Linux Identd (auth)
The identd daemon or service returns user information to a remote host that a user is requesting a service from. This service is misunderstood by many who seem to think that it is used for some kind of user authorization. It is a good idea to support this service which uses port 113. Some remote systems could require it in order to receive requested services. As a system administrator, it can help you identify users who are abusing systems. The identd(1) and authuser(3) man pages give more information about the subject.