Chapter 1 What’s New for FortiOS 5.0 : IPv6 : DHCPv6 relay
  
DHCPv6 relay
You can use the following command to configure a FortiGate interface to relay DHCPv6 queries and responses from one network to a network with a DHCPv6 server and back. The command enables DHCPv6 relay and includes adding the IPv6 address of the DHCP server that the FortiGate unit relays DHCPv6 requests to:
config system interface
edit internal
config ipv6
set dhcp6-relay-service enable
set dhcp6-relay-type regular
set dhcp6-relay-ip 2001:db8:0:2::30
end