<p>Setting both the X-FORWARDED-FOR and CLIENT-IP at the same time (which <a href="https://addons.mozilla.org/en-US/firefox/addon/ipflood/">ipFlood</a> does) results in an HTTP 500.</p>

<p>Here is a demonstration with curl:</p>

<pre><code>$ curl -I -H "X-FORWARDED-FOR: 235.83.227.142" -H "CLIENT-IP: 253.83.227.142" https://www.openstreetmap.org/user/new?cookie_test=true | head -n1
HTTP/1.1 500 Internal Server Error

$ curl -I -H "CLIENT-IP: 253.83.227.142" https://www.openstreetmap.org/user/new?cookie_test=true | head -n1
HTTP/1.1 200 OK

$ curl -I -H "CLIENT-IP: 253.83.227.142" https://www.openstreetmap.org/user/new?cookie_test=true | head -n1
HTTP/1.1 200 OK
</code></pre>

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