Implementing Client Subnet DNS Requests
Published Jun 16, 2015
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In service request C3152801 the F5 support confirmed a bug (ID832769: [DNS::edns0 subnet address] printing random octets when edns client subnet mask is /24 or lower) for TMOS v12 which is fixed in TMOS v14.
In case you are using DNS::return in your TMOS v12 iRule, the EDNS0-ECS information will be inserted automatically. So there is no need to insert this information in the context of DNS_RESPONSE.
But in case the request is handled by gtmd or bind you will need to insert the EDNS0-ECS information the DNS_RESPONSE context.
TMOS v14 has changes in behaviour and fixes. In case you are upgrading, review your iRules, please.