mod_dumpio allows for the logging of all input received by Apache and/or all output sent by Apache to be logged (dumped) to the error.log file.
The data logging is done right after SSL decoding (for input) and right before SSL encoding (for output). As can be expected, this can produce extreme volumes of data, and should only be used when debugging problems.
To configure mod_dumpio you need to enable dump_io.
$ sudo a2enmod dump_io
Next add below mentioned 3 lines in /etc/apache2/apache2.conf
#To capture http request DumpIOInput On #To capture http response DumpIOOutput On #To capture everything DumpIOLogLevel debug
Now change LogLevel in your /etc/apache2/sites-enabled/000-default or /etc/apache2/sites-available/default file to debug.
After doing all above changes restart apache2.
$ service apache2 restart
To view logs you need to check /var/log/apache2/error.log
$ tail -f /var/log/apache2/error.log
To test make a dummy request as shown below and at the same time view error.log file where you can lot of lines related to this request.
$ curl http://127.0.0.1/ -d group=user -d sort=name
Since dump_io will create log of disk io, enabling it in production servers is not a good idea. It will slow down your production server.