I had the need today to come up with an apache rewrite that would, in some cases, change the value of a query string parameter. So for example, for requests coming from anywhere but example.com with a URI beginning “/path” and with a query string parameter named “foo” with the value beginning “bar”, I needed to rewrite the value to be “baz”. I spent some time fooling around with backreferences in the RewriteRule, but I never came up with anything that worked. Eventually, I turned to RewriteMap, which lets you specify text files, hashes, or even external scripts whose output values will be inserted into the destination for the rewrite. So in my example, here’s the apache config:

RewriteMap partner_params prg:/path/to/script.pl
RewriteCond %{HTTP_REFERER} !(.*)example.com(.*)
RewriteCond %{REQUEST_URI} ^/path
RewriteCond %{QUERY_STRING} .*foo=bar.*
RewriteRule ^(.*)$ http://mysite.com$1?${partner_params:%{QUERY_STRING}} [R=301,L]

The RewriteMap line points to a script (source to appear below) that will be executed by the RewriteRule. The first condition specifies that the referring url does not contain “example.com”. The second condition specifies that the URI begins with “/path”. And the third condition specifies that the query string must have a key named “foo” with a value beginning like “bar”. The rule itself takes any matching request and diverts it to http://mysite.com with the same URI as the original request (so something beginning “/path”), then adds a question mark to denote a query string following. Then it passes the query string to the script that RewriteMap knows as “partner_params”. that script reads from STDIN and prints to STDOUT either a newline-terminated result or the four-character response “NULL”. If not NULL, the response is what gets substituted into the RewriteRule.

So now for the script:

#!/usr/bin/perl
$| = 1;
while (<STDIN>) {
if($_ =~/foo=bar/){
$_ =~ s/foo=bar/foo=baz/gi;
}
print $_;
}

Here we simply do a substitution, looking for foo=bar and replacing with foo=baz. And voila, we’ve got custom inline query string munging based on parameters available via pretty standard apache Rewrite data. My particular example probably describes a pretty rare need, but knowing how to have a rewrite call a script to do more complex parsing than is available via apache configuration directives could be handy in a number of ways.