OpenVPN Tunnels and Bridges

Simon Matter

Tom Eastep

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

2023/02/18

Table of Contents

Preliminary Reading
Bridging two Masqueraded Networks
Roadwarrior
Roadwarrior with Duplicate Network Issue
Roadwarrior with IPv6
Bridged Roadwarrior
Bridging Two Networks

Caution

This article applies to Shorewall 3.0 and later and to OpenVPN 2.0 and later. If you are running a version of Shorewall earlier than Shorewall 3.0.0 then please see the documentation for that release.

OpenVPN is a robust and highly configurable VPN (Virtual Private Network) daemon which can be used to securely link two or more private networks using an encrypted tunnel over the Internet. OpenVPN is an Open Source project and is licensed under the GPL. OpenVPN can be downloaded from http://openvpn.net/.

Unless there are interoperability issues (the remote systems do not support OpenVPN), OpenVPN is my choice any time that I need a VPN.

It is widely supported -- I run it on both Linux and Windows.
It requires no kernel patching.
It is very easy to configure.
It just works!

Preliminary Reading

I recommend reading the VPN Basics article if you plan to implement any type of VPN.

Bridging two Masqueraded Networks

Suppose that we have the following situation:

We want systems in the 192.168.1.0/24 subnetwork to be able to communicate with the systems in the 10.0.0.0/8 network. This is accomplished through use of the /etc/shorewall/tunnels file and the /etc/shorewall/policy file and OpenVPN.

While it was possible to use the Shorewall start and stop script to start and stop OpenVPN, I decided to use the init script of OpenVPN to start and stop it.

On each firewall, you will need to declare a zone to represent the remote subnet. We'll assume that this zone is called “vpn” and declare it in /etc/shorewall/zones on both systems as follows.

/etc/shorewall/zones — Systems A & B
#ZONE   TYPE   OPTIONS                 IN_OPTIONS              OUT_OPTIONS
vpn     ipv4

On system A, the 10.0.0.0/8 will comprise the vpn zone.

In /etc/shorewall/interfaces on system A:
#ZONE      INTERFACE        OPTIONS
vpn        tun0

In /etc/shorewall/tunnels on system A, we need the following:

#TYPE         ZONE           GATEWAY        GATEWAY_ZONE
openvpn       net            134.28.54.2

This entry in /etc/shorewall/tunnels opens the firewall so that OpenVPN traffic on the default port 1194/udp will be accepted to/from the remote gateway. If you change the port used by OpenVPN to 7777, you can define /etc/shorewall/tunnels like this:

/etc/shorewall/tunnels with port 7777:

#TYPE             ZONE           GATEWAY         GATEWAY_ZONE
openvpn:7777      net            134.28.54.2

Similarly, if you want to use TCP for your tunnel rather than UDP (the default), then you can define /etc/shorewall/tunnels like this:

/etc/shorewall/tunnels using TCP:

#TYPE             ZONE           GATEWAY         GATEWAY_ZONE
openvpn:tcp       net            134.28.54.2

Finally, if you want to use TCP and port 7777:

/etc/shorewall/tunnels using TCP port 7777:

#TYPE             ZONE           GATEWAY         GATEWAY_ZONE
openvpn:tcp:7777  net            134.28.54.2

This is the OpenVPN config on system A:

dev tun
local 206.162.148.9
remote 134.28.54.2
ifconfig 192.168.99.1 192.168.99.2
route 10.0.0.0 255.0.0.0 192.168.99.2
tls-server
dh dh1024.pem
ca ca.crt
cert my-a.crt
key my-a.key
comp-lzo
verb 5

Similarly, On system B the 192.168.1.0/24 subnet will comprise the vpn zone

In /etc/shorewall/interfaces on system B:
#ZONE      INTERFACE        BROADCAST     OPTIONS
vpn        tun0 

In /etc/shorewall/tunnels on system B, we have:

#TYPE         ZONE           GATEWAY        GATEWAY_ZONE
openvpn       net            206.191.148.9

And in the OpenVPN config on system B:

dev tun
local 134.28.54.2
remote 206.162.148.9
ifconfig 192.168.99.2 192.168.99.1
route 192.168.1.0 255.255.255.0 192.168.99.1
tls-client
ca ca.crt
cert my-b.crt
key my-b.key
comp-lzo
verb 5

You will need to allow traffic between the “vpn” zone and the “loc” zone on both systems -- if you simply want to admit all traffic in both directions, you can use the policy file:

/etc/shorewall/policy on systems A & B

#SOURCE        DEST          POLICY          LOG LEVEL
loc            vpn           ACCEPT
vpn            loc           ACCEPT

On both systems, restart Shorewall and start OpenVPN. The systems in the two masqueraded subnetworks can now talk to each other.

