To setup your own Certificate Authority (CA) and generating certificates and keys for an OpenVPN server and multiple clients 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 change to user root and:

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

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="[email protected]"

Enter the following to generate the master Certificate Authority (CA) certificate and key:

cd /etc/openvpn/easy-rsa/
source vars
./clean-all
./build-ca

Next, we will generate a certificate and private key for the server:

./build-key-server myservername

As in the previous step, most parameters can be defaulted. Two other queries require positive responses, "Sign the certificate? [y/n]" and "1 out of 1 certificate requests certified, commit? [y/n]".

Diffie Hellman parameters must be generated for the OpenVPN server:

./build-dh

All certificates and keys have been generated in the subdirectory keys/. Common practice is to copy them to /etc/openvpn/:

cd keys/
cp myservername.crt myservername.key ca.crt dh1024.pem /etc/openvpn/

The VPN client will also need a certificate to authenticate itself to the server. Usually you create a different certificate for each client. To create the certificate, enter the following in a terminal while being user root:

cd /etc/openvpn/easy-rsa/
source vars
./build-key client1

Copy the following files to the client using a secure method:

As the client certificates and keys are only required on the client machine, you should remove them from the server.

The above is a very simple working VPN. The client can access services on the VPN server machine through an encrypted tunnel. If you want to reach more servers or anything in other networks, push some routes to the clients. E.g. if your company's network can be summarized to the network 192.168.0.0/16, you could push this route to the clients. But you will also have to change the routing for the way back - your servers need to know a route to the VPN client-network.

Or you might push a default gateway to all the clients to send all their internet traffic to the VPN gateway first and from there via the company firewall into the internet. This section shows you some possible options.

Push routes to the client to allow it to reach other private subnets behind the server. Remember that these private subnets will also need to know to route the OpenVPN client address pool (10.8.0.0/24) back to the OpenVPN server.

push "route 10.0.0.0 255.0.0.0"

If enabled, this directive will configure all clients to redirect their default network gateway through the VPN, causing all IP traffic such as web browsing and and DNS lookups to go through the VPN (the OpenVPN server machine or your central firewall may need to NAT the TUN/TAP interface to the internet in order for this to work properly).

push "redirect-gateway def1 bypass-dhcp"

Configure server mode and supply a VPN subnet for OpenVPN to draw client addresses from. The server will take 10.8.0.1 for itself, the rest will be made available to clients. Each client will be able to reach the server on 10.8.0.1. Comment this line out if you are ethernet bridging.

server 10.8.0.0 255.255.255.0

Maintain a record of client to virtual IP address associations in this file. If OpenVPN goes down or is restarted, reconnecting clients can be assigned the same virtual IP address from the pool that was previously assigned.

ifconfig-pool-persist ipp.txt

Push DNS servers to the client.

push "dhcp-option DNS 10.0.0.2"
push "dhcp-option DNS 10.1.0.2"

Allow client to client communication.

client-to-client

Enable compression on the VPN link.

comp-lzo

The keepalive directive causes ping-like messages to be sent back and forth over the link so that each side knows when the other side has gone down. Ping every 1 second, assume that remote peer is down if no ping received during a 3 second time period.

keepalive 1 3

It's a good idea to reduce the OpenVPN daemon's privileges after initialization.

user nobody
group nogroup

OpenVPN 2.0 includes a feature that allows the OpenVPN server to securely obtain a username and password from a connecting client, and to use that information as a basis for authenticating the client. To use this authentication method, first add the auth-user-pass directive to the client configuration. It will direct the OpenVPN client to query the user for a username/password, passing it on to the server over the secure TLS channel.

# client config!
auth-user-pass

This will tell the OpenVPN server to validate the username/password entered by clients using the login PAM module. Useful if you have centralized authentication with e.g. Kerberos.

plugin /usr/lib/openvpn/openvpn-auth-pam.so login

OpenVPN can be setup for either a routed or a bridged VPN mode. Sometimes this is also referred to as OSI layer-2 versus layer-3 VPN. In a bridged VPN all layer-2 frames - e.g. all ethernet frames - are sent to the VPN partners and in a routed VPN only layer-3 packets are sent to VPN partners. In bridged mode all traffic including traffic which was traditionally LAN-local like local network broadcasts, DHCP requests, ARP requests etc. are sent to VPN partners whereas in routed mode this would be filtered.

