As of today Keep-Alive is disabled in nginx configs when using it as a reverse proxy in front of Apache, both installed with Custombuild 2.0 on Directadmin server. Here you can learn how to safely enable it.
1. Copy /usr/local/directadmin/custombuild/configure/nginx_reverse/conf/nginx-defaults.conf
or whatever you want. And run:
cd /usr/local/directadmin/custombuild/ ./build rewrite_confs