Apache HTTP Server Version 2.4

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# Apache Module mod_include

Available Languages: en | fr | ja

Description: | Server-parsed html documents (Server Side Includes)
Status: | Base
Module Identifier: | include_module
Source File: | mod_include.c

### Summary

This module provides a filter which will process files before they are sent to the client. The processing is controlled by specially formatted SGML comments, referred to as elements. These elements allow conditional text, the inclusion of other files or programs, as well as the setting and printing of environment variables.

### Topics

- Enabling Server-Side Includes
- PATH_INFO with Server Side Includes
- Available Elements
- Include Variables
- Variable Substitution
- Flow Control Elements
- Legacy expression syntax

### Directives

### Bugfix checklist

- httpd changelog
- Known issues
- Report a bug

### See also

- `Options`
- `AcceptPathInfo`
- Filters
- SSI Tutorial
- Comments

## Enabling Server-Side Includes

Server Side Includes are implemented by the `INCLUDES` filter. If documents containing server-side include directives are given the extension .shtml, the following directives will make Apache parse them and assign the resulting document the mime type of `text/html`:

    AddType text/html .shtml
    AddOutputFilter INCLUDES .shtml

The following directive must be given for the directories containing the shtml files (typically in a `<Directory>` section, but this directive is also valid in `.htaccess` files if `AllowOverride` `Options` is set):

    Options +Includes

For backwards compatibility, the `server-parsed` handler also activates the INCLUDES filter. As well, Apache will activate the INCLUDES filter for any document with mime type `text/x-server-parsed-html` or `text/x-server-parsed-html3` (and the resulting output will have the mime type `text/html`).

For more information, see our Tutorial on Server Side Includes.

## PATH_INFO with Server Side Includes

Files processed for server-side includes no longer accept requests with `PATH_INFO` (trailing pathname information) by default. You can use the `AcceptPathInfo` directive to configure the server to accept requests with `PATH_INFO`.

## Available Elements

The document is parsed as an HTML document, with special commands embedded as SGML comments. A command has the syntax:

`<!--#element attribute=value attribute=value ... -->`

The value will often be enclosed in double quotes, but single quotes (`'`) and backticks (```) are also possible. Many commands only allow a single attribute-value pair. Note that the comment terminator (`-->`) should be preceded by whitespace to ensure that it isn't considered part of an SSI token. Note that the leading `<!--#` is one token and may not contain any whitespaces.

The allowed elements are listed in the following table:

Element | Description
--- | ---
`comment` | SSI comment
`config` | configure output formats
`echo` | print variables
`exec` | execute external programs
`fsize` | print size of a file
`flastmod` | print last modification time of a file
`include` | include a file
`printenv` | print all available variables
`set` | set a value of a variable

SSI elements may be defined by modules other than `mod_include`. In fact, the `exec` element is provided by `mod_cgi`, and will only be available if this module is loaded.

### The comment Element

This command doesn't output anything. Its only use is to add comments within a file. These comments are not printed.

This syntax is available in version 2.4.21 and later.

`<!--#comment Blah Blah Blah -->
or
<!--#comment text="Blah Blah Blah" -->`

### The config Element

This command controls various aspects of the parsing. The valid attributes are:

`echomsg` (Apache 2.1 and later)

  The value is a message that is sent back to the client if the `echo` element attempts to echo an undefined variable. This overrides any `SSIUndefinedEcho` directives.

  `<!--#config echomsg="[Value Undefined]" -->`

`errmsg`

  The value is a message that is sent back to the client if an error occurs while parsing the document. This overrides any `SSIErrorMsg` directives.

  `<!--#config errmsg="[Oops, something broke.]" -->`

`sizefmt`

  The value sets the format to be used when displaying the size of a file. Valid values are `bytes` for a count in bytes, or `abbrev` for a count in Kb or Mb as appropriate, for example a size of 1024 bytes will be printed as "1K".

  `<!--#config sizefmt="abbrev" -->`

`timefmt`

  The value is a string to be used by the `strftime(3)` library routine when printing dates.

  `<!--#config timefmt=""%R, %B %d, %Y"" -->`

### The echo Element

This command prints one of the include variables defined below. If the variable is unset, the result is determined by the `SSIUndefinedEcho` directive. Any dates printed are subject to the currently configured `timefmt`.

