Environment : Guest users and Instant AP are configured with different DNS servers.
Normally all DNS query from wireless users will be handled by clients DNS server.
However DNS query for host name of the external splash page (cppm.arubanetworks.com) from Guest user will be hijacked by Instant AP and it will be forwarded to DNS server configured for Instant AP. All other DNS query will be handled by client's DNS server.
If DNS server for Instant AP is not configured then Guest user would not be able to resolve host name of the external splash page.