OpenVPN Tunnels and Bridges
Simon
Matter
Tom
Eastep
2003
2004
2005
2006
Simon Mater
Thomas M. 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
.
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
XP.
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 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
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.
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.
/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 br1
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/tunctl -d 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 br1
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/tunctl -d tap0
post-down /sbin/ip link set eth1 down
The Shorewall configuration is just a Simple Bridge.