Roadwarrior

OpenVPN 2.0 provides excellent support for roadwarriors. Consider the setup in the following diagram:

On the gateway system (System A), we need a zone to represent the remote clients — we'll call that zone “road”.

/etc/shorewall/zones — System A:

#ZONE   TYPE   OPTIONS                 IN_OPTIONS              OUT_OPTIONS
road    ipv4

On system A, the remote clients will comprise the road zone.

In /etc/shorewall/interfaces on system A:
#ZONE      INTERFACE        OPTIONS
road       tun+

In /etc/shorewall/tunnels on system A, we need the following:

#TYPE         ZONE           GATEWAY        GATEWAY_ZONE
openvpn:1194  net            0.0.0.0/0

If you are running Shorewall 2.4.3 or later, you might prefer the following in /etc/shorewall/tunnels on system A. Specifying the tunnel type as openvpnserver has the advantage that the VPN connection will still work if the client is behind a gateway/firewall that uses NAT.

#TYPE               ZONE           GATEWAY        GATEWAY_ZONE
openvpnserver:1194  net            0.0.0.0/0

We want the remote systems to have access to the local LAN — we do that with an entry in /etc/shorewall/policy (assume that the local LAN comprises the zone “loc”).

#SOURCE      DESTINATION        POLICY
road         loc                ACCEPT

The OpenVPN configuration file on system A is something like the following:

dev tun

server 192.168.2.0 255.255.255.0

dh dh1024.pem

ca /etc/certs/cacert.pem

crl-verify /etc/certs/crl.pem

cert /etc/certs/SystemA.pem
key /etc/certs/SystemA_key.pem

port 1194

comp-lzo

user nobody

group nogroup

ping 15
ping-restart 45
ping-timer-rem
persist-tun
persist-key

push "route 192.168.1.0 255.255.255.0"

verb 3

Configuration on the remote clients follows a similar line. We define a zone to represent the remote LAN:

/etc/shorewall/zones — System B:

#ZONE   TYPE   OPTIONS                 IN                      OUT
#                                      OPTIONS                 OPTIONS
home    ipv4

On system A, the hosts accessible through the tunnel will comprise the home zone.

In /etc/shorewall/interfaces on system B:
#ZONE      INTERFACE        BROADCAST     OPTIONS
home       tun0

In /etc/shorewall/tunnels on system B, we need the following:

#TYPE         ZONE           GATEWAY        GATEWAY_ZONE
openvpn:1194  net            206.162.148.9

Again, if you are running Shorewall 2.4.3 or later, in /etc/shorewall/tunnels on system B you might prefer:

#TYPE               ZONE           GATEWAY        GATEWAY_ZONE
openvpnclient:1194  net            206.162.148.9

We want the remote client to have access to the local LAN — we do that with an entry in /etc/shorewall/policy.

#SOURCE      DESTINATION        POLICY
$FW          home               ACCEPT

The OpenVPN configuration on the remote clients is along the following line:

dev tun
remote 206.162.148.9
up /etc/openvpn/home.up

tls-client
pull

ca /etc/certs/cacert.pem

cert /etc/certs/SystemB.pem
key /etc/certs/SystemB_key.pem

port 1194

user nobody
group nogroup

comp-lzo

ping 15
ping-restart 45
ping-timer-rem
persist-tun
persist-key

verb 3

If you want multiple remote clients to be able to communicate openly with each other then you must:

Include the client-to-client directive in the server's OpenVPN configuration; or
Specify the routeback option on the tun+ device in /etc/shorewall/interfaces.

Roadwarrior with Duplicate Network Issue

The information in this section was contributed by Nicola Moretti.

If your local lan uses a popular RFC 1918 network like 192.168.1.0/24, there will be times when your roadwarriors need to access your lan from a remote location that uses that same network.

