Browsers (Apache: The Definitive Guide, 3rd Edition)

16.9. Browsers

One complication of the Web is that people are free to choose their own browsers, and not all browsers work alike or even nearly alike. They vary enormously in their capabilities. Some browsers display images; others won't. Some that display images won't display frames, tables, Java, and so on.

You can try to circumvent this problem by asking the customer to go to different parts of your script ("Click here to see the frames version"), but in real life people often do not know what their browser will and won't do. A lot of them will not even understand what question you are asking. To get around this problem, Apache can detect the browser type and set environment variables so that your CGI scripts can detect the type and act accordingly.

SetEnvIf and SetEnvIfNoCase

SetEnvIf attribute regex envar[=value] [..] 
SetEnvIfNoCase attribute regex envar[=value] [..] 
Server config, virtual host, directory, .htaccess (from v 1.3.14)

The attribute can be one of the HTTP request header fields, such as Host, User-Agent, Referer, and/or one of the following:

Remote_Host: The client's hostname, if available
Remote_Addr: The client's IP address
Remote_User: The client's authenticated username, if available
Request_Method: GET, POST, etc.
Request_URI: The part of the URL following the scheme and host

The NoCase version works the same except that regular-expression matching is evaluated without regard to letter case.

BrowserMatch and BrowserMatchNoCase

BrowserMatch regex env1[=value1] env2[=value2] ...
BrowserMatchNoCase regex env1[=value1] env2[=value2] ...
Server config, virtual host, directory, .htaccess (from Apache v 1.3.14)

regex is a regular expression matched against the client's User-Agent header, and env1, env2, ... are environment variables to be set if the regular expression matches. The environment variables are set to value1, value2, etc., if present.

So, for instance, we might say:

BrowserMatch ^Mozilla/[23] tables=3 java

The symbol ^ means start from the beginning of the header and match the string Mozilla/ followed by either a 2 or 3. If this is successful, then Apache creates and, if required, specifies values for the given list of environment variables. These variables are invented by the author of the script, and in this case they are:

tables=3
java

In this CGI script, these variables can be tested and take the appropriate action.

BrowserMatchNoCase is simply a case-blind version of BrowserMatch. That is, it doesn't care whether letters are upper- or lowercase. mOZILLA works as well as MoZiLlA.

Note that there is no difference between BrowserMatch and SetEnvIf User-Agent. BrowserMatch exists for backward compatibility.

nokeepalive

This disables KeepAlive (see Chapter 3). Some versions of Netscape claimed to support KeepAlive, but they actually had a bug that meant the server appeared to hang (in fact, Netscape was attempting to reuse the existing connection, even though the server had closed it). The directive:

BrowserMatch "Mozilla/2" nokeepalive

disables KeepAlive for those buggy versions.[64]

[64]And, incidentally, for early versions of Microsoft Internet Explorer, which unwisely pretended to be Netscape Navigator.

force-response-1.0

This forces Apache to respond with HTTP 1.0 to an HTTP 1.0 client, instead of with HTTP 1.1, as is called for by the HTTP 1.1 spec. This is required to work around certain buggy clients that don't recognize HTTP 1.1 responses. Various clients have this problem. The current recommended settings are as follows:[65]

[65]See http://httpd.apache.org/docs-2.0/env.html.

#
# 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

downgrade-1.0

This forces Apache to downgrade to HTTP 1.0 even though the client is HTTP 1.1 (or higher). Microsoft Internet Explorer 4.0b2 earned the dubious distinction of being the only known client to require all three of these settings:

BrowserMatch "MSIE 4\.0b2;" nokeepalive downgrade-1.0 force-response-1.0