The primary DNS suffix is the DNS namespace which a computer belongs to. This is normally handed out by DHCP server, for example on a windows device DHCP server belonging to a domain mycompany.local on the DHCP server under scope options a domain name of mycompany.local would be included to be assigned to clients.
When running ipconfig /all on a client machine under “Connection-specific DNS Suffix” the primary DNS suffix that has been assigned can be viewed. The client then uses this suffix in DNS queries, for example, if doing a remote desktop connection to only the computer name RDS01 the computer adds the suffix to query the DNS server for the IP address of RDS01.mycompany.local
Adding this DNS suffix via a FortiGate DHCP can be done by
- Create the DHCP scope in GUI
- In command line
- Config sys DHCP server
- Show (verify which DHCP scope to add the DNS suffix)
- Edit 1
- Set domain mycompany.local
- Clients will then get the DNS suffix when assigned an IP address
This is able to be accomplished in the GUI, under DHCP on the interface click advanced > Additional DHCP option add Options Code 15 and put DNS suffix in the Value field.