Suppose you have a apache httpd server working as a reverse proxy. Now suppose that this server has to set a HTTP request header called “token” to be attached to every request made to the backends, and that the header’s value must be copied from a query string parameter called “querytoken”.
This can be simply done with the help of mod_headers + mod_rewrite.
RewriteEngine On
RewriteCond %{QUERY_STRING} (?:^|&)querytoken=([^&]+)
RewriteRule (.*) - [E=QS_TOKEN:%1]
RequestHeader set token %{QS_TOKEN}e env=QS_TOKEN
ProxyPass / http://your_backend/
Will your external client make a request like this:
GET /?querytoken=somestuff HTTP/1.0
Host: example
The request header that apache will do to the backend will be something like this:
GET /?token=somestuff HTTP/1.1
Host: 127.0.0.1:1234
token: somestuff
X-Forwarded-For: 127.0.0.1
X-Forwarded-Host: example
X-Forwarded-Server: myserver.linux
Connection: Keep-Alive
The request header “token” with value “somestuff” is added to the request made to the backend.
Great talk (as usual) of Alberto Massidda of @sourcesense here at @fosdem! 
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