I‘ve 10Mbps server port dedicated to our small business server. The server also act as a backup DNS server and I’d like to slow down outbound traffic on port 80. How do I limit bandwidth allocation to http service 5Mbps (burst to 8Mbps) at peak times so that DNS and other service will not go down due to heavy activity under Linux operating systems?

You need use the tc command which can slow down traffic for given port and services on servers and it is called traffic shaping:

via tc: Linux HTTP Outgoing Traffic Shaping (Port 80 Traffic Shaping).