Many Linux distributions including Ubuntu desktop variants come with Network Manager, a nice GUI to configure your network settings. It also can manage your VPN connections. Make sure you have package network-manager-openvpn installed. Here you see that the installation installs all other required packages as well:

root@client:~# apt-get install network-manager-openvpn
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following extra packages will be installed:
  liblzo2-2 libpkcs11-helper1 network-manager-openvpn-gnome openvpn
Suggested packages:
  resolvconf
The following NEW packages will be installed:
  liblzo2-2 libpkcs11-helper1 network-manager-openvpn
  network-manager-openvpn-gnome openvpn
0 upgraded, 5 newly installed, 0 to remove and 631 not upgraded.
Need to get 700 kB of archives.
After this operation, 3,031 kB of additional disk space will be used.
Do you want to continue [Y/n]? 

To inform network-manager about the new installed packages you will have to restart it:

root@client:~# restart network-manager 
network-manager start/running, process 3078

Open the Network Manager GUI, select the VPN tab and then the 'Add' button. Select OpenVPN as the VPN type in the opening requester and press 'Create'. In the next window add the OpenVPN's server name as the 'Gateway', set 'Type' to 'Certificates (TLS)', point 'User Certificate' to your user certificate, 'CA Certificate' to your CA certificate and 'Private Key' to your private key file. Use the advanced button to enable compression or other special settings you set on the server. Now try to establish your VPN.

Tunnelblick is an excellent free, open source implementation of a GUI for OpenVPN for OS X. The project's homepage is at http://code.google.com/p/tunnelblick/. Download the latest OS X installer from there and install it. Then put your client.ovpn config file together with the certificates and keys in /Users/username/Library/Application Support/Tunnelblick/Configurations/ and lauch Tunnelblick from your Application folder.

# sample client.ovpn for Tunnelblick
client
remote blue.example.com
port 1194
proto udp
dev tun
dev-type tun
ns-cert-type server
reneg-sec 86400
auth-user-pass
auth-nocache
auth-retry interact
comp-lzo yes
verb 3
ca ca.crt
cert client.crt
key client.key

First download and install the latest OpenVPN Windows Installer. OpenVPN 2.2.1 was the latest when this was written. Additionally download an alternative Open VPN Windows GUI. The OpenVPN MI GUI from http://openvpn-mi-gui.inside-security.de seems to be a nice one for Windows 7. Download the latest version. 20110624 was the latest version when this was written.

You need to start the OpenVPN service. Goto Start > Computer > Manage > Services and Applications > Services. Find the OpenVPN service and start it. Set it's startup type to automatic. When you start the OpenVPN MI GUI the first time you need to run it as an administrator. You have to right click on it and you will see that option.

You will have to write your OpenVPN config in a textfile and place it in C:\Program Files\OpenVPN\config\client.ovpn along with the CA certificate. You could put the user certificate in the user's home directory like in the follwing example.

# C:\Program Files\OpenVPN\config\client.ovpn
client
remote server.example.com
port 1194
proto udp
dev tun
dev-type tun
ns-cert-type server
reneg-sec 86400
auth-user-pass
auth-retry interact
comp-lzo yes
verb 3
ca ca.crt
cert "C:\\Users\\username\\My Documents\\openvpn\\client.crt"
key "C:\\Users\\username\\My Documents\\openvpn\\client.key"
management 127.0.0.1 1194
management-hold
management-query-passwords
auth-retry interact

OpenWRT is described as a Linux distribution for embedded devices like WLAN router. There are certain types of WLAN routers who can be flashed to run OpenWRT. Depending on the available memory on your OpenWRT router you can run software like OpenVPN and you could for example build a small inexpensive branch office router with VPN connectivity to the central office. More info on OpenVPN on OpenWRT is here. And here is the OpenWRT project's homepage: http://openwrt.org

Log into your OpenWRT router and install OpenVPN:

opkg update
opkg install openvpn

Check out /etc/config/openvpn and put you client config in there. Copy certificated and keys to /etc/openvpn/

config openvpn client1
        option enable 1                                  
        option client 1                                  
#       option dev tap                                   
        option dev tun  
        option proto udp   
        option ca /etc/openvpn/ca.crt                
        option cert /etc/openvpn/client.crt
        option key /etc/openvpn/client.key
        option comp_lzo 1  

Restart OpenVPN:

service openvpn restart

You will have to see if you need to adjust your router's routing and firewall rules.