Apache mod_rewrite Technical Details

Available Languages: en

+ +

This document discusses some of the technical details of mod_rewrite +and URL matching.

Internal Processing
API Phases
Ruleset Processing

Internal Processing

+ +

The internal processing of this module is very complex but + needs to be explained once even to the average user to avoid + common mistakes and to let you exploit its full + functionality.

First you have to understand that when Apache processes a + HTTP request it does this in phases. A hook for each of these + phases is provided by the Apache API. Mod_rewrite uses two of + these hooks: the URL-to-filename translation hook which is + used after the HTTP request has been read but before any + authorization starts and the Fixup hook which is triggered + after the authorization phases and after the per-directory + config files (.htaccess) have been read, but + before the content handler is activated.

+ +

So, after a request comes in and Apache has determined the + corresponding server (or virtual server) the rewriting engine + starts processing of all mod_rewrite directives from the + per-server configuration in the URL-to-filename phase. A few + steps later when the final data directories are found, the + per-directory configuration directives of mod_rewrite are + triggered in the Fixup phase. In both situations mod_rewrite + rewrites URLs either to new URLs or to filenames, although + there is no obvious distinction between them. This is a usage + of the API which was not intended to be this way when the API + was designed, but as of Apache 1.x this is the only way + mod_rewrite can operate. To make this point more clear + remember the following two points:

+ +

Although mod_rewrite rewrites URLs to URLs, URLs to + filenames and even filenames to filenames, the API + currently provides only a URL-to-filename hook. In Apache + 2.0 the two missing hooks will be added to make the + processing more clear. But this point has no drawbacks for + the user, it is just a fact which should be remembered: + Apache does more in the URL-to-filename hook than the API + intends for it.
+ Unbelievably mod_rewrite provides URL manipulations in + per-directory context, i.e., within + .htaccess files, although these are reached + a very long time after the URLs have been translated to + filenames. It has to be this way because + .htaccess files live in the filesystem, so + processing has already reached this stage. In other + words: According to the API phases at this time it is too + late for any URL manipulations. To overcome this chicken + and egg problem mod_rewrite uses a trick: When you + manipulate a URL/filename in per-directory context + mod_rewrite first rewrites the filename back to its + corresponding URL (which is usually impossible, but see + the RewriteBase directive below for the + trick to achieve this) and then initiates a new internal + sub-request with the new URL. This restarts processing of + the API phases. + +
Again mod_rewrite tries hard to make this complicated + step totally transparent to the user, but you should + remember here: While URL manipulations in per-server + context are really fast and efficient, per-directory + rewrites are slow and inefficient due to this chicken and + egg problem. But on the other hand this is the only way + mod_rewrite can provide (locally restricted) URL + manipulations to the average user.
+

+ +

Don't forget these two points!

Ruleset Processing

+ +

Now when mod_rewrite is triggered in these two API phases, it + reads the configured rulesets from its configuration + structure (which itself was either created on startup for + per-server context or during the directory walk of the Apache + kernel for per-directory context). Then the URL rewriting + engine is started with the contained ruleset (one or more + rules together with their conditions). The operation of the + URL rewriting engine itself is exactly the same for both + configuration contexts. Only the final result processing is + different.

+ +

The order of rules in the ruleset is important because the + rewriting engine processes them in a special (and not very + obvious) order. The rule is this: The rewriting engine loops + through the ruleset rule by rule (RewriteRule directives) and + when a particular rule matches it optionally loops through + existing corresponding conditions (RewriteCond + directives). For historical reasons the conditions are given + first, and so the control flow is a little bit long-winded. See + Figure 1 for more details.

+ [Needs graphics capability to display]
+ Figure 1:The control flow through the rewriting ruleset +

As you can see, first the URL is matched against the + Pattern of each rule. When it fails mod_rewrite + immediately stops processing this rule and continues with the + next rule. If the Pattern matches, mod_rewrite looks + for corresponding rule conditions. If none are present, it + just substitutes the URL with a new value which is + constructed from the string Substitution and goes on + with its rule-looping. But if conditions exist, it starts an + inner loop for processing them in the order that they are + listed. For conditions the logic is different: we don't match + a pattern against the current URL. Instead we first create a + string TestString by expanding variables, + back-references, map lookups, etc. and then we try + to match CondPattern against it. If the pattern + doesn't match, the complete set of conditions and the + corresponding rule fails. If the pattern matches, then the + next condition is processed until no more conditions are + available. If all conditions match, processing is continued + with the substitution of the URL with + Substitution.

+ +

Apache mod_rewrite Technical Details

See also

Internal Processing

API Phases

Ruleset Processing