What it is
https://en.wikipedia.org/wiki/Slowloris_(computer_security)

Tools to check
Slowloris : https://github.com/gkbrk/slowloris
SlowHttpTest : https://github.com/shekyan/slowhttptest

Mitigatation

mod_qos
apt-get update && apt-get -y install libapache2-mod-qos && a2enmod qos && /etc/init.d/apache2 restart
* configuration file in /etc/apache2/mods-enabled/

mod_reqtimeout
a2enmod reqtimeout && /etc/init.d/apache2 restart
* configuration file in /etc/apache2/mods-enabled/