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Friday, January 14, 2011

Exploding myths about mod_rewrite. Part 2.

This article is the anticipated sequel of “Exploding myths about mod_rewrite. Part 1” published more than a year ago. In this two-volume work (which may well become a three-volume one soon) we try to thoroughly explain each tiny issue/option of Helicon Ape mod_rewrite (also true for Apache) to give you all the knowledge necessary to master it in its full. Part 2 devotes itself to contexts processing order.
So, let’s start the story…

Request processing contexts

In the simplest case there are only two request processing contexts:

1. server context (httpd.conf), and
2. “root folder” context (.htaccess)

Server context is executed first and after that, if further processing is allowed (no redirect or proxy happened), root folder config is processed (if present).

server-wide directive
Picture 1. Server-wide configuration (httpd.conf)

per-site directive
Picture 2. Per-site configuration (.htaccess)

Picture 3. Processing order for the configs on Pictures 1 and 2

Now let’s make it more complicated—we’ll have the rules in the root folder and in Directory1. The processing order then becomes:

1. /.htaccess


2. /Directory1/.htaccess


Picture 4. Processing order in case of several .htaccess files

For the request to http://localhost/index.html only the first context is applied, while for http://localhost/Directory1/index.html (and other requests to deeper subfolders) the merged context 1+2 is executed. In our case it’s:


Thus, the child context complements and refines the parent one (but not the server one). This is true for nearly all Apache/Ape modules EXCEPT mod_rewrite. It’s one of a kind and behaves differently.
mod_rewrite behavior

Historically, or for convenience purposes, mod_rewrite contexts do not complement but COMPLETELY OVERRIDE each other. So, if we have two configs

1. /.htaccess

RewriteRule a b

2. /Directory1/.htaccess

RewriteRule b с

the resulting config to be applied to the request will be

RewriteRule b c

and NOT

RewriteRule a b
RewriteRule b с

which may be unobvious for newbies.

For experts! mod_rewrite has an option allowing to change this behavior and inherit the parent rules:

1. /.htaccess

RewriteRule a b

2. /Directory1/.htaccess

# inherit parent rules
RewriteOptions inherit
RewriteRule b с

makes up the following merged config:

RewriteRule b с
# parent rules are appended to the end of the merged config!
RewriteRule a b


section is equivalent in meaning to writing rules in the .htaccess located inside this directory. The only difference is that lives in httpd.conf.
If there are both section and .htaccess for the same directory, they are merged; if the directives inside them interfere, the .htaccess directives are preferred.

Picture 5. Processing order when there are both .htaccess and for the same location

Let’s see how the configs are merged for the request to http://localhost/Directory1/Directory2/index.html if each directory has both section and corresponding .htaccess file.

Picture 6. Processing order when there are several .htaccess files and several sections which are applicable for the same request











The following logics is applied to form the merged config:

1. First to execute is httpd.conf except for tagged sections:

2. Distributed config is built up from the following parts:
a. empty context is created;
b. requested URL is divided into parts like:
c. physical path is defined for each virtual directory:
/ —> C:/inetpub/wwwroot/
/Directory1/ —> C:/inetpub/wwwroot/ Directory1/
/Directory1/Directory2/—> C:/inetpub/wwwroot/ Directory1/ Directory2/
d. corresponding section is found for each path;
e. corresponding .htaccess is found for each path

The resulting sequence of directives will be:


Usually directives’ order is not so important, but not in case with mod_rewrite; that’s why understanding the principles of configs merging may dramatically reduce development and debugging times.

Note! sections are applied not to all parts of the request (see above) but only to the deepest part, and all matches are searched for, for example, if there are two sections:


then both of them get into the merged config.


One should remember that everything written inside server context is applied to all requests and for all sites. Sometimes it may be necessary to limit the scope of directive to one or several sites and that’s the case to use section.
can reside in httpd.conf only. It is merged with server config, i.e. complements it. In case of both .htaccess and section for the specific location, the latter has higher priority and can reject server settings for the specific site (in our case localhost).



Note! mod_rewrite offers another way to restrict scope for the rules to specific host – RewriteCond %{HTTP_HOST}.
The difference is that RewriteCond %{HTTP_HOST} must appear before each RewriteRule, while groups all rules for localhost together and affects all of them. Compare:

RewriteCond %{HTTP_HOST} localhost
RewriteRule . index.php [L]
RewriteCond %{HTTP_HOST} localhost
RewriteRule about$ about.php [L]


RewriteRule . index.php [L]
RewriteRule about$ about.php [L]

On the other hand, the limitation of is that it can’t be used in .htaccess.

Picture 7. Processing order when section is present in httpd.conf

Note! sections are NOT merged together – if there are several s matching the request, the one with the best match is applied. E.g.:

For request to localhost:80/page.html the second line will be executed, whereas for localhost/page.html the first one will fire.

If section is specified inside (which is possible), the processing order is as follows: section of the main server config is accounted first, then inside and after all – .htaccess.

Thus, the use of section outside will lead to application of its () rules to all sites (in case they use this shared folder).

Picture 8. Processing order when there are and sections as well as .htaccess
and sections

These two behave similar to but are used for file names, not for the full path.
E.g., for http://localhost/Directory1/index.html#top they will find the correspondence in file system C:\inetpub\wwwroot\Directory1\index.html and will merge all sections valid for this file name (e.g. and will be merged).
Note! and may reside in .htaccess as well!
and sections

Are applied to the corresponding virtual path, which for http://localhost/Directory1/index.html#top is /Directory1/index.html.

Let’s now put it all together. Here’s the final sequence of sections:

1. httpd.conf
2. httpd.conf
3. .htaccess
4. .htaccess
5. httpd.conf
6. httpd.conf
7. httpd.conf
8. httpd.conf
9. httpd.conf
10. httpd.conf
11. httpd.conf
12. httpd.conf
13. httpd.conf
14. httpd.conf

Seems every aspect of configs processing has been covered. We understand that this article may look somewhat sophisticated, but we are sure there are enthusiasts who’ll find it helpful.

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