Just wrap the following into your before filter or user class:

class User < ActiveRecord::Base
 
  def self.authenticate(username, password)
 
    userid = username
 
    if (password.empty?) then
        password = 'empty'
    end
 
    ldap = Net::LDAP.new(:host => LDAPSERVER, :port => 636, :base => LDAPBASE, :encryption => :simple_tls)
    #or without encryption
    #ldap = Net::LDAP.new(:host => LDAPSERVER, :base => 'ou=users,o=rackspace')
 
    filter = Net::LDAP::Filter.eq('uid', username)
    ldap.search(:filter => filter) {|entry| username = entry.dn}
    ldap.auth(username, password)
 
    if ldap.bind
      # authentication succeeded
      # this will look for the user in the local database also
      #after successfully authenticating
        user = find( :first, :conditions=> { :name => userid })
        if (user)
            return user
        else
           #uncomment this to create the user on successful bind
           #user = User.create (
           #    :active => true,
           #    :is_admin => false,
           #    :name => userid
           #)
           return false
        end
 
    else
      # authentication failed
        return false
    end
 
    end
 
#  protected
 
end