mod_log_ipmask for Apache 2.4

This Apache module allows you to mask parts of the client's IP address before it is written to log files like the AccessLog or CustomLog. The %a and %h placeholders provided by mod_log_config can be amended, so for example %{16}a will only use the first 16 bits (two octets) of the remote IP address.

Since Apache 2.4, the internal data structures distinguish the client's (or user agent's) IP address from the peer's address in the underlying connection. This difference comes into play when forwarding connections, for example in reverse proxy setups or with load balancers.

The %a placeholder in mod_log_config refers to the IP address of the client, that is, the actual user agent. This value can be masked by means of this module.

The additional %{c}a can be used to log the load balancer's IP address. This value is not masked by this module.

The %h placeholder may resolve to either a hostname or an IP address, depending on the HostnameLookups setting and/or the usage of hostname-based access control directives. Be aware that a mask (as in %{16}h) will only be applied when an IP address is found. In other words, when the hostname gets resolved, it will reveal the actual client address.

Why? Who? What?

Masking IP addresses in part or entirely is required by Germany's Telemedia Act. Keeping only partially masked IP addresses in logfiles still allows you to perform web analytics without having to process personal data.

This module was originally funded and developed by Saxonia's data protection officer. Please see https://www.saechsdsb.de/ipmask for details or installation instructions (German only) and the source code for original copyright and author.

Caution This is a fork of the original module, created by webfactory GmbH, Bonn, Germany.

Changes against the original version:

• Minimal change required to make this module work with Apache 2.4.
• Removed the enforced masking of the last octet. By default, it will still be masked, but you can configure your log format with %{32}a to get full IP addresses if you wish.