Riccardo Riva

In some case, you could have more than one server connected to internet which is in Load Balancing.
To ensure that all machines area reachable from the internet you have to correctly configure your DNS entry.

In this case we will take an example of a webservers

To use round robin, each web server must have its own public IP address. A common scenario is to use network address translation and port forwarding at the firewall to assign each web server a public IP address while internally using a private address, usually an IP Address in a DMZ (DeMilitarized Zone).

This example from the DNS zone definition for yourdomain.net assigns the same name to each of the three web servers, but uses different IP addresses for each:

Note : in the HOSTS ADDRESS definition (A) I will use private IP address, but you MUST use public IPs.

;
; Domain database for yourdomain.net
;
yourdomain.net. IN SOA ns1.yourdomain.net. hostmaster.yourdomain.net. (
2009030101 ; serial
10800 ; refresh
3600 ; retry
86400 ; expire
86400 ; default_ttl
)
;
; Name servers
;
yourdomain.net. IN NS ns1.yourdomain.net.
yourdomain.net. IN NS ns2.yourdomain.net.
;
; Web servers
;
www IN A 192.168.1.1
www IN A 192.168.1.2
www IN A 192.168.1.3

When DNS gets a request to resolve the name www.yourdomain.net, it will return one IP address (assumimg the first one), then if a second request is coming, it will return the second one and so on.
Theoretically (in a perfect world), each web server will get one third of all the web traffic.
Due to DNS caching and because some requests may use more resources that others, the load will not be shared equally. However, over time it will come close.

This is a very good example of Load balancing using DNS server (Round Robin).

Hope this help

Bye
Riccardo

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