Is it possible to log all HTTP request headers with Apache?

mod_log_forensic is what you want, but it may not be included/available with your Apache install by default.

Here is how to use it.

LoadModule log_forensic_module /usr/lib64/httpd/modules/mod_log_forensic.so 
<IfModule log_forensic_module> 
ForensicLog ${APACHE_LOG_DIR}/forensic.log
</IfModule> 

Note: Your path may vary, such as /usr/lib/apache2/modules/mod_log_forensic.so

Here is a list of all http-headers: http://en.wikipedia.org/wiki/List_of_HTTP_header_fields

And here is a list of all apache-logformats: http://httpd.apache.org/docs/2.0/mod/mod_log_config.html#formats

As you did write correctly, the code for logging a specific header is %{foobar}i where foobar is the name of the header. So, the only solution is to create a specific format string. When you expect a non-standard header like x-my-nonstandard-header, then use %{x-my-nonstandard-header}i. If your server is going to ignore this non-standard-header, why should you want to write it to your logfile? An unknown header has absolutely no effect to your system.

If you're interested in seeing which specific headers a remote client is sending to your server, and you can cause the request to run a CGI script, then the simplest solution is to have your server script dump the environment variables into a file somewhere.

e.g. run the shell command "env > /tmp/headers" from within your script

Then, look for the environment variables that start with HTTP_...

You will see lines like:

HTTP_ACCEPT=text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
HTTP_ACCEPT_ENCODING=gzip, deflate
HTTP_ACCEPT_LANGUAGE=en-US,en;q=0.5
HTTP_CACHE_CONTROL=max-age=0

Each of those represents a request header.

Note that the header names are modified from the actual request. For example, "Accept-Language" becomes "HTTP_ACCEPT_LANGUAGE", and so on.

In my case easiest way to get browser headers was to use php. It appends headers to file and prints them to test page.

<?php
$fp = fopen('m:/temp/requests.txt', 'a');
$time = $_SERVER['REQUEST_TIME'];
fwrite($fp, $time + "\n");
echo "$time.<br>";
foreach (getallheaders() as $name => $value) {
    $cur_hd = "$name: $value\n";
    fwrite($fp, $cur_hd);
    echo "$cur_hd.<br>";
}
fwrite($fp, "***\n");
fclose($fp);
?>