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Module mod_rewrite: 4 - Special Directives and Examples


26.05.2006. agvozden : aleksinac.net

(Tutorial Part 4)
(bro) In this final part of our tutorial we will take
a look at those special directives we haven't covered

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",

ServerAdmin [email protected]
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 spider
cde2c923.infoseek.com spider spider
cde2c981.infoseek.com spider spider
cde2cb23.infoseek.com spider 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

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

The Rewriting Map will then be available across your

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

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


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:


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:


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.



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