Load balanced host records are similiar to Simple Host (A) records, however, multiple IP addresses can be associated with each name. The Ecessa appliance refers to the Simple Host Records assigned to the load balanced host record and replies with an A record for each IP address associated with the name for all operational WAN links. If the Redundancy Only setting is enabled, only the first IP address associated with the name will be sent (if that WAN line is down, the next address in the list will be sent).
The Ecessa appliance monitors the WAN links and if any WAN is detected to be down, it temporarily removes the effected associated addresses from the load balanced host records. The Time to Live (TTL) should be configured short enough so DNS caching servers look for updates on a regular basis to prevent giving clients incorrect information in the event of a WAN line failure.
Shown below is a diagram depicting the process by which a client machine obtains the location (IP address) of a destination via DNS. The Ecessa applicance is used to aggregate two WAN lines, providing inbound failover and load-balancing for www.example.com.
1. The client machine's browser sends a query to the caching DNS server, designated in its IP configuration, requesting the IP address for "www.example.com"
2. If the caching DNS server has a current address record for www.example.com, it immediately returns the IP address for the client host. If the caching server does not have the information, it contacts a root DNS server and requests the record containing the addresses of the primary and secondary DNS authorities for "example.com"
3. A root name server is one of thirteen name servers that service the Internet. These servers know where the caching server needs to look next to find the information it needs to resolve www.example.com and responds with the list of authorities for example.com.
4. The caching name server sends its request for www.example.com to one of the name servers designated as the authority for the example.com domain. The Ecessa appliance functions as both the primary and secondary authorities for example.com. If the ISP1 link is down, the request will be made to the Ecessa appliance over ISP2.
5. The DNS authority (Ecessa appliance) returns the requested record to the caching DNS server. This record contains the two IP addresses for www.example.com, the Time to Live (TTL), and a sequence number to allow the caching DNS server to determine if the record has changed since it was last updated.
6. The caching DNS server then sends one of the IP addresses of www.example.com to the client machine. The IP addresses for subsequent queries to the caching server will be sent in a "round robin" fashion, causing inbound sessions to www.example.com to be load-balanced across both WAN links.
7. The client establishes a connection directly with www.example.com using one of the WAN links.