For facility / severity level, please refers to https://en.wikipedia.org/wiki/Syslog
vi /etc/rsyslog.conf add local6.* @xxx.xxx.xxx.xxx:514 service rsyslog restart
vi /etc/apache2/site-enabled/xxxx.conf
CustomLog "| /usr/bin/logger -t httpd -p local6.info" "%{X-Forwarded-For}i %l %u %t \"%r\" %s %b \"%{Referer}i\" \"%{User-agent}i\"" CustomLog "| /usr/bin/logger -t httpd -p local6.info" "%h %l %u %t \"%r\" %s %b \"%{Referer}i\" \"%{User-agent}i\""
service apache2 restart