Recipe 3.18 Logging Arbitrary Response Header Fields

Problem

You want to record the values of arbitrary fields the server has included in a response header, probably to debug a script or application.

Solution

Use the %{...}o log format variable in your access log format declaration. For example, to log the Last-Modified header, you would do the following:

%{Last-Modified}o

Discussion

The HTTP response sent by Apache when answering a request can be very complex, according to the server's configuration. Advanced scripts or application servers may add custom fields to the server's response, and knowing what values were set may be of great help when trying to track down an application problem.

Other than the fact that you're recording fields the server is sending rather than receiving, this recipe is analogous to Recipe 3.17 in this chapter; refer to that recipe for more details. The only difference in the syntax of the logging format effectors is that response fields are logged using an o effector, and request fields are logged using i.

See Also

• Recipe 3.17