require 'net/ldap'

Net::LDAP.open(:host => LDAP_HOST,
               :port => 389,
               :encryption => :start_tls,
               :auth => {
                 :method => :simple,
                 :username => LDAP_USER,
                 :password => LDAP_PASSWORD
               }) do |ldap|
  entry = ldap.search(:base => ou=People,#{LDAP_BASE}",
                      :scope => Net::LDAP::SearchScope_WholeSubtree,
                      :filter => "(uid=*)")[0]
  ldap.modify(:dn => entry.dn,
              :operations => [[:replace, "sn", "new-sn"]])
end

require 'active_ldap'

ActiveLdap::Base.establish_connection(:host => LDAP_HOST,
                                      :base => LDAP_BASE,
                                      :method => :tls,
                                      :bind_dn => LDAP_USER,
                                      :password => LDAP_PASSWORD)
class LdapUser < ActiveLdap::Base
  ldap_mapping :dn_attribute => "uid",
               :prefix => 'ou=People',
               :classes => ["person"]
end

entry = LdapUser.find(:first)
entry.sn = "new-sn"
entry.save