mod_setenvif
Allows the setting of environment variables based on characteristics of the request
Base
mod_setenvif.c
setenvif_module

The mod_setenvif module allows you to set internal environment variables according to whether different aspects of the request match regular expressions you specify. These environment variables can be used by other parts of the server to make decisions about actions to be taken, as well as becoming available to CGI scripts and SSI pages.

The directives are considered in the order they appear in the configuration files. So more complex sequences can be used, such as this example, which sets `netscape` if the browser is mozilla but not MSIE.

BrowserMatch ^Mozilla netscape
BrowserMatch MSIE !netscape
Environment Variables in Apache HTTP Server
BrowserMatch
Sets environment variables conditional on HTTP User-Agent
BrowserMatch regex [!]env-variable[=value] [[!]env-variable[=value]] ...
server config
virtual host
directory
.htaccess
FileInfo

The BrowserMatch is a special cases of the SetEnvIf directive that sets environment variables conditional on the `User-Agent` HTTP request header. The following two lines have the same effect:

BrowserMatchNoCase Robot is_a_robot
SetEnvIfNoCase User-Agent Robot is_a_robot

Some additional examples:

BrowserMatch ^Mozilla forms jpeg=yes browser=netscape
BrowserMatch "^Mozilla/[2-3]" tables agif frames javascript
BrowserMatch MSIE !javascript
BrowserMatchNoCase
Sets environment variables conditional on User-Agent without respect to case
BrowserMatchNoCase regex [!]env-variable[=value] [[!]env-variable[=value]] ...
server config
virtual host
directory
.htaccess
FileInfo

The BrowserMatchNoCase directive is semantically identical to the BrowserMatch directive. However, it provides for case-insensitive matching. For example:

BrowserMatchNoCase mac platform=macintosh
BrowserMatchNoCase win platform=windows

The BrowserMatch and BrowserMatchNoCase directives are special cases of the SetEnvIf and SetEnvIfNoCase directives. The following two lines have the same effect:

BrowserMatchNoCase Robot is_a_robot
SetEnvIfNoCase User-Agent Robot is_a_robot
SetEnvIf
Sets environment variables based on attributes of the request
SetEnvIf attribute regex [!]env-variable[=value] [[!]env-variable[=value]] ...
server config
virtual host
directory
.htaccess
FileInfo

The SetEnvIf directive defines environment variables based on attributes of the request. The attribute specified in the first argument can be one of four things:

1. An HTTP request header field (see RFC2616 for more information about these); for example: `Host`, `User-Agent`, `Referer`, and `Accept-Language`. A regular expression may be used to specify a set of request headers.
2. One of the following aspects of the request:
   - `Remote_Host` - the hostname (if available) of the client making the request
   - `Remote_Addr` - the IP address of the client making the request
   - `Server_Addr` - the IP address of the server on which the request was received (only with versions later than 2.0.43)
   - `Request_Method` - the name of the method being used (`GET`, `POST`, et cetera)
   - `Request_Protocol` - the name and version of the protocol with which the request was made (e.g., "HTTP/0.9", "HTTP/1.1", etc.)
   - `Request_URI` - the resource requested on the HTTP request line -- generally the portion of the URL following the scheme and host portion without the query string. See the RewriteCond directive of mod_rewrite for extra information on how to match your query string.
3. The name of an environment variable in the list of those associated with the request. This allows SetEnvIf directives to test against the result of prior matches. Only those environment variables defined by earlier `SetEnvIf[NoCase]` directives are available for testing in this manner. 'Earlier' means that they were defined at a broader scope (such as server-wide) or previously in the current directive's scope. Environment variables will be considered only if there was no match among request characteristics and a regular expression was not used for the attribute.
4. A reference to an extension of an SSL client certificate, located by its object id oid. If a non-SSL request, or in absense of the configured oid, no variable will be set. If the oid is found multiple times, the individual strings will be concatenated, separated by a comma `','`. The oid should reference a string-valued extension.

The second argument (regex) is a

regular expression
. If the regex matches against the attribute, then the remainder of the arguments are evaluated.

The rest of the arguments give the names of variables to set, and optionally values to which they should be set. These take the form of

1. `varname`, or
2. `!varname`, or
3. `varname=value`

In the first form, the value will be set to "1". The second will remove the given variable if already defined, and the third will set the variable to the literal value given by `value`. Since version 2.0.51, Apache httpd will recognize occurrences of `$1`..`$9` within value and replace them by parenthesized subexpressions of regex.

# Example:

SetEnvIf Request_URI "\.gif$" object_is_image=gif
SetEnvIf Request_URI "\.jpg$" object_is_image=jpg
SetEnvIf Request_URI "\.xbm$" object_is_image=xbm
:
SetEnvIf Referer www\.mydomain\.example\.com intra_site_referral
:
SetEnvIf object_is_image xbm XBIT_PROCESSING=1
:
SetEnvIf OID("2.16.840.1.113730.1.13") "(.*)" NetscapeComment=$1
:
SetEnvIf ^TS ^[a-z] HAVE_TS

The first three will set the environment variable `object_is_image` if the request was for an image file, and the fourth sets `intra_site_referral` if the referring page was somewhere on the `www.mydomain.example.com` Web site.

The sixth example will set the `NetscapeComment` environment variable to the string found in the corresponding SSL client certificate field (if found).

The last example will set environment variable `HAVE_TS` if the request contains any headers that begin with "TS" whose values begins with any character in the set [a-z].

Environment Variables in Apache HTTP Server, for additional examples.
SetEnvIfExpr
Sets environment variables based on an ap_expr expression
SetEnvIfExpr expr [!]env-variable[=value] [[!]env-variable[=value]] ...
server config
virtual host
directory
.htaccess
FileInfo

The SetEnvIfExpr directive defines environment variables based on an If `ap_expr`. These expressions will be evaluated at runtime, and applied env-variable in the same fashion as SetEnvIf.

SetEnvIfExpr "tolower(req('X-Sendfile')) == 'd:\images\very_big.iso')" iso_delivered

This would set the environment variable `iso_delivered` every time our application attempts to send it via `X-Sendfile`

A more useful example would be to set the variable rfc1918 if the remote IP address is a private address according to RFC 1918:

SetEnvIfExpr "-R '10.0.0.0/8' || -R '172.16.0.0/12' || -R '192.168.0.0/16'" rfc1918
Expressions in Apache HTTP Server, for a complete reference and more examples.
If
can be used to achive similar results.
mod_filter
SetEnvIfNoCase
Sets environment variables based on attributes of the request without respect to case
SetEnvIfNoCase attribute regex [!]env-variable[=value] [[!]env-variable[=value]] ...
server config
virtual host
directory
.htaccess
FileInfo

The SetEnvIfNoCase is semantically identical to the SetEnvIf directive, and differs only in that the regular expression matching is performed in a case-insensitive manner. For example:

SetEnvIfNoCase Host Example\.Org site=example

This will cause the `site` environment variable to be set to "`example`" if the HTTP request header field `Host:` was included and contained `Example.Org`, `example.org`, or any other combination.