Linux – How to force all internet traffic over a PPTP VPN but still allow local lan access

iptableslinuxlinuxmintpptproute

I have a server running Linux Mint 12 that I want to keep connected to a PPTP VPN all the time. The VPN server is pretty reliable, but it drops on occasion so I just want to make it so all internet activity is disabled if the VPN connection is broken.

I'd also like to figure out a way to restart it automatically, but that's not as big of an issue since this happens pretty rarely.

I also want to always be able to connect to the box from my lan, regardless of whether the VPN is up or not.

Here's what my ifconfig looks like with the VPN connected properly:

eth0      Link encap:Ethernet  HWaddr 00:22:15:21:59:9a  
          inet addr:192.168.0.171  Bcast:192.168.0.255  Mask:255.255.255.0
          inet6 addr: fe80::222:15ff:fe21:599a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:37389 errors:0 dropped:0 overruns:0 frame:0
          TX packets:29028 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:37781384 (37.7 MB)  TX bytes:19281394 (19.2 MB)
          Interrupt:41 Base address:0x8000 

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:16436  Metric:1
          RX packets:1446 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1446 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:472178 (472.1 KB)  TX bytes:472178 (472.1 KB)

tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:10.10.11.10  P-t-P:10.10.11.9  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:14 errors:0 dropped:0 overruns:0 frame:0
          TX packets:23 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:1368 (1.3 KB)  TX bytes:1812 (1.8 KB)

Here's an iptables script I found elsewhere that seemed to be for the problem I'm trying to solve, but it wound up blocking all access, but I'm not sure what I need to change:

#!/bin/bash

#Set variables
IPT=/sbin/iptables
VPN=`ifconfig|perl -nE'/dr:(\S+)/&&say$1'|grep 10.`
LAN=192.168.0.0/24

#Flush rules
$IPT -F
$IPT -X

#Default policies and define chains
$IPT -P OUTPUT DROP
$IPT -P INPUT DROP
$IPT -P FORWARD DROP

#Allow input from LAN and tun0 ONLY
$IPT -A INPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A INPUT -i tun0 -m conntrack --ctstate NEW -j ACCEPT
$IPT -A INPUT -s $LAN -m conntrack --ctstate NEW -j ACCEPT
$IPT -A INPUT -j DROP

#Allow output from lo and tun0 ONLY
$IPT -A OUTPUT -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT
$IPT -A OUTPUT -o tun0 -m conntrack --ctstate NEW -j ACCEPT
$IPT -A OUTPUT -d $VPN -m conntrack --ctstate NEW -j ACCEPT
$IPT -A OUTPUT -j DROP
exit 0

Thanks for your help.

Best Answer

Those iptables rules don't allow traffic to the VPN server, so the VPN can't be established. You need the following rules in the OUTPUT chain before the final DROP rule, where 1.2.3.4 is the IP address of the VPN server. These allow TCP connections to port 1723 (the PPTP control channel) and GRE packets (the data channel).

iptables --append OUTPUT --destination 1.2.3.4 --protocol tcp --dport 1723 --jump ACCEPT
iptables --append OUTPUT --destination 1.2.3.4 --protocol gre --jump ACCEPT
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