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Apache HTTP Server

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  • Environment Variables in Apache

    The Apache HTTP Server provides a mechanism for storing information in named variables that are called environment variables. This information can be used to control various operations such as logging or access control. The variables are also used as a mechanism to communicate with external programs such as CGI scripts. This document discusses different ways to manipulate and use these variables.

    Although these variables are referred to as environment variables, they are not the same as the environment variables controlled by the underlying operating system. Instead, these variables are stored and manipulated in an internal Apache structure. They only become actual operating system environment variables when they are provided to CGI scripts and Server Side Include scripts. If you wish to manipulate the operating system environment under which the server itself runs, you must use the standard environment manipulation mechanisms provided by your operating system shell.


    Setting Environment Variables

    Related Modules


    mod_env
    mod_rewrite
    mod_setenvif
    mod_unique_id
    Related Directives

    BrowserMatch
    BrowserMatchNoCase
    PassEnv
    RewriteRule
    SetEnv
    SetEnvIf
    SetEnvIfNoCase
    UnsetEnv

    Basic Environment Manipulation

    The most basic way to set an environment variable in Apache is using the unconditional SetEnv directive. Variables may also be passed from the environment of the shell which started the server using the PassEnv directive.

    Conditional Per-Request Settings

    For additional flexibility, the directives provided by mod_setenvif allow environment variables to be set on a per-request basis, conditional on characteristics of particular requests. For example, a variable could be set only when a specific browser (User-Agent) is making a request, or only when a specific Referer [sic] header is found. Even more flexibility is available through the mod_rewrite's RewriteRule which uses the [E=...] option to set environment variables.

    Unique Identifiers

    Finally, mod_unique_id sets the environment variable UNIQUE_ID for each request to a value which is guaranteed to be unique across "all" requests under very specific conditions.

    Standard CGI Variables

    In addition to all environment variables set within the Apache configuration and passed from the shell, CGI scripts and SSI pages are provided with a set of environment variables containing meta-information about the request as required by the CGI specification.

    Some Caveats


    Using Environment Variables

    Related Modules

    mod_access
    mod_cgi
    mod_include
    mod_log_config
    mod_rewrite
    Related Directives

    Allow
    CustomLog
    Deny
    LogFormat
    RewriteCond
    RewriteRule

    CGI Scripts

    One of the primary uses of environment variables is to communicate information to CGI scripts. As discussed above, the environment passed to CGI scripts includes standard meta-information about the request in addition to any variables set within the Apache configuration. For more details, see the CGI tutorial.

    SSI Pages

    Server-parsed (SSI) documents processed by mod_include's server-parsed handler can print environment variables using the echo element, and can use environment variables in flow control elements to makes parts of a page conditional on characteristics of a request. Apache also provides SSI pages with the standard CGI environment variables as discussed above. For more details, see the SSI tutorial.

    Access Control

    Access to the server can be controlled based on the value of environment variables using the allow from env= and deny from env= directives. In combination with SetEnvIf, this allows for flexible control of access to the server based on characteristics of the client. For example, you can use these directives to deny access to a particular browser (User-Agent).

    Conditional Logging

    Environment variables can be logged in the access log using the LogFormat option %e. In addition, the decision on whether or not to log requests can be made based on the status of environment variables using the conditional form of the CustomLog directive. In combination with SetEnvIf this allows for flexible control of which requests are logged. For example, you can choose not to log requests for filenames ending in gif, or you can choose to only log requests from clients which are outside your subnet.

    URL Rewriting

    The %{ENV:...} form of TestString in the RewriteCond allows mod_rewrite's rewrite engine to make decisions conditional on environment variables. Note that the variables accessible in mod_rewrite without the ENV: prefix are not actually environment variables. Rather, they are variables special to mod_rewrite which cannot be accessed from other modules.


    Special Purpose Environment Variables

    Interoperability problems have led to the introduction of mechanisms to modify the way Apache behaves when talking to particular clients. To make these mechanisms as flexible as possible, they are invoked by defining environment variables, typically with BrowserMatch, though SetEnv and PassEnv could also be used, for example.

    downgrade-1.0

    This forces the request to be treated as a HTTP/1.0 request even if it was in a later dialect.

    force-no-vary

    This causes any Vary fields to be removed from the response header before it is sent back to the client. Some clients don't interpret this field correctly (see the known client problems page); setting this variable can work around this problem. Setting this variable also implies force-response-1.0.

    force-response-1.0

    This forces an HTTP/1.0 response when set. It was originally implemented as a result of a problem with AOL's proxies. Some clients may not behave correctly when given an HTTP/1.1 response, and this can be used to interoperate with them.

    nokeepalive

    This disables KeepAlive when set.

    suppress-error-charset

    Available in versions after 1.3.26 and 2.0.40

    When Apache issues a redirect in response to a client request, the response includes some actual text to be displayed in case the client can't (or doesn't) automatically follow the redirection. Apache ordinarily labels this text according to the character set which it uses, which is ISO-8859-1.

    However, if the redirection is to a page that uses a different character set, some broken browser versions will try to use the character set from the redirection text rather than the actual page. This can result in Greek, for instance, being incorrectly rendered.

    Setting this environment variable causes Apache to omit the character set for the redirection text, and these broken browsers will then correctly use that of the destination page.


    Examples

    Changing protocol behavior with misbehaving clients

    We recommend that the following lines be included in httpd.conf to deal with known client problems.

    #
    # The following directives modify normal HTTP response behavior.
    # The first directive disables keepalive for Netscape 2.x and browsers that
    # spoof it. There are known problems with these browser implementations.
    # The second directive is for Microsoft Internet Explorer 4.0b2
    # which has a broken HTTP/1.1 implementation and does not properly
    # support keepalive when it is used on 301 or 302 (redirect) responses.
    #
    BrowserMatch "Mozilla/2" nokeepalive
    BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0
    
    #
    # The following directive disables HTTP/1.1 responses to browsers which
    # are in violation of the HTTP/1.0 spec by not being able to grok a
    # basic 1.1 response.
    #
    BrowserMatch "RealPlayer 4\.0" force-response-1.0
    BrowserMatch "Java/1\.0" force-response-1.0
    BrowserMatch "JDK/1\.0" force-response-1.0
    

    Do not log requests for images in the access log

    This example keeps requests for images from appearing in the access log. It can be easily modified to prevent logging of particular directories, or to prevent logging of requests coming from particular hosts.

        SetEnvIf Request_URI \.gif image-request
        SetEnvIf Request_URI \.jpg image-request
        SetEnvIf Request_URI \.png image-request
        CustomLog logs/access_log env=!image-request
    

    Prevent "Image Theft"

    This example shows how to keep people not on your server from using images on your server as inline-images on their pages. This is not a recommended configuration, but it can work in limited circumstances. We assume that all your images are in a directory called /web/images.

        SetEnvIf Referer "^http://www.example.com/" local_referal
        # Allow browsers that do not send Referer info
        SetEnvIf Referer "^$" local_referal
        <Directory /web/images>
           Order Deny,Allow
           Deny from all
           Allow from env=local_referal
        </Directory>
    

    Note: spelling of 'referer' and 'referal' is intentional.

    For more information about this technique, see the ApacheToday tutorial " Keeping Your Images from Adorning Other Sites".


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