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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”.

2006-04-21


Table of Contents

Preliminary Reading
Bridging two Masqueraded Networks
Roadwarrior
Securing a Home Wireless Network with OpenVPN (OpenVPN Bridge)
Configuring the Bridge
Configuring OpenVPN
Firewall (Server) configuration.
Tipper Configuration
Eastepnc6000 (Windows XP) Configuration
Eastepnc6000 (SUSE 10.0) Configuration
Configuring Shorewall
Firewall
Tipper

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.

  1. It is widely supported -- I run it on both Linux and Windows XP.

  2. It requires no kernel patching.

  3. It is very easy to configure.

  4. 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                      OUT
#                                      OPTIONS                 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        BROADCAST     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                      OUT
#                                      OPTIONS                 OPTIONS
road    ipv4

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

In /etc/shorewall/interfaces on system A:

#ZONE      INTERFACE        BROADCAST     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

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 in 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:

  1. Include the client-to-client directive in the server's OpenVPN configuration; and

  2. Specify the routeback option on the tun+ device in /etc/shorewall/interfaces.

If you want to selectively allow communication between the clients, then see this article by Marc Zonzon

Securing a Home Wireless Network with OpenVPN (OpenVPN Bridge)

This section will describe how we once secured our home wireless network using OpenVPN. Our network as it was then[1] is as shown in the following diagram.

The Wireless network is in the lower right of the diagram and consists of two laptops: Eastepnc6000 (Dual Boot Windows XP - SP1, SUSE 10.0) and Tipper (SUSE 10.0). We used OpenVPN to bridge those two laptops with the local LAN shown in the lower left hand corner. The laptops were configured with addresses in the 192.168.3.0/24 network connected to the firewall's eth0 interface which places them in the firewall's Wifi zone. OpenVPN bridging allowed them to be assigned an additional IP address from the 192.168.1.0/24 network and to be securely bridged to the LAN on the lower left.

Note

Eastepnc6000 is shown in both the local LAN and in the Wifi zone with IP address 192.168.1.6 -- clearly, the computer could only be in one place or the other. Tipper could also be in either place and would have the IP address 192.168.1.8 regardless.

Configuring the Bridge

The firewall ran Debian Sarge so the bridge was defined in /etc/network/interfaces.

# LAN interface
auto br0
iface br0 inet static
        address 192.168.1.254
        netmask 255.255.255.0
        pre-up /usr/sbin/openvpn --mktun --dev tap0
        pre-up /sbin/ip link set tap0 up
        pre-up /sbin/ip link set eth3 up
        pre-up /usr/sbin/brctl addbr br0
        pre-up /usr/sbin/brctl addif br0 eth3
        pre-up /usr/sbin/brctl addif br0 tap0
        post-down /usr/sbin/brctl delif br0 eth3
        post-down /usr/sbin/brctl delif br0 tap0
        post-down /usr/sbin/brctl delbr br0
        post-down /usr/sbin/openvpn --rmtun --dev tap0

Note that the IP address assigned to the bridge is 192.168.1.254 -- that was the default gateway address for hosts in the local zone.

Configuring OpenVPN

We used X.509 certificates for authentication.

Firewall (Server) configuration.

/etc/openvpn/server-bridge.conf defined a bridge and reserved IP addresses 192.168.1.64-192.168.1.71 for VPN clients. Note that the bridge server only used local IP address 192.168.3.254. We ran two instances of OpenVPN; this one and a second tunnel-mode instance for remote access (see this article).

dev tap0

local 192.168.3.254

server-bridge 192.168.1.254 255.255.255.0 192.168.1.64 192.168.1.71

client-to-client

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/bridge-clients
ccd-exclusive

verb 3

The files in /etc/openvpn/bridge-clients were used to assign a fixed IP address to each laptop. For example, tipper.shorewall.net:

ifconfig-push 192.168.1.8 255.255.255.0

Tipper Configuration

/etc/openvpn/wireless.conf:

dev tap

remote 192.168.3.254
tls-remote gateway.shorewall.net

client

redirect-gateway

ca /etc/certs/cacert.pem

cert /etc/certs/tipper.pem
key /etc/certs/tipper_key.pem

port 1194

comp-lzo

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

mute-replay-warnings

verb 3

Eastepnc6000 (Windows XP) Configuration

C:\Program Files\Openvpn\config\homewireless.ovpn:

dev tap
remote 192.168.3.254
tls-remote gateway.shorewall.net

tls-client
pull

ca "/Program Files/OpenVPN/certs/cacert.pem"

cert "/Program Files/OpenVPN/certs/eastepnc6000.pem"
key "/Program Files/OpenVPN/certs/eastepnc6000_key.pem"

redirect-gateway

port 1194

comp-lzo

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

verb 3

Eastepnc6000 (SUSE 10.0) Configuration

The configuration was the same as shown above only with "/Program Files/OpenVPN" replaced with "/etc/openvpn" (I love OpenVPN).

Configuring Shorewall

In this configuration, we didn't need any firewalling between the laptops and the local LAN so we set BRIDGING=No in shorewall.conf. The configuration of the bridge then became as described in the Simple Bridge documentation. If you need to control the traffic allowed through the VPN bridge then you will want to configure Shorewall as shown in the Bridge/Firewall documentation.

Firewall

/etc/shorewall/interfaces

Note that the bridge (br0) is defined as the interface to the local zone and has the routeback option.

#ZONE   INTERFACE       BROADCAST               OPTIONS
net     eth2            206.124.146.255         dhcp,norfc1918,logmartians,blacklist,tcpflags,nosmurfs
loc     br0             192.168.1.255           dhcp,routeback
dmz     eth1            -                       logmartians
Wifi    eth0            192.168.3.255           dhcp,maclist
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
/etc/shorewall/tunnels
#TYPE                   ZONE    GATEWAY         GATEWAY
#                                               ZONE
openvpnserver:1194      Wifi    192.168.3.0/24
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE

Tipper

Wireless networks pose a threat to all systems that are connected to them and we therefore ran Firewalls on the two Laptops. Eastepnc6000 ran Sygate™ Security Agent and Tipper ran a Shorewall-based Netfilter firewall.

/etc/shorewall/zones
#ZONE   TYPE    OPTIONS                 IN                      OUT
#                                       OPTIONS                 OPTIONS
lan     ipv4    #Wired LAN at our home
net     ipv4
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE
/etc/shorewall/interfaces
#ZONE    INTERFACE      BROADCAST       OPTIONS
#
net      eth0           detect          routefilter,dhcp,tcpflags
lan      tap0           192.168.1.255
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
/etc/shorewall/policy

Since we didn't expect any traffic between the net zone and the lan zone, we used NONE policies for that traffic. If any such traffic would have occurred, it would have been handled according to the all->all policy.

#SOURCE         DEST            POLICY          LOG             LIMIT:BURST
#                                               LEVEL
fw              net             ACCEPT
fw              lan             ACCEPT
lan             fw              ACCEPT
net             lan             NONE
lan             net             NONE
net             all             DROP            info
# The FOLLOWING POLICY MUST BE LAST
all             all             REJECT          info
#LAST LINE -- DO NOT REMOVE


[1] Our current network uses a similar technique -- see the Xen My Way article.