add-routes.sh

add-routes.sh

#!/bin/bash

# This script adds routes to listed resources via pre-defined gateway or alternatively takes gateway address
# from first argument.
# Usage:
# ./add-routes.sh 
# or to override gateway:
# ./add-routes.sh 234.234.234.234

GATEWAY=${1:-"123.123.123.123"}

ADDRESSES="123.123.123.123 123.123.123.123 \
  123.123.123.123 123.123.123.123"

# To route everything
#ADDRESSES="0.0.0.0"

echo "Adding routes:"

for TARGET in $ADDRESSES; do
  echo " -> $TARGET via $GATEWAY"
  sudo ip route add $TARGET via $GATEWAY
done

To configure a VM for routing we need to edit /etc/sysctl.conf and uncomment the following line:

# net.ipv4.ip_forward=1

Then run sysctl -p from root.

After that we should connect to our VPN and check out which interface it creates.

Suppose that ppp0 is our VPN interface and enp1s0 is our "physical" interface which can interact with the host OS.

Then we should set our firewall in the following way:

# run from root or using sudo
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptables -A FORWARD -i ppp0 -o enp1s0 -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A FORWARD -i enp1s0 -o ppp0 -j ACCEPT

Then you can use add-routes.sh on the host OS using VM ip address as a gateway.