Can I proxy_pass to a variable?

[任晓磊]

config:
server {
        listen [::]:80 default;
        server_name _ ;
        server_name_in_redirect off;
        resolver 74.207.241.5;

        access_log  off;

        auth_basic      "Private proxy";
        auth_basic_user_file /etc/nginx/auth_basic_user;

        location /{
                proxy_pass http://$http_host;
                proxy_set_header Host $http_host;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        }
}

error.log:
2010/05/10 11:02:25 [error] 31032#0: *45 zero length URI to proxy,
client: ::ffff:124.205.200.99, server: _, request: "GET
http://www.whatismyip.com/ HTTP/1.1", host: "www.whatismyip.com"


If a specify an IP address or a domain name in proxy_pass, it worked.
I use $http_host, it doesnt work. How can I do?
# /usr/sbin/nginx -V
nginx version: nginx/0.7.64
TLS SNI support enabled
configure arguments: --conf-path=/etc/nginx/nginx.conf
--error-log-path=/var/log/nginx/error.log
--pid-path=/var/run/nginx.pid --lock-path=/var/lock/nginx.lock
--http-log-path=/var/log/nginx/access.log
--http-client-body-temp-path=/var/lib/nginx/body
--http-proxy-temp-path=/var/lib/nginx/proxy
--http-fastcgi-temp-path=/var/lib/nginx/fastcgi --with-debug
--with-http_stub_status_module --with-http_flv_module
--with-http_ssl_module --with-http_dav_module
--with-http_gzip_static_module --with-mail --with-mail_ssl_module
--with-ipv6 --add-module=/tmp/buildd/nginx-0.7.64/modules/nginx-upstream-fair

-- 
Ren Xiaolei