Chapter 1 What’s New for FortiOS 5.2.1 : Firewall : Dynamic VIP According to DNS Translation
  
Dynamic VIP According to DNS Translation
Dynamic virtual IPs according to DNS translation can now be configured.
When a dynamic virtual IP is used in a policy, the dynamic DNS translation table is installed along with the dynamic NAT translation table into the kernel. All matched DNS responses will be translated and recorded regardless if they hit the policy. When a client request hits the policy, dynamic NAT translation will occur if it matches a record, otherwise the traffic will be blocked.
Syntax
config firewall vip
edit 1
set type dns-translation
set extip 192.168.0.1-192.168.0.100
set extintf dmz
set dns-mapping-ttl 604800
set mappedip 3.3.3.0/24 4.0.0.0/24
end
end