that CONTENT_TYPE
   is "application/x-www-form-urlencoded" [18] or "multipart/form-data"
   [16].  If CONTENT_TYPE is blank, the script can reject the request
   with a 415 'Unsupported Media Type' error, where supported by the
   protocol.

   When parsing PATH_INFO, PATH_TRANSLATED or SCRIPT_NAME the script
   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'.

   When returning header fields, the script should try to send the CGI
   header fields as soon as possible, and should send them before any
   HTTP header fields.  This may help reduce the server's memory
   requirements.

   Script authors should be aware that the REMOTE_ADDR and REMOTE_HOST
   meta-variables (see sections 4.1.8 and 4.1.9) may not identify the
   ultimate source of the request.  They identify the client for the
   immediate request to the server; that client may be a proxy, gateway,
   or other intermediary acting on behalf of the actual source client.

9.  Security Considerations

9.1.  Safe Methods

   As discussed in the security considerations of the HTTP
   specifications [1], [4], the convention has been established that the
   GET and HEAD methods should be 'safe' and 'idempotent' (repeated
   requests have the same effect as a single request).  See section 9.1
   of RFC 2616 [4] for a full discussion.

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9.2.  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 by using
   the Basic authentication scheme, then the client will send an
   Authorization header field containing a username and password.  The
   server validates this information and so it should not pass on the
   password via the HTTP_AUTHORIZATION meta-variable without careful
   consideration.  This also applies to the Proxy-Authorization header
   field and the corresponding HTTP_PROXY_AUTHORIZATION meta-variable.

9.3.  Data Privacy

   Confidential data in a request should be placed in a message-body as
   part of a POST request, and not placed in the URI or message headers.
   On some systems, the environment used to pass meta-variables to a
   script may be visible to other scripts or users.  In addition, many
   existing servers, proxies and clients will permanently record the URI
   where it might be visible to third parties.

9.4.  Information Security Model

   For a client connection using TLS, the security model applies between
   the client and the server, and not between the client and the script.
   It is the server's responsibility to handle the TLS session, and thus
   it is the server which is authenticated to the client, not the CGI
   script.

   This specification provides no mechanism for the script to
   authenticate the server which invoked it.  There is no enforced
   integrity on the CGI request and response messages.

9.5.  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, documents or log files.

   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.

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9.6.  Data Length and Buffering Considerations

   This specification places no limits on the length of the message-body
   presented to the script.  The script 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.  The script 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 Large' (HTTP/1.1).

   Similar considerations apply to the server's handling of the CGI
   response from the script.  There is no limit on the length of the
   header or message-body returned by the script; the server should not
   assume that statically allocated buffers of any size are sufficient
   to contain the entire response.

9.7.  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 cautious 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.).

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9.8.  Relative Paths

   The server should be careful of ".." path segments in the request
   URI.  These should be removed or resolved in the request URI before
   it is split into the script-path and extra-path.  Alternatively, when
   the extra-path is used to find the PATH_TRANSLATED, care should be
   taken to avoid the path resolution from providing translated paths
   outside an expected path hierarchy.

9.9.  Non-parsed Header Output

   If a script returns a non-parsed header output, to be interpreted by
   the client in its native protocol, then the script must address all
   security considerations relating to that protocol.

10.  Acknowledgements

   This work is 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 Chris Adie, Dave Kristol and Mike Meyer; also David
   Morris, Jeremy Madea, Patrick McManus, Adam Donahue, Ross Patterson
   and Harald Alvestrand.

11.  References

11.1  Normative References

   [1]  Berners-Lee, T., Fielding, R. and H. Frystyk, "Hypertext
        Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [2]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI) : Generic Syntax", RFC 2396, August 1998.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirements
        Levels", BCP 14, RFC 2119, March 1997.

   [4]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
        Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
        HTTP/1.1", RFC 2616, June 1999.

   [5]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
        Basic and Digest Access Authentication", RFC 2617, June 1999.

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   [6]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
        Extensions (MIME) Part Two: Media Types", RFC 2046, November
        1996.

   [7]  Hinden, R., Carpenter, B., and L. Masinter, "Format for Literal
        IPv6 Addresses in URL's", RFC 2732, December 1999.

   [8]  "HTTP Status Code Registry",
        http://www.iana.org/assignments/http-status-codes, IANA.

   [9]  "Information Systems -- Coded Character Sets -- 7-bit American
        Standard Code for Information Interchange (7-Bit ASCII)", ANSI
        INCITS.4-1986 (R2002).

   [10] "Information technology -- 8-bit single-byte coded graphic
        character sets -- Part 1: Latin alphabet No. 1", ISO/IEC
        8859-1:1998.

11.2.  Informative References

   [11] Berners-Lee, T., "Universal Resource Identifiers in WWW: A
        Unifying Syntax for the Expression of Names and Addresses of
        Objects on the Network as used in the World-Wide Web", RFC 1630,
        June 1994.

   [12] Braden, R., Ed., "Requirements for Internet Hosts -- Application
        and Support", STD 3, RFC 1123, October 1989.

   [13] Crocker, D., "Standard for the Format of ARPA Internet Text
        Messages", STD 11, RFC 822, August 1982.

   [14] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
        2246, January 1999.

   [15] Hinden R. and S. Deering, "Internet Protocol Version 6 (IPv6)
        Addressing Architecture", RFC 3513, April 2003.

   [16] Masinter, L., "Returning Values from Forms:
        multipart/form-data", RFC 2388, August 1998.

   [17] Mockapetris, P., "Domain Names - Concepts and Facilities", STD
        13, RFC 1034, November 1987.

   [18] Raggett, D., Le Hors, A., and I. Jacobs, Eds., "HTML 4.01
        Specification", W3C Recommendation December 1999,
        http://www.w3.org/TR/html401/.

   [19] Rescola, E. "HTTP Over TLS", RFC 2818, May 2000.

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   [20] St. Johns, M., "Identification Protocol", RFC 1413, February
        1993.

   [21] IBM National Language Support Reference Manual Volume 2,
        SE09-8002-01, March 1990.

   [22] "The Common Gateway Interface",
        http://hoohoo.ncsa.uiuc.edu/cgi/, NCSA, University of Illinois.

12.  Authors' Addresses

   David Robinson
   The Apache Software Foundation

   EMail: drtr@apache.org

   Ken A. L. Coar
   The Apache Software Foundation

   EMail: coar@apache.org

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13.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and at
   www.rfc-editor.org, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

   Intellectual Property

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the ISOC's procedures with respect to rights in ISOC Documents can
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at ietf-
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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