Attributes:

`var`
  The value is the name of the variable to print.
`decoding`

  Specifies whether Apache should strip an encoding from the variable before processing the variable further. The default is `none`, where no decoding will be done. If set to `url`, then URL decoding (also known as %-encoding; this is appropriate for use within URLs in links, etc.) will be performed. If set to `urlencoded`, application/x-www-form-urlencoded compatible encoding (found in query strings) will be stripped. If set to `base64`, base64 will be decoded, and if set to `entity`, HTML entity encoding will be stripped. Decoding is done prior to any further encoding on the variable. Multiple encodings can be stripped by specifying more than one comma separated encoding. The decoding setting will remain in effect until the next decoding attribute is encountered, or the element ends.

  The `decoding` attribute must precede the corresponding `var` attribute to be effective.

`encoding`

  Specifies how Apache should encode special characters contained in the variable before outputting them. If set to `none`, no encoding will be done. If set to `url`, then URL encoding (also known as %-encoding; this is appropriate for use within URLs in links, etc.) will be performed. If set to `urlencoded`, application/x-www-form-urlencoded compatible encoding will be performed instead, and should be used with query strings. If set to `base64`, base64 encoding will be performed. At the start of an `echo` element, the default is set to `entity`, resulting in entity encoding (which is appropriate in the context of a block-level HTML element, e.g. a paragraph of text). This can be changed by adding an `encoding` attribute, which will remain in effect until the next `encoding` attribute is encountered or the element ends, whichever comes first.

  The `encoding` attribute must precede the corresponding `var` attribute to be effective.

  In order to avoid cross-site scripting issues, you should always encode user supplied data.

  ### Example

  `<!--#echo encoding="entity" var="QUERY_STRING" -->`

### The exec Element

The `exec` command executes a given shell command or CGI script. It requires `mod_cgi` to be present in the server. If `Options` `IncludesNOEXEC` is set, this command is completely disabled. The valid attributes are:

`cgi`

  The value specifies a (%-encoded) URL-path to the CGI script. If the path does not begin with a slash (/), then it is taken to be relative to the current document. The document referenced by this path is invoked as a CGI script, even if the server would not normally recognize it as such. However, the directory containing the script must be enabled for CGI scripts (with `ScriptAlias` or `Options` `ExecCGI`).

  The CGI script is given the `PATH_INFO` and query string (`QUERY_STRING`) of the original request from the client; these cannot be specified in the URL path. The include variables will be available to the script in addition to the standard CGI environment.

  ### Example

  `<!--#exec cgi="/cgi-bin/example.cgi" -->`

  If the script returns a `Location:` header instead of output, then this will be translated into an HTML anchor.

  The `include virtual` element should be used in preference to `exec cgi`. In particular, if you need to pass additional arguments to a CGI program, using the query string, this cannot be done with `exec cgi`, but can be done with `include virtual`, as shown here:

  `<!--#include virtual="/cgi-bin/example.cgi?argument=value" -->`

`cmd`

  The server will execute the given string using `/bin/sh`. The include variables are available to the command, in addition to the usual set of CGI variables.

  The use of `#include virtual` is almost always preferred to using either `#exec cgi` or `#exec cmd`. The former (`#include virtual`) uses the standard Apache sub-request mechanism to include files or scripts. It is much better tested and maintained.

  In addition, on some platforms, like Win32, and on unix when using suexec, you cannot pass arguments to a command in an `exec` directive, or otherwise include spaces in the command. Thus, while the following will work under a non-suexec configuration on unix, it will not produce the desired result under Win32, or when running suexec:

  `<!--#exec cmd="perl /path/to/perlscript arg1 arg2" -->`

### The fsize Element

This command prints the size of the specified file, subject to the `sizefmt` format specification. Attributes:

`file`
  The value is a path relative to the directory containing the current document being parsed.

  `This file is <!--#fsize file="mod_include.html" --> bytes.`

  The value of `file` cannot start with a slash (`/`), nor can it contain `../` so as to refer to a file above the current directory or outside of the document root. Attempting to so will result in the error message: `The given path was above the root path`.
`virtual`
  The value is a (%-encoded) URL-path. If it does not begin with a slash (/) then it is taken to be relative to the current document. Note, that this does not print the size of any CGI output, but the size of the CGI script itself.

`This file is <!--#fsize virtual="/docs/mod/mod_include.html" --> bytes.`

Note that in many cases these two are exactly the same thing. However, the `file` attribute doesn't respect URL-space aliases.

