Isolated OpenVPN routing table on Linux

isolated_openvpn_routes.md

Save the route script to /usr/local/lib/openvpn/route on the client. Make it executable with chmod +x.

Remove the push redirect-gateway option from the OpenVPN server config.

Add these options to the OpenVPN client config:

setenv OPENVPN_ROUTE_TABLE 94
route-noexec
route-up /usr/local/lib/openvpn/route
route 0.0.0.0 128.0.0.0
route 128.0.0.0 128.0.0.0

Run these commands as root on the client before starting OpenVPN, replacing VPN_SERVER_IP_ADDRESS as appropriate:

ip rule add table 94
ip rule add to VPN_SERVER_IP_ADDRESS unreachable
ip rule add to VPN_SERVER_IP_ADDRESS table main

echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6

These commands need only be run once per boot. The changes they make will be lost when the system reboots.

route.sh

#!/bin/sh

#
# Environment variables that can be passed in via OpenVPN's --setenv:
#  OPENVPN_ROUTE_TABLE		- the routing table to which all routes are added
#  OPENVPN_ROUTE_SOURCE		- the source IP address for all routes
#

TABLE=$OPENVPN_ROUTE_TABLE
SOURCE=$OPENVPN_ROUTE_SOURCE

if [ -n "$TABLE" -o -n "$SOURCE" ]
then
	# Replace existing routes for our interface with new routes with different table/src
	if [ -n "$SOURCE" ]
	then
		src_arg="src $SOURCE"
	else
		src_arg=
	fi
	/sbin/ip route show dev $dev table main | while read route
	do
		/sbin/ip route delete $route dev $dev table main
		/sbin/ip route add $route dev $dev $src_arg table ${TABLE:-main}
	done
fi

if [ -n "$TABLE" ]
then
	# Replace existing IPv6 routes for our interface with new routes with different table
	/sbin/ip -6 route show dev $dev table main | while read route
	do
		/sbin/ip -6 route delete $route dev $dev table main
		/sbin/ip -6 route add $route dev $dev table $TABLE
	done
fi

# Add IPv4 routes
i=1
while true
do
	network=`eval 'echo $route_network_'$i`
	netmask=`eval 'echo $route_netmask_'$i`
	gateway=`eval 'echo $route_gateway_'$i`
	metric=`eval 'echo $route_metric_'$i`

	if [ "$network" = "" ]
	then
		break
	fi

	command="/sbin/ip route add $network"
	if [ "$netmask" != "" ]
	then
		command="$command/$netmask"
	fi
	if [ "$TABLE" != "" ]
	then
		command="$command table $TABLE"
	fi
	if [ "$metric" != "" ]
	then
		command="$command metric $metric"
	fi
	command="$command dev $dev via $gateway"
	if [ "$SOURCE" != "" ]
	then
		command="$command src $SOURCE"
	fi

	$command

	i=`expr $i + 1`
done

# Add IPv6 routes
i=1
while true
do
	network=`eval 'echo $route_ipv6_network_'$i`
	gateway=`eval 'echo $route_ipv6_gateway_'$i`

	if [ "$network" = "" ]
	then
		break
	fi

	command="/sbin/ip -6 route add $network"
	if [ "$TABLE" != "" ]
	then
		command="$command table $TABLE"
	fi
	command="$command dev $dev via $gateway"

	$command

	i=`expr $i + 1`
done

exit 0