Fix duplicate query string in nginx - nginx

Question

I've got a strange case where my app is now for some reason appending a duplicate query string onto a URL that already has one. I don't have the access or the time to dig into the application code right now, but I do have access to Nginx configs.
What I need is a rewrite rule that will ignore the duplicate query string, ie
http://foo.com?test=bar?test=bar will redirect to http://foo.com?test=bar
Is this even possible? Thank you.

Answer 1

Here, add this inside your server block, before any other location or if:
if ($query_string ~ ^([^?]+)\?\1$) {
set $clean $1;
rewrite . $uri?$clean?;
}
It's quite tricky to get right, because there are more than a few obscure behaviours:
$query_string is the query string without the initial ?
the \1 in the regexp matches a second copy of the first group (...)
you need to save $1 into a new variable because rewrite will overwrite it
you need to use $uri and not $request_uri in the rewrite, because the latter contains a copy of the original query string
you need to add a trailing ? to the rewrite, otherwise the rewrite would append a copy of the original query string! (that's probably the most obscure bit of Nginx syntax, ever)
The simple form of rewrite (without the redirect flag) should be enough for most applications, plus it doesn't require an additional round-trip to the browser.
If it does not work for you, you can try using the $clean query string into the appropriate backend directive (such as fastcgi_param QUERY_STRING and such) or, as a last resort, issuing a browser redirect using the redirect rewrite flag.