Configuring DNS servers per SSL VPN Portal

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