This may be accomplished by configuring a second server on your firewall that uses a different port and by using NETMAP in your Shorewall configuration. The server configuration in the above diagram is modified as shown here:

dev tun

server 192.168.3.0 255.255.255.0

dh dh1024.pem

ca /etc/certs/cacert.pem

crl-verify /etc/certs/crl.pem

cert /etc/certs/SystemA.pem
key /etc/certs/SystemA_key.pem

port 1195

comp-lzo

user nobody

group nogroup

ping 15
ping-restart 45
ping-timer-rem
persist-tun
persist-key

push "route 172.20.1.0 255.255.255.0"

verb 3

In /etc/shorewall/netmap, put these entries:

#TYPE	NET1			INTERFACE	NET2
SNAT	192.168.1.0/24		tun1		172.20.1.0/24
DNAT	172.20.1.0/24		tun1		192.168.1.0/24

The roadwarrior can now connect to port 1195 and access the lan on the right as 172.20.1.0/24.

Roadwarrior with IPv6

While OpenVPN supports tunneling of IPv6 packets, the version of the code that I run under OS X on my Macbook Pro does not support that option. Nevertheless, I am able to take IPv6 on the road with me by creating a 6to4 tunnel through the OpenVPN IPv6 tunnel. In this configuration, the IPv4 address pair (172.20.0.10,172.20.0.11) is used for the OpenVPN tunnel and (2001:470:e857:2::1,2001:470:e857:2::2) is used for the 6to4 tunnel.

Here are my config files:

Server (conventional routed server config):

dev tun

local 70.90.191.121

server 172.20.0.0 255.255.255.128

dh dh1024.pem

ca /etc/certs/cacert.pem

crl-verify /etc/certs/crl.pem

cert /etc/certs/gateway.pem
key /etc/certs/gateway_key.pem

port 1194

comp-lzo

user nobody
group nogroup

keepalive 15 45
ping-timer-rem
persist-tun
persist-key

client-config-dir /etc/openvpn/clients
ccd-exclusive
client-to-client

push "route 172.20.1.0 255.255.255.0"

verb 3
In the CCD file for the Macbook Pro:
ifconfig-push 172.20.0.11 172.20.0.10
From /etc/network/interfaces (very standard 6to4 tunnel configuration):
auto mac
iface mac inet6 v4tunnel
      address 2001:470:e857:2::1
      netmask 64
      endpoint 172.20.0.11
      local 172.20.1.254
Note that while the remote endpoint (172.20.0.11) is also the remote endpoint of the OpenVPN tunnel, the local endpoint (172.20.1.254) of the 6to4 tunnel is not the local endpoint of the OpenVPN tunnel (that;s 172.20.0.10). 172.20.1.254 is the IPv4 address of the Shorewall firewall's LAN interface.
The following excerpts from the Shorewall configuration show the parts of that configuration that are relevant to these two tunnels (bold font). This is not a complete configuration.
/etc/shorewall/zones:
#ZONE           TYPE
fw              firewall
loc             ip              #Local Zone
drct:loc        ipv4            #Direct internet access
net             ipv4            #Internet
vpn             ipv4            #OpenVPN clients
/etc/shorewall/interfaces:
#ZONE  INTERFACE  BROADCAST OPTIONS
loc    INT_IF     detect    dhcp,logmartians=1,routefilter=1,physical=$INT_IF,required,wait=5
net    COM_IF     detect    dhcp,blacklist,optional,routefilter=0,logmartians,proxyarp=0,physical=$COM_IF,nosmurfs
vpn    TUN_IF+    detect    physical=tun+,routeback
-      sit1       -         ignore
-      mac        -         ignore
-      EXT_IF     -         ignore
-      lo         -         ignore
/etc/shorewall/tunnels:
#TYPE                   ZONE    GATEWAY         GATEWAY
#                                               ZONE
openvpnserver:udp       net
6to4                    net
6to4                    vpn
Similarly, here are excerpts from the Shorewall6 configuration.
/etc/shorewall6/zones:
#ZONE     TYPE     OPTIONS        IN            OUT
#                                 OPTIONS       OPTIONS
fw        firewall
net       ipv6
loc       ipv6
rest      ipv6
/etc/shorewall6/interfaces:
#ZONE   INTERFACE       BROADCAST       OPTIONS
net     sit1            detect          tcpflags,forward=1,nosmurfs,routeback
loc     eth4            detect          tcpflags,forward=1
loc     mac             detect          tcpflags,forward=1
rest    eth+
Note that in the IPv6 firewall configuration, the remove Macbook Pro is considered to be part of the local zone (loc).

