Ubuntu – Allow only network connections but not connect to internet

You can add default gateway with command

by the ip address

sudo ip route add default via xxx.xxx.xxx.xxx

xxx.xxx.xxx.xxx - represent ip of vpn server

or by interface

sudo ip route add default dev ppp0  

This can be done manually from terminal when you are connected to vpn.

Second solution is to make script to chance default gw when vpn is connected

Then create script witch will be executed when interface ppp0 go up.

Create script for ppp0 interface

sudo nano /etc/ppp/if-up.d/script

put in script this

    # Check for specific interface if desired
    [ "$IFACE" != "ppp0" ] || exit 0
    # trigger vpn connection
    ip route add default dev ppp0

Save and exit.

Make script called script with execute permissions 755

sudo chmod 755 /etc/network/if-up.d/script

Explanation

When vpn connection will established and ppp0 interface is up script will run and set default device to ppp0

Also you can manually add route to resource on vpn site

sudo ip route add xxx.xxx.xxx.xxx/yy dev ppp0

xxx.xxx.xxx.xxx - represent host/network

yy - represent network mask

example:

add route to host 10.35.5.181 trough ppp0

 sudo ip route add 10.35.5.181/32 dev ppp0

add route to network range 10.35.5.0 with mask 255.255.255.0 trough ppp0

 sudo ip route add 10.35.5.0/24 dev ppp0

This also can be done in Network Manager. On same places where is use this connection only for resources on its network you can add route.

You can modify the gateway for network manager connections from the command line. So while I cannot find this option in the GUI, you can list the connections

nmcli con show

find yours

NAME                     UUID                                  TYPE             DEVICE 
thenameyougaveit         some-id-ksadbf019-aksb821           vpn              wlan1  

change the address (it will be discarded afaict but you cannot change the gateway address without setting it)

nmcli con mod some-id-ksadbf019-aksb821 ipv4.addresses 192.168.1.113/24

and then set the desired ip4 gateway

nmcli con mod some-id-ksadbf019-aksb821 ipv4.gateway 192.168.1.1

And now it works from the GUI. Of course, if anything changes on the other end, it will stop working.