Thank you very much Styopa, I am working on server that was installed by person no longer in the company. I don't know if Map modules was originally installed. I am using Ubuntu 10.04.4 TLS.

here is part of my current configuration:

upstream backend {
server alice.fqdn.com:80;
}

# Note: This following "server" section allows Secure SSL reverse proxy and controlled redirecting.
server {
listen 443;
server_name bob;
ssl on;
ssl_certificate /usr/local/nginx/certs/apps.fqdn.com.crt;
ssl_certificate_key /usr/local/nginx/certs/bob-apps.key;
server_name_in_redirect on;
cust_app_version_routing_data_file /usr/local/nginx/conf/cust_app_version_routing.dat;
cust_app_version_routing_apps app1,app2,app3;
access_log logs/host.access.log main;

proxy_intercept_errors on;
error_page 404 /myportal/Account/LogOn;
# Main location
location / {

my questions is can I add a second "upstream backend" and called "upstream backend2"? I would like to route specific 4 digits number to "upstream backend2"? would this is a possibility?