OpenVPN (212.5)

Candidates should be able to configure a VPN (Virtual Private Network) and create secure point-to-point or site-to-site connections.

References: OpenVPN

Key Knowledge Areas:

OpenVPN

Terms and Utilities:

  • /etc/openvpn/

  • openvpn

OpenVPN

OpenVPN is a free and open source software application that implements virtual private network (VPN) techniques for creating secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities. It uses SSL/TLS security for encryption and is capable of traversing network address translators (NATs) and firewalls.

OpenVPN allows peers to authenticate each other using a pre-shared secret key, certificates, or username/password. When used in a multiclient-server configuration, it allows the server to release an authentication certificate for every client, using signature and Certificate authority. It uses the OpenSSL encryption library extensively, as well as the SSLv3/TLSv1 protocol, and contains many security and control features.

Installing

OpenVPN is available on almost any modern operating system and can be built from source or installed as a pre-built package.

OpenVPN is not compatible with IPsec or any other VPN package. The entire package consists of one binary for both client and server connections, an optional configuration file, and one or more key files depending on the authentication method used.

openvpn options

OpenVPN allows any option to be placed either on the command line or in a configuration file. Though all command line options are preceded by a double-leading-dash (--), this prefix can be removed when an option is placed in a configuration file.

--config file

Load additional config options from file where each line corresponds to one command line option, but with the leading "--" removed.

-dev tunX|tapX|null

TUN/TAP virtual network device (X can be omitted for a dynamic device.).

---nobind bits

Do not bind to local address and port. The IP stack will allocate a dynamic port for returning packets. Since the value of the dynamic port could not be known in advance by a peer, this option is only suitable for peers which will be initiating connections by using the --remote option.

--ifconfig l rn connection_spec

Set TUN/TAP parameters. l is the IP address of the local VPN endpoint. For TUN devices, rn is the IP address of the remote VPN endpoint. For TAP devices, rn is the subnet mask of the virtual ethernet segment which is being created or connected to.

secret file [direction]

Enable Static Key encryption mode (non-TLS). Use pre-shared secret file which was generated with --genkey

Configuration

Simple point-to-point example

This example uses static keys for authentication. This is a very simple setup, ideal for point-to-point networking. In the following example the tun interfaces will be used. Another possibility would be to use the tap interfaces but then the configuration would also be a little bit different. See the man pages for more information about using these interfaces.

A VPN tunnel will be created with a server endpoint of 10.10.10.10 and a client endpoint of 10.10.10.11. The public ipaddress of the server is referenced by vpnserver.example.com. The communication between these endpoints will be encrypted and occur over the default OpenVPN port 1194.

To setup this example a key has to be created: openvpn --genkey --secret static.key. Copy this key (static.key) to both client and server.

Server configuration file (server.conf):

	dev tun
	ifconfig 10.10.10.10 10.10.10.11
	keepalive 10 60
	ping-timer-rem
	persist-tun
	persist-key
	secret static.key
					

Client configuration file (client.conf):

	remote vpnserver.example.com
	dev tun 
	ifconfig 10.10.10.11 10.10.10.10
	keepalive 10 60
	ping-timer-rem
	persist-tun
	persist-key
	secret static.key
					

Start the vpn on the server by running openvpn server.conf and running openvpn client.conf on the client.

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