ServerIron ADX Global Server Load Balancing Guide 189
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GSLB error handling for unsupported DNS requests 1
This process works in topologies where the GSLB ServerIron ADX front-ends a DNS server. However,
not all GSLB topologies require a DNS server. For example, when the GSLB ServerIron ADX is
configured as a DNS cache proxy with DNS override and IP lists. In this case, when the GSLB
ServerIron ADX receives a client query for an unsupported DNS record type, the GSLB ServerIron
ADX cannot forward the client request to a DNS server, so it drops the query without sending a
response to the client, subsequently causing the client to time out.
GSLB error handling enables the GSLB ServerIron ADX to send error messages in response to client
requests for unsupported DNS record types. When clients receive these error messages from the
GSLB ServerIron ADX, the clients query for another DNS record type instead of continuing to query
for the unsupported record type, or timing out on the query altogether.
GSLB ServerIron ADX intercepts queries for unsupported DNS record types, parses them, and
checks if there is a DNS server available to send the query to. If so, it forwards the request to that
DNS server and sends the response, unaltered, to the client. If the GSLB ServerIron ADX
determines that no DNS server is available to process the request, it generates a response with the
appropriate error code and sends it to the client. The response prevents the client from timing out.
Note that i n GSLB t opolog ies that requir e a DNS server, requests for unsupported DNS record types
are always handled by the DNS server, and not by the GSLB ServerIron ADX.
Default settings for GSLB error handling
The configuration default for GSLB error handling differs depending on your GSLB configuration:
•If the GSLB ServerIron ADX is a plain DNS proxy, GSLB error handling is not supported. This is
because a DNS server is always required for this configuration, and client requests always go
to the DNS server.
•If the GSLB ServerIron ADX is a cache proxy, GSLB error handling is enabled by default.
•If the GSLB ServerIron ADX is configured for transparent Intercept mode, GSLB error handling
is disabled by default. If necessary, you can enable error handling (refer to “Disable or
re-enabling GSLB error handling” on page 190).

Using GSLB error handling with transparent intercept mode

In the transparent intercept mode, GSLB error handling is disabled by default.
Enabling error handling with transparent intercept mode can be effective in certain configurations.
For example, when the GSLB ServerIron ADX is configured to intercept and directly respond to
requests for www.gslb.com, and the DNS server that it is intercepting has no record types other
than A records for www.gslb.com. In this example, the GSLB ServerIron ADX responds to client
queries for MX records for www.gslb.com, as follows:
•With error handling enabled, the GSLB ServerIron ADX directly responds with an error handling
message. Since the DNS server does not have any MX records, it would not be efficient if the
GSLB ServerIron ADX forwarded the query to the DNS server. Thus, configuring the GSLB
ServerIron ADX to directly conduct error handling reduces the latency for the response to that
client.
•With error handling disabled, the GSLB ServerIron ADX forwards the query to the DNS server,
which responds with an error handling reply. Without error handling, latency is increased in the
response time to the client.