In Fedora 33 there is a systemd-resolved service enabled by default. Use following steps to make DSN working after establishing FortiSSL VPN connection using Network Manager (using NetworkManager-fortisslvpn-gnome plugin).
Assuming that "ppp0" is you virtual network interface for VPN connection, you can set X.X.X.X and Y.Y.Y.Y as DNS servers:
resolvectl dns ppp0 X.X.X.X Y.Y.Y.Y
Add domain name for this connection:
resolvectl domain ppp0 "exampledomainname.com"
Verify new settings by using:
# resolvectl status
Link 9 (ppp0)
Current Scopes: DNS LLMNR/IPv4
DefaultRoute setting: yes
LLMNR setting: yes
MulticastDNS setting: no
DNSOverTLS setting: no
DNSSEC setting: no
DNSSEC supported: no
Current DNS Server: X.X.X.X
DNS Servers: X.X.X.X
Y.Y.Y.Y
DNS Domain: exampledomainname.com
Link 9 (ppp0)
Current Scopes: DNS LLMNR/IPv4
DefaultRoute setting: yes
LLMNR setting: yes
MulticastDNS setting: no
DNSOverTLS setting: no
DNSSEC setting: no
DNSSEC supported: no
Current DNS Server: X.X.X.X
DNS Servers: X.X.X.X
Y.Y.Y.Y
DNS Domain: exampledomainname.com
To check whether DNS resolving is working and which network interface traffic will be routed by:
# resolvectl query google.pl
google.pl: 216.58.209.3 -- link: wlp2s0
2a00:1450:401b:807::2003 -- link: wlp2s0
-- Information acquired via protocol DNS in 846us.
-- Data is authenticated: no
google.pl: 216.58.209.3 -- link: wlp2s0
2a00:1450:401b:807::2003 -- link: wlp2s0
-- Information acquired via protocol DNS in 846us.
-- Data is authenticated: no
To automate things up (establish FortiVPN connection and set DNS) you can use below script:
https://github.com/openterprise/scripts/blob/master/connectNetworkManagerFortiVPNsetDNS.sh