Round Robin DNS is the simplest way to load balance a webserver between 2 or more servers in different location or to have network redundancy. For load balancing, it does not take in consideration the load on the servers but it's extremely easy to setup. It's also quite effective for network redundancy, you can plug a single server on 2 different network/switch and if one fail, all trafic will go on the available network.

If you need to load balance servers in the same datacenter I suggest reading this article.

Networking setup

• 1 server with multiple NIC

You simply need 2 network cards. Let say you will have eth0 : 64.15.75.216 and eth1 : 209.44.107.14

• 2 servers at different locations

Location 1 IP : 64.15.75.216, location 2 IP : 209.44.107.14

Apache setup

• 1 server with multiple NIC

The apache vhost will look like this :

<VirtualHost 64.15.75.216:80 209.44.107.14:80>
[...]
</VirtualHost>

• 2 servers at different locations

Simply do something like this :

Location 1


<VirtualHost 64.15.75.216:80>
[...]
</VirtualHost>

Location 2


<VirtualHost 209.44.107.14:80>
[...]
</VirtualHost>

DNS server setup

When you have a request on your DNS server, the answer has to send 2 different IP. For example, with Bind, the order of how IP are sent is random.

Try :

dig portafixe.com

and you should see something like :

;; ANSWER SECTION:
portafixe.com.          86400   IN      A       64.15.75.216
portafixe.com.          86400   IN      A       209.44.107.14

if you dig again, IP priority should be different.

This does 2 things.

1. In theory, if one IP doesn't answer, ISP will ask the next IP within a few seconds (redundancy). I did some test with my ISP and the first request took 5 seconds to switch to the available IP.
2. Load balance requests between the 2 IP addresses.

To configure that in bind, you need 2 A records with the differents IP addresses on all DNS servers. Here is an example of the bind zone file of portafixe.com :

[...]
portafixe.com.      A        64.15.75.216
www                 A        64.15.75.216
portafixe.com.      A        209.44.107.14
www                 A        209.44.107.14
[...]

Note : If you do round robin DNS a domain for redundancy, it is also a very good idea to round robin the DNS server.

MySQL replication and web files mirroring

If you have 2 or more servers, I suggest reading those articles :

MySQL replication

Mirroring web & mail files with rsync

They are part of a full tutorial on high availability and load balancing but they can still be used as is with little modifications.