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question about VPN killswitch using Linux iptables rules
 

I guessing its a long shot that I'll get a answer for this one but I figured I'd give it a shot.

I'm in the process of figuring this out with using trial and error. I've yet to find a complete template for setting up iptables for a killswitch online. I'm setting this up on Chromium OS run on vmware.

Chromium already has L2TP/IPsec. As far as I can tell the only way to set up a killswitch is through shell using iptables.

I get the jest of it... but I'm not sure about the exact scripts. Google results give all different answers.

deny all outgoing and incoming by

iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP

make an exception for network adapter by

allow out on ppp0

iptables -A OUTPUT -o ppp0 -p tcp -j ACCEPT ?

allow in on ppp0

iptables -A INPUT -i ppp0 -p tcp -j ACCEPT ?

Make an exception from any ip address to the ip address of the server so I can connect to it

iptables -I OUTPUT -o eth0 -d 123.123.123.123 -j ACCEPT ?

then block all ipv6

?

Hopefully someone here has done this already or has mad networking skills and can help with the scripts. Chrome OS uses iptables v.1.4.21 which is pretty universal although I haven't been able to find an exact example of this configuration online.

Re: question about VPN killswitch using Linux iptables rules
 

I got it to work. But if anyone could tell me if I'm missing anything or if there is a better way to do it I'd appreciate the info. I didnt need to add any ipv6 rules. A test on test-ipv6 shows no ipv6 detected.

Delete all rules in a chain or all chains

iptables –F

Allow all input and output on local host

iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT

Allow traffic input and output from VPN Server

iptables -A INPUT -s 123.123.123.123 -j ACCEPT
iptables -A OUTPUT -d 123.123.123.123 -j ACCEPT

Allow traffic via VPN network adapter (L2TP/IPsec)

iptables -A INPUT -i ppp0 -j ACCEPT
iptables -A OUTPUT -o ppp0 -j ACCEPT

Drop everything else

iptables -A INPUT -j DROP
iptables -A OUTPUT -j DROP
iptables -P INPUT DROP
iptables -P OUTPUT DROP



When I run cmd: iptables -S it shows

-P INPUT DROP
-P FORWARD DROP
-P OUTPUT DROP
-A INPUT -i lo -j ACCEPT
-A INPUT -s 123.123.123.123/32 -j ACCEPT
-A INPUT -i ppp0 -j ACCEPT
-A INPUT -j DROP
-A OUTPUT -o lo -j ACCEPT
-A OUTPUT -d 123.123.123.123/32 -j ACCEPT
-A OUTPUT -o ppp0 -j ACCEPT
-A OUTPUT -j DROP