Each time you add a DNS server to a group, the ProxySG automatically creates a DNS health check for that server IP address and uses a default configuration for the health check.
For example, if you add a DNS server to a primary or alternate DNS group, the created health check has a default hostname of www.bluecoat.com.
If you add a DNS server to a custom group, the longest domain name is used as the default hostname for the health check (in the following example test.griccia.local is the domain for the internal custom group)
From a packet Capture we can see the two default health checks for the primary and for the internal group.
The default hostname for DNS health checks can be modified from Management Console in: Configuration -> Health Checks -> General -> Health Checks