FortiGate
FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Dante_De_Luca_FTNT
Article Id 194558
Description
To configure DNS servers differently for different user groups (or tunnels), configure it uniquely for each SSL VPN portal and then assign user groups a unique portal.

In FortiOS 5.0:

config vpn ssl web portal
edit <portal>
            config widget
                edit 1
                    set name "Tunnel Mode"
                    set type tunnel
...
                    set dns-server1<ip4_addr>
                    set dns-server2 <ip4_addr>

In FortiOS 5.2:

config vpn ssl web portal
edit <portal>
       set dns-server1 <ip4_addr>
       set dns-server2 <ip4_addr>
end

If set, the DNS server settings at the portal level should override those at the global level.
If all SSL VPN portals have DNS settings configured, you can remove the DNS settings at the system level.

config vpn ssl settings
unset dns-server1
unset dns-server2
end
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