VPN between a Raspberry Pi & Linux (Ubuntu) Server

If you want more than just pre-shared keys OpenVPN makes it easy to setup and use a Public Key Infrastructure (PKI) to use SSL/TLS certificates for authentication and key exchange between the VPN server and clients. OpenVPN can be used in a routed or bridged VPN mode and can be configured to use either UDP or TCP. The port number can be configured as well, but port 1194 is the official one. And it is only using that single port for all communication. VPN client implementations are available for almost anything including all Linux distributions, OS X, Windows and OpenWRT based WLAN routers. If the Server is behind a firewall (home router) you need to make a port forward on external port 1194 to your server inside the LAN.

1 Server Installation
2 Public Key Infrastructure Setup
3 Certificate Authority Setup
4 Server Certificates
5 Client Certificates
6 Simple Server Configuration
7 Simple Client Configuration

Server Installation

To install openvpn in a terminal enter:
sudo apt-get install openvpn

Public Key Infrastructure Setup

The first step in building an OpenVPN configuration is to establish a PKI (public key infrastructure). The PKI consists of:
a separate certificate (also known as a public key) and private key for the server and each client, and
- a master Certificate Authority (CA) certificate and key which is used to sign each of the server and client certificates.
OpenVPN supports bidirectional authentication based on certificates, meaning that the client must authenticate the server certificate and the server must authenticate the client certificate before mutual trust is established.
Both server and client will authenticate the other by first verifying that the presented certificate was signed by the master certificate authority (CA), and then by testing information in the now-authenticated certificate header, such as the certificate common name or certificate type (client or server).

Certificate Authority Setup

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_CITY="Bern"
export KEY_ORG="your-company"
export KEY_EMAIL="steve@example.com"

Enter the following to generate the master Certificate Authority (CA) certificate and key:
cd /etc/openvpn/easy-rsa/
source vars

If you get an error message here you probably have to create a link from openssl.cnf >>> openssl-1.0.0.cnf

Server Certificates

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:
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/

Client Certificates

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.

Simple Server Configuration

Along with your OpenVPN installation you got these sample config files (and many more if if you check):
root@server:/# ls -l /usr/share/doc/openvpn/examples/sample-config-files/
total 68
-rw-r--r-- 1 root root 3427 2011-07-04 15:09 client.conf
-rw-r--r-- 1 root root 4141 2011-07-04 15:09 server.conf.gz

Start with copying and unpacking server.conf.gz to /etc/openvpn/server.conf.
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 to make sure the following lines are pointing to the certificates and keys you created in the section above.
ca ca.crt
cert myservername.crt
key myservername.key
dh dh1024.pem

That is the minimum you have to configure to get a working OpenVPN server. You can use all the default settings in the sample server.conf file. Now start the server. You will find logging and error messages in your syslog.

root@server:/etc/openvpn# /etc/init.d/openvpn start
* Starting virtual private network daemon(s)...
* Autostarting VPN 'server' [ OK ]

Now check if OpenVPN created a tun0 interface:

root@server:/etc/openvpn# ifconfig tun0
tun0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr: P-t-P: Mask:

Simple Client Configuration

There are various different OpenVPN client implementations with and without GUIs. You can read more about clients in a later section. For now we use the OpenVPN client for Ubuntu which is the same executable as the server. So you have to install the openvpn package again on the client machine:
sudo apt-get install openvpn
This time copy the client.conf sample config file to /etc/openvpn/.
sudo cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf /etc/openvpn/
Copy the client keys and the certificate of the CA you created in the section above to e.g. /etc/openvpn/ and edit /etc/openvpn/client.conf to make sure the following lines are pointing to those files. If you have the files in /etc/openvpn/ you can omit the path.
ca ca.crt
cert client1.crt
key client1.key

And you have to at least specify the OpenVPN server name or address. Make sure the keyword client is in the config. That's what enables client mode.
remote vpnserver.example.com 1194

Now start the OpenVPN client:

root@client:/etc/openvpn# /etc/init.d/openvpn start
* Starting virtual private network daemon(s)...
* Autostarting VPN 'client' [ OK ]

Check if it created a tun0 interface:

root@client:/etc/openvpn# ifconfig tun0
tun0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr: P-t-P: Mask:

Check if you can ping the OpenVPN server:

root@client:/etc/openvpn# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_req=1 ttl=64 time=0.920 ms

Check out your routes:

root@client:/etc/openvpn# netstat -rn
Kernel IP routing table
Destination ...Gateway .......Genmask............... Flags ...MSS ...Window ...irtt ...Iface ........ .......... ...UH .......0 .........0 .............0..... tun0 ........ ........ ...UGH..... 0 .........0 .............0 .....tun0
192.168.42....0 ....... .......U ..........0 .........0 .............0 .....eth0 ......... . ..................UG .......0 .........0............. 0 .....eth0