### The flastmod Element

This command prints the last modification date of the specified file, subject to the `timefmt` format specification. The attributes are the same as for the `fsize` command.

### The include Element

This command inserts the text of another document or file into the parsed file. Any included file is subject to the usual access control. If the directory containing the parsed file has Options `IncludesNOEXEC` set, then only documents with a text MIME-type (`text/plain`, `text/html` etc.) will be included. Otherwise CGI scripts are invoked as normal using the complete URL given in the command, including any query string.

An attribute defines the location of the document, and may appear more than once in an include element; an inclusion is done for each attribute given to the include command in turn. The valid attributes are:

`file`
  The value is a path relative to the directory containing the current document being parsed. It cannot contain `../`, nor can it be an absolute path. Therefore, you cannot include files that are outside of the document root, or above the current document in the directory structure. The `virtual` attribute should always be used in preference to this one.
`virtual`

  The value is a (%-encoded) URL-path. The URL cannot contain a scheme or hostname, only a path and an optional query string. If it does not begin with a slash (/) then it is taken to be relative to the current document.

  A URL is constructed from the attribute, and the output the server would return if the URL were accessed by the client is included in the parsed output. Thus included files can be nested.

  If the specified URL is a CGI program, the program will be executed and its output inserted in place of the directive in the parsed file. You may include a query string in a CGI url:

  `<!--#include virtual="/cgi-bin/example.cgi?argument=value" -->`

  `include virtual` should be used in preference to `exec cgi` to include the output of CGI programs into an HTML document.

  If the `KeptBodySize` directive is correctly configured and valid for this included file, attempts to POST requests to the enclosing HTML document will be passed through to subrequests as POST requests as well. Without the directive, all subrequests are processed as GET requests.

`onerror`

  The value is a (%-encoded) URL-path which is shown should a previous attempt to include a file or virtual attribute failed. To be effective, this attribute must be specified after the file or virtual attributes being covered. If the attempt to include the onerror path fails, or if onerror is not specified, the default error message will be included.

  `# Simple example
  <!--#include virtual="/not-exist.html" onerror="/error.html" -->`

  `# Dedicated onerror paths
  <!--#include virtual="/path-a.html" onerror="/error-a.html" virtual="/path-b.html" onerror="/error-b.html" -->`

### The printenv Element

This prints out a plain text listing of all existing variables and their values. Special characters are entity encoded (see the `echo` element for details) before being output. There are no attributes.

### Example

`<pre> <!--#printenv --> </pre>`

### The set Element

This sets the value of a variable. Attributes:

`var`
  The name of the variable to set.
`value`
  The value to give a variable.
`decoding`

  Specifies whether Apache should strip an encoding from the variable before processing the variable further. The default is `none`, where no decoding will be done. If set to `url`, `urlencoded`, `base64` or `entity`, URL decoding, application/x-www-form-urlencoded decoding, base64 decoding or HTML entity decoding will be performed respectively. More than one decoding can be specified by separating with commas. The decoding setting will remain in effect until the next decoding attribute is encountered, or the element ends. The `decoding` attribute must precede the corresponding `var` attribute to be effective.

`encoding`

  Specifies how Apache should encode special characters contained in the variable before setting them. The default is `none`, where no encoding will be done. If set to `url`, `urlencoding`, `base64` or `entity`, URL encoding, application/x-www-form-urlencoded encoding, base64 encoding or HTML entity encoding will be performed respectively. More than one encoding can be specified by separating with commas. The encoding setting will remain in effect until the next encoding attribute is encountered, or the element ends. The `encoding` attribute must precede the corresponding `var` attribute to be effective. Encodings are applied after all decodings have been stripped.

### Example

`<!--#set var="category" value="help" -->`

## Include Variables

In addition to the variables in the standard CGI environment, these are available for the `echo` command, for `if` and `elif`, and to any program invoked by the document.

