sudo apt-get install openvpn

Now that the openvpn package is installed, the certificates for the VPN server need to be created.

First, copy the easy-rsa directory to /etc/openvpn. This will ensure that any changes to the scripts will not be lost when the package is updated. From a terminal enter:

sudo mkdir /etc/openvpn/easy-rsa/
sudo cp -r /usr/share/doc/openvpn/examples/easy-rsa/2.0/ /etc/openvpn/

Next, edit /etc/openvpn/easy-rsa/vars adjusting the following to your environment:

export KEY_COUNTRY="US"
export KEY_PROVINCE="NC"
export KEY_CITY="Winston-Salem"
export KEY_ORG="Example Company"
export KEY_EMAIL="steve@example.com"

Enter the following to create the server certificates:

cd /etc/openvpn/easy-rsa/easy-rsa
source vars
./clean-all
./build-dh
./pkitool --initca
./pkitool --server server
cd keys
openvpn --genkey --secret ta.key
sudo cp server.crt server.key ca.crt dh1024.pem ta.key /etc/openvpn/

The VPN client will also need a certificate to authenticate itself to the server. To create the certificate, enter the following in a terminal:

cd /etc/openvpn/easy-rsa/
source vars
./pkitool hostname

Copy the following files to the client:

It is best to use a secure method to copy the certificate and key files. The scp utility is a good choice, but copying the files to removable media then to the client, also works well.

Now configure the openvpn server by creating /etc/openvpn/server.conf from the example file. In a terminal enter:

sudo cp /usr/share/doc/openvpn/examples/sample-config-files/server.conf.gz /etc/openvpn/
sudo gzip -d /etc/openvpn/server.conf.gz

Edit etc/openvpn/server.conf changing the following options to:

local 172.18.100.101
dev tap0
server-bridge 172.18.100.101 255.255.255.0 172.18.100.105 172.18.100.200
push "route 172.18.100.1 255.255.255.0"
push "dhcp-option DNS 172.18.100.20"
push "dhcp-option DOMAIN example.com"
tls-auth ta.key 0 # This file is secret
user nobody
group nogroup

Next, create a couple of helper scripts to add the tap interface to the bridge. Create /etc/openvpn/up.sh:

#!/bin/sh

BR=$1
DEV=$2
MTU=$3
/sbin/ifconfig $DEV mtu $MTU promisc up
/usr/sbin/brctl addif $BR $DEV

And /etc/openvpn/down.sh:

#!/bin/sh

BR=$1
DEV=$2

/usr/sbin/brctl delif $BR $DEV
/sbin/ifconfig $DEV down

Then make them executable:

sudo chmod 755 /etc/openvpn/down.sh
sudo chmod 755 /etc/openvpn/up.sh

After configuring the server, restart openvpn by entering:

sudo /etc/init.d/openvpn restart

With the server configured and the client certificates copied over, create a client configuration file by copying the example. In a terminal on the client machine enter:

sudo cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf /etc/openvpn

Now edit /etc/openvpn/client.conf changing the following options:

dev tap
remote vpn.example.com 1194
cert hostname.crt
key hostname.key
tls-auth ta.key 1

Finally, restart openvpn:

sudo /etc/init.d/openvpn restart

You should now be able to connect to the remote LAN through the VPN.