Client (conventional routed client config):

client

dev tun

proto udp

remote gateway.shorewall.net 1194

resolv-retry infinite

nobind

persist-key
persist-tun

mute-replay-warnings

ca ca.crt
cert mac.crt
key mac.key

ns-cert-type server

comp-lzo

verb 3

up /Users/teastep/bin/up
down /Users/teastep/bin/down

/Users/teastep/bin/up:

#!/bin/bash
LOCAL_IP=172.20.0.11
LOCAL_IPV6=2001:470:e857:2::2
REMOTE_IP=172.20.1.254
REMOTE_IPV6=2001:470:e857:2::1
TUNNEL_IF=gif0

if [ $(ifconfig gif0 | wc -l ) -eq 1 ]; then
    #
    # Tunnel interface is not configured yet
    #
    /sbin/ifconfig $TUNNEL_IF tunnel $LOCAL_IP $REMOTE_IP
    /sbin/ifconfig $TUNNEL_IF inet6 $LOCAL_IPV6 $REMOTE_IPV6 prefixlen 128
else
    /sbin/ifconfig $TUNNEL_IF up
fi

/sbin/route -n add -inet6 default $REMOTE_IPV6 > /dev/null 2>&1

/Users/teastep/bin/down:

#!/bin/bash

TUNNEL_IF=gif0

/sbin/ifconfig $TUNNEL_IF down
/sbin/route -n delete -inet6 default > /dev/null 2>&1

Bridged Roadwarrior

If you want to use a bridged OpenVPN configuration rather than a routed configuration, then follow any of the available HOWTOs to set up the bridged configuration. Then:

In your current Shorewall two-interface configuration, replace references to your internal interface with the name of the bridge; and
Set the routeback option in the bridge's entry in /etc/shorewall/interfaces; end

Add this entry to /etc/shorewall/tunnels:

#TYPE               ZONE           GATEWAY        GATEWAY_ZONE
openvpnserver:1194  net            0.0.0.0/0

This will make the roadwarrior part of your local zone.

Bridging Two Networks

Occasionally, the need arises to have a single LAN span two different geographical locations. OpenVPN allows that to be done easily.

Consider the following case:

Part of the 192.168.1.0/24 network is in one location and part in another. The two LANs can be bridged with OpenVPN as described in this section. This example uses a fixed shared key for encryption.

OpenVPN configuration on left-hand firewall:

remote 130.252.100.109
dev tap0
secret /etc/openvpn/bridgekey

OpenVPN configuration on right-hand firewall:

remote 206.124.146.176
dev tap0
secret /etc/openvpn/bridgekey

The bridges can be created by manually making the tap device tap0 and bridgeing it with the local ethernet interface. Assuming that the local interface on both sides is eth1, the following stanzas in /etc/network/interfaces (Debian and derivatives) will create the bridged interfaces.

Note

The stanzas below were written before bridges could be defined in /etc/network/interfaces. For current usage, see bridge-utils-interfaces (5).

/etc/network/interfaces on the left-hand firewall:

iface br0 inet static
      pre-up /usr/sbin/openvpn --mktun --dev tap0
      pre-up /usr/sbin/brctl addbr br0
      address 192.168.1.254
      network 192.168.1.0
      broadcast 192.168.1.255
      netmask 255.255.255.0
      post-up /sbin/ip link set tap0 up
      post-up /usr/sbin/brctl addif br0 tap0
      post-up /sbin/ip link set eth1 up
      post-up /usr/sbin/brctl addif br0 eth1
      post-down /usr/sbin/brctl delbr br0
      post-down /usr/sbin/openvpn --rmtun --dev tap0
      post-down /sbin/ip link set eth1 down

/etc/network/interfaces on the right-hand firewall:

iface br0 inet static
      pre-up /usr/sbin/openvpn --mktun --dev tap0
      pre-up /usr/sbin/brctl addbr br0
      address 192.168.1.253
      network 192.168.1.0
      broadcast 192.168.1.255
      netmask 255.255.255.0
      post-up /sbin/ip link set tap0 up
      post-up /usr/sbin/brctl addif br0 tap0
      post-up /sbin/ip link set eth1 up
      post-up /usr/sbin/brctl addif br0 eth1
      post-down /usr/sbin/brctl delbr br0
      post-down /usr/sbin/openvpn --rmtun --dev tap0
      post-down /sbin/ip link set eth1 down

The Shorewall configuration is just a Simple Bridge.