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Module mod_rewrite Tutorial (Part 4)

by Dirk Brockhausen    

These directives cannot be defined on directory level. This means that you will have to be able to edit the Apache 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.:

<VirtualHost 192.168.1.1>

ServerAdmin webmaster@yourdomain.com

DocumentRoot /usr/www/htdocs/yourdomain

ServerName yourdomain.com

RewriteLog /usr/apache/logs/yourdomain_mod_rewrite_log

RewriteLogLevel 1

</VirtualHost>

(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.

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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:%{REMOTE_HOST}} =spider [OR]

RewriteCond ${botBase:%{REMOTE_ADDR}} =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.

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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-%{TIME_MON}-%{TIME_DAY}.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.

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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.