Module mod_rewrite Tutorial (Part 4)

Special Directives and Examples
by Dirk Brockhausen

In this final part of our tutorial we will take a look at those special directives we
haven't covered yet.

These directives cannot be defined on directory level.

This means that you will have to be able to edit theApache webserver's
configuration file (httpd.conf). These permissions will usually only be assigned to
users "root" or "admin".

If you wish to log all operations effected by mod_rewrite you can activate logging
with the following entries:

RewriteLog /usr/local/apache/logs/mod_rewrite_log
RewriteLogLevel 1

These entries are not written into the file ".htaccess" but in "Section 2: 'Main' server
configuration" of file "httpd.conf".

All mod_rewrite manipulations will be logged in this file. The log file can have any
name you prefer. It can be referenced as an absolute path or relative to
ServerRoot.

If you wish to maintain separate log files for individual virtual hosts, you will have
to place the pertinent entries in "Section 3: Virtual Hosts", e.g.:

ServerAdmin -email-
DocumentRoot /usr/www/htdocs/yourdomain
ServerName yourdomain.com
RewriteLog /usr/apache/logs/yourdomain_mod_rewrite_log
RewriteLogLevel 1

(Note: If your email reader or browser wraps these lines take care to enter them
unwrapped in your file!)

The RewriteLogLevel can be defined within a range of 1 to 8. Normally, 1 will do
fine. Higher levels are only required for debugging purposes.


Another directive which is very handy for cloaking purposes are the so-called
Rewriting Maps. These are files consisting of key/value pairs, e.g. in the simple
format of an ordinary text file:

cde2c920.infoseek.com spider
205.226.201.32 spider
cde2c923.infoseek.com spider
205.226.201.35 spider
cde2c981.infoseek.com spider
205.226.201.129 spider
cde2cb23.infoseek.com spider
205.226.203.35 spider

These keys are, as you can see, hostnames or IPs. In this simplistic example the
value is always the same, namely "spider".

This directive is entered either in the server section 2 or in the virtual host section
3 in file "httpd.conf":

RewriteMap botBase txt:/www/yourdomain/spiderspy.txt

The Rewriting Map will then be available across your server.

The other directives are entered in file ".htaccess":

RewriteCond ${botBase:%} =spider [OR]
RewriteCond ${botBase:%} =spider
RewriteRule ^(.*).htm$ $1.htm [L]
RewriteRule ^.*.htm$ index.html [L]

The conditions will make the system check whether the required access is
generated by a spider. To this effect a lookup of file "spiderspy.txt" is triggered.

If the key is found, the value "spider" is returned and the condition is rendered as
true.

Next, the first RewriteRule will be executed. This one determines that the called
for ".htm" page will be fed to the spider. The variable $1 is equal to the part in
parentheses of "^(.*).htm$", i.e. the file name will remain the same.

If the URL is called by a normal human visitor, rule 2 applies: the user will be
redirected to page "index.html".

As the ".htm" pages will only be read by spiders, they can be optimized accordingly
for the search engines.

You may also use a file in dbm format instead of an ordinary text file. The binary
data base format helps accelerate the lookup which is particularly important
if you are operating from very large spider lists.

This example given above offers a simple cloaking functionality. All ordinary
visitors will always be redirected to the site's "index.html" page and there is no
access logging beyond the mod_rewrite logs.

However, it does go to show how you can effectively replace several lines of Perl
code with just a few lines of mod_rewrite.

Our last example will illustrate this in some greater detail.

The objective is to present site visitors with your "Picture of the Day". Visitors will
click a link, e.g.: < http://www.yourdomain.com/pic.html > which will display a
different picture every day.

We will work from these server variables:

TIME_MON
TIME_DAY

In file ".htaccess" we will enter the following single code line:

RewriteRule ^pic.html$ pic-%-%.html

(Note: If your email reader or browser wraps this line take care to enter it
unwrapped in your file!)

The URL called for will be rewritten, e.g. to:

pic-08-28.html
pic-08-29.html
pic-08-30.html
etc.

So all you have to do is upload the pertinent files once, after which you won't need
to tend to their daily assignation anymore.

Obviously the time variables can also be used for other periodicities.


With this final example our mod_rewrite tutorial has come to its end.

Of course, we have not tackled each and every directive, variable, etc. here.

Rather, we suggest you view this tutorial as a general introduction intended to
help you as a start off point towards a more in-depth study of the mod_rewrite
module, enabling you to customize it according to your specific requirements.

Dirk Brockhausen is the co-founder and principal of fantomaster.com Ltd. (UK)
and fantomaster.com GmbH (Belgium), a company specializing in webmasters
software development, industrial-strength cloaking and search engine positioning
services. He holds a doctorate in physics and has worked as an SAP consultant
and software developer since 1994. He is also Technical Editor of fantomNews, a
free newsletter focusing on search engine optimization, available at:
< http://fantomaster.com/fantomnews-sub.html >
You can contact him at mailto:-email-
(c) copyright 2000 by fantomaster.com

The Apache mod_rewrite Tutorials

Module mod_rewrite Tutorial — Part 1/4
The Apache Server Power Commander

Module mod_rewrite Tutorial — Part 2/4
Rule Conditions

Module mod_rewrite Tutorial — Part 3/4
Rewriting URLs

Module mod_rewrite Tutorial — Part 4/4
Special Directives and Examples

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