reslog resolves IPs in Apache(8) log files. The result can then be analyzed
by another program, like Analog. You can think of it as a replacement of the
Apache(8) HostNameLookups directive, in the sense that it resolves client
IPs altogether once a day.

Resolving takes long time. This is mainly caused by resolving: Network
packets may be filtered by firewalls; DNS servers may not be correctly
configured; may not be up working; may sit in slow network sections; may be
old slow machines; may have traffic jam... etc. All these reasons are out of
our control.

WWW: http://search.cpan.org/dist/reslog/