Apache Log Format: What does "%{c}a" mean?

Question

I've been tasked with parsing Apache web log files to extract data into another format. The customer provided me with the format line which includes the directive "%{c}a".

While I can find a definition for "%a" (the remote IP address), I cannot find any reference as to what the bracketed part means.

Does anyone recognize it and let me know what it represents?

Answer 1

From https://httpd.apache.org/docs/2.4/mod/mod_log_config.html#formats:

%{c}a Underlying peer IP address of the connection (see the mod_remoteip module).