`DATE_GMT`
  The current date in Greenwich Mean Time.
`DATE_LOCAL`
  The current date in the local time zone.
`DOCUMENT_ARGS`
  This variable contains the query string of the active SSI document, or the empty string if a query string is not included. For subrequests invoked through the `include` SSI directive, `QUERY_STRING` will represent the query string of the subrequest and `DOCUMENT_ARGS` will represent the query string of the SSI document. (Available in Apache HTTP Server 2.4.19 and later.)
`DOCUMENT_NAME`
  The filename (excluding directories) of the document requested by the user.
`DOCUMENT_PATH_INFO`
  The trailing pathname information. See directive `AcceptPathInfo` for more information about `PATH_INFO`.
`DOCUMENT_URI`
  The (%-decoded) URL path of the document requested by the user. Note that in the case of nested include files, this is not the URL for the current document. Note also that if the URL is modified internally (e.g. by an `alias` or `directoryindex`), the modified URL is shown.
`LAST_MODIFIED`
  The last modification date of the document requested by the user.
`QUERY_STRING_UNESCAPED`
  If a query string is present in the request for the active SSI document, this variable contains the (%-decoded) query string, which is escaped for shell usage (special characters like `&` etc. are preceded by backslashes). It is not set if a query string is not present. Use `DOCUMENT_ARGS` if shell escaping is not desired.
`USER_NAME`
  The user name of the owner of the file.

## Variable Substitution

Variable substitution is done within quoted strings in most cases where they may reasonably occur as an argument to an SSI directive. This includes the `config`, `exec`, `flastmod`, `fsize`, `include`, `echo`, and `set` directives. If `SSILegacyExprParser` is set to `on`, substitution also occurs in the arguments to conditional operators. You can insert a literal dollar sign into the string using backslash quoting:

`<!--#set var="cur" value="\$test" -->`

If a variable reference needs to be substituted in the middle of a character sequence that might otherwise be considered a valid identifier in its own right, it can be disambiguated by enclosing the reference in braces, a la shell substitution:

`<!--#set var="Zed" value="${REMOTE_HOST}_${REQUEST_METHOD}" -->`

This will result in the `Zed` variable being set to "`X_Y`" if `REMOTE_HOST` is "`X`" and `REQUEST_METHOD` is "`Y`".

## Flow Control Elements

The basic flow control elements are:

`<!--#if expr="test_condition" -->
<!--#elif expr="test_condition" -->
<!--#else -->
<!--#endif -->`

The `if` element works like an if statement in a programming language. The test condition is evaluated and if the result is true, then the text until the next `elif`, `else` or `endif` element is included in the output stream.

The `elif` or `else` statements are used to put text into the output stream if the original test_condition was false. These elements are optional.

The `endif` element ends the `if` element and is required.

test_condition is a boolean expression which follows the ap_expr syntax. The syntax can be changed to be compatible with Apache HTTPD 2.2.x using `SSILegacyExprParser`.

The SSI variables set with the `var` element are exported into the request environment and can be accessed with the `reqenv` function. As a short-cut, the function name `v` is also available inside `mod_include`.

The below example will print "from local net" if client IP address belongs to the 10.0.0.0/8 subnet.

`<!--#if expr='-R "10.0.0.0/8"' -->
from local net
<!--#else -->
from somewhere else
<!--#endif -->`

The below example will print "foo is bar" if the variable `foo` is set to the value "bar".

`<!--#if expr='v("foo") = "bar"' -->
foo is bar
<!--#endif -->`

### Reference Documentation

See also: Expressions in Apache HTTP Server, for a complete reference and examples. The restricted functions are not available inside `mod_include`

## Legacy expression syntax

This section describes the syntax of the `#if expr` element if `SSILegacyExprParser` is set to `on`.

`string`
  true if string is not empty
`-A string`

  true if the URL represented by the string is accessible by configuration, false otherwise. This is useful where content on a page is to be hidden from users who are not authorized to view the URL, such as a link to that URL. Note that the URL is only tested for whether access would be granted, not whether the URL exists.

  ### Example

  `<!--#if expr="-A /private" -->
  Click <a href="/private">here</a> to access private information.
  <!--#endif -->`

`string1 = string2
string1 == string2
string1 != string2`

  Compare string1 with string2. If string2 has the form `/string2/` then it is treated as a regular expression. Regular expressions are implemented by the PCRE engine and have the same syntax as those in perl 5. Note that `==` is just an alias for `=` and behaves exactly the same way.

