Etienne Kneuss

Table of Content

1. Introduction
2. Truth about the Last modifier
3. How mod_rewrite works
4. Pitfalls
5. Where L shouldn't be used
6. Conclusions

Introduction

From what I have seen in many tutorials or in IRC discussions, I must say that the usage of the [L] (or Last) modifier has become some sort of a standard, a flag that everyone should put there "just in case", as it can't be harmful. This often demonstrates bad understanding on how apache's mod_rewrite internally works. I'll try to explain why [L] should be used wisely, and especially in which cases you shouldn't use it at all.

Truth about the Last modifier

Let's focus on rewriteRules in a per-directory context (a .htaccess file for example), which sadly represents the majority of mod_rewrite's uses. In this context, the [L] flag won't force the rewrite to stop, it will only triggers the internal sub-request at this point. But the sub-request may get through a set of rewriteRules defined in a per-directory context again, meaning that the rewrite of the current URL may not be finished.

Let's consider a simple example:

Most people would think that when requesting /a, the internally requested file would be /b because of the [L] flag. Well, it's not. The last requested file will actually be /c. To understand why, you need to understand how mod_rewrite handles a request in a per-directory context.

How mod_rewrite works

First, there is an important difference between a set of rewriteRules in a per-directory context and one placed in a more general context like in the httpd.conf file. The way mod_rewrite will work is determined by the context in which the rewriteRules are defined. Because mod_rewrite was ment to rewrite URLs to files, it won't be able to directly handle requests in a .htaccess file, where the requests will have already been translated to files. For that reason, mod_rewrite will try to translate them back to URLs and use them in an internal sub request, which will restart the API phases. Read API Phases for more details on that subject.

It means that when an url is rewritten in a per-directory context, it will trigger a sub-request, wich may be rewritten again, and again.

Applying that to the example, we get:

Pitfalls

A common pitfall is a rewriteRule that ends in an infinite loop. For example:

Let's say we request bar, it will get rewritten to page_bar.php which will get rewritten to page_page_bar.php.php and so on. Adding a [L] flag here won't help at all.
There are multiple solutions here. One would be to write your rewriteRules in a more general context. Another solution would be to ensure that mod_rewrite won't get in an infinite loop. You could either use a rewriteCond or simply modify the pattern so it can't match the rewritten url.

Where [L] shouldn't be used

As a general rule, don't use [L] flag when you're defining only one rewriteRule, it's useless. Additionally, don't use [L] when another rewriteRule in the file is able to match the rewritten url. Doing so will only give more work to the rewrite engine. To demonstrate that, let's consider the first example without any [L] flag:

It would require only two sub-requests for the same result. In that case, the [L] flag is better left out.

Conclusions

I hope this little explanantion will help people in understanding how the [L] (Last) flag and more generally mod_rewrite works in a per-directory context. Additionnaly you should never define a set of rewriteRules in a per-directory context, as it will be slower and you may run into the kind of troubles explained above.