Why does this dynamic Nginx proxy configuration not work? -

i have nginx configuration looks this:

location /textproxy {   proxy_pass http://$arg_url;   proxy_connect_timeout 1s;   proxy_redirect off;   proxy_hide_header content-type;   proxy_hide_header content-disposition;   add_header content-type "text/plain";   proxy_set_header host $host; } 

the idea proxies remote url, , rewrites content header text/plain.

for example call:

http://nx/textproxy?url=http://foo:50070/a/b/c?arg=abc:123 

and return contents of http://foo:50070/a/b/c?arg=abc:123, wrapped text/plain header.

this doesn't seem work though, 'invalid upstream port' errors:

2013/07/23 19:05:10 [error] 25292#0: *963177 invalid port in upstream "http://foo:50070/a/b/c?arg=abc:123", client: xx.xxx.xx.xx, server: ~^(?<h>nx)(\..+|)$, request: "get /textproxy?url=http://foo:50070/a/b/c?arg=abc:123 http/1.1", host: "nx" 

any ideas? i'm having hard time trying work out.