Map lighttpd to specific action?

Rails
1

I have a multi-city application set up and right now the city is set by
a session variable:

        if @user.city.to_s=='3'
             session[:city]="nyc"
            redirect_to(:controller => "users", :action =>

“nycindex”)

Is it possible to map a specific URL to that same action? e.g.
ny.foo.com would set @user.city.to_s == ‘3’ and so head over to the
users/nycindex page?

Can that be on in lighttpd or is there another way to look for it?

Thanks…