  If you are matching positive (`=` or `==`), you can capture grouped parts of the regular expression. The captured parts are stored in the special variables `$1` .. `$9`. The whole string matched by the regular expression is stored in the special variable `$0`

  ### Example

  `<!--#if expr="$QUERY_STRING = /^sid=([a-zA-Z0-9]+)/" -->
  <!--#set var="session" value="$1" -->
  <!--#endif -->`

`string1 < string2
string1 <= string2
string1 > string2
string1 >= string2`
  Compare string1 with string2. Note, that strings are compared literally (using `strcmp(3)`). Therefore the string "100" is less than "20".
`( test_condition )`
  true if test_condition is true
`! test_condition`
  true if test_condition is false
`test_condition1 && test_condition2`
  true if both test_condition1 and test_condition2 are true
`test_condition1 || test_condition2`
  true if either test_condition1 or test_condition2 is true

"`=`" and "`!=`" bind more tightly than "`&&`" and "`||`". "`!`" binds most tightly. Thus, the following are equivalent:

`<!--#if expr="$a = test1 && $b = test2" -->
<!--#if expr="($a = test1) && ($b = test2)" -->`

The boolean operators `&&` and `||` share the same priority. So if you want to bind such an operator more tightly, you should use parentheses.

Anything that's not recognized as a variable or an operator is treated as a string. Strings can also be quoted: `'string'`. Unquoted strings can't contain whitespace (blanks and tabs) because it is used to separate tokens such as variables. If multiple strings are found in a row, they are concatenated using blanks. So,

`string1 string2` results in `string1 string2`
and
`'string1 string2'` results in `string1 string2`.

### Optimization of Boolean Expressions

If the expressions become more complex and slow down processing significantly, you can try to optimize them according to the evaluation rules:

- Expressions are evaluated from left to right
- Binary boolean operators (`&&` and `||`) are short circuited wherever possible. In conclusion with the rule above that means, `mod_include` evaluates at first the left expression. If the left result is sufficient to determine the end result, processing stops here. Otherwise it evaluates the right side and computes the end result from both left and right results.
- Short circuit evaluation is turned off as long as there are regular expressions to deal with. These must be evaluated to fill in the backreference variables (`$1` .. `$9`).

If you want to look how a particular expression is handled, you can recompile `mod_include` using the `-DDEBUG_INCLUDE` compiler option. This inserts for every parsed expression tokenizer information, the parse tree and how it is evaluated into the output sent to the client.

### Escaping slashes in regex strings

All slashes which are not intended to act as delimiters in your regex must be escaped. This is regardless of their meaning to the regex engine.

## SSIEndTag Directive

Description: | String that ends an include element
Syntax: | `SSIEndTag tag`
Default: | `SSIEndTag "-->"`
Context: | server config, virtual host
Status: | Base
Module: | mod_include

This directive changes the string that `mod_include` looks for to mark the end of an include element.

    SSIEndTag "%>"

### See also

- `SSIStartTag`

## SSIErrorMsg Directive

Description: | Error message displayed when there is an SSI error
Syntax: | `SSIErrorMsg message`
Default: | `SSIErrorMsg "[an error occurred while processing this directive]"`
Context: | server config, virtual host, directory, .htaccess
Override: | All
Status: | Base
Module: | mod_include

The `SSIErrorMsg` directive changes the error message displayed when `mod_include` encounters an error. For production servers you may consider changing the default error message to `"<!-- Error -->"` so that the message is not presented to the user.

This directive has the same effect as the `<!--#config errmsg=message -->` element.

    SSIErrorMsg "<!-- Error -->"

## SSIETag Directive

Description: | Controls whether ETags are generated by the server.
Syntax: | `SSIETag on|off`
Default: | `SSIETag off`
Context: | directory, .htaccess
Status: | Base
Module: | mod_include
Compatibility: | Available in version 2.2.15 and later.

Under normal circumstances, a file filtered by `mod_include` may contain elements that are either dynamically generated, or that may have changed independently of the original file. As a result, by default the server is asked not to generate an `ETag` header for the response by adding `no-etag` to the request notes.

The `SSIETag` directive suppresses this behaviour, and allows the server to generate an `ETag` header. This can be used to enable caching of the output. Note that a backend server or dynamic content generator may generate an ETag of its own, ignoring `no-etag`, and this ETag will be passed by `mod_include` regardless of the value of this setting. `SSIETag` can take on the following values:

`off`
  `no-etag` will be added to the request notes, and the server is asked not to generate an ETag. Where a server ignores the value of `no-etag` and generates an ETag anyway, the ETag will be respected.
`on`
  Existing ETags will be respected, and ETags generated by the server will be passed on in the response.

