Apache Cookbook, 2nd Edition

11.5. Avoiding DNS Lookups

Problem

You want to avoid situations where you have to do DNS lookups of client addresses, as this is a very slow process.

Solution

Always set the HostNameLookups directive to Off:

HostNameLookups Off

Make sure that, whenever possible, Allow from and/or Deny from directives use the IP address, rather than the hostname of the hosts in question.

Discussion

DNS lookups can take a very long time—anywhere from 0 to 60 seconds—and should be avoided at all costs. In the event that a client address cannot be looked up at all, it can take up to a minute for the lookup to time out, during which time the child process that is doing the lookup cannot do anything else.

There are a number of cases in which Apache will need to do DNS lookups, and so the goal here is to completely avoid those situations.

HostNameLookups

Before Apache 1.3, HostNameLookups, which determines whether Apache logs client IP addresses or hostnames, defaulted to on, meaning that each Apache log entry required a DNS lookup to convert the client IP address to a hostname to put in the logfile. Fortunately, that directive now defaults to off, and so this is primarily an admonition to leave it alone.

If you need to have these addresses converted to hostnames, this should be done by another program, preferably running on a machine other than your production Web server. That is, you really should copy the file to some other machine for the purpose of processing, so that the effort required to do this processing ...