Proxy GET requests to arbitrary hosts with nginx and secure_link

proxy.conf

#$ URI=www.google.com/images/errors/robot.png
#$ SECRET=secret
#$ echo -n $URI$SECRET | md5sum
#649465bd9b4c3cf30ed8a24a89dbd203  -
#$ curl -sI http://proxy.example.net/proxy/$(echo -n $URI$SECRET | md5sum | sed 's/[^0-9a-f]//g')/$URI | grep Content-Length:
#Content-Length: 6327

server {
        server_name proxy.example.net;
        listen [::]:80;
        resolver 127.0.0.1;

        location /proxy {
                expires max;
                limit_except GET {
                        deny all;
                }

                secure_link_secret secret;

                if ($secure_link = "") {
                        return 403;
                }

                if ($secure_link ~ "([^/]+)(/.+)") {
                        set $next_host $1;
                        set $next_uri $2;
                }

                #proxy_cache cache;
                #proxy_cache_key "proxy:$next_host$next_uri";
                #proxy_cache_valid 1h;
                proxy_set_header  Host $next_host;
                proxy_pass http://$next_host$next_uri?;
        }
}