Tips for debugging .htaccess rewrite rules

This is a repost of one of my articles on StackOverflow.

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:
  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
  1. 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.
  7. 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 regexpCheck.php below for a simple script that you can add to a private/test directory in your site to help you do this. I’ve kept this brief rather than pretty. Just past this into a file regexpCheck.php in a test directory to use it on your website. This will help you build up any regexp and test it against a list of test cases as you do so. I am using the PHP PCRE engine here, but having had a look at the Apache source, this is basically identical to the one used in Apache. There are many HowTos and tutorials which provide templates and can help you build your regexp skills.

Any more comment or suggestions?

Listing 1 — phpinfo


Listing 2 — regexpCheck.php

<html><head><title>Regexp checker</title></head><body>
    $a_pattern= isset($_POST['pattern']) ? $_POST['pattern'] : "";
    $a_ntests = isset($_POST['ntests']) ? $_POST['ntests'] : 1;
    $a_test   = isset($_POST['test']) ? $_POST['test'] : array();

    $res = array(); $maxM=-1; 
    foreach($a_test as $t ){
        $rtn = @preg_match('#'.$a_pattern.'#',$t,$m);
        if($rtn == 1){
            $res[]=array_merge( array('matched'),  $m );
        } else {
            $res[]=array(($rtn === FALSE ? 'invalid' : 'non-matched'));
?> <p>&nbsp; </p>
<form method="post" action="<?php echo $_SERVER['SCRIPT_NAME'];?>">
    <label for="pl">Regexp Pattern: </label>
    <input id="p" name="pattern" size="50" value="<?php echo htmlentities($a_pattern,ENT_QUOTES,"UTF-8");;?>" />
    <label for="n">&nbsp; &nbsp; Number of test vectors: </label>
    <input id="n" name="ntests"  size="3" value="<?php echo $a_ntests;?>"/>
    <input type="submit" name="go" value="OK"/><hr/><p>&nbsp;</p>
    <table><thead><tr><td><b>Test Vector</b></td><td>&nbsp; &nbsp; <b>Result</b></td>
    for ( $i=0; $i<$maxM; $i++ ) echo "<td>&nbsp; &nbsp; <b>\$$i</b></td>";
    echo "</tr><tbody>\n";
    for( $i=0; $i<$a_ntests; $i++ ){
        echo '<tr><td>&nbsp;<input name="test[]" value="', 
            htmlentities($a_test[$i], ENT_QUOTES,"UTF-8"),'" /></td>';
        foreach ($res[$i] as $v) { echo '<td>&nbsp; &nbsp; ',htmlentities($v, ENT_QUOTES,"UTF-8"),'&nbsp; &nbsp; </td>';}
        echo "</tr>\n";
?> </table></form></body></html>


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