Tips for debugging .htaccess rewrite rules

Question

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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Online .htaccess rewrite testing

I found this Googling for RegEx help, it saved me a lot of time from having to upload new .htaccess files every time I make a small modification.

from the site:

htaccess tester

To test your htaccess rewrite rules, simply fill in the url that you're applying the rules to, place the contents of your htaccess on the larger input area and press "Check Now" button.

Don't forget that in .htaccess files it is a relative URL that is matched.

In a .htaccess file the following RewriteRule will never match:

RewriteRule ^/(.*)     /something/$s

Set environment variables and use headers to receive them:

You can create new environment variables with RewriteRule lines, as mentioned by OP:

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

But if you can't get a server-side script to work, how can you then read this environment variable? One solution is to set a header:

Header set TEST_FOOBAR "%{REDIRECT_TEST0}e"

The value accepts format specifiers, including the %{NAME}e specifier for environment variables (don't forget the lowercase e). Sometimes, you'll need to add the REDIRECT_ prefix, but I haven't worked out when the prefix gets added and when it doesn't.