Using the EDNS0 Client Subnet Option for Global Server Load Balancing

EDNS Client Subnet (ECS) is a DNS header extension that provides the client subnet details. You can use these details to improve the accuracy of NetScaler Global Server Load Balancing (GSLB) by using the client network location rather than the DNS resolver location to determine the topological closeness of the client.

Note

NetScaler supports only EDNS0.

Important:

Make sure that the LDNS in your deployment supports EDNS0 Client Subnet so that the incoming DNS queries contains the EDNS0 Client Subnet option and the NetScaler appliance uses the ECS address while processing the DNS query.

In a typical GSLB deployment, when you use proximity-based load balancing methods like static proximity or dynamic round-trip time (RTT), the NetScaler appliance uses the local DNS (LDNS) IP address for determining the topological closeness of the client and performs GSLB accordingly. But when a centralized DNS resolver, such as Google DNS or OpenDNS, is involved in the deployment, the NetScaler appliance sends the DNS request to a datacenter close to the centralized DNS resolver, which might not be close to the client. For example, in a typical NetScaler GSLB deployment using the static proximity load balancing method, an end-user request from Japan is sent to a datacenter in Japan and an end user request from California is sent to a datacenter in California. But if a centralized DNS resolver is involved, the NetScaler appliance might send a request from Japan to a datacenter in California.

You can use the ECS option in deployments that include the NetScaler appliance configured as Authoritative DNS (ADNS) server for a GSLB domain. If you use static proximity as the load balancing method, you can use the IP subnet in the EDNS header instead of the LDNS IP address to determine the geographical proximity of the client. In the case of proxy mode deployment, the NetScaler appliance forwards an ECS-enabled DNS query as-is to the back-end servers, and the appliance does not cache ECS-enabled DNS responses.

Note

The ECS option is not applicable for all other deployment modes, such as ADNS mode for non-GSLB domains, resolver mode, and forwarder mode. In all these modes, the ECS option is ignored by the NetScaler appliance. Also, by default, ECS is disabled for GSLB deployment.

To enable EDNS0 Client Subnet option by using the command line interface:

At the command prompt, type:

set gslb vserver <vserver_name> **-ECS ENABLED

set gslb vserver vserver-GSLB-1 -ECS ENABLED

Address validation

You can configure a GSLB virtual server to verify that the address returned by the EDNS0 Client Subnet (ECS) option of the DNS query is not a private or an unroutable IP address. With address validation enabled, the NetScaler appliance ignores the ECS address in the DNS query if it is listed in the following table, and instead uses the LDNS IP address for global server load balancing.

Note

By default, address validation is disabled.

To enable address validation by using the command line interface

At the command prompt, type:

set gslb vserver <vserver_name> -ecsAddressValidation ENABLED

set gslb vserver vserver-GSLB-1 -ecsAddressValidation ENABLED