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Server load balancing and failover requires the instant modification
of DNS records in order to re-direct inbound sessions to the
correct network interface. Our ActiveDNS & ISP Load Balancing
module provide this capability along with advanced smart balancing
capabilities.
How Does It Work:
The ActiveDNS and ISP Load Balancer incorporates a complete
DNS server. This DNS server is dynamically updated with the
latest IP address and active interface information.
The DNS server's purpose is to respond to remote clients
inbound requests for IP address information based on the queried
domain. By changing how responses to these requests are handled,
the Vector Routing module can determine on which interface the
inbound traffic is received from the remote client. This is
a very effective method for load balancing and redirecting inbound
traffic during a network outage.
In order for this method of "inbound routing" to work, the
Edge appliance, and the ActiveDNS module, must be configured
as the domain primary DNS server.
The method used determine how the DNS responds to remote
clients is based on the interface address information, active
path status (as determined by the Vector Routing module), and
changes made to the dynamic DNS database based on those methods.
As the DNS responses are made to the remote clients, they
have a limited TTL (time to live) value and include all of the
IP addresses of the network interfaces which are associated
with the active network paths. These addresses are provided
in an order defined in RFC 1034 / 1035 / 1794 and BIND 4.9,
September 1998. An example of how ActiveDNS has implemented
these standards is given below:
Equal Round-Robin Response
www IN A 10 10.0.0.100 5 1
www IN A 10 10.0.0.101 5 1
www IN A 10 10.1.1.100 5 1
(where 5 is the TTL specified in seconds)
Dynamically Weighted Response
www IN A 10 10.0.0.100 1 WAN1 (the lower the weighting the
more preferred)
www IN A 20 10.0.0.102 1 WAN1 (where the "20" is less preferred)
www IN A 10 10.1.1.100 0 WAN2 (where the "0" represents a DOWN
interface and is not provided in the DNS response)
Some BIND servers considers any TTL under 300 seconds as
"irrational", and substitutes in the value of 300 instead. This
greatly hampers the functionality of volatile zones. In the
fastest of all cases - a 0 TTL - information would be used once,
and then thrown away. Many the new server allow for the RR information
to be calculated every 5 seconds, and the RRs handed out with
a TTL of 0. It must be considered that one limitation of the
speed of a zone is going to be the ability of a machine to calculate
new information fast enough.
Weighted Route Selection
As seen in the above example, weighted route selection is
performed for both outgoing and incoming connections.
Outbound connections can be routed directly, or load balanced
between two or more interfaces and their gateways. The method
used by the ActiveDNS and ISP Load Balancer is to increase the
weight of each default route, and thus increase the likelihood
that the route will be used.
Inbound connections are similarly load balanced using the
ActiveDNS module's dynamic DNS server. In this case the IP addresses
provided in response to DNS requests are similarly weighted
so that the more highly weighted addresses are provided as the
first address in the response.
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XRoads
Networks ActiveDNS