Name

HostNameLookups

Synopsis

HostNameLookups [on|off|double]
Default: off    
Server config, virtual host

If this directive is on,^[6] then every incoming connection is reverse DNS resolved, which means that, starting with the IP number, Apache finds the hostname of the client by consulting the DNS system on the Internet. The hostname is then used in the logs. If switched off, the IP address is used instead. It can take a significant amount of time to reverse-resolve an IP address, so for performance reasons it is often best to leave this off, particularly on busy servers. Note that the support program logresolve is supplied with Apache to reverse-resolve the logs at a later date.^[7]

The new double keyword supports the double-reverse DNS test. An IP address passes this test if the forward map of the reverse map includes the original IP. Regardless of the setting here, mod_access access lists using DNS names require all the names to pass the double-reverse test.

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^[6] Before Apache v1.3, the default was on. Upgraders please note.

^[7] Dynamically allocated IP addresses may not resolve correctly at any time other than when they are in use. If it is really important to know the exact name of the client, HostNameLookups should be set to on.