Apparently there was once a Stack Overflow question of the same name, but was deleted. So when I wanted to add a log file just for redirects to an apache httpd server, I had to dig a little through the mod_log_config and expressions documentation.
Starting in version 2.4, apache supports an expr argument to the CustomLog directive to specify an expression that must evaluate to true in order for the request to be logged to the custom log file. So to capture only redirects (responses with a status code in the 300s) for GET requests, I set up the following directive:
CustomLog /var/log/apache2/redirect.log redirect "expr=%{REQUEST_STATUS} -ge 300 && %{REQUEST_STATUS} -lt 400 && %{REQUEST_METHOD} == 'GET'"The second argument to the directive — redirect — is just the name of the custom log format I defined. It's not a special keyword or anything — it could have been foo or bar or whatever. This is the format I ended up using, capturing the host (%V:%p) and path (%r) requested, the location (%{Location}o) to which the server redirected the request, and the referrer (%{Referer}i) to the original request:
LogFormat "%V:%p %a %t \"%r\" %>s Location: %{Location}o Referer: %{Referer}i" redirect
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