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Routing in Ubuntu

Dear Team,

I have two network and connected by VPN connection. At the moment, users in network 1.0 can go internet by pass router and this router has the rule forwarded all internal application connection will through VPN tunnel.
For permit the users not to go to internet except VPN application, I built the Ubuntu server had IP and set the default gateway of clients to this IP.
Please advise me how to do the routing in Ubuntu to forward all VPN connection to network 2 and block any internet connection.

Thank you.
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Mohammed Rahman
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You can probably block entire internet traffic and allow only VPN (specific IP) on the router itself. Which router are you using? Make and model?
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I used Cisco 1921.
Use access control list (ACL) to deny all http (port 80) and https (port 443) traffic. Doing so, none of the users will be able to access internet from inside out.

deny tcp any host  eq 80
deny tcp any host  eq 443

Router01#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router01(config)#access-list 101 deny ip any any eq 80
Router01(config)#access-list 101 deny ip any any eq 443

Router01#configure terminal
Router01(config)#interface (select your wan interface)
Router01(config-if)#ip access-group 101 out  

wr - will save the running configuration to startup configuration.

** ip access-group 101 out will deny all http (80) and https (443) from inside network to outside network. Hence, users will not be able to access internet.

Doing so, you probably will not require a separate Ubuntu server to block internet access.

** If your VPN users need to use any http/https service, you can add "permit <IP address> commands and then Deny any any.
This will allow users to access only specified websites and block remaining all.

Hope this helps. If not, I would advice you to post this question in Cisco Routing/Firewall/Security category. You can then get best solution.
Thanks for your quick respond.

But the problem is I did not have the permission to access the router. That why I built the this Ubuntu server.

Do you have any other advise ?!
I apologize, I am not good with Ubuntu and its features. However, I came across a solution on link below. Looks like you have to change the DNS in conf file and point it to fake DNS server, so that the website name cannot be resolved. But, if users try to access internet using the site's IP address, they may be lucky to get onto internet.

sudo su
echo "nameserver" > /etc/resolv.conf


Check this out. Seems to be perfect solution for your setup.

Another perfect solution below.
Sorry for the late reply,

All of these solutions could not resolve my issue.
Any idea ?!

Thank you.
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Mohammed Rahman
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Seem did not have any solutions you posted can resolved my issue.
I asked network admin to permit it on router.
Anyway, thank you for your support.