Hi,
thank you for reply...

first i remove proxy_rediretct and add www.mydomain.com as you told;
in this case my address bar change to 192.168.1.1 and as i saw the users directly connect to 192.168.1.1
but i don't want this; i need address bar always be as www.mydomain.com

then i add my proxy_redirect code and the browser fell into endless loop;

thank you again;