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Simple Overview of OpenVPN

OpenVPN: Simple Overview

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. Open VPN 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.

Server Installation

To install open vpn 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 theeasy-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="Winston-Salem"
export KEY_ORG="Example Company"
export KEY_EMAIL=""

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

cd /etc/openvpn/easy-rsa/
source vars

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 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:

  • /etc/openvpn/ca.crt
  • /etc/openvpn/easy-rsa/keys/client1.crt
  • /etc/openvpn/easy-rsa/keys/client1.key

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 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 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.confto 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 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

The OpenVPN server always uses the first usable IP address in the client network and only that IP is pingable. E.g. if you configured a /24 for the client network mask, the .1 address will be used. The P-t-P address you see in the ifconfig output above is usually not answering ping requests.

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   U         0 0          0 eth0         UG        0 0          0 eth0

First trouble shooting

If the above didn’t work for you, check this:

  • Check your syslog, e.g. grep -i vpn /var/log/syslog
  • Can the client connect to the server machine? Maybe a firewall is blocking access? Check syslog on server.
  • Client and server must use same protocol and port, e.g. UDP port 1194, see port and proto config option
  • Client and server must use same config regarding compression, see comp-lzo config option
  • Client and server must use same config regarding bridged vs routed mode, see server vs server-bridge config option

Advanced configuration


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, 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 ( back to the OpenVPN server.

push "route"

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 for itself, the rest will be made available to clients. Each client will be able to reach the server on Comment this line out if you are ethernet bridging.


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"
push "dhcp-option DNS"

Allow client to client communication.


Enable compression on the VPN link.


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!

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/ login

Please read the OpenVPN hardening security guide for further security advice.


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.


Make sure you have the bridge-utils package installed:

sudo apt-get install bridge-utils

Before you setup OpenVPN in bridged mode you need to change your interface configuration. Let’s assume your server has an interface eth0 connected to the internet and an interface eth1 connected to the LAN you want to bridge. Your /etc/network/interfaces would like this:

auto eth0
iface eth0 inet static

auto eth1
iface eth1 inet static

This straight forward interface config needs to be changed into a bridged mode like where the config of interface eth1 moves to the new br0 interface. Plus we configure that br0 should bridge interface eth1. We also need to make sure that interface eth1 is always in promiscuous mode – this tells the interface to forward all ethernet frames to the IP stack.

auto eth0
iface eth0 inet static

auto eth1
iface eth1 inet manual
  up ip link set $IFACE up promisc on

auto br0
iface br0 inet static
  bridge_ports eth1

At this point you need to restart networking. Be prepared that this might not work as expected and that you will lose remote connectivity. Make sure you can solve problems having local access.

sudo /etc/init.d/network restart


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

;dev tun
dev tap
up "/etc/openvpn/ br0 eth1"

Next, create a helper script to add the tap interface to the bridge and to ensure that eth1 is promiscuous mode. Create/etc/openvpn/



/sbin/ip link set "$TAPDEV" up
/sbin/ip link set "$ETHDEV" promisc on
/sbin/brctl addif $BR $TAPDEV

Then make it executable:

sudo chmod 755 /etc/openvpn/

After configuring the server, restart openvpn by entering:

sudo /etc/init.d/openvpn restart

Client Configuration

First, install openvpn on the client:

sudo apt-get install openvpn

Then with the server configured and the client certificates copied to the /etc/openvpn/ directory, 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
;dev tun

Finally, restart openvpn:

sudo /etc/init.d/openvpn restart

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

Client software implementations


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:
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 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
port 1194
proto udp
dev tun
dev-type tun
ns-cert-type server
reneg-sec 86400
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 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
port 1194
proto udp
dev tun
dev-type tun
ns-cert-type server
reneg-sec 86400
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 1194
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 Open VPN on OpenWRT is here. And here is the OpenWRT project’s homepage:

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 server:

/etc/init.d/openvpn restart

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


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