Can I make Apache or IIS add a specific header if a certain query string is present?

I want every URL with "download=1" in the query string to be served with Content-Disposition: attachment.

Weird question title because originally this was about Apache only, but now I've added IIS too.


mod_rewrite with mod_headers make this possible as follows:

RewriteEngine on
RewriteCond %{QUERY_STRING} (^|&)download=1(&|$)
RewriteRule ^(.*?([^/]*))$ $1 [E=DOWNLOAD:$2]
Header set "Content-Disposition" "attachment; filename=\"%{DOWNLOAD}e\"" env=DOWNLOAD

Haven't tested this with anything containing non-ASCII or spaces. Those will probably end up showing in the file name URL-escaped, i.e. "%20" etc.
Edit: Special characters should work just fine.

I had to do the same on IIS 7.5, so here's the way to do it: place the following XML into a site's <system.webServer><rewrite> ... section:

    <rule name="DownloadAnything">
        <match serverVariable="RESPONSE_Content_Disposition" pattern=".*" />
        <action type="Rewrite" value="attachment" />
            <add input="{QUERY_STRING}" pattern="(^|&amp;)download=1(&amp;|$)" />

(obviously if you already have an <outboundRules> element, place only the rule inside the existing element)

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