IdentityCheck

The IdentityCheck directive causes the server to attempt to identify the client’s user by querying the identd daemon of the client host. (See RFC 1413 for details, but the short explanation is that identd will, when given a socket number, reveal which user created that socket — that is, the username of the client on his home machine.)

IdentityCheck [on|off]

If successful, the user ID is logged in the access log. However, as the Apache manual austerely remarks, you should “not trust this information in any way except for rudimentary usage tracking.” Furthermore (or perhaps, furtherless), this extra logging slows Apache down, and many machines do not run an identd daemon, or if they do, they prevent external access to it. Even if the client’s machine is running identd, the information it provides is entirely under the control of the remote machine. Many providers find that it is not worth the trouble to use IdentityCheck.