incoming X-Forwarded-For in conditional evaulation

[Andrey Semyonov]

Hi.

I've set up nginx as a front-end for apache with mod_rpaf.

My config (parts that are meaningful for this thread) is:
--------------------------------------------------------------------
http {
    proxy_set_header Host $host;
    proxy_set_header X-Forwarded-For $remote_addr;

    server {
        listen 80;
        server_name ~^www\.domain\.com$;
        location / {
            proxy_pass http://127.0.0.1:80;
        }
    }
}
--------------------------------------------------------------------
This works fine. But now I need to bypass original incoming
X-Forwarded-For without change if it equals to '1.2.3.4' or '2.3.4.5'.

I've tried to filter like this:
        if ($http_X-Forwarded-For = "1.2.3.4") { proxy_set_header
X-Forwarded-For "1.2.3.4"; }
but that didn't work. How do I evaluate incoming custom HTTP headers
(those not having their own dedicated variables like $content_length
and others) in config file?