Brocade Communications Systems 12.4.00 Manual De Usuario

GSLB error handling for unsupported DNS requests

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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 in GSLB topologies that require 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:

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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. 

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If the GSLB ServerIron ADX is a cache proxy, GSLB error handling is enabled by default.

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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 

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:

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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.

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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.