Tips for debugging .htaccess rewrite rules

Question

Many posters have problems debugging their RewriteRule and RewriteCond statements within their .htaccess files. Most of these are using a shared hosting service and therefore don't have access to the root server configuration. They cannot avoid using .htaccess files for rewriting and cannot enable a RewriteLogLevel" as many respondents suggest. Also there are many .htaccess-specific pitfalls and constraints are aren't covered well. Setting up a local test LAMP stack involves too much of a learning curve for most.

So my Q here is how would we recommend that they debug their rules themselves. I provide a few suggestions below. Other suggestions would be appreciated.

1. Understand that the mod_rewrite engine cycles through .htaccess files. The engine runs this loop:

   do
     execute server and vhost rewrites (in the Apache Virtual Host Config)
     find the lowest "Per Dir" .htaccess file on the file path with rewrites enabled
     if found(.htaccess)
        execute .htaccess rewrites (in the user's directory)
   while rewrite occurred
   

   So your rules will get executed repeatedly and if you change the URI path then it may end up executing other .htaccessfiles if they exist. So make sure that you terminate this loop, if necessary by adding extra RewriteCond to stop rules firing. Also delete any lower level .htaccess rewrite rulesets unless explicitly intent to use multi-level rulesets.

2. Make sure that the syntax of each Regexp is correct by testing against a set of test patterns to make sure that is a valid syntax and does what you intend with a fully range of test URIs. See answer below for more details.

3. Build up your rules incrementally in a test directory. You can make use of the "execute the deepest .htaccess file on the path feature" to set up a separate test directory (tree) and debug rulesets here without screwing up your main rules and stopping your site working. You have to add them one at a time because this is the only way to localise failures to individual rules.

4. Use a dummy script stub to dump out server and environment variables. (See Listing 2)If your app uses, say, blog/index.php then you can copy this into test/blog/index.php and use it to test out your blog rules in the test subdirectory. You can also use environment variables to make sure that the rewrite engine in interpreting substitution strings correctly, e.g.

   RewriteRule ^(.*) - [E=TEST0:%{DOCUMENT_ROOT}/blog/html_cache/$1.html]
   

   and look for these REDIRECT_* variables in the phpinfo dump. BTW, I used this one and discovered on my site that I had to use %{ENV:DOCUMENT_ROOT_REAL} instead. In the case of redirector looping REDIRECT_REDIRECT_* variables list the previous pass. Etc..

5. Make sure that you don't get bitten by your browser caching incorrect 301 redirects. See answer below. My thanks to Ulrich Palha for this.

6. The rewrite engine seems sensitive to cascaded rules within an .htaccess context, (that is where a RewriteRule results in a substitution and this falls though to further rules), as I found bugs with internal sub-requests (1), and incorrect PATH_INFO processing which can often be prevents by use of the [NS], [L] and [PT] flags.

Any more comment or suggestions?

Listing 1 -- phpinfo

<?php phpinfo(INFO_ENVIRONMENT|INFO_VARIABLES);

Answer 1

Here are a few additional tips on testing rules that may ease the debugging for users on shared hosting

1. Use a Fake-user agent

When testing a new rule, add a condition to only execute it with a fake user-agent that you will use for your requests. This way it will not affect anyone else on your site.

e.g

#protect with a fake user agent
RewriteCond %{HTTP_USER_AGENT}  ^my-fake-user-agent$
#Here is the actual rule I am testing
RewriteCond %{HTTP_HOST} !^www\.domain\.com$ [NC] 
RewriteRule ^ http://www.domain.com%{REQUEST_URI} [L,R=302] 

If you are using Firefox, you can use the User Agent Switcher to create the fake user agent string and test.

2. Do not use 301 until you are done testing

I have seen so many posts where people are still testing their rules and they are using 301's. DON'T.

If you are not using suggestion 1 on your site, not only you, but anyone visiting your site at the time will be affected by the 301.

Remember that they are permanent, and aggressively cached by your browser. Use a 302 instead till you are sure, then change it to a 301.

3. Remember that 301's are aggressively cached in your browser

If your rule does not work and it looks right to you, and you were not using suggestions 1 and 2, then re-test after clearing your browser cache or while in private browsing.

4. Use a HTTP Capture tool

Use a HTTP capture tool like Fiddler to see the actual HTTP traffic between your browser and the server.

While others might say that your site does not look right, you could instead see and report that all of the images, css and js are returning 404 errors, quickly narrowing down the problem.

While others will report that you started at URL A and ended at URL C, you will be able to see that they started at URL A, were 302 redirected to URL B and 301 redirected to URL C. Even if URL C was the ultimate goal, you will know that this is bad for SEO and needs to be fixed.

You will be able to see cache headers that were set on the server side, replay requests, modify request headers to test ....

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