Apache · Configuration and operations
How does mod_rewrite work at a high level?
Short answer
mod_rewrite matches the request URL against RewriteRule patterns and can rewrite, redirect or proxy it, optionally guarded by RewriteCond conditions. Rules run in order, so flags and context determine the final result.
RewriteEngine On enables processing. RewriteCond adds conditions that must all pass for the following RewriteRule. The rule pattern matches the URL path, and the substitution can be a new path, an absolute URL for a redirect or a proxy target with the P flag.
RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ index.php [L]Context matters
- In server or virtual host context the pattern sees the path starting with a slash
- In directory and .htaccess context the leading slash is stripped
- RewriteBase sets the base for relative substitutions in directory context
- L stops the current rule pass, but may not stop all processing; END does
- PT passes the rewritten URI back through the URL-to-filename mapping
Use the simplest tool that works. A single permanent redirect belongs in Redirect or RedirectMatch from mod_alias. Rewrite rules are justified when the decision depends on headers, query strings, filesystem existence or a chain of conditions.
Follow-ups you should expect
Interviewers use these to check whether the first answer was understanding or memorisation.
- 1What is the difference between RewriteRule and RewriteCond?
- 2Why does a pattern behave differently in .htaccess?
- 3When would you use Redirect instead of mod_rewrite?
- 4How do you debug a rewrite loop?
Say it in the interview
mod_rewrite evaluates conditions and rules in order. Conditions must all match, then the rule can rewrite, redirect or proxy. The context changes what the pattern sees, especially the leading slash in .htaccess. I use mod_alias for simple redirects and reserve rewrite for conditional routing, and I debug with rewrite trace logging.
Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.
Related questions
Reading is step one
Now say it out loud.
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