## SSILastModified Directive

Description: | Controls whether `Last-Modified` headers are generated by the server.
Syntax: | `SSILastModified on|off`
Default: | `SSILastModified off`
Context: | directory, .htaccess
Status: | Base
Module: | mod_include
Compatibility: | Available in version 2.2.15 and later.

Under normal circumstances, a file filtered by `mod_include` may contain elements that are either dynamically generated, or that may have changed independently of the original file. As a result, by default the `Last-Modified` header is stripped from the response.

The `SSILastModified` directive overrides this behaviour, and allows the `Last-Modified` header to be respected if already present, or set if the header is not already present. This can be used to enable caching of the output. `SSILastModified` can take on the following values:

`off`
  The `Last-Modified` header will be stripped from responses, unless the `XBitHack` directive is set to `full` as described below.
`on`
  The `Last-Modified` header will be respected if already present in a response, and added to the response if the response is a file and the header is missing. The `SSILastModified` directive takes precedence over `XBitHack`.

## SSILegacyExprParser Directive

Description: | Enable compatibility mode for conditional expressions.
Syntax: | `SSILegacyExprParser on|off`
Default: | `SSILegacyExprParser off`
Context: | directory, .htaccess
Status: | Base
Module: | mod_include
Compatibility: | Available in version 2.3.13 and later.

As of version 2.3.13, `mod_include` has switched to the new ap_expr syntax for conditional expressions in `#if` flow control elements. This directive allows to switch to the old syntax which is compatible with Apache HTTPD version 2.2.x and earlier.

## SSIStartTag Directive

Description: | String that starts an include element
Syntax: | `SSIStartTag tag`
Default: | `SSIStartTag "<!--#"`
Context: | server config, virtual host
Status: | Base
Module: | mod_include

This directive changes the string that `mod_include` looks for to mark an include element to process.

You may want to use this option if you have 2 servers parsing the output of a file each processing different commands (possibly at different times).

    SSIStartTag "<%"
    SSIEndTag   "%>"

The example given above, which also specifies a matching `SSIEndTag`, will allow you to use SSI directives as shown in the example below:

### SSI directives with alternate start and end tags

`<%printenv %>`

### See also

- `SSIEndTag`

## SSITimeFormat Directive

Description: | Configures the format in which date strings are displayed
Syntax: | `SSITimeFormat formatstring`
Default: | `SSITimeFormat "%A, %d-%b-%Y %H:%M:%S %Z"`
Context: | server config, virtual host, directory, .htaccess
Override: | All
Status: | Base
Module: | mod_include

This directive changes the format in which date strings are displayed when echoing `DATE` environment variables. The formatstring is as in `strftime(3)` from the C standard library.

This directive has the same effect as the `<!--#config timefmt=formatstring -->` element.

    SSITimeFormat "%R, %B %d, %Y"

The above directive would cause times to be displayed in the format "22:26, June 14, 2002".

## SSIUndefinedEcho Directive

Description: | String displayed when an unset variable is echoed
Syntax: | `SSIUndefinedEcho string`
Default: | `SSIUndefinedEcho "(none)"`
Context: | server config, virtual host, directory, .htaccess
Override: | All
Status: | Base
Module: | mod_include

This directive changes the string that `mod_include` displays when a variable is not set and "echoed".

    SSIUndefinedEcho "<!-- undef -->"

## XBitHack Directive

Description: | Parse SSI directives in files with the execute bit set
Syntax: | `XBitHack on|off|full`
Default: | `XBitHack off`
Context: | server config, virtual host, directory, .htaccess
Override: | Options
Status: | Base
Module: | mod_include

The `XBitHack` directive controls the parsing of ordinary html documents. This directive only affects files associated with the MIME-type `text/html`. `XBitHack` can take on the following values:

`off`
  No special treatment of executable files.
`on`
  Any `text/html` file that has the user-execute bit set will be treated as a server-parsed html document.
`full`
  As for `on` but also test the group-execute bit. If it is set, then set the `Last-modified` date of the returned file to be the last modified time of the file. If it is not set, then no last-modified date is sent. Setting this bit allows clients and proxies to cache the result of the request.

  ### Note

  You would not want to use the full option, unless you assure the group-execute bit is unset for every SSI script which might `#include` a CGI or otherwise produces different output on each hit (or could potentially change on subsequent requests).

  The `SSILastModified` directive takes precedence over the `XBitHack` directive when `SSILastModified` is set to `on`.

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